Editorial

5 pages, 189 KiB

Open AccessEditorial

Recent Advances in Fluid Mechanics: Feature Papers, 2022

by Mehrdad Massoudi

Fluids 2023, 8(10), 262; https://doi.org/10.3390/fluids8100262 - 26 Sep 2023

Viewed by 2020

This Special Issue is a collection of papers from some of the leading researchers discussing new findings or cutting-edge developments relating to all aspects of fluid mechanics [...] Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

Research

Jump to: Editorial, Review

18 pages, 425 KiB

Open AccessArticle

Modal Representation of Inertial Effects in Fluid–Particle Interactions and the Regularity of the Memory Kernels

by Giuseppe Procopio and Massimiliano Giona

Fluids 2023, 8(3), 84; https://doi.org/10.3390/fluids8030084 - 28 Feb 2023

Cited by 3 | Viewed by 1222

Abstract

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 [...] Read more.

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–particle interactions. One of the main results is that viscoelastic effects induce the regularization of the inertial memory kernels at

t = 0

, eliminating the

1 / \sqrt{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. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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24 pages, 43961 KiB

Open AccessArticle

Investigation of Flow-Induced Instabilities in a Francis Turbine Operating in Non-Cavitating and Cavitating Part-Load Conditions

by Mohammad Hossein Arabnejad, Håkan Nilsson and Rickard E. Bensow

Fluids 2023, 8(2), 61; https://doi.org/10.3390/fluids8020061 - 10 Feb 2023

Cited by 3 | Viewed by 1635

Abstract

The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft [...] Read more.

The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft tube. In this paper, we investigate the formation of this vortex using the scale-resolving methods SST-SAS, wall-modeled LES (WMLES), and zonal WMLES. The numerical results are first validated against the available experimental data, and then analyzed to explain the effect of using different scale-resolving methods in detail. It is revealed that although all methods can capture the main features of the RVRs, the WMLES method provides the best quantitative agreement between the simulation results and experiment. Furthermore, cavitating simulations are performed using WMLES method to study the effect of cavitation on the flow in the turbine. These effects of cavitation are shown to be highly dependent on the amount of vapor in the RVR. If the amount of vapor is small, cavitation induces broadband high-frequency fluctuations in the pressure and forces exerted on the turbine. As the amount of cavitation increases, these fluctuations tend to have a distinct dominant frequency which is different from the frequency of the RVR. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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13 pages, 489 KiB

Open AccessArticle

A Generalized Diffusion Equation: Solutions and Anomalous Diffusion

by Ervin K. Lenzi, Aloisi Somer, Rafael S. Zola, Luciano R. da Silva and Marcelo K. Lenzi

Fluids 2023, 8(2), 34; https://doi.org/10.3390/fluids8020034 - 17 Jan 2023

Cited by 3 | Viewed by 1594

Abstract

We investigate the solutions of a generalized diffusion-like equation by considering a spatial and time fractional derivative and the presence of non-local terms, which can be related to reaction or adsorption–desorption processes. We use the Green function approach to obtain solutions and evaluate [...] Read more.

We investigate the solutions of a generalized diffusion-like equation by considering a spatial and time fractional derivative and the presence of non-local terms, which can be related to reaction or adsorption–desorption processes. We use the Green function approach to obtain solutions and evaluate the spreading of the system to show a rich class of behaviors. We also connect the results obtained with the anomalous diffusion processes. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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23 pages, 4128 KiB

Open AccessFeature PaperArticle

Assessment of a RANS Transition Model with Flapping Foils at Moderate Reynolds Numbers

by Luca Alberti, Emanuele Carnevali and Andrea Crivellini

Fluids 2023, 8(1), 23; https://doi.org/10.3390/fluids8010023 - 08 Jan 2023

Cited by 2 | Viewed by 1542

Abstract

Numerical simulations based on a high-order discontinuous Galerkin solver were performed to investigate two-dimensional flapping foils at moderate Reynolds numbers, moving with different prescribed harmonic motion laws. A Spalart–Allmaras RANS model with and without an algebraic local transition modification was employed for the [...] Read more.

