Fluids doi: 10.3390/fluids8060177

Authors: Daniel Costero Federico Piscaglia

In computations of unsteady flow problems by the arbitrary Lagrangian&ndash;Eulerian (ALE) method, the introduction of the grid velocity in the transport terms of the governing equations is not a sufficient condition for conservativeness if topology changes in the dynamic mesh are present and the number of mesh cells changes. We discuss an extension to second-order time differencing schemes (Implicit Euler and Crank&ndash;Nicolson) in the finite volume framework, to achieve second-order time-accuracy of the solution. Numerical experiments are given to illustrate the effectiveness of the presented method.

]]>Fluids doi: 10.3390/fluids8060176

Authors: Chen Xu Wei Zhang

This study investigated the diffusion impact on the chemical perturbation of NOx and O3 caused by the streamer and leader parts of a blue jet in the low stratosphere (18&ndash;30 km), using the coupling of a detailed stratospheric chemistry model and a typical diffusion model. The study found that diffusion significantly impacted the evolution of chemical perturbations at both short-term and long-term time scales after the blue jet discharge, with changes in NOx and O3 concentrations observed at different altitudes (18&ndash;28 km). At 18 km, the concentrations of NOx and N2O that account for diffusion start to decrease after 1 s, whereas those without diffusion remain at their peak concentrations. Meanwhile, O3 is slowly destroyed with less NOx, rather than dropping to an unrealistic low value immediately after the discharge without diffusion. The perturbation caused by the blue jet discharge disappears within a few tens of seconds at 18 km when molecular diffusion is considered. At 30 km, the chemical perturbation from four point sources was observed through changes in NO2 concentrations. However, the total concentration of NO2 perturbed by the streamer part discharge at the given surface was negligible when considering diffusion. Overall, this study provided a useful model tool for a more accurate assessment of the chemical effects of individual blue jets.

]]>Fluids doi: 10.3390/fluids8060175

Authors: Masih Zolghadr Seyed Mohammad Ali Zomorodian Abazar Fathi Ravi Prakash Tripathi Neda Jafari Darshan Mehta Parveen Sihag Hazi Mohammad Azamathulla

The causes of many bridge failures have been reported to be local scour around abutments. This study examines roughening elements as devices with which to intercept the downflow responsible for the formation of the principal vortex, which is what triggers local scour around abutments. Two vertical wall abutments with different widths were examined under four different hydraulic conditions in a clear-water regime. Elements with different thicknesses (t) and protrusions (P) with the same dimensions, (P = t = 0.05 L, 0.1 L, 0.2 L, and 0.3 L, where L is the length of the abutment) and with varying depths of installation (Z) were considered. Elements were installed in two positions: between the sediment surface and water elevation and buried within the sediment. To determine the optimum depth of installation, one element was first installed on the sediment surface, and the number of elements was increased in each subsequent test. The results show that installing elements between water surface elevation and the sediment&rsquo;s initial level did not show any defined trend on scour depth reduction. However, the optimum installation depth of the elements is 0.6&ndash;0.8 L below the initial bed level. Moreover, the roughening elements with thickness and protrusion of P = t = 0.2 L resulted in the most effective protection of the foundation. The best arrangement, (P = t = 0.2 L and Z = &gt;0.6&ndash;0.8 L) reduced the maximum scour depth by up to 30.4% and 32.8% for the abutment with smaller and larger widths, respectively.

]]>Fluids doi: 10.3390/fluids8060174

Authors: Yupeng Sun Hafiz Muhammad Adeel Hassan Joe Alexandersen

Stacked plate heat exchangers are widely used in thermal energy storage systems and a comprehensive and accurate analysis is necessary for their application and optimization. The fluid flow distribution between the plates is important to ensure even and full usage of the thermal energy storage potential. However, due to the complex topography of the plate surface, it would be computationally expensive to simulate the flow distribution in the multiple channels using a full three-dimensional model, so this work applies a reduced-dimensional model to significantly reduce the computational cost of the simulation and provides a comprehensive analysis of the effect of the internal structure on the internal flow distribution. The work extends a previously presented model to consider transient flow and a multichannel height distribution strategy to allow for simulating multiple channels between stacks of plates. Based on fully-developed flow assumptions, the three-dimensional model is reduced to a planar model, thus obtaining simulation results with satisfactory accuracy at a significantly lower computational cost. The model is verified by a three-dimensional simulation of a sliced two-channel model representing the considered system. The reduced-dimensional model gives similar results to the three-dimensional model for different geometrical and physical parameters. Lastly, the extended reduced-dimensional model is used to simulate the flow of a full two-channel model and the influence of the plate topography on the internal flow distribution is investigated through a comprehensive parametric analysis. The analysis shows that the complex topography of the plate surface eliminates the variation in inlet velocity and significantly changes the internal fluid flow, eventually resulting in a consistent velocity distribution.

]]>Fluids doi: 10.3390/fluids8060173

Authors: Arash Ghahraman Gyula Bene

Viscous linear surface waves are studied at arbitrary wavelength, layer thickness, viscosity, and surface tension. We find that in shallow enough fluids no surface waves can propagate. This layer thickness is determined for some fluids, water, glycerin, and mercury. Even in any thicker fluid layers, propagation of very short and very long waves is forbidden. When wave propagation is possible, only a single propagating mode exists for a given horizontal wave number. In contrast, there are two types of non-propagating modes. One kind of them exists at all wavelength and material parameters, and there are infinitely many such modes for a given wave number, distinguished by their decay rates. The other kind of non-propagating mode that is less attenuated may appear in zero, one, or two specimens. We notice the presence of two length scales as material parameters, one related to viscosity and the other to surface tension. We consider possible modes for a given material on the parameter plane layer thickness versus wave number and discuss bifurcations among different mode types. Motion of surface particles and time evolution of surface elevation is also studied at various parameters in glycerin, and a great variety of behaviour is found, including counterclockwise surface particle motion and negative group velocity in wave propagation.

]]>Fluids doi: 10.3390/fluids8060172

Authors: Denis Kuimov Maxim Minkin Alexandr Yurov Alexandr Lukyanov

Cavitation, as a unique technology for influencing liquid substances, has attracted much attention in the oil refining industry. The unique capabilities of cavitation impact can initiate the destruction of molecular compounds in the liquid. At the same time with a large number of successful experimental studies on the treatment of liquid hydrocarbon raw materials, cavitation has not been introduced in the oil refining industry. Often the impossibility of implementation is based on the lack of a unified methodology for assessing the intensity and threshold of cavitation creation. The lack of a unified methodology does not allow for predicting the intensity and threshold of cavitation generation in different fluids and cavitation-generating devices. In this review, the effect of cavitation on various rheological properties and fractional composition of liquid hydrocarbons is investigated in detail. The possibility of using the cavitation number as a single parameter for evaluating the intensity and threshold of cavitation generation is analyzed, and the limitations of its application are evaluated. The prospects of introducing the technology into the industry are discussed and a new vision of calculating the analog of cavitation numbers based on the analysis of the mutual influence of feedstock parameters and geometry of cavitators on each other is presented.

]]>Fluids doi: 10.3390/fluids8060171

Authors: Ian Adams Julian Simeonov Samuel Bateman Nathan Keane

We have developed and tested a numerical model for turbulence resolving simulations of dense mud&ndash;water mixtures in oscillatory bottom boundary layers, based on a low Stokes number formulation of the two-phase equations. The resulting non-Boussinesq equation for the fluid momentum is coupled to a transport equation for the mud volumetric concentration, giving rise to a volume-averaged fluid velocity that is non-solenoidal, and the model was implemented as a new compressible flow solver. An oscillating pressure gradient force was implemented in the correction step of the standard semi-implicit method for pressure linked equations (SIMPLE), for consistency with the treatment of other volume forces (e.g., gravity). The flow solver was further coupled to a new library for Bingham plastic materials, in order to model the rheological properties of dense mud mixtures using empirically determined concentration-dependent yield stress and viscosity. We present three direct numerical simulation tests to validate the new MudMixtureFoam solver against previous numerical solutions and experimental data. The first considered steady flow of Bingham plastic fluid with uniform concentration around a sphere, with Bingham numbers ranging from 1 to 100 and Reynolds numbers ranging from 0.1 to 100. The second considered the development of turbulence in oscillatory bottom boundary layer flow, and showed the formation of an intermittently turbulent layer with peak velocity perturbations exceeding 10 percent of the freestream flow velocity and occurring at a distance from the bottom comparable to the Stokes boundary layer thickness. The third considered the effects of density stratification due to resuspended sediment on turbulence in oscillatory bottom boundary layer flow with a bulk Richardson number of 1&times;10&minus;4 and a Stokes&ndash;Reynolds number of 1000, and showed the formation of a lutocline between 20 and 40 Stokes boundary layer depths. In all cases, the new solver produced excellent agreement with the previous results.

]]>Fluids doi: 10.3390/fluids8060170

Authors: Raed Alrdadi Michael H. Meylan

In this study, a numerical simulation of fluid flow through railway ballast in the time domain is presented, providing a model for unsteady-state flow. It is demonstrated that the position of the free surface with respect to time can also be used to solve the steady flow case. The effect of ballast fouling is included in the model to capture the realistic behavior of railway ballast, which is critical to understanding the impact of flooding. A thorough comparison with a range of previous studies, including theoretical and experimental approaches, is made, and very close agreement is obtained. The significant impact of ballast fouling on fluid flow and its potential consequences for railway infrastructure are highlighted by the simulation. Valuable insights into the behavior of water flow through porous media and its relevance to railway ballast management are offered by this study.

]]>Fluids doi: 10.3390/fluids8060169

Authors: Victor Bolobov Yana Martynenko Sergey Yurtaev

Reduction of energy expenditures required for various technological processes is a pressing issue in today&rsquo;s economy. One of the ways to solve this issue in regard to liquefied natural gas (LNG) storage is the recovery of its vapours from LNG tanks using an ejector system. In that respect, studies on the outflow of the real gas through the nozzle, the main element of the ejector, and identifying differences from the ideal gas outflow, are of high relevance. Particularly, this concerns the determination of the discharge coefficient &micro; as the ratio of the actual flowrate to the ideal one, taking into account the energy losses at gas outflow through the nozzle. The discharge coefficient values determined to date for various nozzle geometries are, as a rule, evaluated empirically and contradictory in some cases. The authors suggest determining the discharge coefficient by means of an experiment. This paper includes &micro; values determined using this method for the critical outflow of air to atmosphere through constrictor nozzles with different outlet diameters (0.003 m; 0.004 m; 0.005 m) in the pressure range at the nozzle inlet of 0.5&ndash;0.9 MPa. The obtained results may be used for the design of an ejector system for the recovery of the boil-off gas from LNG tanks, as well as in other fields of industry, for the design of technical and experimental devices with nozzles for various applications.

]]>Fluids doi: 10.3390/fluids8060168

Authors: Bulat Yakupov Ivan Smirnov

The acoustic cavitation of fluids, as well as related physical and chemical phenomena, causes a variety of effects that are highly important in technological processes and medicine. Therefore, it is important to be able to control the conditions that allow cavitation to begin and progress. However, the accurate prediction of acoustic cavitation is dependent on a complex relationship between external influence parameters and fluid characteristics. The multiparameter problem restricts the development of successful theoretical models. As a result, it is critical to identify the most important parameters influencing the onset of the cavitation process. In this paper, the ultrasonic frequency, hydrostatic pressure, temperature, degassing, density, viscosity, volume, and surface tension of a fluid were investigated using machine learning to determine their significance in predicting acoustic cavitation strength. Three machine learning models based on support vector regression (SVR), ridge regression (RR), and random forest (RF) algorithms with different input parameters were trained. The results showed that the SVM algorithm performed better than the other two algorithms. The parameters affecting the active cavitation nuclei, namely hydrostatic pressure, ultrasound frequency, and outgassing degree, were found to be the most important input parameters influencing the prediction of the cavitation threshold. Other parameters have a minor impact when compared to the first three, and their role can be compensated for by alternative variables. The further development of the obtained results provides a new way to optimize and improve existing theoretical models.

]]>Fluids doi: 10.3390/fluids8060167

Authors: Anastasia S. Vanina Alexander V. Sychev Anastasia I. Lavrova Pavel V. Gavrilov Polina L. Andropova Elena V. Grekhnyova Tatiana N. Kudryavtseva Eugene B. Postnikov

Studying transport processes in the brain&rsquo;s extracellular space is a complicated problem when considering the brain&rsquo;s tissue. Tests of corresponding physical and mathematical problems, as well as the need for materials with cheap but realistic properties to allow for testing of drug delivery systems, lead to the development of artificial phantom media, one kind of which is explored in this work. We report results from quantifying the spread of a standard contrast agent used in clinical computed tomography, Iopromide, in samples of collagen-based hydrogels. Its pure variant as well as samples supplied with lipid and surfactant additives were explored. By comparing to solutions of the diffusion equation which reproduce these data, the respective diffusion coefficients were determined. It was shown that they are relevant to the range typical for living tissue, grow with elevation in the lipid content and diminish with growth in surfactant concentration.

]]>Fluids doi: 10.3390/fluids8060166

Authors: Sergey E. Yakush Yuli D. Chashechkin Andrey Y. Ilinykh Vladislav A. Usanov

The impingement of a short-duration water jet on a pool of molten Rose&rsquo;s metal is studied experimentally herein. Short-duration water jet impacting on the free surface of a molten metal pool with a temperature of 300 &deg;C are generated with a pneumatic water delivery system, with two-camera high-speed video registration. A total of 14 experimental series, each containing 5 repeated tests, are performed for a water volume of 0.2&ndash;1 mL and a jet impact velocity of 4.1&ndash;9.0 m/s. The cavity development in the melt layer is studied, with the main stages described herein. Despite the significantly higher density of melt in comparison with water, the cavity can reach the melt pool bottom; furthermore, its further collapse results in the formation of a central jet rising to the height of a few centimeters. The maximum height of the central jet is shown to depend linearly on the total momentum of the water jet, and a semi-logarithmic correlation is found for the maximum diameter of the cavity. Repeatability analysis is performed within each experimental series, and the relative standard deviation for the melt splash height is shown to be from 8.8% to 26.8%. The effects of the pool depth, the vessel shape, and the water temperature are weaker in the range of the experimental parameters used here.

]]>Fluids doi: 10.3390/fluids8060165

Authors: Victor L. Mironov Sergey V. Mironov

We present the theoretical description of plane Couette flow based on the previously proposed equations of vortex fluid, which take into account both the longitudinal flow and the vortex tubes rotation. It is shown that the considered equations have several stationary solutions describing different types of laminar flow. We also discuss the simple model of turbulent flow consisting of vortex tubes, which are moving chaotically and simultaneously rotating with different phases. Using the Boussinesq approximation, we obtain an analytical expression for the stationary profile of mean velocity in turbulent Couette flow, which is in good agreement with experimental data and results of direct numerical simulations. Our model demonstrates that near-wall turbulence can be described by a coordinates-independent coefficient of eddy viscosity. In contrast to the viscosity of the fluid itself, this parameter characterizes the turbulent flow and depends on Reynolds number and roughness of the channel walls. Potentially, the proposed model can be considered as a theoretical basis for the experimental measurement of the eddy viscosity coefficient.

