Search

Inhalation exposes individuals to a diverse range of airborne particles carrying potential health risks. Airflow and particle transport behavior is highly sensitive to airway geometry. Therefore, it is a huge source of inter-subject variability — in both disease development as well as inhalation therapy outcomes. Numerical simulations of multi-phase flow of particle-laden air within the respiratory system has emerged as a key tool in assessing these complex physical phenomena. This study investigates the implications of bifurcation asymmetry in airway geometry on the fate of inhaled particles by employing Computational Fluid Dynamics (CFD) calculations. To that end, we use single generation 'Y-units'. Then we extend the study to multi-generation (G 5 − G 11) bronchial tree networks, both symmetric and asymmetric. An Eulerian–Eulerian approach was employed to solve for the coupled air-flow and particle transport equations, utilizing the OpenFOAM® CFD package. The relationship between lung structure and particle dynamics is revealed by analyzing the effects of asymmetry on particle transport and deposition. Our results show that although geometric asymmetry may not have a significant impact on the deposition fractions of single Y-units, its effects keep accumulating over multiple generations. This study also sheds light on local deposition patterns, identifying the specific deposition hot spots, leading to a comprehensive understanding of inhaled particle behavior in distinct lung geometries. [Display omitted] • Airflow and particle transport in the central airways were simulated using CFD. • Symmetric and asymmetric bifurcations (Y-units) as well as multi-generation bronchial trees (G5–G11) were compared. • Branching asymmetry in human airways influences particle deposition. • Mechanisms through which asymmetry has an impact on particle deposition was explained. [ABSTRACT FROM AUTHOR]

Predicting two-phase flow pattern characteristics and flow transition is fundamental to address several industrial problems, e.g. for the oil and gas industry. In order to fulfil the upscaling from the laboratory to the industrial scale, resource-efficient flow testing facilities which closely replicate industrial flow characteristics are needed. To address this, we introduce a new experimental device named as the Rocking and Rolling Ring Flow Loop (3RFL), which is size, cost, and time-efficient. In the present work, we employ the 3RFL apparatus at atmospheric pressure and temperature, with air and water as working fluids. However, the ultimate goal of our work is to build an experimental set-up capable of capturing the under-pressure reactive multiphase flow (gas-liquid–solid) dynamics typical of flow assurance in oil production and transportation. At the moment, the 3RFL can induce different flow regimes by adjusting the system control parameters such as rocking angle, rocking rate, and liquid volume fraction, all without requiring a pump. We observe three flow regimes, and analyse the impact of control parameters on their emergence. Our findings reveal that flow regime transitions are influenced by the competition between gravitational and centrifugal forces, which arise due to the curvature of the tube. Among three employed modelling strategies, namely mechanistic modelling, total energy minimization and a combined approach, we find that the total energy minimization model best compares to the experimental liquid height. [Display omitted] • A novel flow-testing benchtop without pumping is designed. • Backlight imaging is adopted to characterize flow behaviour. • The influence of centrifugal force on flow regimes and flow transitions are analysed. • Energy minimization, mechanistic and combined models are applied to predict the flow. [ABSTRACT FROM AUTHOR]

A recently proposed hyperbolic granular model (Balakrishnan and Bellan, 2024) has been used to investigate the problem of supersonic-jet induced cratering on a solid-particle bed. This model relies upon the concept of added mass and a fluid-mediated particle pressure to render the system of equations hyperbolic. The jet is modeled using Large Eddy Simulation (LES) and the solid phase is modeled in an Eulerian framework using a Kinetic-Theory-based model modified for dense particle collections. The formulation also incorporates pseudo-turbulent kinetic energy (PTKE) which has been shown to modify a flow field laden with particles. The results explore the influence of the jet-to-ambient fluid density ratio, of the ambient fluid density, and of PTKE. A detailed analysis of the local and time-wise evolution of the added mass is presented through quantification of convection, added mass source and velocity-difference contributions. A quantitative assessment of the PTKE equation shows that while production is mostly effective at the crater base and walls, the dissipation and source term, both of which are also effective in the ejecta, nearly balance each other. The influence of the PTKE is mostly observed in the dilute particle regions (soil/gas interface and ejecta), with no effect on the macroscopic length scales of the flow. Both the jet-to-ambient fluid density ratio and the ambient fluid density affect the macroscopic crater features through entertainment into the jet that is determined by the former, and through the density of the fluid entrained, determined by the latter. This complex interaction governs the evolution of the crater diameter, visible top-view depth, and lower depth of the compacted region. [Display omitted] • Supersonic plume-induced soil cratering modeled using a hyperbolic granular model. • Effects of added mass (contributing to hyperbolicity) and PTKE are investigated. • Producing added mass are: convection; solid/fluid velocity difference; source term. • The above contributions are quantified spatially for several cratering scenarios. • PTKE affects the parabolic-craters floor shape and ejecta spatial distribution. [ABSTRACT FROM AUTHOR]

• A method for decomposing the pressure force on the immersed body is derived for RANS/LES. • The new method keeps good accuracy in single-phase and multiphase flows. • The effect of different cavitation types on lift and drag is quantified. • Tip clearance region is nonnegligible, where the lift is generated by the interaction of shear and rotating flows. In this paper, the force decomposition method (FDM) is proposed for decomposing the pressure forces acting on the immersed body. The major improvement in FDM is the applicability in RANS and LES simulations with non-constant density flows, e.g. cavitating flow. In the single-phase flow over a circular cylinder (Re = 3900), the FDM results show excellent agreement with the pressure force given by conventional method. In the cavitating flow over a hydrofoil with tip clearance, FDM can also reproduce the same tendency of the pressure force with an average deviation below 17%. In this case, the vorticity and kinematic effect induced force dominates the pressure force. In order to isolate the effect of attached cavitation near the suction side and tip clearance cavitation, a method combing domain cutting (manually) and cell extracting (by a threshold of vorticity magnitude) is proposed. The lift caused by vorticity force is mainly affected by the vortices shedding from the suction side of the hydrofoil. Also, the tip clearance region should not be ignored, where the lift generation by TLV is the interaction of strong shear and rotating flows. [Display omitted] [ABSTRACT FROM AUTHOR]

The accurate reconstruction of immiscible fluid–fluid interfaces from the volume fraction field is a critical component of geometric Volume of Fluid methods. A common strategy is the Piecewise Linear Interface Calculation (PLIC), which fits a plane in each mixed-phase computational cell. However, recent work goes beyond PLIC by using two planes or even a paraboloid. To select such planes or paraboloids, complex optimization algorithms as well as carefully crafted heuristics are necessary. Yet, the potential exists for a well-trained machine learning model to efficiently provide broadly applicable solutions to the interface reconstruction problem at lower costs. In this work, the viability of a machine learning approach is demonstrated in the context of a single plane reconstruction. A feed-forward deep neural network is used to predict the normal vector of a PLIC plane given volume fraction and phasic barycenter data in a 3 × 3 × 3 stencil. The PLIC plane is then translated in its cell to ensure exact volume conservation. Our proposed neural network PLIC reconstruction (PLIC-Net) is equivariant to reflections about the Cartesian planes. Training data is analytically generated with O (1 0 6) randomized paraboloid surfaces, which allows for the sampling a broad range of interface shapes. PLIC-Net is tested in multiphase flow simulations where it is compared to standard LVIRA and ELVIRA reconstruction algorithms, and the impact of training data statistics on PLIC-Net's performance is also explored. It is found that PLIC-Net greatly limits the formation of spurious planes and generates cleaner numerical break-up of the interface. Additionally, the computational cost of PLIC-Net is lower than that of LVIRA and ELVIRA. These results establish that machine learning is a viable approach to Volume of Fluid interface reconstruction and is superior to current reconstruction algorithms for some cases. [Display omitted] • A neural network method for 3D PLIC reconstructions deployed in VOF simulations. • Analytical paraboloid training data prepares the neural network for curved fluid–fluid interfaces. • Reflection equivariance about Cartesian planes is enforced. • Neural network reconstructions reduce spurious planes and computational cost when compared to (E)LVIRA. [ABSTRACT FROM AUTHOR]