Numerical simulations based on a high-order discontinuous Galerkin solver were performed to investigate two-dimensional flapping foils at moderate Reynolds numbers, moving with different prescribed harmonic motion laws. A Spalart–Allmaras RANS model with and without an algebraic local transition modification was employed for the resolution of multiple kinematic configurations, considering both moderate-frequency large-amplitude flapping and high-frequency small-amplitude pure heaving. The propulsive performance of the airfoils with the two modelling approaches were tested by referring to experimental and (scale-resolving) numerical data available in the literature. The results show an increase in effectiveness in predicting loads when applying the transition model. This is particularly true at low Strouhal numbers when, after laminar separation at the leading edge, vorticity dynamics appears to have a strong effect on the forces exerted on the profile. Specifically, the transition model more accurately predicts the wake topology emerging in the flow field, which is the primary influence on thrust/drag generation. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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20 pages, 4456 KiB

Open AccessArticle

CFD Simulation of a Hybrid Solar/Electric Reactor for Hydrogen and Carbon Production from Methane Cracking

by Malek Msheik, Sylvain Rodat and Stéphane Abanades

Fluids 2023, 8(1), 18; https://doi.org/10.3390/fluids8010018 - 02 Jan 2023

Cited by 5 | Viewed by 1976

Abstract

Methane pyrolysis is a transitional technology for environmentally benign hydrogen production with zero greenhouse gas emissions, especially when concentrated solar energy is the heating source for supplying high-temperature process heat. This study is focused on solar methane pyrolysis as an attractive decarbonization process [...] Read more.

Methane pyrolysis is a transitional technology for environmentally benign hydrogen production with zero greenhouse gas emissions, especially when concentrated solar energy is the heating source for supplying high-temperature process heat. This study is focused on solar methane pyrolysis as an attractive decarbonization process to produce both hydrogen gas and solid carbon with zero CO₂ emissions. Direct normal irradiance (DNI) variations arising from inherent solar resource variability (clouds, fog, day-night cycle, etc.) generally hinder continuity and stability of the solar process. Therefore, a novel hybrid solar/electric reactor was designed at PROMES-CNRS laboratory to cope with DNI variations. Such a design features electric heating when the DNI is low and can potentially boost the thermochemical performance of the process when coupled solar/electric heating is applied thanks to an enlarged heated zone. Computational fluid dynamics (CFD) simulations through ANSYS Fluent were performed to investigate the performance of this reactor under different operating conditions. More particularly, the influence of various process parameters including temperature, gas residence time, methane dilution, and hybridization on the methane conversion was assessed. The model combined fluid flow hydrodynamics and heat and mass transfer coupled with gas-phase pyrolysis reactions. Increasing the heating temperature was found to boost methane conversion (91% at 1473 K against ~100% at 1573 K for a coupled solar-electric heating). The increase of inlet gas flow rate Q₀ lowered methane conversion since it affected the gas space-time (91% at Q₀ = 0.42 NL/min vs. 67% at Q₀ = 0.84 NL/min). A coupled heating also resulted in significantly better performance than with only electric heating, because it broadened the hot zone (91% vs. 75% methane conversion for coupled heating and only electric heating, respectively). The model was further validated with experimental results of methane pyrolysis. This study demonstrates the potential of the hybrid reactor for solar-driven methane pyrolysis as a promising route toward clean hydrogen and carbon production and further highlights the role of key parameters to improve the process performance. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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19 pages, 4762 KiB

Open AccessArticle

The Role of Inertia in the Onset of Turbulence in a Vortex Filament

by Jean-Paul Caltagirone

Fluids 2023, 8(1), 16; https://doi.org/10.3390/fluids8010016 - 02 Jan 2023

Cited by 2 | Viewed by 1382

Abstract

The decay of the kinetic energy of a turbulent flow with time is not necessarily monotonic. This is revealed by simulations performed in the framework of discrete mechanics, where the kinetic energy can be transformed into pressure energy or vice versa; this persistent [...] Read more.