]]>Fluids doi: 10.3390/fluids8060164

Authors: Mouh Assoul Abdelouahab El jaouahiry Jamila Bouchgl Mourad Echchadli Saïd Aniss

We investigate the effect of horizontal quasi-periodic oscillation on the stability of two superimposed immiscible fluid layers confined in a horizontal Hele-Shaw cell. To approximate real oscillations, a quasi-periodic oscillation with two incommensurate frequencies is considered. Thus, the linear stability analysis leads to a quasi-periodic oscillator, with damping, which describes the evolution of the amplitude of the interface. Two types of quasi-periodic instabilities occur: the low-wavenumber Kelvin-Helmholtz instability and the large-wavenumber resonances. We mainly show that, for equal amplitudes of the superimposed accelerations, and for a low irrational frequency ratio, there is competition between several resonance modes allowing a very large selection of the wavenumber from lower to higher values. This is a way to control the sizes of the waves. Furthermore, increasing the frequency ratio has a stabilizing effect for both types of instability whose thresholds are found to correspond to quasi-periodic solutions using the frequency spectrum. For a ratio of the two superimposed displacement amplitudes equal to unity and less than unity, the number of resonances and competition between their modes also become significant for the intermediate values of the ratio of frequencies. The effects of other physical and geometrical parameters, such as the damping coefficient, density ratio, and heights of the two fluid layers, are also examined.

]]>Fluids doi: 10.3390/fluids8060163

Authors: Stoyan Nedeltchev

This article focuses on the prediction of the small bubble holdups (assuming the existence of two major bubble classes) in two bubble columns (0.289 m in ID and 0.102 m in ID), operated with organic liquids under various conditions (including high temperature and pressure). A new correction factor has been established in the existing model for the prediction of the gas holdups in the homogeneous regime. The correction parameter is a single function of the E&ouml;tv&ouml;s number (gravitational forces to surface tension forces), which characterizes the bubble shape. In addition, the behavior of small bubble holdups in 1-butanol (selected as a frequently researched alcohol) aerated with nitrogen, in a smaller BC (0.102 m in ID), at various operating pressures, is presented and discussed. The ratio of small bubble holdup to overall gas holdup, as a function of superficial gas velocity and operating pressure, has been investigated. All small bubble holdups in this work have been measured by means of the dynamic gas disengagement technique.

]]>Fluids doi: 10.3390/fluids8060162

Authors: Yuxing Lin Ebrahim Kadivar Ould el Moctar

In this work, we experimentally investigated the cavitation effects on the hydrodynamic behavior of a circular cylinder at different cavitating flows. We analyzed the cavitation dynamics behind the circular cylinder using a high-speed camera and also measured the associated hydrodynamic forces on the circular cylinder using a load cell. We studied the cavitation dynamics around the cylinder at various types of the cavitating regimes such as cloud cavitation, partial cavitation and cavitation inception. In addition, we analyzed the cavitation dynamics at three different Reynolds numbers: 1 &times; 105, 1.25 &times; 105 and 1.5 &times; 105. The results showed that the hydrodynamics force on the circular cylinder can be increased with the formation of the cavitation behind the cylinder compared with the cylinder at cavitation inception regime. The three-dimensional flow caused complex cavitation behavior behind the cylinder and a strong interaction between vortex structures and cavity shedding mechanism. In addition, the results revealed that the effects of the Reynolds number on the cavitation dynamics and amplitude of the shedding frequency is significant. However the effects of the cavitation number on the enhancement of the amplitude of the shedding frequency in the cavitating flow with a constant velocity is slightly higher than the effects of Reynolds number on the enhancement of the amplitude of the shedding frequency at a constant cavitation number.

]]>Fluids doi: 10.3390/fluids8050161

Authors: Jiangbo Wu Yao Lv Yongqing He Xiaoze Du Jie Liu Wenyu Zhang

Erythrocyte enrichment is needed for blood disease diagnosis and research. DLD arrays with an I-shaped pillar (I-pillar) sort erythrocytes in a unique, accurate, and low-reagent method. However, the existing I-shaped pillar DLD arrays for erythrocyte sorting have the drawbacks of higher flow resistance and more challenging fabrication. A two-dimensional erythrocyte simulation model and the arbitrary Lagrangian&ndash;Euler equations at the cell&ndash;fluid boundary were built based on the fluid&ndash;solid coupling method to investigate the influencing factors of the erythrocyte flow path in an I-pillar DLD array and find its optimization method. Three different sizes of I-pillars were built and multiple sets of corresponding arrays were constructed, followed by finite element simulations to separately investigate the effects of these arrays on the induction of erythrocyte motion paths. This work demonstrates the motion paths of erythrocyte models in a series of I-pillar arrays with different design parameters, aiming to summarize the variation modes of erythrocyte motion paths, which in turn provides some reference for designing and optimizing the pillar size and array arrangement methods for I-pillar array DLD chips.

]]>Fluids doi: 10.3390/fluids8050159

Authors: Abdulaziz Al Baraikan Krzysztof Czechowicz Paul D. Morris Ian Halliday Rebecca C. Gosling Julian P. Gunn Andrew J. Narracott Gareth Williams Pankaj Garg Maciej Malawski Frans van de Vosse Angela Lungu Dan Rafiroiu David Rodney Hose

Acting upon clinical patient data, acquired in the pathway of percutaneous intervention, we deploy hierarchical, multi-stage, data-handling protocols and interacting low- and high-order mathematical models (chamber elastance, state-space system and CFD models), to establish and then validate a framework to quantify the burden of ischaemia. Our core tool is a compartmental, zero-dimensional model of the coupled circulation with four heart chambers, systemic and pulmonary circulations and an optimally adapted windkessel model of the coronary arteries that reflects the diastolic dominance of coronary flow. We guide the parallel development of protocols and models by appealing to foundational physiological principles of cardiac energetics and a parameterisation (stenotic Bernoulli resistance and micro-vascular resistance) of patients&rsquo; coronary flow. We validate our process first with results which substantiate our protocols and, second, we demonstrate good correspondence between model operation and patient data. We conclude that our core model is capable of representing (patho)physiological states and discuss how it can potentially be deployed, on clinical data, to provide a quantitative assessment of the impact, on the individual, of coronary artery disease.

]]>Fluids doi: 10.3390/fluids8050160

Authors: Manal Alotaibi Shoug Alotaibi Ruud Weijermars

Gaussian solutions of the diffusion equation can be applied to visualize the flow paths in subsurface reservoirs due to the spatial advance of the pressure gradient caused by engineering interventions (vertical wells, horizontal wells) in subsurface reservoirs for the extraction of natural resources (e.g., water, oil, gas, and geothermal fluids). Having solved the temporal and spatial changes in the pressure field caused by the lowered pressure of a well&rsquo;s production system, the Gaussian method is extended and applied to compute and visualize velocity magnitude contours, streamlines, and other relevant flow attributes in the vicinity of well systems that are depleting the pressure in a reservoir. We derive stream function and potential function solutions that allow instantaneous modeling of flow paths and pressure contour solutions for transient flows. Such analytical solutions for transient flows have not been derived before without time-stepping. The new closed-form solutions avoid the computational complexity of time-stepping, required when time-dependent flows are modeled by superposing steady-state solutions using complex analysis methods.

]]>Fluids doi: 10.3390/fluids8050158

Authors: Henry Francis Annapeh Victoria Kurushina

Evaluating the hydrodynamic force fluctuations acting on each structure in a group of subsea objects of different cross-section shapes, sizes and relative positions represents a challenge due to the sensitivity of the vortex shedding process, especially for a variety of sheared flows. The present study uses the numerical 2D computational fluid dynamics model to estimate the flow-induced forces on a group of small circular and D-shaped cylinders in the linear and parabolic sheared flow, which are placed in proximity to a larger structure of the squared cross-section. This allows us to evaluate loads, which are affected by the presence of subsea equipment located on the seabed. The average Reynolds number of the considered linear flow profile is 3900, while the parabolic flow profile has the maximum Reynolds number of 3900. The k-&omega; SST turbulence model is used for simulations. The work demonstrates the effect of the cross-sectional shape of smaller cylinders on hydrodynamic coefficients, explores the effect from the spacing in between the structures and highlights differences between loads in the linearly sheared and parabolic flow. The results obtained show that the presence of the squared cylinder notably influences the mean drag coefficient on the first cylinder, for both circular and D-shaped cylinders. The parabolic sheared flow profile in this series leads to the highest mean drag and the highest amplitudes of the fluctuating drag and lift coefficients.

]]>Fluids doi: 10.3390/fluids8050156

Authors: Antonin Leprevost Vincent Faucher Maria Adela Puscas

The present article addresses the topic of grid motion computation in Arbitrary Lagrange&ndash;Euler (ALE) simulations, where a fluid mesh must be updated to follow the displacements of Lagrangian boundaries. A widespread practice is to deduce the motion for the internal mesh nodes from a parabolic equation, such as the harmonic equation, introducing an extra computational cost to the fluid solver. An alternative strategy is proposed to minimize that cost by changing from the parabolic equation to a hyperbolic equation, implementing an additional time derivative term allowing an explicit solution of the grid motion problem. A fictitious dynamic problem is thus obtained for the grid, with dedicated material parameters to be carefully chosen to enhance the computational efficiency and preserve the mesh quality and the accuracy of the physical problem solution. After reminding the basics of the ALE expression of the Navier&ndash;Stokes equations and describing the proposed hyperbolic equation for the grid motion problem, the paper provides the necessary characterization of the influence of the fictitious grid parameters and the analysis of the robustness of the new approach compared to the harmonic reference equation on a significant 2D test case. A 3D test case is finally extensively studied in terms of computational performance to highlight and discuss the benefits of the hyperbolic equation for ALE grid motion.

]]>Fluids doi: 10.3390/fluids8050157

Authors: Adit Misar Phillip Davis Mesbah Uddin

Racecar aerodynamic development requires well-correlated simulation data for rapid and incremental development cycles. Computational Fluid Dynamics (CFD) simulations and wind tunnel testing are industry-wide tools to perform such development, and the best use of these tools can define a race team&rsquo;s ability to compete. With CFD usage being limited by the sanctioning bodies, large-scale mesh and large-time-step CFD simulations based on Reynolds-Averaged Navier&ndash;Stokes (RANS) approaches are popular. In order to provide the necessary aerodynamic performance advantages sought by CFD development, increasing confidence in the validity of CFD simulations is required. A previous study on a Scale-Averaged Simulation (SAS) approach using RANS simulations of a Gen-6 NASCAR, validated against moving-ground, open-jet wind tunnel data at multiple configurations, produced a framework with good wind tunnel correlation (within 2%) in aerodynamic coefficients of lift and drag predictions, but significant error in front-to-rear downforce balance (negative lift) predictions. A subsequent author&rsquo;s publication on a Scale-Resolved Simulation (SRS) approach using Improved Delayed Detached Eddy Simulation (IDDES) for the same geometry showed a good correlation in front-to-rear downforce balance, but lift and drag were overpredicted relative to wind tunnel data. The current study compares the surface pressure distribution collected from a full-scale wind tunnel test on a Gen-6 NASCAR to the SAS and SRS predictions (both utilizing SST k&minus;&omega; turbulence models). CFD simulations were performed with a finite-volume commercial CFD code, Star-CCM+ by Siemens, utilizing a high-resolution CAD model of the same vehicle. A direct comparison of the surface pressure distributions from the wind tunnel and CFD data clearly showed regions of high and low correlations. The associated flow features were studied to further explore the strengths and areas of improvement needed in the CFD predictions. While RANS was seen to be more accurate in terms of lift and drag, it was a result of the cancellation of positive and negative errors. Whereas IDDES overpredicted lift and drag and requires an order of magnitude more computational resources, it was able to capture the trend of surface pressure seen in the wind tunnel measurements.

]]>Fluids doi: 10.3390/fluids8050155

Authors: Francesco Duronio Angelo De Vita Alessandro Montanaro Luigi Allocca

Among the most relevant fields of research recently investigated for improving the performance of gasoline direct injection (GDI) engines, there are ultrahigh injection pressures and the flash-boiling phenomenon. Both perform relevant roles in improving the air/fuel mixing process, reducing tailpipe emissions and implementing new combustion methods. When a high-temperature fuel is released into an environment with a pressure lower than the fuel&rsquo;s saturation pressure, flash boiling occurs. Due to complex two-phase flow dynamics and quick droplet vaporization, flash boiling can significantly modify spray formation. Specifically, if properly controlled, flash boiling produces important benefits for the fuel&ndash;air mixture formation, the combustion quality and, in general, for overall engine operation. Flash boiling was broadly investigated for classical injection pressure, but few works concern ultrahigh injection pressure. Here, the investigation of the spray produced by a multihole injector was performed using both experimental imaging techniques and CFD simulations aiming to highlight the combined impact of the injection pressure and the flash boiling occurrence on the spray morphology. The shadowgraph method was employed to observe the spray experimentally. The information gathered allows for assessing the performances of an Eulerian&ndash;Lagrangian algorithm purposely developed. Breakup and evaporation models, appropriate for flashing sprays, were implemented in a CFD (Computational Fluid Dynamics) code. The experimental results and the CFD simulations demonstrate a good agreement, demonstrating that through adoption of a flash-boiling breakup model, it is possible to reproduce non-evaporating and superheated sprays while changing few simulation parameters. Finally, the results also show the significance of injection pressure in preventing spray collapse.

]]>Fluids doi: 10.3390/fluids8050154

Authors: Ebrahim Kadivar Ali Rajabpour Ould El Moctar

In this work, we performed molecular dynamics simulations to study the dynamics of a shock wave-induced single nanobubble collapsing near one flexible and two rigid boundaries. The flexible boundary consisted of polyethylene, and the rigid boundaries were made of aluminum and iron. The shock waves impinging on the nanobubble inside a molecular system were generated using a momentum mirror approach. For two relative wall distances, we studied the dynamics of the shock-induced single nanobubble and its collapse near the flexible and the rigid boundaries. The atomic velocity contours surrounding the single nanobubble and the collapse-induced damage on the boundaries were analyzed. We obtained this collapse-induced damage from ten collapsing nanobubbles. Results showed that the relative wall distance affected the single nanobubble&rsquo;s collapse dynamics near the boundaries. A generated nanojet was directed on the surfaces during the collapse process. From the collapse-induced damage point of view, the depth damage of the polyethylene, iron, and aluminum boundaries for the relative wall distance of &gamma; = 1.3 were 6.0, 0.47 and 0.63 nm, respectively. It was observed that the extensive collapse-induced damage occurred only on the polyethylene boundary.

]]>Fluids doi: 10.3390/fluids8050153

Authors: Hani Alahmadi Shailesh Naire

We theoretically considered two-dimensional flow in a vertically aligned thick molten liquid film to investigate the competition between cooling and gravity-driven draining, which is relevant in the formation of metallic foams. Molten liquid in films cools as it drains, losing its heat to the surrounding colder air and substrate. We extended our previous model to include non-isothermal effects, resulting in coupled non-linear evolution equations for the film&rsquo;s thickness, extensional flow speed and temperature. The coupling between the flow and cooling effect was via a constitutive relationship for temperature-dependent viscosity and surface tension. This model was parameterized by the heat transfer coefficients at the film&ndash;air free surface and film&ndash;substrate interface, the P&eacute;clet number, the viscosity&ndash;temperature coupling parameter and the slope of the linear surface tension&ndash;temperature relationship. A systematic exploration of the parameter space revealed that at low P&eacute;clet numbers, increasing the heat transfer coefficient and gradually reducing the viscosity with temperature was conducive to cooling and could slow down the draining and thinning of the film. The effect of increasing the slope of the surface tension&ndash;temperature relationship on the draining and thinning of the film was observed to be more effective at lower P&eacute;clet numbers, where surface tension gradients in the lamella region opposed the gravity-driven flow. At higher P&eacute;clet numbers, though, the surface tension gradients tended to enhance the draining flow in the lamella region, resulting in the dramatic thinning of the film in the later stages.