We show that a pipe transporting gas and liquid can be operated at conditions where both stratified flow and slug flow are stable. In this bi-stable flow regime, it is possible to observe stratified flow or regular hydrodynamic slug flow, depending on the inlet conditions. However, it is also possible to get one single slug at a time in the pipe. The distinctive characteristic of such "solitary" slugs is that they grow continuously as they travel along the pipe and therefore, in long pipelines, they can reach lengths up to thousands of pipe diameters and flood the downstream process installations. We first show how the length of solitary slugs can be predicted using slug tracking simulations. Based on those transient simulation results, we provide an explanation for the slug growth and we propose a simple quasi-steady-state model allowing us to predict the slug length in good agreement with the transient simulations. Finally, we explain how we can determine the boundaries of the bi-stable flow regime in which solitary slugs occur. [Display omitted] • In long flowlines transporting gas and liquid a single slug can occur at a time. • The slug can grow very large and potentially flood downstream processes. • There can be a bi-stable flow regime where slug and stratified flow are both stable. • Solitary slugs occur only in the bi-stable regime, whose boundaries are discussed. • Existence of solitary slugs is demonstrated with slug tracking simulations. • Slug growth is caused by mismatch of upstream and downstream conditions. • A simple quasi-steady-state model shows very good agreement with simulations. [ABSTRACT FROM AUTHOR]

• As the cavitation number of the projectiles launched successively decreases, the vortex structure is gradually enriched. The cavitation flow can have a great impact on the projectile's water-exit attitude and stability. At present, the research on cavitation flow is mainly focused on the single projectile, while less research has been conducted on the cavitation flow of projectiles successively launched underwater. In this paper, a verification of the flow simulation method and validation of the cavitation model is presented. The multiphase flow associated with cavitation flows, interference characteristics of the cavitation vortex structure, and load characteristics of the projectiles successively launched underwater are studied. The results show that owing to the flow interference, the attitude of the projectile is deflected to the outside. A large-scale cavity shedding phenomenon of the inside cavity occurs under the incoming flow. A large number of small-scale vortex rings appear above the water surface in the projectile that successively exited the water. As the cavitation number of the projectiles launched successively decreases, the turbulent vortex structure is gradually enriched. In addition, both the inside and the outside are subject to extremely high peak collapse loads. Remarkably, the peak pulsating pressure generated by the collapse of the cavity is strongly correlated with the state of the cavity when the head of the projectile touches the water surface. [Display omitted] [ABSTRACT FROM AUTHOR]

Multiphase compressible flow systems can exhibit unsteady and fast-transient dynamics, marked by sharp gradients and discontinuities, and material boundaries that interact with the evolving flow. The transient nature of the dynamics presents challenges to employing artificial intelligence (AI) and data-driven models for predicting flow behaviors. In this study, we explore the potential of physics-aware recurrent convolutional neural networks (PARC) to model the spatiotemporal dynamics of multiphase flows in the presence of shocks and reaction fronts. PARC is a neural network model that incorporates the generic form of the diffusion–advection–reaction equation in its network architecture, which mimics the process of solving the governing equations of fluid flows. In contrast to physics-informed machine learning approaches such as physics-informed neural networks (PINNs) where models are trained to directly minimize the residual of governing equations, PARC takes a dynamical systems viewpoint and does not seek to minimize potentially nonconvex and nonlinear loss terms. To assess the ability of PARC to accurately learn and simulate the physics of multiphase flows, we train and test PARC on various flow simulation problems, including the Burgers' equation, fluid flow behind a cylindrical cross-section, and unsteady shock interactions with a particle at varying Mach numbers. We analyze PARC's performance and examine sources of error in its prediction, in terms of differentiation and integration schemes and different weighting strategies for the model update. Based on our observations, we discuss PARC's capabilities and limitations in multiphase flow applications and propose future research directions. [Display omitted] • We assess the physics-aware recurrent convolution (PARC) network in multiphase flows. • Multiphase flows present machine learning challenges such as sharp gradients. • PARC predictions closely match physics-based solvers, with less computation time. • PARC can generalize to various unseen initial conditions. • PARC outperforms other machine learning models in complex multiphase flow settings. [ABSTRACT FROM AUTHOR]

Spectral analysis of dispersed two-phase flows is highly desirable to reveal the interplay of the various flow scales, much larger or much smaller than the size of the dispersed bodies. This is a challenging task as the matching conditions at the body interfaces generate singularities in the fields describing the two-phase mixture. The nature of these singularities and their consequences on the spectra are theoretically analyzed for bubble or droplet flows. Results of direct numerical simulations are reported and spatial spectra of the mixture velocity, the flow forces and their power are examined. The regular part of the spectral densities of energy production, dissipation and transfers between scales are separated from their singular part. The resulting spectral energy balance, free of the footprint of the singularities, is found in agreement with coarse-grained simulations where the interfaces are filtered out before solving the Navier–Stokes equations. These results pave the way for the spectral analysis of more complex turbulent dispersed flows. [Display omitted] • Spectral analysis of dispersed two-phase flows is complicated by discontinuities at interfaces. • The theoretical footprints of various field singularities at sharp interfaces are determined. • Energy spectrum and spectral energy budget calculated from DNS of a swarm of rising bubbles. • A method for separating the regular and singular parts of the spectra is proposed. • The regular part of DNS spectra is in good agreement with that of Coarse-Grained Simulations. [ABSTRACT FROM AUTHOR]

The intricate process of bubble growth dynamics involves a broad spectrum of physical phenomena from microscale mechanics of bubble formation to macroscale interplay between bubbles and surrounding thermo-hydrodynamics. Traditional bubble dynamics models including atomistic approaches and continuum-based methods segment the bubble dynamics into distinct scale-specific models. To bridge the gap between microscale stochastic fluid models and continuum-based fluid models for bubble dynamics, we develop a composite neural operator model to unify the analysis of nonlinear bubble dynamics across microscale and macroscale regimes by integrating a many-body dissipative particle dynamics (mDPD) model with a continuum-based Rayleigh–Plesset (RP) model through a novel neural network architecture, which consists of a deep operator network for learning the mean behavior of bubble growth subject to pressure variations and a long short-term memory network for learning the statistical features of correlated fluctuations in microscale bubble dynamics. Training and testing data are generated by conducting mDPD and RP simulations for nonlinear bubble dynamics with initial bubble radii ranging from 0.1 to 1.5 micrometers. The results show that the trained composite neural operator model can accurately predict bubble dynamics across scales, with a 99% predictive accuracy for the time evolution of the bubble radius under varying external pressure while containing correct size-dependent stochastic fluctuations in microscale bubble growth dynamics. The composite neural operator is the first deep learning surrogate for multiscale bubble growth dynamics that can capture correct stochastic fluctuations in microscopic fluid phenomena, which sets a new direction for future research in multiscale fluid dynamics modeling. [Display omitted] • We developed a composite neural network model for multiscale bubble dynamics. • Our model offers seamless scale-bridging in multiscale bubble dynamics. • It enables learning of scale-dependent stochastic processes on top of mean-field bubble growth. [ABSTRACT FROM AUTHOR]