The decay of the kinetic energy of a turbulent flow with time is not necessarily monotonic. This is revealed by simulations performed in the framework of discrete mechanics, where the kinetic energy can be transformed into pressure energy or vice versa; this persistent phenomenon is also observed for inviscid fluids. Different types of viscous vortex filaments generated by initial velocity conditions show that vortex stretching phenomena precede an abrupt onset of vortex bursting in high-shear regions. In all cases, the kinetic energy starts to grow by borrowing energy from the pressure before the transfer phase to the small turbulent structures. The result observed on the vortex filament is also found for the Taylor–Green vortex, which significantly differs from the previous results on this same case simulated from the Navier–Stokes equations. This disagreement is attributed to the physical model used, that of discrete mechanics, where the formulation is based on the conservation of acceleration. The reasons for this divergence are analyzed in depth; however, a spectral analysis allows finding the established laws on the decay of kinetic energy as a function of the wave number. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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30 pages, 7002 KiB

Open AccessArticle

Patterning Behavior of Hybrid Buoyancy-Marangoni Convection in Inclined Layers Heated from Below

by Wasim Waris and Marcello Lappa

Fluids 2023, 8(1), 12; https://doi.org/10.3390/fluids8010012 - 29 Dec 2022

Cited by 3 | Viewed by 1925

Abstract

Alongside classical effects driven by gravity or surface tension in non-isothermal fluids, the present experimental study concentrates on other exotic (poorly known) modes of convection, which are enabled in a fluid layer delimited from below by a hot plate and unbounded from above [...] Read more.

Alongside classical effects driven by gravity or surface tension in non-isothermal fluids, the present experimental study concentrates on other exotic (poorly known) modes of convection, which are enabled in a fluid layer delimited from below by a hot plate and unbounded from above when its container is inclined to the horizontal direction. By means of a concerted approach based on the application of a thermographic visualization technique, multiple temperature measurements at different points and a posteriori computer-based reconstruction of the spatial distribution of wavelengths, it is shown that this fluid-dynamic system is prone to develop a rich set of patterns. These include (but are not limited to), spatially localized (compact) cells, longitudinal wavy rolls, various defects produced by other instabilities and finger-like structures resulting from an interesting roll pinching mechanism (by which a single longitudinal roll can be split into two neighboring rolls with smaller wavelength). Through parametric variation of the tilt angle, the imposed temperature difference and the volume of liquid employed, it is inferred that the observable dynamics are driven by the ability of gravity-induced shear flow to break the in-plane isotropy of the system, the relative importance of surface-tension-driven and buoyancy effects, and the spatially varying depth of the layer. Some effort is provided to identify universality classes and similarities with other out-of-equilibrium thermal systems, which have attracted significant attention in the literature. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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16 pages, 1839 KiB

Open AccessArticle

Wall-Modeled and Hybrid Large-Eddy Simulations of the Flow over Roughness Strips

by Teresa Salomone, Ugo Piomelli and Giuliano De Stefano

Fluids 2023, 8(1), 10; https://doi.org/10.3390/fluids8010010 - 27 Dec 2022

Cited by 4 | Viewed by 2118

Abstract

The flow over alternating roughness strips oriented normally to the mean stream is studied using wall-modeled large-eddy simulations (WMLES) and improved delayed detached-eddy simulations (IDDES) (a hybrid method solving the Reynolds-averaged Navier–Stokes (RANS) equations near the wall and switching to large-eddy simulations (LES) [...] Read more.

The flow over alternating roughness strips oriented normally to the mean stream is studied using wall-modeled large-eddy simulations (WMLES) and improved delayed detached-eddy simulations (IDDES) (a hybrid method solving the Reynolds-averaged Navier–Stokes (RANS) equations near the wall and switching to large-eddy simulations (LES) in the core of the flow). The calculations are performed in an open-channel configuration. Various approaches are used to account for roughness by either modifying the wall boundary condition for WMLES or the model itself for IDDES or by adding a drag forcing term to the momentum equations. By comparing the numerical results with the experimental data, both methods with both roughness modifications are shown to reproduce the non-equilibrium effects, but noticeable differences are observed. The WMLES, although affected by the underlying equilibrium assumption, predicts the return to equilibrium of the skin friction in good agreement with the experiments. The velocity predicted by the IDDES does not have memory of the upstream conditions and recovers to the equilibrium conditions faster. Memory of the upstream conditions appears to be a critical factor for the accurate computational modeling of this flow. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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21 pages, 5984 KiB

Open AccessArticle

Evaluation of SPH and FVM Models of Kinematically Prescribed Peristalsis-like Flow in a Tube

by Xinying Liu, Simon M. Harrison, Paul W. Cleary and David F. Fletcher

Fluids 2023, 8(1), 6; https://doi.org/10.3390/fluids8010006 - 23 Dec 2022

Cited by 5 | Viewed by 2097

Abstract

Peristaltic flow is important in many biological processes, including digestion, and forms an important component of any in silico model of the stomach. There is a clear need to verify the simulations of such flows. An analytical solution was identified that can be [...] Read more.