]]>Fluids doi: 10.3390/fluids8050152

Authors: Max Quissek Thomas Lauer

The understanding of impingement processes is crucial for optimizing automotive selective catalytic reduction (SCR) systems. An accurate description of this behavior helps design exhaust systems and increases the validity of modeling approaches. A component test bench was set up, featuring a droplet chain generator for producing droplet sizes typically found in the urea&ndash;water solution sprays of SCR systems. A heatable impingement plate with an interchangeable surface enabled investigation of the influence of surface roughness. Data were acquired using a high-speed camera and image postprocessing. The droplet&ndash;wall interaction could be described using different regimes. An approach to characterizing impingement behavior based on weighted-regime superposition enabled gradual transitions between regimes, instead of step-like changes. It was observed that the surface roughness increased the droplet&ndash;solid contact area and generated thermal-induced secondary droplets at lower temperatures. A region of enhanced mechanical disintegration of the droplet was found, caused by peaks of the surface shearing off parts of the droplet. The probability of a droplet rebounding from the wall was reduced on a rough surface, due to the interference of the surface spikes with the droplet&rsquo;s spreading and contracting motion. Additionally, the influence of surface topography was investigated using a shot-peened surface. Caused by this surface&rsquo;s reduced root mean square slope, the aforementioned enhancement of mechanical disintegration was not observed.

]]>Fluids doi: 10.3390/fluids8050151

Authors: Finn Knüppel Inga Thomas Frank-Hendrik Wurm Benjamin Torner

Ventricular assist devices (VADs) are implantable turbomachines that save and improve the lives of patients with severe heart failure. In the preclinical evaluation, a VAD design must be experimentally or numerically tested regarding its pump characteristics, primarily for its pressure buildup (pressure head&nbsp;H) since it must provide the cardiovascular system with a sufficient blood flow rate&nbsp;Q. Those pump characteristics are determined on a test bench. Here, a glycerol-water mixture is almost exclusively used as blood-analogous fluid, which should reflect the properties (density, viscosity) of blood as close as possible. However, glycerol water has some disadvantages, such as a higher density compared to real blood and a relatively high cost. Therefore, the study aimed to analyze six different blood analogous fluids to select the most suitable one in consideration of fluid handling, costs, and, most importantly, fluid properties (material and rheological). First, all fluids were mixed to achieve reference values of blood density and viscosity from the literature. Afterwards, the pump characteristics (pressure heads and efficiencies via the VAD) were experimentally and numerically determined and compared among each other and with literature values. Of all six investigated fluids, only the aqueous&ndash;polyethylene glycol 200 (PEG 200) solution matches exactly the desired blood properties, and the pump characteristics of this fluid are in the expected range for the analyzed operation point of the VAD. Another advantage is that the cost of the mixture is 35% lower compared to glycerol water. Additionally, we demonstrate that non-Newtonian flow behavior has little effect on the pump characteristics in our VAD.

]]>Fluids doi: 10.3390/fluids8050150

Authors: Md. Mahbubur Rahman Djiby Bal Keishi Kariya Akio Miyara

Due to its low global warming potential (GWP) and good environmental properties, CF3I can be a suitable component of refrigerant mixtures in the field of refrigeration and air conditioning. In this work, the local condensation heat transfer characteristics of CF3I were experimentally investigated in a plate heat exchanger (PHE). The condensation heat transfer experiments were carried out under conditions of vapor qualities from 1.0 to 0.0, at saturation temperatures of 25&ndash;30 &deg;C, mass fluxes of 20&ndash;50 kg/m2s, and heat fluxes of 10.4&ndash;13.7 kW/m2. Local heat transfer coefficients were found to vary in both the horizontal and vertical directions of the plate heat exchanger showing similar trends in all mass fluxes. In addition, the characteristics of local heat flux and wall temperature distribution as a function of distance from the inlet to the outlet of the refrigerant channel were explored in detail. The comparison of the experimental data of CF3I with that of R1234yf in the same test facility showed that the heat transfer coefficients of CF3I were comparable to R1234yf at a low vapor quality and a mass flux of 20 kg/m2s. However, R1234yf exhibited a transfer coefficient about 1.5 times higher at all vapor qualities and a mass flux of 50 kg/m2s. The newly developed correlation predicts well the experimentally obtained data for both CF3I and R1234yf within &plusmn;30%.

]]>Fluids doi: 10.3390/fluids8050149

Authors: Elizaveta Kolesnik Evgueni Smirnov Elena Babich

The results of a numerical solution of the problem of supersonic flow past a blunt fin mounted on a plate with a developing boundary layer are presented. The initial formulation of the problem is based on the presented in the literature computational and experimental investigation, in which the laminar flow regime was studied for the fin perpendicular to the plate at the free-stream Mach number equal to 6.7. Earlier, the authors showed (2020) that under these conditions there exist two stable solutions to the problem. These solutions correspond to the metastable states of flow with different configurations of the vortex structure and different patterns of local heat transfer. In the present study, the influence of a temperature ratio on the vortex structure in the separation region, local heat transfer, and the possibility of obtaining a dual solution are investigated. The ability to switch between solutions of two types using a short-time change in the plate temperature ratio are shown.

]]>Fluids doi: 10.3390/fluids8050148

Authors: Gracjan M. Skonecki James M. Buick

Simulations are presented for flow around pairs of circular cylinders at a Reynolds number of 3900. The flow is assumed to be two-dimensional and incompressible in nature and the simulations are performed using a RANS (Reynolds Averaged Navier Stokes) approach with a k-&epsilon; model. Simulations are performed for three different configurations of the cylinders: A tandem configuration where the line joining the centre of the cylinders is parallel to the mean flow direction; side-by-side, where the centre line is perpendicular to the mean flow direction; and staggered where the centre line is an angle &alpha; to the flow direction. Simulation results are presented for cylinder separations ranging from 1.125 to 4 diameters and for values of &alpha; between 10&deg; and 60&deg;. The results are presented and discussed in terms of the lift and drag coefficients, the Strouhal number, the vorticity field and the flow regimes observed. The results and flow regimes are also compared to previous observations at lower Reynolds numbers to investigate the Reynolds number dependence of the phenomena.

]]>Fluids doi: 10.3390/fluids8050147

Authors: József Bakosi Mátyás Constans Zoltán Horváth Ákos Kovács László Környei Marc Charest Aditya Pandare Paula Rutherford Jacob Waltz

We present an open-source code, Xyst, intended for the simulation of complex-geometry 3D compressible flows. The software implementation facilitates the effective use of the largest distributed-memory machines, combining data-, and task-parallelism on top of the Charm++ runtime system. Charm++&rsquo;s execution model is asynchronous by default, allowing arbitrary overlap of computation and communication. Built-in automatic load balancing enables redistribution of arbitrarily heterogeneous computational load based on real-time CPU load measurement at negligible cost. The runtime system also features automatic checkpointing, fault tolerance, resilience against hardware failure, and supports power- and energy-aware computation. We verify and validate the numerical method and demonstrate the benefits of automatic load balancing for irregular workloads.

]]>Fluids doi: 10.3390/fluids8050146

Authors: Omar Selim Christoph Brücker

A novel approach for sensing and characterising the flow over an aerofoil is introduced. Arrays of flexible wind-hair-like sensors distributed over an aerofoil, which are tracked remotely using high-speed imaging and processing, acting as &ldquo;digital tufts&rdquo;, are used to provide real-time readings of local flow information with high temporal resolution. The use case presented in this paper has the sensors embedded within the suction side of a NACA0012 aerofoil and tested in a wind tunnel for varying angles of attack in static and dynamic tests. The time-averaged signals were able to provide information pertaining to the free-stream velocity and instantaneous angle of attack. The capability of the sensor type to provide temporal flow information is also explored. The sensors were used to detect low-frequency oscillations, which are pre-cursory to stall. These are hypothesised to be linked to breathing modes of the laminar separation bubble, causing a shear-layer flapping observed on the sensors. Such low-frequency oscillations were also detected shortly before separation in the ramp-up studies.

]]>Fluids doi: 10.3390/fluids8050145

Authors: Grazia Lo Sciuto Paweł Kowol Giacomo Capizzi

In this paper, the characterization of a new clutch control strategy by means of a magnetorheological fluid (MR) has been investigated. The clutch system was designed and manufactured in the laboratory in order to determine its static and dynamic characteristics. As a result of experimental measurements, the torque control of the developed MR clutch was determined in the frequency and time domains, as were the analytical relationships describing the connection between the control and controlled variables. The obtained results demonstrate that the analytical models are in good agreement with the experimental data, with an overall error of about 7%.

]]>Fluids doi: 10.3390/fluids8050144

Authors: Michael Landl René Prieler Ernesto Monaco Christoph Hochenauer

To enable the lattice-Boltzmann method (LBM) to account for temporally constant but spatially varying thermophysical properties, modifications must be made. Recently, many methods have emerged that can account for conjugate heat transfer (CHT). However, there still is a lack of information on the possible physical property range regarding realistic properties. Therefore, two test cases were investigated to gain further insight. First, a differentially heated cavity filled with blocks was used to investigate the influence of CHT on the error and stability of the LBM simulations. Reference finite volume method (FVM) simulations were carried out to estimate the error. It was found that a range between 0.5 to 1.5 is recommended for the fluid relaxation time to balance computational effort, stability, and accuracy. In addition, realistic thermophysical properties of fluids and solids were selected to test whether the lattice-Boltzmann method is suitable for simulating relevant industry-related applications. For a stable simulation, a mesh with 64 times more lattices was needed for the most extreme test case. The second test case was an insulated cavity with a heating pad as the local heat source, which was investigated in terms of the accuracy of a transient simulation and compared to a FVM simulation. It was found that the fluid-phase relaxation time mainly determines the error and that large thermal relaxation times for the solid improve accuracy. Observed deviations from the FVM reference simulations ranged from approximately 20% to below 1%, depending on collision operator and combination of relaxation times. For processes with a large temperature spread, the temporally constant thermophysical properties of the LBM are the primary constraint.

]]>Fluids doi: 10.3390/fluids8050143

Authors: Clément Loiseau Stéphane Mimouni Didier Colmont Stéphane Vincent

The CFD numerical study of the flash boiling phenomenon of a water film was conducted using an Euler&ndash;Euler method, and compared to the experiments on the flashing of a water film. The water film is initially heated at temperatures ranging from 34 to 74 &#8728;C (frim 1 to 41 &#8728;C superheat), and the pressure is decreased from 1 bar to 50 mbar during the experiments. This paper shows that the experiments could not be correctly modelled by a simple liquid/bubble model because of the overestimation of the drag force above the water film (in the gas/droplet region). The generalised large interface model (GLIM), however, a multi-regime approach implemented in the version 7.0 of the neptune_cfd software, is able to differentiate the water film, where liquid/bubble interactions are predominant from the gas region where gas/droplet interactions are predominant, and gives nice qualitative results. Finally, this paper shows that the interfacial heat transfer model of Berne for superheated liquids could accurately predict the evolution of the water temperature over time.

]]>Fluids doi: 10.3390/fluids8050142

Authors: Elena Popova Alexandre Lazarian

Magnetohydrodynamical (MHD) turbulence is ubiquitous in magnetized astrophysical plasmas, and it radically changes a great variety of astrophysical processes. In this review, we introduce the concept of MHD turbulence and explain the origin of its scaling. We consider the implications of MHD turbulence for various problems: dynamo in different types of stars, flare activity, solar and stellar wind from different stars, the propagation of cosmic rays, and star formation. We also discuss how the properties of MHD turbulence provide a new means of tracing magnetic fields in interstellar and intracluster media.

]]>Fluids doi: 10.3390/fluids8050141

Authors: Sergey Dmitriev Andrey Kurkin Aleksandr Dobrov Denis Doronkov Aleksey Pronin Dmitry Solntsev

The efficiency of heat transfer in air-cooled heat exchangers of various industrial facilities depends on the flow rate of the coolant, its inlet temperature and ambient temperature. These parameters are transient and depend both on the features of the technological process and on weather conditions. One option for a compact design of heat exchangers is the use of close-packed coils with a small bending radius. In this case, heat transfer in the complex geometry of the annular space cannot be described by simple one-dimensional dependencies. To solve this problem, it is necessary to consider the three-dimensional spatial structure of the heat exchange surface. Since the size of the grid elements will be several orders of magnitude less than the size of the facility, the size of the computational grids for CFD modeling full-scale heat exchangers will be billions of finite volumes, and even on powerful supercomputers, the solution time will be about a month. One way to reduce computational costs is to use reduced order models, in which the computational domain is not modeled directly; instead, simplified models, such as a porous medium model, are used to describe it. However, such models require additional closing relations and coefficients that characterize the actual channel geometry. This paper presents a technique for creating a digital twin of a heat exchanger with small bend radius coils based on a porous medium model. The values of heat transfer coefficients and hydraulic resistance depend on the speed of air movement in the space between the coils. The calculated value of the thermal power obtained using the strengthened model was 529 kW, which corresponds to the passport data of 500 kW, with less than 6% deviation for the heat exchanger under study. This confirms the correctness of the calculation with accepted simplifications. The calculation time in this case was only a few minutes when using a personal computer. The developed numerical model allows for the resolution of performance characteristics based on the temperature of the cooled medium at the inlet, air temperature, and fan speed. Analyzing the different modes of turning on the cooling fans made it possible to determine the values of the thermal power when turning off the fans or reducing the number of revolutions.

]]>Fluids doi: 10.3390/fluids8050140

Authors: Suhaib M. Hayajneh Jamal Naser

The purpose of this paper is to investigate the fire performance in a multi-storey cross-laminated timber (CLT) structure by the computational fluid dynamics (CFD) technique using the Fire Dynamics Simulator (FDS v.6.7). The study investigates fire temperature, heat release rate (HRR), and gas concentration (O2, CO2). The importance of this research is to ensure that the fire performance of timber buildings is adequate for occupant safety and property protection. Moreover, the proposed technique provides safety measures in advance for engineers when designing buildings with sufficient fire protection by predicting the fire temperature, time to flashover and fire behaviour. The present numerical modelling is designed to represent a 10-storey CLT residential building where each floor has an apartment with 9.14 m length by 9.14 width dimensions. The pyrolysis model was performed with thermal and kinetic parameters where the furniture, wood cribs and CLT were allowed to burn by themselves in simulation. This research is based on a full-scale experiment of a two-storey CLT building. The present results were validated by comparing them with the experimental data. Numerical simulation of CLT building models show a very close accuracy to the experiment performed in the benchmark paper. The results show that the CFD tools such as FDS can be used for predicting fire scenarios in multi-storey CLT buildings.