This work presents the development of a novel solver tailored for simulating multiphase flows within heterogeneous porous media. Leveraging the Eulerian multi-fluid model coupled with Darcy's law, the solver demonstrates adaptability across diverse scales, effectively handling heterogeneous porosity and permeability fields. The proposed solver, called upstreamFoam , extends the capabilities of OpenFOAM framework, specifically the multiphaseEulerFoam , by incorporating models for porous media simulations. This integration introduces new features and formulations, allowing for the simulation of compressible multiphase flows in porous media with intricate properties. The approach presented here provides a robust framework for characterizing reservoirs and treating heterogeneous porous systems at different scales. A successful validation of the introduced solver for classical problems with analytical, semi-analytical, and reference solutions is presented. Then, applications on a wide range of multiphase flows in heterogeneous porous media at different scales have been studied, demonstrating the potential of the solver to simulate complex multiphase problems. [Display omitted] • OpenFOAM-based solver for simulating multiphase flows in heterogeneous porous media. • Combines the Eulerian multi-fluid model with Darcy's law, employing PIMPLE strategy. • Addresses fluid and matrix compressibility. • Great versatility for simulating a wide range of multiphase flows in porous media. • Applications for challenging multiphase flows in porous media at different scales. [ABSTRACT FROM AUTHOR]

The present work studies bubble breakup in gas–liquid centrifugal pumps. Different inlet gas-fraction conditions are considered to provoke bubble-to-bubble, slug-to-bubble, and slug-to-slug pattern transitions. Results are presented for different mixture velocities and pump rotation frequencies. Data on mean equivalent bubble diameter and slug unit properties (length, passage frequency) are introduced. For small bubbles, a simple phenomenological theory is developed for the prediction of mean diameters. A discussion on the dynamics of small bubbles in the impeller region is also presented. [Display omitted] • The present work studies the fragmentation of large and small bubbles by a centrifugal pump impeller. • The flow conditions were selected to lie outside the range of other previous works. • Data on mean equivalent bubble diameter and slug unit properties are introduced. • For small bubbles, a simple phenomenological theory is developed for the prediction of mean diameters. • The splitting of large bubbles was shown to be a very complex phenomenon. [ABSTRACT FROM AUTHOR]

Electrostatic rotary bell atomizers are commonly used in several engineering applications, including the automobile industry. A high-speed rotating nozzle operating in a strong background electric field atomizes paint into charged droplets that range from a few micrometers to tens of micrometers in diameter. The atomization process directly determines the droplet size and droplet charge distributions which subsequently control the transfer efficiency and the surface finish quality. We have previously developed a tool to perform high fidelity simulations of near-bell atomization with electrohydrodynamic effects. In this work, we perform simulations employed with a droplet ancestry extraction tool to analyze previously inaccessible information and understand the physical processes driving atomization. We find that the electric field accelerates breakup processes and enhances secondary atomization. The total number of droplets, the ratio of secondary to primary droplets, and the ratio of coalescence to breakup activity are all much higher when operating in an electric field. We analyze the droplet velocity, local Weber number and charge density statistics to understand the complex physics in electrically assisted breakup. The results of the study have helped us gain insights into the physics of atomization in electrostatic rotary sprays. [Display omitted] • We simulate near-bell atomization in electrostatic rotary bell atomizers. • We extract droplet ancestry and local flow statistics. • Statistics show that the electric field has a minor impact on the atomization. [ABSTRACT FROM AUTHOR]

We present recent experimental results on the dynamics of atomization of a liquid column by a coaxial turbulent gas jet, under varying air angular momentum (swirl) conditions and densities. We compare between atomization under atmospheric conditions and in a pressurized environment where the atomizing and ambient air density is 5 times the atmospheric level. For both conditions, the parameter space includes a gas-to-liquid momentum ratio in the range of M = 25 − 56 and swirl ratios of S R = 0 − 1. High-speed shadowgraphy images in the spray near-field are used to quantify the spatial and temporal evolution of the liquid–gas interface. In the mid-field, Phase Doppler Interferometry, collected radially across the spray, quantify droplet size, velocity, and number density distributions. At high pressure and with increasing M, we observe a decrease in spray angle when S R = 0 but a spray angle increase when S R > 0. 5 (critical swirl). We reconcile these observations with the crown de-wetting mechanism, first observed in X-ray shadowgraphs of the spray. Crown de-wetting depends on gas inertia relative to the liquid, which increases linearly with atomization environment pressure, leading to this switch in behavior. This mechanism also explains the modified droplet radial distribution in the mid-field, with critical transition from concave to convex-shaped trends in the mean droplet size profile. Finally, we reconstruct the full spray droplet population from radial measurements, showing that swirl not only modifies the transport of droplets in the spray cross-section, but also enhances break-up and reduces the Sauter Mean Diameter globally, for all momentum ratios and pressure conditions tested. [Display omitted] • We explore the effect of swirl on gas–liquid atomization with varying pressures. • Higher swirl in the gas phase results in larger spreading angles. • At critical swirl ratios the temporal dynamics of the spray are also affected. • The overall spray droplet sizes decrease with increasing swirl ratios. [ABSTRACT FROM AUTHOR]

The proper design of the manifold, distributing the flow to the individual nozzles, is crucial for the Pelton turbine performance. The generation of both energy losses and secondary flows must be reduced to a minimum to provide optimal water jet quality and, hence, efficiency. However, the inflow state due to the penstock is commonly disregarded at the design stage. By numerical flow simulation, we show how such penstock perturbations affect the water jet cross-section and axis deviation by considering different pipe bends upstream of the manifold. The two-phase flows resulting in a typical Pelton turbine manifold are predicted using the Volume-Of-Fluid (VOF) approach in conjunction with the k − ω SST turbulence closure model. Pipe bends are attached five manifold inlet diameters upstream of the manifold itself. The consequences on the secondary flows in the nozzles and the free water jet quality are reported quantitatively. The analysis shows that even small inflow perturbations into the manifold due to penstock piping significantly affect the water jet quality. A yet unreported mechanism is revealed, where the secondary flows mitigate the impact of the flow rate imbalance across the nozzle on the water jet deviation. [Display omitted] • Even small perturbations in the penstock piping affect the water jet quality. • The secondary flow ratio is a good estimator for the free water jet deformation. • Secondary flows can reduce the water jet deviation. [ABSTRACT FROM AUTHOR]

During the landing and recycling on the sea surface of a spacecraft or rocket, a concave structure may impact on the water surface. In the present paper, we experimentally and numerically analyze the multiphase flows and transient impact load during the oblique water entry of the concave-nose projectile. The projectiles used in the experiments consist of a main body that embeds an acceleration measurement system, and a transparent concave-nose. Firstly, free surface evolutions and cavity dynamics are given for the concave-nose projectile, which shows significant changes compared with the projectile without concave-nose. A special jet inside the concave-nose during the formation of the air-entertainment cavity is observed. Subsequently, it is found that compared with the projectile without concave-nose in the early stage. The increase rate of the axial force of the concave-nose projectile is slower and the force curve has a steep drop stage, then they enter the uniform reduction stage. As the concave depth increases, a 'double-peak' phenomenon on the axial force curve appears. It is found that the force peak is transferred to a single peak with increasing the impact velocity. The force peak of the concave-nose projectile is greater than the projectile without concave-nose. Besides, with increasing the concave depth, the maximum axial force of the concave-nose projectile increases firstly, and then decreases. When the ratio of concave depth to inner diameter is greater than 2, the influence of concave depth on the peak value of axial force is small. Moreover, the pressure distributions of the fluid domain and the concave-nose are analyzed to reveal the mechanism of the variation of the force peak. The study is able to provide a basic reference for the analysis of the recycling of the rocket, aircraft, etc. [Display omitted] • The free surface evolution and transient impact load of the oblique water entry of the concave-nose projectile are studied experimentally and numerically. • The experimental and numerical results of the complicated multiphase flow characteristics inside the concave-nose are given. • The characteristics of the impact load of the concave-nose projectile are investigated quantitatively. • The mechanism of the effect of the concave depth on the force peak of the projectile is revealed. [ABSTRACT FROM AUTHOR]