Peristaltic flow is important in many biological processes, including digestion, and forms an important component of any in silico model of the stomach. There is a clear need to verify the simulations of such flows. An analytical solution was identified that can be used for model verification, which gives an equation for the net volumetric flow over a cycle for an applied sinusoidal wall motion. Both a smooth particle hydrodynamics (SPH) code (from the CSIRO), which is being used to develop a stomach model that includes wall motion, buoyancy, acid secretion and food breakdown, and the Ansys Fluent Finite Volume Method (FVM) solver, that is widely used in industry for complex engineering flows, are used in this exercise. Both give excellent agreement with the analytic solution for the net flow over a cycle for a range of occlusion ratios of 0.1–0.6. Very similar velocity fields are obtained with the two methods. The impact of parameters affecting solution stability and accuracy are described and investigated. Having validated the moving wall capability of the SPH model it can be used with confidence in stomach simulations that include wall motion. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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17 pages, 4548 KiB

Open AccessArticle

Jet Velocity and Acoustic Excitation Characteristics of a Synthetic Jet Actuator

by Nadim Arafa, Pierre E. Sullivan and Alis Ekmekci

Fluids 2022, 7(12), 387; https://doi.org/10.3390/fluids7120387 - 16 Dec 2022

Cited by 4 | Viewed by 1554

Abstract

The effect of the excitation frequency of synthetic jet actuators on the mean jet velocity issuing from an array of circular orifices is investigated experimentally, focusing on the acoustic excitation characteristics of the actuator’s cavity. Two cavity configurations are considered. In the first [...] Read more.

The effect of the excitation frequency of synthetic jet actuators on the mean jet velocity issuing from an array of circular orifices is investigated experimentally, focusing on the acoustic excitation characteristics of the actuator’s cavity. Two cavity configurations are considered. In the first configuration, synthetic jets are generated by exciting a single, large cavity having an array of sixteen orifices via sixteen piezoelectric elements. In the second configuration, the cavity volume of the first configuration is divided into eight isolated compartments, each with two orifices and two piezoelectric elements. Several distinct resonant peaks were observed in the frequency response of the synthetic jet actuator built with a single large-aspect-ratio cavity, whereas the case of compartmentalised cavities exhibited a single resonant peak. Acoustic simulations of the large-aspect-ratio-cavity volume showed that the multiple peaks in its frequency response correspond to the acoustic standing-wave mode shapes of the cavity. Due to its large aspect ratio, several acoustic mode shapes coexist in the excitation frequency range aside from the Helmholtz resonance frequency. When the actuator’s cavity volume is compartmentalised, only the Helmholtz resonance frequency is observed within the excitation frequency range. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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13 pages, 848 KiB

Open AccessArticle

Stability of a Regularized Casson Flow down an Incline: Comparison with the Bingham Case

by Benedetta Calusi, Angiolo Farina, Lorenzo Fusi and Liviu Iulian Palade

Fluids 2022, 7(12), 380; https://doi.org/10.3390/fluids7120380 - 09 Dec 2022

Cited by 4 | Viewed by 1199

Abstract

In this paper, we study the two-dimensional linear stability of a regularized Casson fluid (i.e., a fluid whose constitutive equation is a regularization of the Casson obtained through the introduction of a smoothing parameter) flowing down an incline. The stability analysis has been [...] Read more.

