Fluids, Volume 8, Issue 5 (May 2023) – 25 articles

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–Euler equations at the cell–fluid boundary were built based on the fluid–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. Full article

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’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. Full article

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’ 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. Full article

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-ω 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. Full article

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’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–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’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 $k-\omega$ 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. Full article

The present article addresses the topic of grid motion computation in Arbitrary Lagrange–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–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. Full article

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’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–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–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. Full article

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’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. Full article

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’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–air free surface and film–substrate interface, the Péclet number, the viscosity–temperature coupling parameter and the slope of the linear surface tension–temperature relationship. A systematic exploration of the parameter space revealed that at low Péclet 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–temperature relationship on the draining and thinning of the film was observed to be more effective at lower Péclet numbers, where surface tension gradients in the lamella region opposed the gravity-driven flow. At higher Péclet 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. Full article

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–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–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–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’s spreading and contracting motion. Additionally, the influence of surface topography was investigated using a shot-peened surface. Caused by this surface’s reduced root mean square slope, the aforementioned enhancement of mechanical disintegration was not observed. Full article

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 $H$) since it must provide the cardiovascular system with a sufficient blood flow rate $Q$. 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–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. Full article

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 25–30 °C, mass fluxes of 20–50 kg/m²s, and heat fluxes of 10.4–13.7 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 ±30%. Full article

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. Full article

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-ε 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 α to the flow direction. Simulation results are presented for cylinder separations ranging from 1.125 to 4 diameters and for values of α between 10° and 60°. 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. Full article

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++’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. Full article

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 “digital tufts”, 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. Full article

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%. Full article

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. Full article

The CFD numerical study of the flash boiling phenomenon of a water film was conducted using an Euler–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 ${}^{\circ }$C (frim 1 to 41 ${}^{\circ }$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. Full article

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. Full article

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. Full article

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 (${\mathrm{O}}_2$, ${\mathrm{CO}}_2$). 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. Full article

The two-fluid (Euler–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’s performance. Turbulent energy spectra in different regions of the hydrocyclone were analyzed. Small-scale turbulence spectra at near-wall points exhibit $f^{-4}$ law, where f is frequency. Whereas for points at the air-column interface, the energy spectra show an inertial subrange $f^{-5/3}$ followed by a dissipative range of $f^{-7}$ law. Full article

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–liquid system as well as the Plateau–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. Full article

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–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’s improvement. Full article