The Phase Field Method stands as a promising technique for modeling complex two-phase and multi-phase flow systems, with the advective Cahn–Hilliard equation adept at capturing the evolution of intricate interfacial structures. However, achieving simulation accuracy necessitates the preservation of the diffuse interface profile, which is often challenged by the convection term within the Cahn–Hilliard equation, leading to deviations from the equilibrium interface state. To address this challenge, we introduce an innovative approach aimed at iteratively restoring the equilibrium interface profile after each time step. This method combines a level-set profile-corrected equation with an algebraic relation between the phase field and the signed distance function, resulting in a preservation equation that relies on the phase-field-related signed distance function rather than the phase field function itself. Quantitative computational tests affirm the effectiveness of this approach in minimizing discretization errors, reducing dependence on the numerical P e ́ clet number, and improving mass conservation accuracy for each phase, all while reducing computational time by approximately 20-fold due to the coarser grid used. Validation through simulations across various scenarios demonstrates the approach's reliability, with results closely aligning with analytical solutions, prior numerical findings, and experimental data, thereby underscoring the efficiency and precision of our proposed approach. [Display omitted] • Profile-preserving formulation for phase field methods in simulating two-phase flows. • Reduced computational cost and improved mass conservation on a coarser grid. • Lower sensitivity to numerical Péclet numbers, minimal computational expense. • Rigorous validation in surface tension-dominant scenarios affirms reliability. [ABSTRACT FROM AUTHOR]

• Effective LES and wave generation for water entry with sliding mesh are established. • Water entry at wave phases are analyzed with theoretical correction model developed. • Cavity feature and deep-seal in following and against wave conditions are revealed. • Vortex structures and pressure signals resolved are analyzed with cavity evolution. • Water entry dynamics in different wave conditions are discussed with flow mechanism. Objects entering water into a wave instead of calm water involve complex multiphase flow phenomena. Large eddy simulation (LES) is used to numerically investigate the water entry of a small-scale cylinder in a regular wave field. A free-motion model of the cylinder is established using the sliding grid technique. The velocity boundary wave-generation method is employed, where a second-order Stokes wave analytical solution is applied at the inlet, and wave-absorbing sources are added in regions far from the water entry region. By comparison with the experimental results, the numerical scheme in this study can capture the cavity evolution, movement of the entering body, and even the closure of splashes. The influence of the four wave phases on the vertical entry and various water entry angles are investigated. The cavities are primarily influenced by the velocity field direction and wave slope in different wave phases. Based on this, a theoretical correction approach has been proposed, showing good agreement with the numerical results. The different characteristics of the deep seal for the against-wave entry and following-wave entry cases are discovered. The multiphase flow field caused by vertical entry is more complex than inclined entry because of the splash difference. The cylinder drag is primarily derived from the contact between the cylinder and asymmetrical cavity. [Display omitted] [ABSTRACT FROM AUTHOR]

High-fidelity numerical simulation of compressible multi-phase flows is of great challenge due to its competing requirements for resolving complex flow structures with low dissipation and capturing moving interfaces as well as other discontinuities sharply. Recently, a novel hybrid scheme, combining the standard targeted essentially non-oscillatory (TENO) scheme with Tangent of Hyperbola for INterface Capturing (THINC) scheme as two building blocks for smooth and non-smooth regions respectively and thus named as TENO-THINC, has been proposed and shows great potential for resolving complex single-phase fluids. In this work, a high-order finite-volume method, based on the TENO-THINC scheme for spatial reconstruction and the Harten-Lax-van Leer contact (HLLC) approximate Riemann solver for flux evaluation, is developed for simulating compressible multi-phase flows with a reduced five-equation formulation of the diffuse-interface model. The TENO-THINC scheme deploys the THINC reconstruction to resolve the physical discontinuities as well as the material interfaces within a few cells, and is desired to resolve the interface evolution more reliably in multi-phase flow simulations. Several algorithms have been implemented and elaborated for ensuring the numerical robustness of extreme simulations with high density and pressure ratios. Numerical results of the 1D and 2D challenging benchmark tests show that the TENO-THINC scheme is more robust than the standard TENO scheme and less dissipative than both the TENO and WENO-JS schemes. This property is essential for the long-term simulations of compressible multi-phase flows. [Display omitted] • A high-order TENO-THINC scheme is developed to simulate multi-phase flows. • The numerical framework does not generate spurious oscillations. • The scheme will invoke THINC reconstruction near discontinuities and interfaces. • In the smooth region, the TENO-THINC scheme will restore the high-order accuracy. • A set of numerical techniques is developed for guaranteeing the numerical stability. [ABSTRACT FROM AUTHOR]

The presence of bubbles in a turbulent flow changes the flow drastically and enhances the mixing. Adding salt to the bubbly aqueous flow changes the bubble coalescence properties as compared to regular demineralized water. Here we provide direct experimental evidence that also the turbulent thermal energy spectra are strongly changed. Experiments were performed in the Twente Mass and Heat Transfer water tunnel, in which we can measure the thermal spectra in bubbly turbulence in salty water. We find that the mean bubble diameter decreases with increasing concentration of salt (NaCl), due to the inhibition of bubble coalescence. With increasing salinity, the transition frequency from the classical − 5 / 3 scaling of the thermal energy spectrum to the bubble induced − 3 scaling shifts to higher frequencies, thus enhancing the overall thermal energy. We relate this frequency shift to the smaller size of the bubbles for the salty bubbly flow. Finally we measure the heat transport in the bubbly flow, and show how it varies with changing void fraction and salinity: Increases in both result into increases in the number of extreme events. [Display omitted] • The presence of bubbles changes mixing properties in a flow. • The classical − 5 / 3 scaling changes to a − 3 bubble induced scaling in kinetic and thermal spectrum. • The increase in heat transfer in turbulent bubbly flow increases with both increasing void fraction and salt concentration. [ABSTRACT FROM AUTHOR]

The current state-of-the-art for computational modeling of polydisperse multiphase flows is reviewed and future research directions are discussed. Physics-based computational models at three distinct levels of abstraction: the microscale, mesoscale and macroscale; are discussed and compared. Special emphasis is placed on the relationship between the models at different scales and on how information from the finer scales is used to provide closures at the coarser scales. For disperse multiphase flows, it is argued that the passage from a direct-numerical simulation at the microscale to the kinetic description at the mesoscale is the crucial step for ensuring the validity of the macroscale model. In particular, the passage from the microscale to the mesoscale requires physics-based closures, while the passage from the mesoscale to the macroscale requires mathematical closures. The choices made in the physical and mathematical closures of the spatial fluxes and coupling terms will determine whether the Eulerian model is well-posed. In addition, the use of quadrature-based moment methods for polydisperse particles is presented as an efficient macroscale closure when dealing with a distribution of particle sizes. Examples of monodisperse and polydisperse multiphase flows are provided for cases where the fluid phase is compressible (high speed) and incompressible (low speed). [Display omitted] • Kinetic-based modeling of polydisperse multiphase flows is reviewed. • A multiscale modeling framework consisting of micro-, meso- and macroscale models is proposed. • The links between the micro- and mesoscale and the meso- and macroscale models are described using gas–particle flow. • The importance of verifying that the model is well-posed as each scale is emphasized. • The multiscale modeling approach is shown to reproduce the correct physics over a wide range of parameters. [ABSTRACT FROM AUTHOR]