In this paper, we study the two-dimensional linear stability of a regularized Casson fluid (i.e., a fluid whose constitutive equation is a regularization of the Casson obtained through the introduction of a smoothing parameter) flowing down an incline. The stability analysis has been performed theoretically by using the long-wave approximation method. The critical Reynolds number at which the instability arises depends on the material parameters, on the tilt angle as well as on the prescribed inlet discharge. In particular, the results show that the regularized Casson flow has stability characteristics different from the regularized Bingham. Indeed, for the regularized Casson flow an increase in the yield stress of the fluid induces a stabilizing effect, while for the Bingham case an increase in the yield stress entails flow destabilization. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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21 pages, 9876 KiB

Open AccessArticle

Single-Mode Solutions for Convection and Double-Diffusive Convection in Porous Media

by Chang Liu and Edgar Knobloch

Fluids 2022, 7(12), 373; https://doi.org/10.3390/fluids7120373 - 05 Dec 2022

Cited by 3 | Viewed by 1283

Abstract

This work employs single-mode equations to study convection and double-diffusive convection in a porous medium where the Darcy law provides large-scale damping. We first consider thermal convection with salinity as a passive scalar. The single-mode solutions resembling steady convection rolls reproduce the qualitative [...] Read more.

This work employs single-mode equations to study convection and double-diffusive convection in a porous medium where the Darcy law provides large-scale damping. We first consider thermal convection with salinity as a passive scalar. The single-mode solutions resembling steady convection rolls reproduce the qualitative behavior of root-mean-square and mean temperature profiles of time-dependent states at high Rayleigh numbers from direct numerical simulations (DNS). We also show that the single-mode solutions are consistent with the heat-exchanger model that describes well the mean temperature gradient in the interior. The Nusselt number predicted from the single-mode solutions exhibits a scaling law with Rayleigh number close to that followed by exact 2D steady convection rolls, although large aspect ratio DNS results indicate a faster increase. However, the single-mode solutions at a high wavenumber predict Nusselt numbers close to the DNS results in narrow domains. We also employ the single-mode equations to analyze the influence of active salinity, introducing a salinity contribution to the buoyancy, but with a smaller diffusivity than the temperature. The single-mode solutions are able to capture the stabilizing effect of an imposed salinity gradient and describe the standing and traveling wave behaviors observed in DNS. The Sherwood numbers obtained from single-mode solutions show a scaling law with the Lewis number that is close to the DNS computations with passive or active salinity. This work demonstrates that single-mode solutions can be successfully applied to this system whenever periodic or no-flux boundary conditions apply in the horizontal. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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11 pages, 3636 KiB

Open AccessArticle

Blade Drag Resistance in Windage Operating of Low Pressure Steam Turbines

by Antonio Mambro, Francesco Congiu, Enzo Galloni and Davide Lanni

Fluids 2022, 7(12), 372; https://doi.org/10.3390/fluids7120372 - 04 Dec 2022

Cited by 1 | Viewed by 1655

Abstract

On the basis of previous experimental and numerical studies, the windage operation of low-pressure turbine rear stage is investigated. The state of the steam within the rotor channel was correlated to measurements carried out downstream of the blades for different ventilation regimes. Considering [...] Read more.

On the basis of previous experimental and numerical studies, the windage operation of low-pressure turbine rear stage is investigated. The state of the steam within the rotor channel was correlated to measurements carried out downstream of the blades for different ventilation regimes. Considering very-low-volume flow conditions, the ventilation power was related to the drag force acting on the moving blades. A correlation was identified between the drag coefficient and a Reynolds number relative to the reverse flow height. This correlation can be used in order to predict the power loss of a last-stage moving blade operating at low load. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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20 pages, 710 KiB

Open AccessArticle

Mass Transport in Membrane Systems: Flow Regime Identification by Fourier Analysis

by Stefan Heinz, Jakob Heinz and Jonathan A. Brant

Fluids 2022, 7(12), 369; https://doi.org/10.3390/fluids7120369 - 30 Nov 2022

Cited by 6 | Viewed by 1144

Abstract

The numerical calculation of local mass distributions in membrane systems by computational fluid dynamics (CFD) offers indispensable benefits. However, the concept to calculate such distributions in response to separate variations of operation conditions (OCs) makes it difficult to address overall, flow-physics-related questions, which [...] Read more.