]]>Fluids doi: 10.3390/fluids8050139

Authors: Mustafa Bukhari Hassan Fayed Saad Ragab

The two-fluid (Euler&ndash;Euler) model and large-eddy simulation are used to compute the turbulent two-phase flow of air and water in a cyclonic flotation device known as an Air-Sparged Hydrocyclone (ASH). In the operation of ASH, air is injected through a porous cylindrical wall. The study considers a 48 mm diameter hydrocyclone and uses a block-structured fine mesh of 10.5 million hexagonal elements. The air-to-water injection ratio is 4, and a uniform air bubble diameter of 0.5 mm is specified. The flow field in ASH was investigated for the inlet flow rate of water of 30.6 L/min at different values of underflow exit pressure. The current simulations quantify the effects of the underflow exit pressure on the split ratio and the overall flow physics in ASH, including the distribution of the air volume fraction, water axial velocity, tangential velocity, and swirling-layer thickness. The loci of zero-axial velocity surfaces were determined for different exit pressures. The water split ratio through the overflow opening varies with underflow exit pressure as 6%, 8%, 16%, and 26% for 3, 4, 5, and 6 kPa, respectively. These results indicate that regulating the pressure at the underflow exit can be used to optimize the ASH&rsquo;s performance. Turbulent energy spectra in different regions of the hydrocyclone were analyzed. Small-scale turbulence spectra at near-wall points exhibit f&minus;4 law, where f is frequency. Whereas for points at the air-column interface, the energy spectra show an inertial subrange f&minus;5/3 followed by a dissipative range of f&minus;7 law.

]]>Fluids doi: 10.3390/fluids8050138

Authors: Koji Hasegawa Yuya Kishimoto

The interfacial instability of a complex fluid in a multiphase flow system is ubiquitous in both nature and industry. We experimentally investigated the spreading and interfacial instability dynamics of a binary droplet (a water and 2-propanol (IPA) mixture) on an immiscible (sunflower oil) pool. For droplets of 40 wt% IPA solution on sunflower oil, fingering instability occurred at the spreading liquid front. To reveal the interfacial characteristics of the spreading and fingering processes, we analyzed the interplay among the speed, diameter, and number of fingers on the spreading front. Based on our observations, the finger length, wavelength between the fingers, head length, and neck length were quantified. Our experimental results clearly demonstrate that fingering instability can be driven by the capillary effect for a liquid&ndash;liquid system as well as the Plateau&ndash;Rayleigh instability. We hope that our results will inspire further experimental and numerical investigations to provide deeper insights into the interfacial dynamics of multicomponent droplets in a liquid pool.

]]>Fluids doi: 10.3390/fluids8050137

Authors: Efstathios Chatzoglou Antonios Liakopoulos Filippos Sofos

In this study, we investigate the performance of the smoothed particle hydrodynamics (SPH) method regarding the computation of confined flows in microchannels. Modeling and numerical simulation with SPH involve the representation of flowing matter as distinct mass points, leading to particle discretization of the Navier&ndash;Stokes equations. The computational methodology exhibits similarities with other well-established particle methods, such as molecular dynamics (MD), dissipative particle dynamics (DPD), and smooth dissipative particle dynamics (SDPD). SPH has been extensively tested in the simulation of free-surface flows. However, studies on the performance of the method in internal flow computations are limited. In this work, we study flows in microchannels of variable cross-sections with a weakly compressible SPH formulation. After preliminary studies of flows in straight constant cross-section ducts, we focus on channels with sudden expansion and/or contraction. Flow models based on periodic or various inlet/outlet boundary conditions and their implementations are discussed in the context of 2D and 3D simulations. Numerical experiments are conducted to evaluate the accuracy of the method in terms of flowrate, velocity profiles, and wall shear stress. The relation between f and Re for constant cross-section channels is computed with excellent accuracy. SPH captured the flow characteristics and achieved very good accuracy. Compressibility effects due to the weakly compressible smoothed particle hydrodynamics (WCSPH) formulation are negligible for the flows considered. Several typical difficulties and pitfalls in the application of the SPH method in closed conduits are highlighted as well as some of the immediate needs for the method&rsquo;s improvement.

]]>Fluids doi: 10.3390/fluids8040136

Authors: Aleksey V. Dengaev Aydar A. Kayumov Andrey A. Getalov Firdavs A. Aliev Gadel F. Baimukhametov Boris V. Sargin Alexander F. Maksimenko Alexey V. Vakhin

Ultrasound technologies are well-known for their ability to intensify the heat and mass transfer processes. Hence, ultrasonic treatment processes are widely applied for the separation of oil&ndash;water emulsions, optimization of oil pumping processes, cleaning the bottomhole zone, etc. However, the main phenomenon under the positive influence of ultrasonic waves on such processes is the cavitation bubbles implosion on the water&ndash;oil boundary. It is well-known that ultrasound energy contributes to the reversible viscosity reduction in heavy oil systems. However, it is possible to exhibit chemical destruction of the weakest carbon&ndash;heteroatom bonds in the structure of the asphaltenes. This study investigates the influences of controllable ultrasound waves with frequency ranges of 20&ndash;60 kHz under the exposure time of 60 s on the rheology of a heavy crude oil sample produced from the Ashalcha reservoir (Tatarstan Republic, Russia). The specific feature of this study is the application of multi-frequency ultrasonic exposure with a wide spectrum of side harmonics with the frequency up to 400 kHz. The results of the Saturates, Aromatics, Resins and Asphaltenes (SARA) analysis method support the chemical consequences of ultrasonication of crude oil. The content of resins under the irradiation of ultrasound waves altered from 32.5 wt.% to 29.4 wt.%, while the number of aromatics hydrocarbons raised from 24.3 wt.% to 34.1 wt.%. The Gas Chromatography&mdash;Mass Spectroscopy (GC-MS) analytical analysis method was applied to qualitatively compare the composition of saturated and aromatics fractions between the initial and upgraded heavy crude oil in order to show the chemical destruction of asphaltene bonds after the ultrasonic treatment. It was demonstrated that ultrasound waves allowed chemical conversion of asphaltene fragments that led to irreversible viscosity reduction. The viscosity of the heavy oil sample under the favorable ultrasonic irradiation conditions reduced from 661.2 mPa&middot;s to 178.8 mPa&middot;s. This advantage can be used to develop enhanced oil recovery methods and partial upgrading processes in downstream conditions.

]]>Fluids doi: 10.3390/fluids8040135

Authors: James K. Arthur

Many open-channel turbulent flow studies have been focused on highly constrained conditions. Thus, it is rather conventional to note such flows as being fully developed, fully turbulent, and unaffected by sidewalls and free surface disturbances. However, many real-life flow phenomena in natural water bodies and artificially installed drain channels are not as ideal. This work is aimed at studying some of these unconstrained conditions. This is achieved by using particle image velocimetry measurements of a developing turbulent open-channel flow over a smooth wall. The tested flow effects are low values of the Reynolds number based on the momentum thickness Re&theta; (ranging from 165 to 930), low aspect ratio AR (ranging from 1.1 to 1.5), and Froude number Fr (ranging from 0.1 to 0.8). The results show that the mean flow has an inner region with a logarithmic layer with a von K&aacute;rm&aacute;n constant of 0.40&ndash;0.41, and a log law constant ranging from 5.0 to 6.0. The friction velocity and coefficient of skin friction are predictable using the formulations of Fr and Re&theta; presented in this work. The outer region is also characterized by a dip location, which is predictable using an equation associated with Re&theta;. The higher-order turbulence statistics, on the other hand, show distinguishing traits, such as correlation coefficients ranging from &minus;0.1 to 0.5. Overall, this work demonstrates that for the unconstrained conditions studied, friction evaluations associated with Reynolds shear stress and some notable turbulence modelling functions used in conventional open-channel flows are inapplicable.

]]>Fluids doi: 10.3390/fluids8040134

Authors: Mikhail Shur Mikhail Strelets Andrey Travin

This paper addresses one of the major obstacles arising in the high-fidelity scale-resolving simulations of turbulent flows inside ducts with the walls covered by acoustic liners in order to attenuate the sound radiated from the duct. It consists of the development of spatial hydrodynamic (convective) instability over the treated walls at the low values of the acoustic resistance of the liner. For reasons that remain unclear, the growth rate of this instability and its effect on sound propagation through the duct is strongly overestimated by the CFD simulations using the macroscopic concept of the locally reacting acoustic impedance. A new damping volume source term (&ldquo;body force&rdquo;) is proposed, whose introduction into the momentum equation resolves this issue by means of artificially suppressing the instability while remaining within the framework of the computationally efficient model of the impedance wall, i.e., without trying to simulate the liner microscopically. Examples are presented of the application of the developed methodology to the flows in the grazing impedance tubes with two different liners. They suggest that the proposed form of the damping source term can be considered universal and that the suppression of the hydrodynamic instability ensured by this term is not accompanied by any significant distortion of the propagation of the sound waves and the turbulence statistics, except for a very narrow near-wall region.

]]>Fluids doi: 10.3390/fluids8040133

Authors: Ahmed Faraj Alarbi Alsharief Xili Duan Yuri S. Muzychka

Drag reduction (DR) using superhydrophobic surfaces (SHSs) has received intensive interest due to the emergence of SH coating technology. The air layer (plastron &ldquo;&delta;&rdquo;) trapped between the SHS and the water controls the flow slip over the SHSs. We demonstrate slippage over three fabricated SHSs in laminar and low turbulent Taylor&ndash;Couette flows. We experimentally investigate how the slip length increases with a higher Reynolds number (Re) over the tested SHSs; simultaneously, the air plastron thickness investigates using a viscous model. The mean skin friction coefficient (Cf) can be fitted to a modified semi-empirical logarithmic law expressed in the Prandtl&ndash;von K&aacute;rm&aacute;n coordinate. An effective slip length is estimated in the 35&ndash;41 &micro;m range with an achieved 7&ndash;11% DR for the tested surfaces. Statistical analysis is used to develop a regression model from the experimental data. The model shows an R2 of 0.87 and good agreement with the experimental data. This shows the relation between the dimensionless slip length (b+), the dimensionless plastron thickness (&delta;+), and the Reynolds number, which is directly proportional. The regression model shows that b+ and Reynolds numbers have a higher impact on the &delta;+ than the surface wettability, which attribute to the small difference in the wetting degree between the three tested surfaces. The practical importance of the work lies in its ability to provide a deep understanding of the reduction in viscous drag in numerous industrial applications. Furthermore, this research serves as a groundwork for future studies on hydrophobic applications in internal flows.

]]>Fluids doi: 10.3390/fluids8040132

Authors: Mikhail V. Chernyshov Karina E. Savelova

The supersonic flow of a reactive gas mixture with Mach reflection of oblique shocks and pulsed energy supply at the Mach stem is considered within the framework of the Chapman&ndash;Jouguet theory. An approximate analytical model is proposed that quickly determines the shape and size of the shock-wave structure as well as the flow parameters in various flow regions. As an example of the application of the proposed analytical model, the &ldquo;first barrel&rdquo; of a highly overexpanded jet flow of an air-methane mixture with a high supersonic velocity, is studied. Flows of hydrogen&ndash;air and hydrogen&ndash;oxygen mixtures were also considered for comparison with preceding numerical results. The height of the triple point of the Mach reflection is determined in the presence of a change in the chemical composition of the mixture and an isobaric pulsed energy supply at the main shock.

]]>Fluids doi: 10.3390/fluids8040131

Authors: Lingxi Han Tianyuan Zhang Di Yang Rui Han Shuai Li

The jet impact from a collapsing bubble is an important mechanism of structural damage in underwater explosions and cavitation erosion. The Boundary Integral Method (BIM) is widely used to simulate nonspherical bubble dynamic behaviors due to its high accuracy and efficiency. However, conventional BIM cannot simulate toroidal bubble dynamics, as the flow field transforms from single-connected into double-connected. To overcome this problem, vortex cut and vortex ring models can be used to handle the discontinuous potential on the toroidal bubble surface. In this work, we compare these two models applied to toroidal bubble dynamics in a free field and near a rigid wall in terms of bubble profile, bubble gas pressure, and dynamic pressure induced by the bubble, etc. Our results show that the two models produce comparable outcomes with a sufficient number of nodes in each. In the axisymmetric case, the vortex cut model is more efficient than the vortex ring model. Moreover, we found that both models improve in self-consistency as the number of bubble surface elements (N) increases, with N=300 representing an optimal value. Our findings provide insights into the numerical study of toroidal bubble dynamics, which can enhance the selection and application of numerical models in research and engineering applications.

]]>Fluids doi: 10.3390/fluids8040130

Authors: Armands Gritsans Valentina Koliskina Andrei Kolyshkin Felix Sadyrbaev

Linear stability analysis of a combined convective flow in an annulus is performed in the paper. The base flow is generated by two factors: (a) different constant wall temperatures and (b) heat release as a result of a chemical reaction that takes place in the fluid. The nonlinear boundary value problem for the distribution of the base flow temperature is analyzed using bifurcation analysis. The linear stability problem is solved numerically using a collocation method. Two separate cases are considered: Case 1 (non-zero different constant wall temperatures) and Case 2 (zero wall temperatures). Numerical calculations show that the development of instability is different for Cases 1 and 2. Multiple minima on the marginal stability curves are found for Case 1 as the Prandtl number increases. Concurrence between local minima leads to the selection of the global minimum in such a way that a finite jump in the value of the wave number is observed for some values of the Prandtl number. All marginal stability curves for Case 2 have one minimum in the range of the Prandtl numbers considered. The corresponding critical values of the Grashof number decrease monotonically as the Prandtl number grows.

]]>Fluids doi: 10.3390/fluids8040129

Authors: Iosif Moulinos Christos Manopoulos Sokrates Tsangaris

Balloon pumps are employed to assist cardiac function in cases of acute myocardial infarction, ventricular arrhythmias, cardiogenic shock, unstable angina, refractory ventricular failure, or cardiac surgery. Counterpulsation, through increasing the diastolic pressure and reducing the systolic pressure, increases coronary perfusion and assists the heart to pump more blood at each contraction. An expanding-contracting balloon, modifying the Poiseuille flow in a straight circular duct, is examined in this study. The balloon is spheroid-shaped, with the length of its minor axis, which is perpendicular to the flow direction, changing in time following a sinusoidal law. The inlet flow volume rate is steady while the rate that the fluid volume leaves the duct varies in time due to the presence of the balloon. For a pulsation frequency of 60 pulses/min, the pressure difference across the pulsating balloon exhibits significant phase lagging behind the outflow volume waveform. The outlet pressure depends on the balloon radius oscillation amplitude and is computed for a range of such. The flow field around the spheroid, periodically expanding-contracting balloon in the steady flow stream is presented, in which the exact pattern of the gradual downstream intensification of the flow pulsation alongside the spheroid body is also identified.