• We introduced the inclined slits in purging plugs to improve fluid flow characteristics and control bubble behavior. • A a file acting as a user-defined function was compiled to define the parameters of the ejected bubble. • This study provides an orientation for enhancing the performance of the multi-hole nozzle. Inclined slits are introduced in purging plugs to improve fluid flow characteristics and control bubble behavior. In this study, the discrete phase model was used to describe bubble behavior, and a file acting as a user-defined function was compiled to define the location and inclined angle of the injected bubble, the flow rate, and the initial bubble diameter. Because of the variable density and pressure in a high-temperature melt, bubble expansion considerably affects the behavior of bubbles, which was considered in our numerical simulation. The results showed that the divergent slit reduces the collision probability of generated bubbles as they rise, decreasing the average size of bubbles in the ladle. Moreover, the converging slit intensified the turbulent kinetic energy dissipation caused by bubble collisions. It is essential to improve the flow characteristics of molten steel and control bubble size by varying the inclined slit angles of the purging plug in a ladle, which is also applicable to gas stirring reactors with a multi-hole nozzle. [Display omitted] [ABSTRACT FROM AUTHOR]

• Interfacial Area Transport Equation (IATE) enabled Two-Fluid Model for gas influx simulation. • Transient simulation of bubble breakage and coalescence during gas influx management. • A novel practice in implementing IATE in wellbore-scale simulation for well control. • Validation of the IATE and two-fluid model based on distributed fiber-optic sensing data. Gas influx management is a critical aspect of petroleum drilling operations, which aims to prevent the uncontrolled outflow of formation gas and the potential consequences caused by blowouts. This study investigates the distribution of gas influx and the interfacial area concentration (IAC) during gas influx circulation operations through a combination of full-scale experiments and advanced numerical simulations. Experiments were conducted to simulate downhole gas influxes by injecting Nitrogen gas into the base of a 5,160-foot-deep experimental wellbore, which was filled with water. A Distributed Fiber-Optic Sensing (DFOS) system was installed in the wellbore to obtain high-resolution acoustic and temperature data and monitor the gas influx behaviors when circulated out of the annulus. Various types of measurements were obtained to analyze the gas influx length, slip velocities, void fraction distribution, and dispersion behaviors. In addition, an advanced numerical simulator was developed based on a modified Two-Fluid Model and the Interfacial Area Transport Equation (IATE) to simulate the gas influx behaviors. The model estimations were compared to the experimental measurements for validation of accuracy and further insights. The numerical modeling framework, based on the interfacial area transport equation, effectively captured bubble interactions, including mechanisms including bubble breakage and coalescence, by estimating the interfacial area distribution. The gas influx dispersion during circulation was analyzed using both model estimations and experimental data. Good agreement was observed between the model predictions and experimental data, including multi-depths gauge measurements and the DFOS data. This is a novel application of the IATE in well-scale multiphase flow simulations, which has proved to be an effective tool for predicting the dynamics of gas influx distributions. The outcomes of this study provide critical insights into the design and optimization of gas influx management during Managed Pressure Drilling (MPD). [Display omitted] [ABSTRACT FROM AUTHOR]

• Nano-electrosprayed droplets transfer momentum to ambient gas, producing a gas jet. • Gas jets produced by nano-electrospray (nES) have a narrow, high-velocity core. • The high-velocity gas jet core is produced in the nES liquid jet breakup region. • Droplet size is a significant parameter affecting the velocity of the gas jet. Electrospray (ES) is production of a charged droplet plume that transits from an electrically biased supply capillary to a counter electrode under the influence of an electric field. Drag interactions with the surrounding gas decelerate the droplets while transferring the momentum from droplets to accelerate the surrounding gas, resulting in an induced gas flow. This work seeks to characterize the structure of the resulting gas jet and identify the key mechanisms defining the flow structure. The phenomenon of gas jets produced by momentum transfer from nano-electrospray (nES) plumes is explored with schlieren visualization, thermal anemometry, and numerical simulations. Schlieren visualization experiments provide information on the flow structure in support of simulation predictions, and the hot thermistor anemometry measurements of gas velocities outside the spray demonstrate quantitatively validated simulation results. The study reveals the formation of a moderately high velocity coaxial gas jet within the nES plume and provides insight into the overall flow structure of the induced flow. The multiphase electrohydrodynamic simulations enable numerical experimentation to explore the fundamental physics of coupled droplet-gas transport and the resulting flow structure. The simulations, confirmed by the experiments, reveal gas jetting induced by nES with a narrow (sub hundred micrometers in diameter) core originating from the nES liquid-jet breakup region and a surrounding larger-in-extent zone (several hundred micrometers in diameter) of lower velocity gas flow within the electrospray plume. This behavior is due to nES ejecting a stream of droplets from the tip of a narrow liquid cone-jet, where the combined effect of many small droplets transferring momentum to a confined region of gas yields a narrow but high-velocity gas stream. The micro-jets produced by nES have practical utility for mass spectrometry, 3D printing and fabrication, and thermal management. [Display omitted] [ABSTRACT FROM AUTHOR]

Hyperbolic conservation equations for polydisperse, high-speed, compressible, fluid–particle flows with added mass and fluid-phase pseudoturbulence are derived. First, the kinetic-based model for monodisperse particles that accounts for collisional and frictional pressure, as well as added mass and internal energy, is reviewed. Then, a kinetic-based model for polydisperse particles is formulated in terms of the moments of the particle size distribution, and velocity moments conditioned on the particle mass or size. Transport equations for velocity moments up to second order (or total kinetic energy) are closed using the hyperbolic quadrature method of moments. In the numerical implementation for the spatial fluxes and source terms, the particle mass distribution is treated using the generalized quadrature method of moments and the size-conditioned moments are found with the conditional quadrature method of moments. Example results for spatially 1-D test cases demonstrate the ability of the polydisperse model to capture a wide range of particle-size-dependent multiphase flow physics. [Display omitted] • A kinetic model for polydisperse particles that includes added mass and particle–fluid–particle stresses is formulated. • Kinetic-based model is coupled to modified Euler equations for high-speed fluid flow, and includes pseudo-turbulent kinetic energy. • For collisional flows, moment equations for particle phase are derived and closed using quadrature-based moment methods. • Macroscale model is well-posed and solved numerically using standard solvers for hyperbolic conservation laws. • Using numerical examples, polydisperse model reproduces the correct physics over a wide range of volume fractions. [ABSTRACT FROM AUTHOR]

The settling behavior of individual spheres in a quiescent fluid was studied experimentally. The dynamics of the spheres was analyzed in the parameter space of particle-to-fluid density ratio (Γ) and Galileo number (Ga), with Γ ∈ (1. 1 , 7. 9) and Ga ∈ (100 , 340). The experimental results showed for the first time that the mean trajectory angle with the vertical exhibits a complex behavior as Ga and Γ are varied. Numerically predicted regimes such as Vertical Periodic for low Γ values, and Planar Rotating for high Γ values were validated. In particular, for the denser spheres, a clear transition from planar to non-planar trajectories was observed, accompanied by the emergence of semi-helical trajectories corresponding to the Planar Rotating Regime. The spectra of trajectory oscillations were also quantified as a function of Ga , confirming the existence of oblique oscillating regimes at both low and high frequencies. The amplitudes of the perpendicular velocities in these regimes were also quantified and compared with numerical simulations in the literature. The terminal velocity and drag of the spheres were found to depend on the particle-to-fluid density ratio, and correlations between the drag coefficient and particle Reynolds number (R e p ) as a function of Ga were established, allowing for the estimation of drag and settling velocity using Ga , a control parameter, rather than the response parameter R e p . [Display omitted] • Regime Identification: Settling regimes, terminal velocity, and drag coefficient are studied. • Density Ratio Impact Explicit dependence of particle density in terminal velocity and drag is found. • Chaotic Behavior: The emergence of chaotic particle trajectories at high Galileo number is confirmed. [ABSTRACT FROM AUTHOR]