The numerical calculation of local mass distributions in membrane systems by computational fluid dynamics (CFD) offers indispensable benefits. However, the concept to calculate such distributions in response to separate variations of operation conditions (OCs) makes it difficult to address overall, flow-physics-related questions, which require the consideration of the collective interaction of OCs. It is shown that such understanding-related relationships can be obtained by the analytical solution of the advection–diffusion equation considered. A Fourier series model (FSM) is presented, which provides exact solutions of an advection–diffusion equation for a wide range of OCs. On this basis, a new zeroth-order model is developed, which is very simple and as accurate as the complete FSM for all conditions of practical relevance. Advection-dominated blocked and diffusion-dominated unblocked flow regimes are identified (depending on a Péclet number which compares the flow geometry with a length scale imposed by the flow), which implies relevant requirements for the use of lab results for pilot- and full-scale applications. Analyses reveal the equivalence of variations of OCs, which offers a variety of options to accomplish desired flow regime changes. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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17 pages, 983 KiB

Open AccessFeature PaperArticle

From Two-Equation Turbulence Models to Minimal Error Resolving Simulation Methods for Complex Turbulent Flows

by Stefan Heinz

Fluids 2022, 7(12), 368; https://doi.org/10.3390/fluids7120368 - 29 Nov 2022

Cited by 5 | Viewed by 1696

Abstract

Hybrid RANS-LES methods are supposed to provide major contributions to future turbulent flow simulations, in particular for reliable flow predictions under conditions where validation data are unavailable. However, existing hybrid RANS-LES methods suffer from essential problems. A solution to these problems is presented [...] Read more.

Hybrid RANS-LES methods are supposed to provide major contributions to future turbulent flow simulations, in particular for reliable flow predictions under conditions where validation data are unavailable. However, existing hybrid RANS-LES methods suffer from essential problems. A solution to these problems is presented as a generalization of previously introduced continuous eddy simulation (CES) methods. These methods, obtained by relatively minor extensions of standard two-equation turbulence models, represent minimal error simulation methods. An essential observation presented here is that minimal error methods for incompressible flows can be extended to stratified and compressible flows, which opens the way to addressing relevant atmospheric science problems (mesoscale to microscale coupling) and aerospace problems (supersonic or hypersonic flow predictions). It is also reported that minimal error methods can provide valuable contributions to the design of consistent turbulence models under conditions of significant modeling uncertainties. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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12 pages, 625 KiB

Open AccessArticle

Measurement of Mass Transfer Intensity in Gas–Liquid Medium of Bioreactor Circuit Using the Thermometry Method

by Ilya Starodumov, Irina Nizovtseva, Sergey Lezhnin, Sergey Vikharev, Vladislav Svitich, Pavel Mikushin, Dmitri Alexandrov, Nikolay Kuznetsov and Dmitri Chernushkin

Fluids 2022, 7(12), 366; https://doi.org/10.3390/fluids7120366 - 25 Nov 2022

Cited by 4 | Viewed by 1374

Abstract

The development of energy-efficient solutions for large-scale fermenters demands a deep and comprehensive understanding of hydrodynamic and heat and mass transfer processes. Despite a wide variety of research dedicated to measurements of mass transfer intensity in bubble flows, this research subject faces new [...] Read more.

The development of energy-efficient solutions for large-scale fermenters demands a deep and comprehensive understanding of hydrodynamic and heat and mass transfer processes. Despite a wide variety of research dedicated to measurements of mass transfer intensity in bubble flows, this research subject faces new challenges due to the topical development of new innovative bioreactor designs. In order to understand the fluid dynamics of the gas–liquid medium, researchers need to develop verified CFD models describing flows in the bioreactor loop using a progressive physical and mathematical apparatus. In the current paper, we represent the results of evaluating the key performance indicator of the bioreactor, namely the volumetric mass transfer coefficient (

k_{L} a

) known as a parameter of dominant importance for the design, operation, scale-up, and optimization of bioreactors, using the developed thermometry method. The thermometry method under consideration was examined within a series of experiments, and a comparative analysis was provided for a number of various regimes also being matched with the classical approaches. The methodology, experiment results, and data verification are given, which allow the evaluation of the effectiveness and prediction of the fluid flows dynamics in bioreactors circuits and ultimately the operational capabilities of the fermenter line. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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17 pages, 3148 KiB

Open AccessArticle

Bubble Growth in Supersaturated Liquids

by Raj Kumar Nayak Maloth, Roger E. Khayat and Christopher T. DeGroot

Fluids 2022, 7(12), 365; https://doi.org/10.3390/fluids7120365 - 25 Nov 2022

Cited by 5 | Viewed by 1876

Abstract

Bubble formation and dissolution have a wide range of industrial applications, from the production of beverages to foam manufacturing processes. The rate at which the bubble expands or contracts has a significant effect on these processes. In the current work, the hydrodynamics of [...] Read more.