]]>Fluids doi: 10.3390/fluids8040128

Authors: Andres Reyes-Urrutia Juan Pablo Capossio Cesar Venier Erick Torres Rosa Rodriguez Germán Mazza

The fluidization of certain biomasses used in thermal processes, such as sawdust, is particularly difficult due to their irregular shapes, varied sizes, and low densities, causing high minimum fluidization velocities (Umf). The addition of an inert material causes its Umf to drop significantly. The determination of the Umf of the binary mixture is however hard to obtain. Generally, predictive correlations are based on a small number of specific experiments, and sphericity is seldom included. In the present work, three models, i.e., an empirical correlation and two artificial neural networks (ANN) models were used to predict the Umf of biomass-inert mixtures. An extensive bibliographical survey of more than 200 datasets was conducted with complete data about densities, particle diameters, sphericities, biomass fraction, and Umf. With the combined application of the partial dependence plot (PDP) and the ANN models, the average effect of sphericity on Umf was quantitatively determined (inverse relationship) together with the average impact of the biomass fraction on Umf (direct relationship). In comparison with the empirical correlations, the results showed that both ANN models can accurately predict the Umf of the presented binary mixtures with errors lower than 25%.

]]>Fluids doi: 10.3390/fluids8040127

Authors: Mohammad I. Inam Wenxian Lin Steven W. Armfield Mehdi Khatamifar

The flow behavior of weak symmetric plane fountains in linearly stratified fluids is studied numerically with three-dimensional simulations over a range of the Froude (Fr), Reynolds (Re), and stratification numbers (s). The two main parameters describing the fountain characterization are the dimensionless maximum fountain penetration height (zm) and intrusion velocity (uint), which differ significantly at different flow development stages. It was found that the stratification stabilizes the symmetry of the weak fountains, which makes the fountain become asymmetric at a larger Fr value, and zm at the fully developed stage continues to increase as a result of the intrusion, which continually changes the ambient fluid stratification features, thus the buoyant force. The evolution of intrusion experiences three distinct stages. Both Fr and s have effects on zm and uint, with the effect of Fr usually larger than that of s. The overall impacts of Fr and s can be quantified in terms of Frasb, with a and b varying for different parameters. With numerical results, empirical correlations are produced in terms of Frasb for each relevant parameter, which generally predict the results very well.

]]>Fluids doi: 10.3390/fluids8040126

Authors: Ermiyas Lakew Amirhosein Sarchami Giovanni Giustini Hyungdae Kim Kishan Bellur

Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This paper presents analyses of the evaporation of &ldquo;microlayers&rdquo;, i.e., ultra-thin layers of liquid present beneath steam bubbles growing at the heated surface in the atmospheric pressure nucleate of boiling water. Evaporation of the microlayer is believed to be a major contributor to the phase change heat transfer, but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still poorly understood. Mass, momentum, and energy transfer in the microlayer are modeled with a lubrication theory approach that accounts for capillary and intermolecular forces and interfacial mass transfer. The model is embodied in a third-order nonlinear film evolution equation, which is solved numerically. Variable wall-temperature boundary conditions are applied at the solid&ndash;liquid interface to account for conjugate heat transfer due to evaporative heat loss at the liquid&ndash;vapor interface. Predictions obtained with the current approach compare favorably with experimental measurements of microlayer evaporation. By comparing film profiles at a sequence of times into the ebullition cycle of a single bubble, likely values of evaporative heat transfer coefficients were inferred and found to fall within the range of previously reported estimates. The result suggests that the coefficients may not be a constant, as previously assumed, but instead something that varies with time during the ebullition cycle.

]]>Fluids doi: 10.3390/fluids8040125

Authors: Bo Liao Guohai Dong Yuxiang Ma Xiaozhou Ma

Head-on collisions between two solitary waves in the framework of the nonlinear Schr&ouml;dinger (NLS) equation were investigated using the Fourier spectral method. When solitary waves undergo collision, the peak value of surface elevation (hereafter referred to as &zeta;max) exhibits fluctuations with increasing relative water depths k0h (where k0 is the wave number and h is the water depth). &zeta;max is approximately equal to the sum of the peak values of the two solitary waves with smaller wave steepness &epsilon;0 (&epsilon;0 = k0a0, a0 is the free background amplitude parameter), and it exhibits fluctuations for &epsilon;0 &gt; 0.10. Similar results have been observed in the study of head-on collisions for four solitary waves. These results show that the water depth and wave steepness play important roles in the collision of solitary waves, and the effects of the interactions of intense wave groups are important in studies of the mechanisms and manifestations of freak oceanic waves.

]]>Fluids doi: 10.3390/fluids8040124

Authors: Kannangara D. C. R. Dissanayaka Norio Tanaka

There are multiple initiatives aimed at strengthening coastal communities against tsunami disaster risks, such as growing vegetation belts, construction of embankments, moats, and different hybrid alternatives. To find a solution for strengthening the coastal buildings themselves, we firstly reviewed the flow phenomena around a single emergent (circular and rectangular) cylinder (case C1), which was considered as a piloti-type column under different Froude conditions, and evaluated the formation of surface bow-waves, hydraulic jump detachment, and wall-jet-like bow-waves. Secondly, the flow characteristics were investigated under the same Froude conditions with side-by-side two-cylinder (case C2) and four-cylinder (case C4) arrays in an open channel. Surface bow-wave length (LBw) increased by 7&ndash;12% over the rectangular cylinders (RCs) compared to the circular cylinders (CCs) with a subcritical flow. For the supercritical flow with a 1/200 bed slope, hydraulic jump detachment was observed in relation to the Froude number. The observed length of the hydraulic jump detachment (Ljump) varied between 3.1&ndash;8.5% and 4.2&ndash;12.9% for the CCs and RCs in the supercritical flow with a 1/200 bed slope. In addition, the wall-jet-like bow-wave height (hjet) over the CCs was increased by 37% and 29% compared to the RCs with a supercritical flow and zero bed slope (orifice-type flow). For case C4, a hydraulic jump was observed for the supercritical flow over the horizontal channel bed. Finally, empirical equations were defined concerning the geometrical shape and arrangement based on the experiment data for the single and side-by-side configurations of the cylinders to validate the height of the wall-jet-like bow-wave as the most critical flow property.

]]>Fluids doi: 10.3390/fluids8040123

Authors: Natalya Burmasheva Sergey Ershkov Evgeniy Prosviryakov Dmytro Leshchenko

To solve the problems of geophysical hydrodynamics, it is necessary to integrally take into account the unevenness of the bottom and the free boundary for a large-scale flow of a viscous incompressible fluid. The unevenness of the bottom can be taken into account by setting a new force in the Navier&ndash;Stokes equations (the Rayleigh friction force). For solving problems of geophysical hydrodynamics, the velocity field is two-dimensional. In fact, a model representation of a thin (bottom) baroclinic layer is used. Analysis of such flows leads to the redefinition of the system of equations. A compatibility condition is constructed, the fulfillment of which guarantees the existence of a nontrivial solution of the overdetermined system under consideration. A non-trivial exact solution of the overdetermined system is found in the class of Lin&ndash;Sidorov&ndash;Aristov exact solutions. In this case, the flow velocities are described by linear forms from horizontal (longitudinal) coordinates. Several variants of the pressure representation that do not contradict the form of the equation system are considered. The article presents an algebraic condition for the existence of a non-trivial exact solution with functional arbitrariness for the Lin&ndash;Sidorov&ndash;Aristov class. The isobaric and gradient flows of a viscous incompressible fluid are considered in detail.

]]>Fluids doi: 10.3390/fluids8040122

Authors: Roberta Caruana Stefano De Antonellis Luca Marocco Paolo Liberati Manfredo Guilizzoni

Indirect Evaporative Cooling (IEC) is a very promising technology to substitute and/or integrate traditional air conditioning systems, due to its ability to provide cooling capacity with limited power consumption. Literature studies proved that a higher wettability of the IEC plates corresponds to better performance of the system. In this work, wettability of three different surfaces used for IEC systems plates&mdash;uncoated aluminum alloy (AL), standard epoxy coating (STD), and a hydrophilic lacquer (HPHI)&mdash;is studied and characterized in terms of static and dynamic contact angles. The static contact angle resulted to be the lowest for the HPHI surface (average 69&deg;), intermediate for the STD surface (average 75&deg;), and the highest for the AL surface (average 89&deg;). The analysis of the dynamic contact angles showed that their transient behavior is similar for all the surfaces, and the advancing and receding contact angles obtained are consistent with the results of the static analysis. These results will be useful as input parameters in models aimed at predicting the IEC system performance, also using computational fluid dynamics.

]]>Fluids doi: 10.3390/fluids8040121

Authors: Marlene Crone Michael Türk

Supercritical fluid reactive deposition is an environmentally friendly technique for the synthesis of supported mono- or bimetallic nanoparticles. Experimental results show that the adsorption of a precursor on a substrate is the crucial process step that controls the loading and the size of the deposited metal nanoparticles. In this review, an overview of experimental and modeling work is given and selected experimental data were correlated with the following adsorption isotherm models: Henry, Freundlich, Langmuir, Toth, and Langmuir&ndash;Freundlich equations. As a result, in the case of precursors with a low CO2 solubility and therewith low uptake, the adsorption behavior can be described with sufficient accuracy by the Henry approach. Furthermore, the Freundlich and Langmuir equations enable sufficiently accurate descriptions of the experimental data. In the end, strategies for overcoming the knowledge gaps for essential future research directions are suggested.

]]>Fluids doi: 10.3390/fluids8040120

Authors: Atul Bhattad Boggarapu Nageswara Rao Vinay Atgur Ibham Veza Mohd Faiz Muaz Ahmad Zamri Islam Md Rizwanul Fattah

This paper aims to develop models for the thermal conductivity and viscosity of hybrid nanofluids of aluminium oxide and titanium dioxide (Al2O3-TiO2). The study investigates the impact of fluid temperature (283 K&ndash;298 K) on the performance of a plate heat exchanger using Al2O3-TiO2 hybrid nanofluids with different particle volume ratios (0:5, 1:4, 2:3, 3:2, 4:1, and 5:0) prepared with a 0.1% concentration in deionised water. Experimental evaluations were conducted to assess the heat transfer rate, Nusselt number, heat transfer coefficient, Prandtl number, pressure drop, and performance index. Due to the lower thermal conductivity of TiO2 nanoparticles compared to Al2O3, a rise in the TiO2 ratio decreased the heat transfer coefficient, Nusselt number, and heat transfer rate. Inlet temperature was found to decrease pressure drop and performance index. The Al2O3 (5:0) nanofluid demonstrated the maximum enhancement of around 16.9%, 16.9%, 3.44%, and 3.41% for the heat transfer coefficient, Nusselt number, heat transfer rate, and performance index, respectively. Additionally, the TiO2 (0:5) hybrid nanofluid exhibited enhancements of 0.61% and 2.3% for pressure drop and Prandtl number, respectively. The developed hybrid nanofluids enhanced the performance of the heat exchanger when used as a cold fluid.

]]>Fluids doi: 10.3390/fluids8040119

Authors: Pietro Renato Avallone Simona Russo Spena Stefano Acierno Maria Giovanna Esposito Andrea Sarrica Marco Delmonte Rossana Pasquino Nino Grizzuti

Hydrocolloids are long-chain biopolymers that can form viscous solutions or gels when dissolved in water. They are employed as rheological modifiers in various manufacturing processes or finished products. Due to its unique gelation properties, animal gelatin is one of the most widely used hydrocolloids, finding applications in several fields such as food, pharmaceutical, and photographic. Nowadays, the challenge of finding valid alternatives to animal products has become a crucial issue, for both ethical and environmental reasons. The aim of this work, is to propose a green hydrocolloidal network, able to reproduce the gelation features of animal gelatin gels. &kappa;-carrageenan gels may be an interesting alternative to gelatin, due to their attractive gelling features. We investigate the thermorheological behavior of &kappa;-carrageenan aqueous solutions at various concentrations, focusing on gel features such as transition temperature and gel strength. To improve the viscoelastic response of such gels, we add a viscosity-enhancing hydrocolloid, i.e., xanthan gum. The results show that the gel strength increases exponentially with xanthan concentration, thus suggesting a synergistic interaction between the two networks. We also study the effect of sucrose on the thermal and mechanical properties of modified gels, finding a marked increase in transition temperatures and gel elasticity. In recent years, three-dimensional (3D) food printing has been extensively studied in the food industry, due to its many advantages, such as customized food design, personalized nutrition, simplified supply chain, and the expansion of available food materials. In view of this growing interest for additive manufacturing, we also study the printability of the complete formulation composed of &kappa;-carrageenan, xanthan gum and sucrose.

]]>Fluids doi: 10.3390/fluids8040118

Authors: Ying Wu Fue-Sang Lien Eugene Yee Guang Chen

The transverse flow-induced vibration (FIV) of an elastically-supported cylinder-plate assembly (viz., a rigid splitter-plate attached to the downstream side of a circular cylinder) with a low mass ratio of 10 and zero structural damping is investigated using numerical simulations at a Reynolds number of 100. The structural oscillations and characteristics of the flow around the structure are analyzed in terms of the vibration characteristics and the fluid forces as a function of the plate length LSP and the reduced velocity Ur. These investigations involve a wide range of plate lengths LSP/D = 0&ndash;4 (where D is the cylinder diameter) over an extensive span of reduced velocities Ur = 2&ndash;30. For LSP/D &le; 0.5, self-limiting oscillations are induced in the assembly&mdash;these oscillations correspond to either a vortex-induced vibration (VIV) or an integrated VIV-galloping response. For LSP/D &ge; 0.75, the amplitude response is no longer self-limiting in the sense that the oscillation amplitude increases linearly with increasing Ur&mdash;these oscillations correspond to either a strongly correlated VIV-galloping regime (for LSP/D = 0.75), or two clearly separated regimes: namely, a VIV regime with small-amplitude oscillation and a non-limiting galloping regime (for LSP/D &gt; 0.75).

]]>Fluids doi: 10.3390/fluids8040117

Authors: Edoardo Mantecca Alessandro Colombo Antonio Ghidoni Gianmaria Noventa David Pasquale Stefano Rebay

The aim of this work is to describe an efficient implementation of cubic and multiparameter real gas models in an existing discontinuous Galerkin solver to extend its capabilities to the simulation of turbulent real gas flows. The adopted thermodynamic models are van der Waals, Peng&ndash;Robinson, and Span&ndash;Wagner, which differ from each other in terms of accuracy and computational cost. Convective numerical fluxes across elements interfaces are calculated with a thermodynamic consistent linearized Riemann solver, whereas for boundary conditions, a linearized expression of the generalized Riemann invariants is employed. Transport properties are treated as temperature- and density-dependent quantities through multiparameter correlations. An implicit time integration is adopted; Jacobian matrix and thermodynamic derivatives are obtained with the automatic differentiation tool Tapenade. The solver accuracy is assessed by computing both steady and unsteady real gas test cases available in the literature, and the effect of the mesh size and polynomial degree of approximation on the solution accuracy is investigated. A good agreement with experimental and numerical reference data is observed and specific non-classical phenomena are well reproduced by the solver.