The present paper introduces a numerical approach based on the level-set framework to simulate multiphase flow in the context of multiple emulsion formation. The present technique, designed for the flow of n-phases, is applicable for the production of multiphase emulsions and also for the modeling of two and three-dimensional systems. This method is utilized to simulate the formation of single, double, and triple emulsions in a microfluidic cross-junction. The accuracy of the findings is confirmed by conducting an investigation of a single emulsion at home, as well as by referencing experimental and numerical literature on multiple emulsions. Until recently, the prospect of a shift in the double droplet formation regime at a constant capillary ratio between phases had not been taken into account. The present study yielded a flow regime mapping diagram that exhibits a transition in the double droplet formation regime under a constant capillary number ratio between the inner and middle phases. Using a morphological diagram, our method takes into consideration the effect of varying values of interfacial tension between the phases on the formation of double droplets. A comparison of two-dimensional and three-dimensional modeling strategies for double emulsion is also conducted, with the aim of clarifying the dissimilar results obtained from these two techniques. [Display omitted] • A level-set-based numerical method is used to explore multiphase flow. • Using this method, multiple emulsion formation in a microfluidic device is simulated. • Phase-morphological interactions are included. • Simulations of 2-D and 3-D higher-order emulsion generation are incorporated. [ABSTRACT FROM AUTHOR]

• The SO 2 absorption into a droplet falling freely is investigated numerically. • The deformation and internal circulation occur during droplet falling and facilitate the mass transfer of gas-droplet. • A modified mass transfer coefficient of gas phase is proposed. The SO 2 absorption into a droplet falling freely is investigated numerically based on a detailed three-dimensional mode in this work. Gas-liquid multiphase flow simulations coupled with interphase mass transfer are performed using the coupled Level-set and Volume of Fluid method (CLSVOF) to trace the gas-droplet interface. The results of simulation reveal that droplet would deform and deviate from the standard sphere during falling, with an approximately 5.6 % increase in surface area under the discussed working conditions. The evolution mechanism of the internal circulation in the droplet is clarified: the increase in the relative velocity of gas-droplet facilitates the formation and intensification of the internal circulation in the droplet. As the internal circulation develops into the stable stage, the convective mass transport would dominate the mass transfer process and promote the mass transfer. The distribution laws of the local mass transfer rate and coefficient over the droplet surface are found analogous to the typical case of heat transfer around a sphere. Finally, considering the effects of both droplet deformation and internal circulation on mass transfer concurrently, the variation of Sh as a function of Reynolds number is obtained, which appears the same trend but yields slightly higher values compared to the results of empirical correlations. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper presents an advanced Volume of Fluid (VOF) method that enables performant three dimensional Direct Numerical Simulations (DNS) of the interaction of two immiscible fluids in a gaseous environment with large topology changes, e.g., binary droplet collisions. One of the challenges associated with the introduction of a third immiscible phase into the VOF method is the reconstruction of the phase boundaries near the triple line in arbitrary arrangements. For this purpose, an efficient method based on a Piecewise Linear Interface Calculation (PLIC) is shown. Moreover, the surface force modeling with the robust Continuous Surface Stress (CSS) model was enhanced to treat such three-phase situations with large topology changes and thin films. A consistent scaling of the fluid properties at the interfaces ensures energy conservation. The implementation of these methods in the multi-phase flow solver Free Surface 3D (FS3D) allowed a successful validation. A qualitative comparison of the morphology in binary collisions of immiscible droplets as well as a quantitative comparison regarding the threshold velocities that distinguish different collision regimes shows excellent agreement with experimental results. These simulations enable the evaluation of experimentally inaccessible data like the contributions of kinetic, surface and dissipative energy of both immiscible liquids during the collision process. Furthermore, the comparison against binary collisions of the same liquids highlights similarities and differences between immiscible and equal droplet collisions. Both can support the modeling of the immiscible liquid interaction in the future. [Display omitted] • Predictive and energy-conserving DNS of immiscible liquids' interaction. • Framework for simulation of large topology changes, thin films and triple lines. • Extension of the VOF method with three-phase PLIC and a modified CSS model. • Implementation into multi-phase solver shows excellent agreement with experiments. • Comparison with single-liquid collisions reveals the dominant liquid phase. [ABSTRACT FROM AUTHOR]

A predictive, point-cloud tracer is presented that determines with a quantified uncertainty the Lagrangian motion of a group of point-particles within a finite region. The tracer assumes a random forcing within confidence intervals to account for the empiricism of data-driven force models and stochasticity related to the chaotic nature of the subcloud scale dynamics. It builds on the closed Subgrid Particle-Averaged Reynolds Stress-Equivalent (SPARSE) formulation presented in Domínguez-Vázquez et al. (2023) that assumes a deterministic forcing. SPARSE–R describes the first two moments of particle clouds with moment equations in closed-form, with a theoretical third-order convergence rate with respect to the standard deviations of the cloud variables. The cloud model alleviates computational cost and enhances the convergence rate as compared to Monte Carlo (MC) based point-particle methods. The randomness in the forcing model leads to virtual stresses that correlate random forcing and field fluctuations. These stresses strain and rotate the random cloud as compared to a deterministically forced cloud and thus determine to what extent the random forcing propagates into the confidence intervals of the dispersed solution. In symmetric flows the magnitude of the virtual stress is zero. Tests in analytical carrier fields and in a decaying homogeneous isotropic turbulence flow computed with a discontinuous Galerkin (DG) compressible DNS solver are performed to verify and validate the SPARSE–R method for randomly forced particles. [Display omitted] • A predictive point-cloud tracer is developed that models groups of randomly forced particles. • A polynomial chaos expansion models random forcing functions with confidence intervals. • The point-cloud tracer propagates the uncertainty in the forcing function into its solution. • A "virtual stress" is dubbed. It strains the cloud through a mixing of the cloud's random samples. • The virtual stress is negligible in directions of symmetry or in isotropic flows. [ABSTRACT FROM AUTHOR]

• A first numerical study on gas-focused non-Newtonian micro-jets, described by power-law rheology. • New findings on stability, shape, and velocities of such jets. • A detailed analysis of fluid flow patterns in the nozzle and in the jet as a function of the power-law index. The present numerical study assesses the jet length, diameter and velocity of various non-Newtonian power-law fluids of a gas dynamic virtual nozzle. A related two-phase flow problem is formulated within the mixture framework and solved with the finite volume method and volume of fluid interface treatment in axisymmetry. The process parametric range allows the incompressible laminar flow assumption. A comprehensive jet characteristics analysis is carried out in a typical micro-nozzle configuration for a range of shear-thinning to shear-thickening fluids with power law indices 0.5 ≤ n < 1.5, gas mass flow rate of 10 mg/min and liquid volumetric flow rate of 43 µl/min, resulting in a gas Reynolds number of 130 with Reynolds and Weber numbers for a reference water jet being 90 and 10, respectively. It is observed that jets from shear-thinning fluids (0.5 ≤ n < 1.0) tend to be thicker, longer, and slower when compared with the shear-thickening fluids (1.0 < n ≤ 1.5). A dripping-jetting phase diagram of the nozzle is constructed by varying the power law index, gas and liquid flow rates in the range 0.9–1.1, 5–15 mg/min and 5–50 µl/min, respectively. It is observed that the area of stable jetting decreases with the increase of the power law index. The obtained novel information on the behaviour of non-Newtonian gas-focused micro-jets provides a possible new dimension for tailoring the serial crystallography sample delivery systems where the micro-jets carry dispersed crystals into an X-ray beam. [Display omitted] [ABSTRACT FROM AUTHOR]