Bubble formation and dissolution have a wide range of industrial applications, from the production of beverages to foam manufacturing processes. The rate at which the bubble expands or contracts has a significant effect on these processes. In the current work, the hydrodynamics of an isolated bubble expanding due to mass transfer in a pool of supersaturated gas–liquid solution is investigated. The complete scalar transportation equation (advection–diffusion) is solved numerically. It is observed that the present model accurately predicted bubble growth when compared with existing approximated models and experiments. The effect of gas–liquid solution parameters such as inertia, viscosity, surface tension, diffusion coefficient, system pressure, and solubility of the gas has been investigated. It is found that the surface tension and inertia have a very minimal effect during the bubble expansion. However, it is observed that the viscosity, system pressure, diffusion, and solubility have a considerable effect on bubble growth. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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18 pages, 12231 KiB

Open AccessArticle

Computational Fluid Dynamics Approach for Oscillating and Interacting Convective Flows

by Attila Gergely and Zoltán Néda

Fluids 2022, 7(11), 339; https://doi.org/10.3390/fluids7110339 - 24 Oct 2022

Cited by 1 | Viewed by 1737

Abstract

The oscillation and collective behavior of convective flows is studied by a computational fluid dynamics approach. More specifically, the rising dynamics of heated fluid columns is simulated in gravitational field using a simplified 2D geometry. The numerical method uses the FEniCS package for [...] Read more.

The oscillation and collective behavior of convective flows is studied by a computational fluid dynamics approach. More specifically, the rising dynamics of heated fluid columns is simulated in gravitational field using a simplified 2D geometry. The numerical method uses the FEniCS package for solving the coupled Navier–Stokes and heat-diffusion equations. For the flow of a single heated fluid column, the effect of the inflow yield and the nozzle diameter is studied. In agreement with the experiments, for a constant nozzle diameter the oscillation frequency increases approximately linearly as a function of the the flow rate, while for a constant flow rate the frequency decreases as a power law with the increased nozzle diameter. For the collective behavior of two nearby flows, we find a counter-phase synchronization and a decreasing trend of the common oscillation frequency with the distance between the jets. These results are in agreement with the experiments, and our computational study also suggests that the phenomenon is present on largely different length-scales. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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20 pages, 8195 KiB

Open AccessArticle

Effect of the Pore Geometry on the Driving Pressure across a Bubble Penetrating a Single Pore

by Shadi Ansari and David S. Nobes

Fluids 2022, 7(10), 333; https://doi.org/10.3390/fluids7100333 - 20 Oct 2022

Cited by 5 | Viewed by 1788

Abstract

The passage of a bubble and the required energy for its motion through a confining pore can potentially be affected by the surface roughness and geometry of the pore. The motion of an isolated bubble passing through four different pore geometries (three circular [...] Read more.

The passage of a bubble and the required energy for its motion through a confining pore can potentially be affected by the surface roughness and geometry of the pore. The motion of an isolated bubble passing through four different pore geometries (three circular pores, a smooth pore and 2 with different roughness, and a sharp triangular pore) is investigated. The shape of the deformed bubble passing these geometries was evaluated to determine the pressure drop across the bubble and hence the driving force to cause motion. The results of investigating the motion of the bubbles and the change in the pressure and velocity of the bubbles showed that the pore shape and surface roughness have a significant effect on the passage of the isolated phase. The motion of the bubble entering the entrance of the circular pores was similar for all circular cases. On exiting, however, a clear difference between the cases due to the presence of the peaks of the roughness was observed. These results indicate that, in addition to the critical pressure at the entrance of the pore, extra resistance will be introduced due to bubble phase pinning at the exit caused by roughness of the pore. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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22 pages, 8514 KiB

Open AccessArticle

CFD Investigation into the Effects of Surrounding Particle Location on the Drag Coefficient

by David Dodds, Abd Alhamid R. Sarhan and Jamal Naser

Fluids 2022, 7(10), 331; https://doi.org/10.3390/fluids7100331 - 17 Oct 2022

Cited by 3 | Viewed by 1554

Abstract

In the simulation of dilute gas-solid flows such as those seen in many industrial applications, the Lagrangian Particle Tracking method is used to track packets of individual particles through a converged fluid field. In the tracking of these particles, the most dominant forces [...] Read more.