]]>Fluids doi: 10.3390/fluids8040116

Authors: Stefan Schoder Jakob Schmidt Andreas Fürlinger Roppert Klaus Maurerlehner Paul

New innovative green concepts in electrified vertical take-off and landing vehicles are currently emerging as a revolution in urban mobility going into the third dimension (vertically). The high population density of cities makes the market share highly attractive while posing an extraordinary challenge in terms of community acceptance due to the increasing and possibly noisier commuter traffic. In addition to passenger transport, package deliveries to customers by drones may enter the market. The new challenges associated with this increasing transportation need in urban, rural, and populated areas pose challenges for established companies and startups to deliver low-noise emission products. The article&rsquo;s objective is to revisit the benefits and drawbacks of an affordable acoustic measurement campaign focused on early prototyping. In the very early phase of product development, available resources are often considerably limited. With this in mind, this article discusses the sound power results using the enveloping surface method in a typically available low-reflection room with a reflecting floor according to DIN EN ISO 3744:2011-02. The method is applied to a subsonic electric ducted fan (EDF) unit of a 1:2 scaled electrified vertical take-off and landing vehicle. The results show that considerable information at low costs can be gained for the early prototyping stage, despite this easy-to-use, easy-to-realize, and non-fine-tuned measurement setup. Furthermore, the limitations and improvements to a possible experimental setup are presented to discuss a potentially more ideal measurement environment. Measurements at discrete operating points and transient measurements across the total operating range were conducted to provide complete information on the EDF&rsquo;s acoustic behavior. The rotor-self noise and the rotor&ndash;stator interaction were identified as primary tonal sound sources, along with the highest broadband noise sources located on the rotor. Based on engineering experience, a first acoustic improvement treatment was also quantified with a sound power level reduction of 4 dB(A). In conclusion, the presented method is a beneficial first measurement campaign to quantify the acoustic properties of an electric ducted fan unit under minimal resources in a reasonable time of several weeks when starting from scratch.

]]>Fluids doi: 10.3390/fluids8040115

Authors: Jiří Primas Michal Malík Pavel Pokorný Josef Novák Petr Parma Filip Sanetrník Petr Schovanec

This paper is focused on the research of airflow generating through the use of high-voltage electrohydrodynamic devices. For this purpose, the authors built several electrohydrodynamic airflow generators with one point electrode and one tube electrode of varying dimensions and compared their efficiency in generating the airflow in order to find an optimal design. The character of the flow was also analyzed with the help of particle image velocimetry, and velocity vector maps and velocity profile were acquired. In addition, a possible practical cooling application was proposed and realized with positive results. Lastly, the products present in the generated airflow were tested for ozone and nitrogen oxides, which could have detrimental effects on human health and material integrity. In both cases, the concentration has been found to be below permissible limits.

]]>Fluids doi: 10.3390/fluids8040114

Authors: Alberto Carpinteri Gianni Niccolini Federico Accornero

By using complex potentials, some light is shed on the analogy between the singularity problems arising in fluid and fracture mechanics&mdash;in particular, those concerning plane irrotational flows around sharp obstacles and plane elasticity in cracked bodies. Applications to two equivalent geometries are shown: a thin plate transversally immersed in a uniform flow and a crack subjected to uniform out-of-plane shearing stress at infinity (Mode III). The matching between the fluid velocity field and the shearing stress field is consistent with the hydrodynamic analogy. Aside from the Reynolds criterion for the natural laminar-to-turbulent transition, a velocity-intensity factor criterion is defined to predict the forced turbulent-to-vortex-shedding fluid-flow transition (forced transitional flow) generated by a transversal plate obstacle. It is interesting to remark that the velocity-intensity factor presents physical dimensions intermediate between those of a velocity and a kinematic viscosity. In addition, it will be demonstrated that size affects the occurrence of natural-to-forced transitional phenomena in fluids, in a strict analogy to the scale-dependent ductile-to-brittle failure transitions in solids.

]]>Fluids doi: 10.3390/fluids8040113

Authors: Nastaran Rezaee John Aunna Jamal Naser

Experimental investigations of Marangoni flow in micro-foams have faced challenges due to the inherent difficulties in detecting and measuring this flow. The Marangoni flow manifests as small spots within the lamellae films, which makes it hard to accurately analyze. Hence, to elucidate Marangoni flow characteristics, this study introduces and investigates comprehensive three-dimensional models of flow in microscale foams. The geometric models contained Plateau Borders (PB), nodes, and films. The recirculating Marangoni flow was simulated and studied for different interfacial mobilities. Inside the foams, the Marangoni flow velocities were at the same scale with the PB flow velocity for mobile interfaces. However, for a more rigid interface, the magnitude of the Marangoni flow was considerably less than that of the PB owing to the combined effect of high surface hydraulic resistance on the Marangoni flows and nature of the Marangoni flow as a surface-only flow type. Furthermore, the effect of the film thickness on the Marangoni flow was analyzed. Thicker films have a stronger effect in reducing the Marangoni flow than PB flow. This is due to the higher ratio of gravity body force to the Marangoni-driven surface force for thicker films. Finally, the combined effect of the liquid&ndash;air interfacial mobility and film thickness on the Marangoni velocity was studied.

]]>Fluids doi: 10.3390/fluids8040112

Authors: Raul Alberto Bernal-Orozco Ignacio Carvajal-Mariscal Oliver Marcel Huerta-Chavez

Simulation is a valuable tool for the study of DBD actuators, therefore accurate, computationally efficient, and robust numerical models are required. The performance of three DBD actuator models was studied: the phenomenological Shyy and Suzen models, and the empirical D&ouml;rr and Kloker model. The first objective of this work is to determine the ability of these models to reproduce the force and induced flow by comparing the numerical results with experimental reference data reported in the literature. As a second objective, modifications have been proposed to improve these models. Several simulations were performed in OpenFOAM with different geometrical parameters, voltages, and frequencies. Discrepancies and limitations of the models were identified. The modified D&ouml;rr and Kloker model allows more consistent use of this model by considering a factor that relates it to voltage and frequency. Shyy&rsquo;s modified model reduces the overestimation of force and velocity. Suzen&rsquo;s modified model is the one that fits the reference data better, so its use is suggested over the other models. The proposed modifications are easy to implement and allow significant improvements in the capacity of the models to reproduce the effects of a DBD actuator.

]]>Fluids doi: 10.3390/fluids8040111

Authors: Saket Kapse Dena Rahman Eldad J. Avital Nithya Venkatesan Taylor Smith Lidia Cantero-Garcia Fariborz Motallebi Abdus Samad Clive B. Beggs

Existing indoor closed ultraviolet-C (UVC) air purifiers (UVC in a box) have faced technological challenges during the COVID-19 breakout, owing to demands of low energy consumption, high flow rates, and high kill rates at the same time. A new conceptual design of a novel UVC-LED (light-emitting diode) air purifier for a low-cost solution to mitigate airborne diseases is proposed. The concept focuses on performance and robustness. It contains a dust-filter assembly, an innovative UVC chamber, and a fan. The low-cost dust filter aims to suppress dust accumulation in the UVC chamber to ensure durability and is conceptually shown to be easily replaced while mitigating any possible contamination. The chamber includes novel turbulence-generating grids and a novel LED arrangement. The turbulent generator promotes air mixing, while the LEDs inactivate the pathogens at a high flow rate and sufficient kill rate. The conceptual design is portable and can fit into ventilation ducts. Computational fluid dynamics and UVC ray methods were used for analysis. The design produces a kill rate above 97% for COVID and tuberculosis and above 92% for influenza A at a flow rate of 100 L/s and power consumption of less than 300 W. An analysis of the dust-filter performance yields the irradiation and flow fields.

]]>Fluids doi: 10.3390/fluids8040110

Authors: Jairo Murillo-Rincón Carlos Duque-Daza

The use of a synthetic jet as the flow control technique to modulate a turbulent incompressible round jet was explored and assessed by numerical simulations. The flow response was characterised in terms of turbulent statistics and acoustic response in the far-field. A quasi-Direct Numerical Simulation (qDNS) strategy was used to predict the turbulent effects. The Ffowcs-Williams and Hawkings (FWH) acoustic analogy was employed to compute the far-field acoustic response. An amplification effect of the instabilities induced by the control jet was observed for some of the parameters explored. It was observed that the control technique allows controlling the axial distribution of the production and dissipation of turbulent kinetic energy, but with respect to the acoustic aspects, the appearance of a greater number of noise sources was observed, which in the far-field, resulted in an increase from 1 to 20 dB of the equivalent noise for the different operating parameters of the control technique studied.

]]>Fluids doi: 10.3390/fluids8040109

Authors: Olga A. Azarova

High speed gas flows occur during the movement of aircrafts, rockets, and descent vehicles, as well as in combustion chambers, nozzles, and many other technological applications [...]

]]>Fluids doi: 10.3390/fluids8040108

Authors: Aleksey V. Dengaev Mohammed A. Khelkhal Andrey A. Getalov Gadel F. Baimukhametov Aydar A. Kayumov Alexey V. Vakhin Marat R. Gafurov

The present review paper discusses the different aspects related to the chemical transformation of oil components through ultrasound assistance. Ultrasound intensifies heat and mass transfer processes in oil production and treatment, which is used to separate water&ndash;oil emulsions, optimize pumping, clean the bottomhole zone, and more. The main reason for the positive effect of ultrasound is the cavitation phenomenon, which forms vapor&ndash;gas bubbles that cause changes in the structure and properties of dispersed phases, intensifying processes such as dissolution, extraction, and emulsification. The inhomogeneities in the medium being processed also reduce resistance to bubble formation and increase the intensity of technological processes. It is believed that ultrasonic treatment of heavy oil influences the colloid structure of oil. Such effects were observed in several studies. Despite the widespread use of ultrasound in oil processing, the chemical transformation of hydrocarbons during ultrasonic treatment remains an understudied area, particularly for heavy oil. Furthermore, the transformation mechanism of high-molecular-weight fragments of oil under ultrasonic energy is still poorly understood. Heavy oil can benefit greatly from ultrasonic treatment, both after production for pipeline transportation or plant processing and in the reservoir. This is due to the improved mobility of oil in rock and the chemical transformation of high-molecular components, such as resins, asphaltenes, and paraffins. These transformations contribute to the overall improvement of heavy oil processing, making it a crucial area for further research and development. In this review paper, we will explore the latest innovations in oil processing, specifically focusing on the chemical transformation of oil components through ultrasound assistance. This will include a comprehensive analysis of the underlying mechanisms of ultrasonic treatment and their impact on the chemical composition of oil. The review will also include a discussion of the current state of the art and future directions for research in this field, highlighting the potential for further advancements in the use of ultrasound in oil processing.

]]>Fluids doi: 10.3390/fluids8040107

Authors: John V. Shebalin

Transition of ideal, homogeneous, incompressible, magnetohydrodynamic (MHD) turbulence to near-equilibrium from non-equilibrium initial conditions is examined through new long-time numerical simulations on a 1283 periodic grid. Here, we neglect dissipation because we are primarily concerned with behavior at the largest scale which has been shown to be essentially the same for ideal and real (forced and dissipative) MHD turbulence. A Fourier spectral transform method is used to numerically integrate the dynamical equations forward in time and results from six computer runs are presented with various combinations of imposed rotation and mean magnetic field. There are five separate cases of ideal, homogeneous, incompressible, MHD turbulence: Case I, with no rotation or mean field; Case II, where only rotation is imposed; Case III, which has only a mean magnetic field; Case IV, where rotation vector and mean magnetic field direction are aligned; and Case V, which has nonaligned rotation vector and mean field directions. Dynamic coefficients are predicted by statistical mechanics to be zero-mean random variables, but largest-scale coherent magnetic structures emerge in all cases during transition; this implies dynamo action is inherent in ideal MHD turbulence. These coherent structures are expected to occur in Cases I, II and IV, but not in Cases III and V; future studies will determine whether they persist.

]]>Fluids doi: 10.3390/fluids8040106

Authors: Emily K. Hawkins Jonathan S. Cheng Jewel A. Abbate Timothy Pilegard Stephan Stellmach Keith Julien Jonathan M. Aurnou

The connection between the heat transfer and characteristic flow velocities of planetary core-style convection remains poorly understood. To address this, we present novel laboratory models of rotating Rayleigh&ndash;B&eacute;nard convection in which heat and momentum transfer are simultaneously measured. Using water (Prandtl number, Pr&#8771;6) and cylindrical containers of diameter-to-height aspect ratios of &Gamma;&#8771;3,1.5,0.75, the non-dimensional rotation period (Ekman number, E) is varied between 10&minus;7&#8818;E&#8818;3&times;10&minus;5 and the non-dimensional convective forcing (Rayleigh number, Ra) ranges from 107&#8818;Ra&#8818;1012. Our heat transfer data agree with those of previous studies and are largely controlled by boundary layer dynamics. We utilize laser Doppler velocimetry (LDV) to obtain experimental point measurements of bulk axial velocities, resulting in estimates of the non-dimensional momentum transfer (Reynolds number, Re) with values between 4&times;102&#8818;Re&#8818;5&times;104. Behavioral transitions in the velocity data do not exist where transitions in heat transfer behaviors occur, indicating that bulk dynamics are not controlled by the boundary layers of the system. Instead, the LDV data agree well with the diffusion-free Coriolis&ndash;Inertia&ndash;Archimedian (CIA) scaling over the range of Ra explored. Furthermore, the CIA scaling approximately co-scales with the Viscous&ndash;Archimedian&ndash;Coriolis (VAC) scaling over the parameter space studied. We explain this observation by demonstrating that the VAC and CIA relations will co-scale when the local Reynolds number in the fluid bulk is of order unity. We conclude that in our experiments and similar laboratory and numerical investigations with E&#8819;10&minus;7, Ra&#8818;1012, Pr&#8771;7, heat transfer is controlled by boundary layer physics while quasi-geostrophically turbulent dynamics relevant to core flows robustly exist in the fluid bulk.

]]>Fluids doi: 10.3390/fluids8030105

Authors: Rita M. Carvalho Cândida Neto Luís M. N. B. F. Santos Margarida Bastos José C. S. Costa

The wetting behavior of ionic liquids (ILs) on the mesoscopic scale considerably impacts a wide range of scientific fields and technologies. Particularly under vacuum conditions, these materials exhibit unique characteristics. This work explores the effect of the deposition rate and substrate temperature on the nucleation, droplet formation, and droplet spreading of ILs films obtained by thermal evaporation. Four ILs were studied, encompassing an alkylimidazolium cation (CnC1im) and either bis(trifluoromethylsulfonyl)imide (NTf2) or the triflate (OTf) as the anion. Each IL sample was simultaneously deposited on surfaces of indium tin oxide (ITO) and silver (Ag). The mass flow rate was reproducibly controlled using a Knudsen cell as an evaporation source, and the film morphology (micro- and nanodroplets) was evaluated by scanning electron microscopy (SEM). The wettability of the substrates by the ILs was notably affected by changes in mass flow rate and substrate temperature. Specifically, the results indicated that an increase in the deposition rate and/or substrate temperature intensified the droplet coalescence of [C2C1im][NTf2] and [C2C1im][OTf] on ITO surfaces. Conversely, a smaller impact was observed on the Ag surface due to the strong adhesion between the ILs and the metallic film. Furthermore, modifying the deposition parameters resulted in a noticeable differentiation in the droplet morphology obtained for [C8C1im][NTf2] and [C8C1im][OTf]. Nevertheless, droplets from long-chain ILs deposited on ITO surfaces showed intensified coalescence, regardless of the deposition rate or substrate temperature.