• Experimental study on the surface-piercing hydrofoil at high speed. • The coupling of the ventilated cavity and vaporous cavity. • A prediction model of the maximum depression depth of the water surface. • A criterion of cavitation-induced ventilation. In this study, the cavitating flow and cavitation-induced ventilation flow around a surface-piercing hydrofoil were investigated to gain in-depth understanding of the interaction mechanism between the vaporous cavity and free surface at low cavitation numbers. Experiments were conducted in a constrained-launching water tank to visualize the cavity using a high-speed camera. Unsteady cloud cavitation and cavitation-induced ventilation at atmospheric pressure were observed and analyzed while piercing the free surface. The flow regime map was summarized at a fixed aspect ratio of AR h = 1.5. Subsequently, a physical model was proposed to predict the maximum depression depth of the water surface (H) at the trailing edge of the hydrofoil. Both the physical model and experimental results reveal that the non-dimensional depth H / c has a linear relation to Fn 2 c × Re c × sin 2 α. Finally, a criterion for cavitation-induced ventilation based on the improved lifting-line theory and a physical model were proposed. A new relation H / L c ∼ α 0.5 was obtained, where L c is the maximum cavity length. The results of this study can guide the design and application of hydrofoils for ventilation and cavitation processes. [Display omitted] [ABSTRACT FROM AUTHOR]

The solution of a variable coefficient Poisson equation, where the same coefficient, the unknown function and its spatial derivatives may be discontinuous, is an interesting and challenging problem, whose solution plays a fundamental role in the simulation of many physical phenomena and, in particular, of an incompressible multiphase flow. On the other hand, the Ghost Fluid Method (GFM) represents an effective numerical technique to manage derivative terms in presence of an interface (that is, a discontinuity) and to preserve its own sharp nature. Then, the method is a natural candidate to deal with the above mentioned problem. The equation, however, may be characterized by a significant jump of the coefficient across the interface, a condition which still leaves room for open questions and in which the original GFM-based approach seems not to work properly. The paper provides a comprehensive answer to these questions and proposes a novel, simpler implementation of the method (named GFMxP), testing its efficiency and robustness by a lot of well-known numerical examples. [Display omitted] • A new Ghost Fluid Method for a variable coefficient Poisson equation is proposed. • Some inconsistencies affecting the original coding of the method are discussed. • All results and solutions are presented with a detailed and critical error analysis. • A comparison with alternative methods and numerical procedures is presented. • The results of the paper could be very useful for the whole CFD community. [ABSTRACT FROM AUTHOR]

• The presence of solid beds in turbulent pipe flows results in secondary fluid motions which impacts particle transport dynamics. • Streamwise vorticity due to corners in the cross-sectional plane of pipes is increased as the pipe bed height increases. • Particles are influenced by the secondary flows, with particles in a half-pipe less likely to concentrate near the upper walls. • The secondary flows also modify the deposition and resuspension rates of particles, particularly near the corners of the bed. Turbulent particle-laden flows in pipes can result in particle deposition leading to the formation of solid beds. The presence of such beds modifies the flow field, resulting in secondary motions in the plane of the pipe cross-section, which in turn impact particle transport. In this work turbulent pipe flows with equal mass flow rates and solid beds of height H b = 0 (full pipe), 0.5 R (three-quarter pipe), and R (half pipe) are predicted using direct numerical simulation, with the beds represented simplistically as flat surfaces. The particulate phase is one-way coupled to the flow at a volume fraction of 10−3 and particle motion is solved for using a Lagrangian point-particle approach. The Reynolds numbers computed based on bulk velocity and equivalent pipe diameter for the full, ¾ and, ½ pipes are 5,300, 5,909 and 7,494, respectively. The same particle size is used in all the simulations and their respective Stokes numbers, based on the shear timescale, are 0.5, 1.2 and 1.9, respectively. The results for flows with beds show that the fluid flow exhibits secondary vortices and an increase in the mean streamwise vorticity caused by corners in the cross-sectional plane of the pipes, with their intensity near the upper curved wall increasing with H b. However, the upper vortices remain relative weak compared to those in lower regions of the pipes. The increase in mean streamwise vorticity in the half pipe is larger than that in the three-quarter pipe near the upper curved wall, while similar near the flat pipe floor due to the resistance of the curved wall to secondary motions. The movement of the particles in the cross-sectional plane is consistent with that of the secondary flows, but with slightly lower velocities. In regions near the wall away from the pipe corners, particle concentration in the half pipe is lower than in the three-quarter pipe, most likely due to its thinner boundary layer. This is reversed for concentration maxima near the pipe corners because of the magnitude of the secondary flows. Finally, the secondary flow changes the deposition or resuspension rate of the particles, particularly near the pipe corners, but these are always less than equivalent rates in the full pipe flow, which is likely caused by the magnitude of the wall unit. [Display omitted] [ABSTRACT FROM AUTHOR]

• The article presents an experimental study on two-phase gas-liquid slug flows in pipes with a slight direction change. • High-speed cameras and four resistive sensor-equipped metering stations were used to collect data. • A noteworthy difference between some experimental points that actually showed stratified flow and the flow regimes predicted by a traditional flow pattern map was observed. • Slug-to-stratified transitional flow in the downward section was observed under some test conditions. • Interpretation of the results were based upon sound statistical analyses. An experimental study on two-phase gas-liquid slug flows in pipes with a slight direction change is presented. A lab rig composed of a horizontal tubular section followed by a downward inclined pipe of the same diameter but at different inclinations was assembled. Resistive sensors were installed at four different pipe locations (herein referred to as "test sections"), so as to measure characteristic flow parameters. Two high-speed cameras were used to visualize and identify flow phenomena. Experiments under slug flow conditions in the horizontal section at different gas and liquid flow rates were carried out. In some test conditions the slug-to-stratified transition in the downward section was observed. The mechanism causing flow stratification was associated to the liquid film acceleration during its transit through the elbow, as the film flows at velocities occasionally higher than those of the liquid slug. Thus, the liquid slug sheds liquid to the film ahead, making the elongated bubble to penetrate into the liquid slug region, leading to the onset of a stratified flow. A noteworthy difference between the number of experimental points that actually showed stratified flow and the flow patterns predicted for the same points by a downward flow pattern map was observed. Therefore, the model presented by Taitel et al. (Slug flow modeling for downward inclined pipe flow: theoretical considerations. International Journal of Multiphase Flow, Vol. 26, No. 5, pp. 833–844, 2000) was selected for predicting transitions in this specific flow condition – that is, from horizontal to downward flow with a direction change. The results obtained with this kind of modelling showed to be consistent with the experiments. [ABSTRACT FROM AUTHOR]

• Two-phase gas-liquid bubbly flow experiments were conducted in a 212 m long near-horizontal pipe with an inner diameter of 56.3 mm and a pipe angle of 0.13°. • Gas concentration profiles, bubble sizes, and pressure drops were measured. • A model based on the balance between turbulent diffusion and gravitational drift was implemented and compared to the measured gas concentration profiles. This work aims to establish a fast and robust mechanistic model framework for predicting phase fraction profiles in bubbly flows. The main motivation is that the associated closure laws can in the future be used as a basis for a more general gas entrainment model in multiphase flows. Two-phase gas-liquid experiments were conducted in a 212 meter long pipe with an inner diameter of 56.3 mm and a pipe angle of 0.13°. The experiments were conducted with Exxsol D60 oil and Argon gas. The system was pressurized to 12 bara, yielding a gas density of 19 kg/m3. Phase fraction profiles were measured using a vertically traversing gamma densitometer, and bubble sizes were obtained using a CANTY InFlow Particle Sizer system. In addition, pressure transmitters were connected to the pipe facilitating pressure drop measurements. The experiments were performed in the bubbly flow region, where the gas was dispersed as bubbles in the liquid. The model presented in this paper is based on a gravity/turbulent diffusion balance, where the measured bubble size distributions are used as input to the model. The agreement between the model and the data was found to be good, although some discrepancies could be seen. A key model ingredient was found to be the effect of turbulence on the bubble drag. It was also found that the Sauter mean bubble size could be used in place of the bubble size distribution without significant loss of accuracy. [Display omitted] [ABSTRACT FROM AUTHOR]