In the simulation of dilute gas-solid flows such as those seen in many industrial applications, the Lagrangian Particle Tracking method is used to track packets of individual particles through a converged fluid field. In the tracking of these particles, the most dominant forces acting upon the particles are those of gravity and drag. In order to accurately predict particle motion, the determination of the aforementioned forces become of the upmost importance, and hence an improved drag force formula was developed to incorporate the effects of particle concentration and particle Reynolds number. The present CFD study examines the individual effects of particles located both perpendicular and parallel to the flow direction, as well as the effect of a particle entrain within an infinite matrix of evenly distributed particles. Results show that neighbouring particles perpendicular to the flow (Model 2) have an effect of increasing the drag force at close separation distances, but this becomes negligible between 5–10 particle diameters depending on particle Reynolds number (Re_p). When entrained in an infinite line of particles co-aligned with the flow (Model 1), the drag force is remarkably reduced at close separation distances and increases as the distance increases. The results of the infinite matrix of particles (Model 3) show that, although not apparent in the individual model, the effect of side particles is experienced many particle diameters downstream. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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12 pages, 1816 KiB

Open AccessArticle

The Influence of Mitral Valve Asymmetry for an Improved Choice of Valve Repair or Replacement

by Dario Collia and Gianni Pedrizzetti

Fluids 2022, 7(9), 293; https://doi.org/10.3390/fluids7090293 - 05 Sep 2022

Cited by 2 | Viewed by 1299

Abstract

The study of valve asymmetry represents an important avenue for modern cardiac surgery. The correct choice of leaflet reconstruction may indicate a new path in the quality and long-term survival of patients. A systematic investigation was performed with a total of 25 numerical [...] Read more.

The study of valve asymmetry represents an important avenue for modern cardiac surgery. The correct choice of leaflet reconstruction may indicate a new path in the quality and long-term survival of patients. A systematic investigation was performed with a total of 25 numerical simulations using a healthy ventricle and an ideal valve with varying degrees of valve asymmetry. An overall assessment is made in terms of vorticity, kinetic energy, dissipated energy, and hemodynamic forces. The results indicate that the optimal asymmetry to consider for a valve repair or prosthetic design is between 0.2 and 0.4 with an optimal point of about 0.3. Out of this range, the heart is subjected to an excessive workload, which can only worsen the patient’s state of health. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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Review

Jump to: Editorial, Research

22 pages, 2426 KiB

Open AccessReview

Dilational Rheology of Fluid/Fluid Interfaces: Foundations and Tools

by Eduardo Guzmán, Armando Maestro, Carlo Carbone, Francisco Ortega and Ramón G. Rubio

Fluids 2022, 7(10), 335; https://doi.org/10.3390/fluids7100335 - 20 Oct 2022

Cited by 9 | Viewed by 1925

Abstract

Fluid/fluid interfaces are ubiquitous in science and technology, and hence, the understanding of their properties presents a paramount importance for developing a broad range of soft interface dominated materials, but also for the elucidation of different problems with biological and medical relevance. However, [...] Read more.

Fluid/fluid interfaces are ubiquitous in science and technology, and hence, the understanding of their properties presents a paramount importance for developing a broad range of soft interface dominated materials, but also for the elucidation of different problems with biological and medical relevance. However, the highly dynamic character of fluid/fluid interfaces makes shedding light on fundamental features guiding the performance of the interfaces very complicated. Therefore, the study of fluid/fluid interfaces cannot be limited to an equilibrium perspective, as there exists an undeniable necessity to face the study of the deformation and flow of these systems under the application of mechanical stresses, i.e., their interfacial rheology. This is a multidisciplinary challenge that has been evolving fast in recent years, and there is currently available a broad range of experimental and theoretical methodologies providing accurate information of the response of fluid/fluid interfaces under the application of mechanical stresses, mainly dilational and shear. This review focused on providing an updated perspective on the study of the response of fluid/fluid interfaces to dilational stresses; to open up new avenues that enable the exploitation of interfacial dilational rheology and to shed light on different problems in the interest of science and technology. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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