]]>Fluids doi: 10.3390/fluids8030104

Authors: Andrey Kozelkov Andrey Kurkin Vadim Kurulin Kseniya Plygunova Olga Krutyakova

Verification problems and numeric simulation of cavitation processes with the help of LOGOS computational fluid dynamics software are presented in this article. The Volume of Fluid method realized within LOGOS allowing numerical simulation of double-phase problems with a free surface is used for numeric simulation. Cavitation is resolved by updating the method with the account for interphase mass exchange; its condensation and evaporation parameters are calculated with the use of the Schnerr&ndash;Sauer and Zwart&ndash;Gerber&ndash;Belamri cavitation models. Numerical simulation results of most actual test problems considering turbulence and having reliable numerical data are presented, including simulations of flow around cylinders with flat and hemispherical end surfaces for various cavitation numbers. Numerical simulation results are presented for the process of rotation of a VP1304 screw propeller in the cavitational mode. Numerical experiments prove the operability of the implemented method.

]]>Fluids doi: 10.3390/fluids8030103

Authors: Alexander Schukmann Andreas Schneider Viktor Haas Martin Böhle

Over the last few decades, several grid coupling techniques for hierarchically refined Cartesian grids have been developed to provide the possibility of varying mesh resolution in lattice Boltzmann methods. The proposed schemes can be roughly categorized based on the individual grid transition interface layout they are adapted to, namely cell-vertex or cell-centered approaches, as well as a combination of both. It stands to reason that the specific properties of each of these grid-coupling algorithms influence the stability and accuracy of the numerical scheme. Consequently, this naturally leads to a curiosity regarding the extent to which this is the case. The present study compares three established grid-coupling techniques regarding their stability ranges by conducting a series of numerical experiments for a square duct flow, including various collision models. Furthermore the hybrid-recursive regularized collision model, originally introduced for cell-vertex algorithms with co-located coarse and fine grid nodes, has been adapted to cell-centered and combined methods.

]]>Fluids doi: 10.3390/fluids8030102

Authors: Indro Pranoto Muhammad Aulia Rahman Cahya Dhika Wicaksana Alan Eksi Wibisono Fauzun Arif Widyatama

The trend of miniaturisation in recent decades has led to the development of compact electronic devices. The reduction in the required dimension leads to the exponential rise in the heat flux dissipated from such a system. A proper thermal management system is necessary to keep the temperature of a computer chip&rsquo;s junction within acceptable limits and maintain its performance. Flow boiling modification using straight fins in microchannels has proven to be an effective passive enhancement of the cooling system. The core interest of this research is figuring out the optimal configuration of the fin shapes and configurations. Hence, it is crucial to gain a comprehensive understanding of the flow boiling phenomenon to establish a more general approach. In this study, the boiling heat transfer performance of fin microchannels with various shapes and dimensions is investigated experimentally. The study has shown that the choice of fin geometry has a significant impact on the thermal performance of a heat transfer system. Specifically, the results indicate that a rectangular cross-section fin performs better than a trapezoidal one with the same fin gap. The rectangular cross-section fin exhibits the highest heat transfer coefficient of 5066.84 W/m2&#8729;K, outperforming the trapezoidal fin in terms of heat transfer capability. As the hydraulic diameter reduces, the thermal boundary layer becomes denser, providing a more distributed saturated region. This leads to the increase in the heat transfer coefficient up to 22.5% and 17.1% for rectangular and trapezoidal fins, respectively. Additionally, the efficiency analysis shows that, albeit increasing the mass flux and reducing the gap increase the average cooling performance, but the pressure drop jumps up to 48%, reducing the efficiency of the heat removal system.

]]>Fluids doi: 10.3390/fluids8030101

Authors: Rafail V. Abramov

Turbulence in fluids is an ubiquitous phenomenon, characterized by spontaneous transition of a smooth, laminar flow to rapidly changing, chaotic dynamics. In 1883, Reynolds experimentally demonstrated that, in an initially laminar flow of water, turbulent motions emerge without any measurable external disturbance. To this day, turbulence remains a major unresolved phenomenon in fluid mechanics; in particular, there is a lack of a mathematical model where turbulent dynamics emerge naturally from a laminar flow. Recently, we proposed a new theory of turbulence in gases, according to which turbulent motions are created in an inertial gas flow by the mean field effect of the intermolecular potential. In the current work, we investigate the effect of viscosity in our turbulence model by numerically simulating the air flow at normal conditions in a straight pipe for different values of the Reynolds number. We find that the transition between laminar and turbulent flow in our model occurs, without any deliberate perturbations, as the Reynolds number increases from 2000 to 4000. As the simulated flow becomes turbulent, the decay rate of the time averaged Fourier spectrum of the kinetic energy in our model approaches Kolmogorov&rsquo;s inverse five-thirds law. Both results are consistent with experiments and observations.

]]>Fluids doi: 10.3390/fluids8030100

Authors: Van-Tu Nguyen Warn-Gyu Park

This review paper aims to summarize recent advancements in time-marching schemes for solving Navier&ndash;Stokes (NS) equations in multiphase flow simulations. The focus is on dual-time stepping, local preconditioning, and artificial compressibility methods. These methods have proven to be effective in achieving high time accuracy in simulations, as well as converting the incompressible NS equations into a hyperbolic form that can be solved using compact schemes, thereby accelerating the solution convergence and allowing for the simulation of compressible flows at all Mach numbers. The literature on these methods continues to grow, providing a deeper understanding of the underlying physical processes and supporting technological advancements. This paper also highlights the imposition of dual-time stepping on both incompressible and compressible NS equations. This paper provides an updated overview of advanced methods for the CFD community to continue developing methods and select the most suitable two-phase flow solver for their respective applications.

]]>Fluids doi: 10.3390/fluids8030099

Authors: Nicolas Hafen Jan E. Marquardt Achim Dittler Mathias J. Krause

Rearrangement events in wall-flow filters lead to the formation of specific deposition patterns, which affect a filter&rsquo;s pressure drop, its loading capacity and the separation efficiency. A universal and consistent formulation of probable causes and influence factors does not exist and appropriate calculation models that enable a quantification of respective influence factors are missing. In this work, a previously developed lattice Boltzmann method, which has been used with inflow velocities of up to 2&nbsp;m&nbsp;s&minus;1, is applied to elevated velocities of up to 60&nbsp;m&nbsp;s&minus;1. The particle-free flow, a single layer fragment and a deposition layer during break-up are investigated as three different scenarios. One goal of this work is a comprehensive quantification of the stability and accuracy of both particle-free and particle-including flows, considering static, impermeable deposition-layer fragments. A second goal is the determination of the hydrodynamic surface forces and the deduction of the local detachment likelihood of individual layer fragments. Satisfactory stability and accuracy can be shown for fluid velocity, fluid pressure and the hydrodynamic forces. When considering layer fragments, the parameter domain turns out to be limited to inflow velocities of 28&nbsp;m&nbsp;s&minus;1. It is shown that fragment detachment rather occurs consecutively and regions of no possible detachment are identified. The work contributes to an understanding of rearrangement events and respective deposition pattern predictions and enables potential optimizations in engine performance, fuel consumption and the service life of wall-flow filters.

]]>Fluids doi: 10.3390/fluids8030098

Authors: Elena Mosheva Ramil Siraev Dmitry Bratsun

Recently, we found that a two-layer miscible system placed in a vertical slab reactor shows an occurrence of a density shock-wave-like pattern. This wave resembles a turbulent bore separating immobile fluid and an area of intense mixing. It travels away from the convective core of the system and is highly dependent on the intensity of a gravity-dependent chemoconvection in the cocurrent flow. The novelty of this work is that we demonstrate that the change in angle between gravity and wave direction allows controlling the chemoconvection intensity and, consequently, the rate of a spatially-extended reaction. We study both experimentally and numerically the effect of the spatial orientation of a slab reactor to a gravity field on a flow structure induced by a neutralization reaction. In experiments, we use aqueous mixtures of nitric acid and sodium hydroxide. We apply the Fizeau interferometry to visualize the flow and use the PIV method to measure the fluid velocity. The mathematical model includes reaction&ndash;diffusion&ndash;convection equations that describe 3D flows. We study the flow modifications with a change in the inclination angle from 0 to 90 degrees. At small angles (up to 30), the cocurrent flow becomes spatially heterogeneous, and the fields of salt and acid are separated. If the inclination exceeds 50 degrees, the wavefront is deformed, and the wave breaks up, resulting in a sharp decrease in the reaction rate.

]]>Fluids doi: 10.3390/fluids8030097

Authors: Carmine Di Nucci Rafik Absi

We focus on the fully developed turbulent flow in circular pipes and channels. We provide a comparison of the mean velocity profiles, and we compute the values of the global indicators, such as the skin friction, the mean velocity, the centerline velocity, the displacement thickness, and the momentum thickness. The comparison is done at low-to-moderate Reynolds numbers. For channel flow, we deduced the mean velocity profiles using an indirect turbulent model; for pipe flow, we extracted the needed information from a direct numerical simulation database available in the open literature. A one-to-one comparison of these values at identical Reynolds numbers provides a deep insight into the difference between pipe and channel flows. This line of reasoning allows us to highlight some deviations among the mean velocity profiles extracted from different pipe databases.

]]>Fluids doi: 10.3390/fluids8030096

Authors: Toshio Tagawa

In rotating Rayleigh&ndash;B&eacute;nard problems, convection with traveling waves may occur near the sidewalls. The Rayleigh number, Taylor number and Prandtl number are involved in this phenomenon, and the convection mode is determined depending on their values. We focused on the onset of this convection with traveling waves under the assumption that centrifugal force is neglected. By conducting two-dimensional linear stability analyses assuming periodicity of the flow and temperature fields along the sidewall direction, we investigated the effect of the Taylor number and the Prandtl number on the critical Rayleigh number and also attempted to understand the phenomenon qualitatively through three-dimensional visualizations. It was exhibited that as the Taylor number increases, the wave number, the Rayleigh number and the phase speed are found to increase. On the other hand, as the Prandtl number decreases, the wavenumber and the Rayleigh number decrease, but the phase velocity increases. The present analyses suggest that convection modes localized near the sidewalls are unlikely to emerge for low Prandtl number cases, which are comparable to those of liquid metals.

]]>Fluids doi: 10.3390/fluids8030095

Authors: Dinara Kurmanova Nurbolat Jaichibekov Anton Karpenko Konstantin Volkov

The heating of oil and oil products is widely used to reduce energy losses during transportation. An approach is developed to determine the effective length of the heat exchanger and the temperature of the cold coolant (oil) at its outlet in the case of a strong dependence of oil viscosity on temperature. Oil from the Uzen field (Kazakhstan) is considered as a heated coolant, and water is considered as a heating component. The method of the log&ndash;mean temperature difference, modified for the case of variable viscosity, and the methods of computational fluid dynamics (CFD) are used for calculations. The results of the numerical calculations are compared with the data obtained on the basis of a theoretical approach at a constant viscosity. When using a theoretical approach with a constant or variable viscosity, the heat transfer coefficients to cold and hot coolants are found using criterion dependencies. The Reynolds-averaged Navier&ndash;Stokes (RANS) and a turbulence model that takes into account the laminar&ndash;turbulent transition are applied. In the case of variable oil viscosity, a transition from the laminar flow regime to the turbulent one is manifested, which has a significant effect on the effective length of the heat exchanger. The obtained results of the CFD calculations are of interest for the design of heat exchangers of a new type, for example, helicoid ones.

]]>Fluids doi: 10.3390/fluids8030094

Authors: Rémi Tailleux

Explicit expressions of the 3D velocity field in terms of the conserved quantities of ideal fluid thermocline theory, namely the Bernoulli function, density, and potential vorticity, are generalised in this paper to a compressible ocean with a realistic nonlinear equation of state. The most general such expression is the &lsquo;inactive wind&rsquo; solution, an exact nonlinear solution of the inviscid compressible Navier&ndash;Stokes equation that satisfies the continuity equation as a consequence of Ertel&rsquo;s potential vorticity theorem. However, due to the non-uniqueness of the choice of the Bernoulli function, such expressions are not unique and primarily differ in the magnitude of their vertical velocity component. Due to the thermobaric nonlinearity of the equation of state, the expression for the 3D velocity field of a compressible ocean is found to resemble its ideal fluid counterpart only if constructed using the available form of the Bernoulli function, the Bernoulli equivalent of Lorenz&rsquo;s available potential energy (APE). APE theory also naturally defines a quasi-material, approximately neutral density variable known as the Lorenz reference density. This density variable, in turn, defines a potential vorticity variable that is minimally affected by thermobaric production, thus providing all the necessary tools for extending most results of ideal fluid thermocline theory to a compressible ocean.

]]>Fluids doi: 10.3390/fluids8030093

Authors: Miloš Kocić Živojin Stamenković Jelena Petrović Jasmina Bogdanović-Jovanović

The problem considered in this paper is a steady micropolar fluid flow in porous media between two plates. This model can be used to describe the flow of some types of fluids with microstructures, such as human and animal blood, muddy water, colloidal fluids, lubricants and chemical suspensions. Fluid flow is a consequence of the constant pressure gradient along the flow, while two parallel plates are fixed and have different constant temperatures during the fluid flow. Perpendicular to the flow, an external magnetic field is applied. General equations of the problem are reduced to ordinary differential equations and solved in the closed form. Solutions for velocity, microrotation and temperature are used to explain the influence of the external magnetic field (Hartmann number), the characteristics of the micropolar fluid (coupling and spin gradient viscosity parameter) and the characteristics of the porous medium (porous parameter) using graphs. The results obtained in the paper show that the increase in the additional viscosity of micropolar fluids emphasizes the microrotation vector. Moreover, the analysis of the effect of the porosity parameter shows how the permeability of a porous medium can influence the fluid flow and heat transfer of a micropolar fluid. Finally, it is shown that the influence of the external magnetic field reduces the characteristics of micropolar fluids and tends to reduce the velocity field and make it uniform along the cross-section of the channel.

]]>Fluids doi: 10.3390/fluids8030092

Authors: Vadim Kramar Aleksey Kabanov Oleg Kramar Sergey Fateev Valerii Karapetian

The article discusses approaches to solving the problems of detecting, recognizing, and localizing an object with given distinctive features in an aquatic environment using a technical stereo vision system, taking into account restrictions. The stereo vision system is being developed as part of the task in which the AUV, for the purpose of conducting a monitoring mission, follows from the starting point of its route along a given trajectory in order to detect and classify an object with known characteristics and determine its coordinates using a technical stereo vision system at a distance up to 5 m from it with appropriate water clarity. The developed program for the system of the technical stereo vision should provide the AUV with the following information: video sequence; a frame with an image of the detected object; previously unknown characteristics of the object if it is possible to detect them (color, size or shape); distance to the object from the technical stereo vision system; and linear coordinates relative to the technical stereo vision system. Testing of the developed software was carried out on the operating module of the stereo vision installed on the AUV in the underbody compartment. The study was carried out in the pool and in open water. The experiments performed have shown the effectiveness of the developed system when used in conjunction with an underwater robot.

]]>Fluids doi: 10.3390/fluids8030091

Authors: Giorgio Besagni Nicolò Varallo Riccardo Mereu

Bubble columns are used in many different industrial applications, and their design and characterisation have always been very complex. In recent years, the use of Computational Fluid Dynamics (CFD) has become very popular in the field of multiphase flows, with the final goal of developing a predictive tool that can track the complex dynamic phenomena occurring in these types of reactors. For this reason, we present a detailed literature review on the numerical simulation of two-phase bubble columns. First, after a brief introduction to bubble column technology and flow regimes, we discuss the state-of-the-art modelling approaches, presenting the models describing the momentum exchange between the phases (i.e., drag, lift, turbulent dispersion, wall lubrication, and virtual mass forces), Bubble-Induced Turbulence (BIT), and bubble coalescence and breakup, along with an overview of the Population Balance Model (PBM). Second, we present different numerical studies from the literature highlighting different model settings, performance levels, and limitations. In addition, we provide the errors between numerical predictions and experimental results concerning global (gas holdup) and local (void fraction and liquid velocity) flow properties. Finally, we outline the major issues to be solved in future studies.

]]>Fluids doi: 10.3390/fluids8030090

Authors: Mirza M. Shah

Boiling of mixtures in channels is involved in many chemical processes, and refrigerant mixtures are finding a rapidly increasing use in the refrigeration industries. Hence, reliable prediction of CHF (critical heat flux) is needed. There is no published method that has been verified over a wide range of parameters. In this study, available data for CHF in tubes and annuli were compared to general correlations for tubes and annuli, which are well-verified for single-component fluids. This paper presents the results of this comparison. It was found that general correlations for single-component fluids were in satisfactory agreement with data for mixtures. The data included mixtures of refrigerants and chemicals, reduced pressure 0.02 to 0.8597, mass flux 200 to 3798 kgm&minus;2s&minus;1, temperature glide 2.1 to 21.9 K, and critical quality &minus;0.46 to 0.99. The data for annuli were predicted with a MAD (mean absolute deviation) of 13.6%, and the data for tubes had a MAD of 13.9% when compared to the Shah correlations for single-component fluids. Based on the results of the present data analysis and results of research on pool boiling, a range is identified in which pure fluid correlations can be used for mixtures. This will be useful in the design of heat exchangers involving the boiling of mixtures in channels.

]]>Fluids doi: 10.3390/fluids8030089

Authors: William Ferretto Igor Matteo Carraretto Andrea Tiozzo Marco Montini Luigi Pietro Maria Colombo

Water accumulation is a major problem in the flow assurance of gas pipelines. To limit liquid loading issues, deliquification by means of surfactant injection is a promising alternative to the consolidated mechanical methods. However, the macroscopic behavior of foam pipe flow in the presence of other phases has barely been explored. The goal of this work was to propose an approach to simulate air&ndash;water&ndash;foam flows in horizontal pipes using OLGA by Schlumberger, an industry standard tool for the transient simulation of multiphase flow. The simulation results were compared with experimental data for 60 mm and 30 mm ID (Inner Diameter) horizontal pipelines. Preliminary validation for two-phase air&ndash;water flow was carried out, which showed that correct flow pattern recognition is essential to accurately reproduce the experimental data. Then, stratified air&ndash;foam&ndash;water flows were investigated, assuming different models for the foam local velocity distribution. Foam rheology was considered through the Herschel&ndash;Bulkley model with the yield stress varying in time due to foam decay. The results showed good agreement for a uniform velocity profile and fresh foam properties in the case of the 60 mm ID pipeline, whereas for the 30 mm ID, which was characterized by significantly higher velocities, a linear velocity profile and 2000 s foam aging provided the best agreement. In both cases, the pressure gradient was overestimated, and the mean absolute prediction error ranged from about 5% to 30%.

]]>Fluids doi: 10.3390/fluids8030088

Authors: S. G. Mallinson G. D. McBain B. R. Brown

The mass of individual droplets ejected from a thermal inkjet printhead increases with increasing local temperature near the ejector nozzles. The amount of ink deposited on the page and so the printed image density depends on the droplet mass. Thus, printhead temperature nonuniformity results in printed image density variations that can be unacceptable to the end users of the printed output. Such temperature variations arise from a combination of the ink fluid flow and the heat transfer in both the ink and the solid components in the printhead. Conjugate heat transfer (CHT) in thermal inkjet printheads is investigated here using validated numerical simulations. A typical thermal inkjet printhead is considered here for the first time, with cold ink drawn through the solid structural components by the ejector nozzle refill. The effect of the width of the feedhole above the printhead chip on the temperature field within the chip is analyzed. Validation of the simulation model required the derivation of novel analytical solutions for the relatively simple problems of fully developed forced convection in a differentially heated planar channel and conduction against convection in plug flow. The results from numerical simulations of these two problems are found to compare well with the newly derived analytical solutions. CHT in flow over a backward-facing step with a heated downstream wall was also simulated as part of the validation process, and good agreement was observed with earlier numerical studies. For the main part of the study, it was found that increasing the width of the feedhole reduces the gradients in temperature on the surface of the printhead chip, thus reducing temperature-related printing defects.

]]>Fluids doi: 10.3390/fluids8030087

Authors: Mingyang Li Zhixin Liu Jin Yao Ho Teck Neng Wong

The 3D cementitious material printing method is an extrusion-based additive manufacturing strategy in which cementitious materials are extruded through a dynamic nozzle system to form filaments. Despite its ability to fabricate structures with high complexity and efficiency, the uneven material distribution during the extrusion and deposition process is often encountered when a radial toolpath is introduced. This limits the design freedom and printing parameters that can be utilized during radial toolpath printing. Here, we report a facile strategy to overcome the existing challenges of cementitious material non-homogeneity by rationally developing new nozzle geometries that passively compensate the differential deposition rate encountered in conventional rectangular nozzles. Using two-phase numerical study, we showed that our strategy has the potential of achieving a homogeneous mass distribution even when the nozzle travel speed is unfavorably high, while filament from a rectangular nozzle remains highly non-homogenous. The material distribution unevenness can be reduced from 1.35 to 1.23 and to 0.98 after adopting trapezoid and gaussian nozzles, indicating improvements of 34.3% and 94.2%, respectively. This work not only outlines the methodology for improving the quality of corner/curved features in 3DCMP, but also introduces a new strategy which can be adopted for other extrusion-based fabrication techniques with high material inertia.

]]>Fluids doi: 10.3390/fluids8030086

Authors: Antoine Morente Aashish Goyal Anthony Wachs

We implement the Direction-Splitting solver originally proposed by Keating and Minev in 2013 and allow complex geometries to be described by a triangulation defined in STL files. We develop an algorithm that computes intersections and distances between the regular Cartesian grid and the surface triangulation using a ray-tracing method. We thoroughly validate the implementation on assorted flow configurations. Finally, we illustrate the scalability of our implementation on a test case of a steady flow through 144,327 spherical obstacles randomly distributed in a tri-periodic box at Re=19.2. The grid comprises 6.8 billion cells and the computation runs on 6800 cores of a supercomputer in less than 48 h.

]]>Fluids doi: 10.3390/fluids8030085

Authors: Sagidolla Batay Bagdaulet Kamalov Dinmukhamed Zhangaskanov Yong Zhao Dongming Wei Tongming Zhou Xiaohui Su

To evaluate novel turbine designs, the wind energy sector extensively depends on computational fluid dynamics (CFD). To use CFD in the design optimization process, where lower-fidelity approaches such as blade element momentum (BEM) are more popular, new tools to increase the accuracy must be developed as the latest wind turbines are larger and the aerodynamics and structural dynamics become more complex. In the present study, a new concurrent aerodynamic shape optimization approach towards multidisciplinary design optimization (MDO) that uses a Reynolds-averaged Navier&ndash;Stokes solver in conjunction with a numerical optimization methodology is introduced. A multidisciplinary design optimization tool called DAFoam is used for the NREL phase VI turbine as a baseline geometry. Aerodynamic design optimizations in terms of five different schemes, namely, cross-sectional shape, pitch angle, twist, chord length, and dihedral optimization are conducted. Pointwise, a commercial mesh generator is used to create the numerical meshes. As the adjoint approach is strongly reliant on the mesh quality, up to 17.8 million mesh cells were employed during the mesh convergence and result validation processes, whereas 2.65 million mesh cells were used throughout the design optimization due to the computational cost. The Sparse Nonlinear OPTimizer (SNOPT) is used for the optimization process in the adjoint solver. The torque in the tangential direction is the optimization&rsquo;s merit function and excellent results are achieved, which shows the promising prospect of applying this approach for transient MDO. This work represents the first attempt to implement DAFoam for wind turbine aerodynamic design optimization.

]]>Fluids doi: 10.3390/fluids8030084

Authors: Giuseppe Procopio Massimiliano Giona

This article develops a modal expansion (in terms of functions exponentially decaying with time) of the force acting on a micrometric particle and stemming from fluid inertial effects (usually referred to as the Basset force) deriving from the application of the time-dependent Stokes equation to model fluid&ndash;particle interactions. One of the main results is that viscoelastic effects induce the regularization of the inertial memory kernels at&nbsp;t=0, eliminating the&nbsp;1/t-singularity characterizing Newtonian fluids. The physical origin of this regularization stems from the finite propagation velocity of the internal shear stresses characterizing viscoelastic constitutive equations. The analytical expression for the fluid inertial kernel is derived for a Maxwell fluid, and a general method is proposed to obtain accurate approximations of it for generic complex viscoelastic fluids, characterized by a spectrum of relaxation times.

]]>Fluids doi: 10.3390/fluids8030083

Authors: Elisabetta Sieni Simonetta Geninatti Crich Maria Rosaria Ruggiero Lucia Del Bianco Federico Spizzo Roberta Bertani Mirto Mozzon Marco Barozzi Michele Forzan Paolo Sgarbossa

The paper aims to compare different methods able to estimate the specific loss power (SLP) generated by three different types of magnetic nanoparticles, MNPs, dispersed in a suspension fluid, e.g., octane or water. The nanoparticles were characterized morphologically in terms of shape and size, chemically for composition and their physical properties like magnetization and SLP were studied. We evidenced the differences in SLP evaluation due to the applied method, particularly in the presence of thermally induced phenomena such as aggregation or precipitation of MNPs that can affect the heating curve of the samples. Then, the SLP determination methods less sensible to this phenomenon appear to be the ones that use the initial slope when the sample is in quasi-adiabatic condition. Finally, we propose a comparison of those methods based on the pros and cons of their use for the SLP determination of magnetic nanofluids. In particular, the analysis of the behavior of the heating curve is useful to evaluate the useful amplitude of the interval analysis for the initial slope methods.

]]>Fluids doi: 10.3390/fluids8030082

Authors: Shubham Gupta Subhodip Chatterjee Arnab Chanda

Accidental injuries due to slips and falls are considered serious threats to public safety. Sufficient friction at the footwear and flooring interface is essential to reduce slip-related risks. The presence of slippery fluidic contaminants, such as water, further reduces friction and increases the risks of slip-related accidents drastically. While the effect of floorings and contaminants on footwear traction has been measured extensively across a variety of footwear designs, only a few studies have explored the science of the outsole design and its role in providing sufficient traction. In this work, the tread design of a commonly encountered outsole pattern, i.e., with vertically oriented tread channels, was parametrically altered across its width and gap. Based on the impressions of an original footwear design, nine outsoles were fabricated. The induced fluid pressures, mass flow rates, and traction were quantified by using a computational fluid dynamics (CFD) framework and through slip testing experiments. Outsoles that had wide treads with small gaps decreased the overall slipping risk on dry floorings. As compared to the tread area, tread gaps were found to be a dominating parameter in providing adequate shoe&ndash;floor traction in wet slipping conditions. The methods, including the outcomes presented in this work, are anticipated to advance the understanding of the science behind footwear friction and help footwear manufacturers optimize outsole designs to reduce slip and fall risks.

]]>Fluids doi: 10.3390/fluids8030081

Authors: Artur V. Dmitrenko Vladislav M. Ovsyannikov

The aim of this investigation is to show the solution for the critical Reynolds number in the flow around the sphere on the basis of theory of stochastic equations and equivalence of measures between turbulent and laminar motions. Solutions obtained by numerical methods (DNS, LES, RANS) require verification and in this case the theoretical results have special value. For today in the scientific literature, there is J. Talor&rsquo;s implicit formula connecting the critical Reynolds number with the parameters of the initial fluctuations in the flow around the sphere. Here the derivation of the explicit formula is presented. The results show a satisfactory correspondence of the obtained theoretical dependence for the critical Reynolds number to the experiments in the flow around the sphere.

]]>Fluids doi: 10.3390/fluids8030080

Authors: Simon Knecht Denislav Zdravkov Albert Albers

Simulative optimization methods often build on an iterative scheme, where a simulation model is solved in each iteration. To reduce the time needed for an optimization, finding the right balance between simulation model quality, and simulation time is essential. This is especially true for transient problems, such as fluid flow within a hydromechanical system. Therefore, we present an approach to building steady-state surrogate models for oscillating flow in a pipe with a local heat source. The main aspect is to model the fluid as a solid with an orthotropic heat transfer coefficient. The values of this coefficient are fitted to reproduce the temperature distribution of the transient case by parametric optimization. It is shown that the presented approach is feasible for different sets of parameters and creates suitable surrogate models for oscillating flow within a pipe with a local heat source. In future works, the presented approach will be transferred from the simplified geometry under investigation to industrial problems.

]]>Fluids doi: 10.3390/fluids8030079

Authors: Georgi Djambazov

The numerically simulated method of using electromagnetic field from an alternating current is a patented method to create in liquid metal, under the conditions of resonance, acoustic waves of sufficient strength to cause cavitation and implosion of gas bubbles, leading to beneficial degassing and grain refinement. The modelling stages of electromagnetics are described below along with acoustics in liquids, bubble dynamics, and their interactions. Sample results are presented for a cylindrical container with liquid aluminium surrounded by an induction coil. The possibility of establishing acoustic resonance and sustaining the bubble oscillation at a useful level is demonstrated. Limitations of the time-dependent approach to this multi-physics modelling problem are also discussed.

]]>Fluids doi: 10.3390/fluids8030078

Authors: Andres Santiago Espinosa-Moreno Carlos Alberto Duque-Daza Diego Alexander Garzón-Alvarado

Anatomical airways parameters, such as length, diameter and angles, have a strong effect on the flow dynamics. Aiming to explore the effect of variations of the bifurcation angle (BA) and carina rounding radius (CRR) of lower human airways on respiratory processes, numerical simulations of airflow during inhalation and exhalation were performed using synthetic bifurcation models. Geometries for the airways models were parameterized based on a set of different BA&rsquo;s and several CRR&rsquo;s. A range of Reynolds numbers (Re), relevant to the human breathing process, were selected to analyze airflow behavior. The numerical results showed a significant influence of BA and the CRR on the development of the airflow within the airways, and, therefore, affecting the following relevant features of the flow: the deformation of velocity profiles, alterations of pressure drop, flow patterns, and, finally, enhancement or attenuation of wall shear stresses (WSS) appearing during the regular respiratory process. The numerical results showed that increases in the bifurcation angle value were accompanied by pressure increases of about 20%, especially in the regions close to the bifurcation. Similarly, increases in the BA value led to a reduction in peak shear stresses of up to 70%. For the ranges of angles and radii explored, an increase in pressure of about 20% and a reduction in wall shear stress of more than 400% were obtained by increasing the carina rounding radius. Analysis of the coherent structures and secondary flow patterns also revealed a direct relationship between the location of the vortical structures, the local maxima of the velocity profiles and the local vorticity minima. This relationship was observed for all branches analyzed, for both the inhalation and exhalation processes of the respiratory cycle.

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