• Head-on ternary droplet collision is investigated numerically. • The shape evolution and energy change of head-on ternary droplet collision are described. • The axial and radial geometrical characteristic of head-on ternary droplet collision are analyzed. • The effect of the Weber number, central droplet size and distance offset on the head-on ternary droplet collision are investigated. In recent years, ternary droplet collision has been proposed in some new application prospects, such as three-dimensional (3D) reactive inkjet printer. To better apply ternary droplet collision, head-on ternary droplet collision of same liquid in the gaseous environment is investigated numerically. The investigation is based on the finite volume numerical solution of the Navier–Stokes equations in axisymmetric form. Volume of Fluid (VOF) methodology and adaptive grid technique are used to capture the liquid-gas interface and the reliability of the methodology is verified by published experimental data. Head-on ternary droplet collision for a wide range of Weber number is studied. The shape and velocity field evolution of three colliding droplets (coalescence and separation) is described in detail. Moreover, the time evolution of the kinetic energy and the surface energy of three colliding droplets, the viscous dissipation and the maximum deformation are evaluated. Furthermore, based on the dimensionless flow parameters, relevant correlations determining the length of ligament, the maximum diameter of ligament, the length of bridge and the maximum radial length are also presented quantitatively. Finally, the effect of central droplet size and distance offset on the head-on ternary droplet collision are investigated, the results are also compared and analyzed. It is believed that the present work is meaningful for better applying ternary droplet collision. [ABSTRACT FROM AUTHOR]

• The macroscopic equations with numerical dissipation terms of three representative weakly compressible multiphase models are derived. • General mechanisms for stabilizing computations of those weakly compressible multiphase models are revealed. • A general weakly compressible multiphase solver with good numerical stability and accuracy for challenging conditions is proposed. The present paper analyzes three representative weakly compressible multiphase models. It is found that these models contain some identical numerical dissipation terms, i.e., pressure diffusion and bulk viscosity terms. Numerical investigations demonstrate that these identical numerical dissipation terms provide general mechanisms for stabilizing computations. The generality of mechanisms is reflected in (a) they widely exist in many weakly compressible multiphase models and (b) they are interpretable physical mechanisms that do not depend on numerical schemes. Many weakly compressible multiphase models can be incorporated into a general theoretical framework based on the general mechanisms. Consequently, a general weakly compressible solver for multiphase flows (GWCS-MF) is proposed. It is derived from standard governing equations and can be applied to nonuniform meshes easily. Detailed numerical tests show that it can achieve good numerical stability and accuracy for challenging conditions (inviscid fluids, large density ratios, high Weber numbers, and high Reynolds numbers) and simulate complex interface evolution well. These good performances exhibit the advantages of GWCS-MF and further validate those general mechanisms. [Display omitted] [ABSTRACT FROM AUTHOR]

• A non-orthogonal moment set is used to construct the multiple relaxation time collision operator, which improves numerical stability and efficiency. • A local gradient calculation scheme is developed which has been proven it can resolve the discontinuity at the phase interface. • The achievable simulation parameters are extended by using the current model, where the tested maximum density ratio can reach to 10 6. • The proposed model is able to reproduce various complex multiphase problems and achieve good agreement with previous experiments. This study proposes a three-dimensional non-orthogonal multiple-relaxation-time (NMRT) phase-field multiphase lattice Boltzmann (PFLB) model within a recently established unified lattice Boltzmann model (ULBM) framework [Luo et al., Phil. Trans. R. Soc. A 379, 20200397, 2021]. The conservative Allen-Cahn equation and the incompressible Navier-Stokes (NS) equations are solved. In addition, a local gradient calculation scheme for the order parameter of the Allen-Cahn equation is constructed with the non-equilibrium part of the distribution function. A series of benchmark cases are conducted to validate the proposed model, including the two-phase Poiseuille flow, Rayleigh-Taylor instability, binary liquid/metal droplet collision, and a bubble rise in water. The present simulation results are in good agreement with existing simulation and experimental data. In the simulation of the co-current two-phase Poiseuille flow, the present model is proven to resolve the discontinuity at the phase interface and provide accurate results at extremely high density ratios (i.e., up to 10 6). Finally, the proposed model is adopted to simulate two challenging cases: (1) water droplet splashing during its impacting on a thin liquid film and (2) liquid jet breakup. The simulation results demonstrate an excellent agreement with previous experimental results, both qualitatively and quantitatively. In these simulations, the Weber number and Reynolds number reach 105 and 6000, respectively, and the viscosity can be as low as ∼ 10 − 4 , in the lattice unit. [Display omitted] [ABSTRACT FROM AUTHOR]

• Multiphase modelling of gas-particle mixtures to study and replicate large-scale experiments on granular flows. • The non-uniformity of solid phases allows to describe the segregation dynamics active in real granular flows. • The polydispersity highly affects the velocities of simulated granular flows. Granular flows consist of discrete macroscopic particles. If they are non-cohesive, their status is determined by the interaction of particle-particle frictional forces, external boundaries and gravity. In particular, the understanding of the transport mechanisms of granular materials is of paramount importance for the characterization of volcanic granular flows and for hazard assessments associated with these flows. In order to investigate dynamics of these kinds of flows, we replicated large-scale experiments with multiphase computational fluid dynamic (CFD) simulations using the Two-Fluid Model approach, with an emphasis on the polydispersity effect on the flow behaviour. The CFD simulations were run using the software MFIX. The present work consists of: 1) investigations on the drag force relationships implemented in MFIX; 2) applications of MFIX to replicate large-scale experiments on volcanic dry granular flows sliding on an inclined channel; 3) comparisons between experimental and simulated data with particular emphasis on the velocity of the granular flow front. Simulations on polydisperse granular flows demonstrated the simulated flows capability to replicate segregation dynamics active in real granular flows, and the polydispersity effects on velocities and shapes of granular flows. The non-uniformity of solid phases highly affects the dynamic of the whole flow and results in a better agreement between simulated and experimental flow velocities than the simplest monodisperse particles systems. In particular, the greater the number of the solid phases, the lower the velocity of granular flows and the mean square error, which decreases by ca. 50%. [Display omitted] [ABSTRACT FROM AUTHOR]

A large amount of published data show that particles with diameter above 10% of the turbulence integral length scale (D / l > 0. 1) tend to increase the turbulent kinetic energy of the carrier fluid above the single-phase value, and smaller particles tend to suppress it. We attempted to remove limitations in earlier modeling efforts for solids on the coupling between the particles and turbulence, and better fits to the turbulence modulation amplitude as function of D / l was achieved for a number of data sets. Explicit algebraic forms of the full model were derived using asymptotic analysis, and these are general enough for application to emulsions, bubbles and solids in bulk regions of multiphase turbulent flow. Rigorous particle-kinetic theory was used to derive the work exchanged between the particles and the fluid due to both drag and added mass forces, where the latter is essential for low or moderate particle/fluid density ratios, enabling a well justified model also for emulsions and bubbles. A novel sub-model for turbulence production by vortex shedding due to turbulence-generated slip velocity was incorporated, where earlier models took the slip velocity as an input parameter. The correct asymptotic limit of vanishing turbulence modulation for small tracer particles was also provided, giving better fit to the data for small particles. We found that turbulence augmentation for large diameter solids is due to vortex shedding, and turbulence suppression for small diameters is due to mainly to turbulent drag forces and extra fluid dissipation – a conclusion that agrees with earlier models for solids, despite their possible shortcomings. An important finding is that the mechanisms for turbulence suppression for bubbles and emulsion droplets are similar to those of solids, but with the addition of added mass scaling factors. Another important observation is that augmentation may not occur at all for bubbles or emulsion droplets since the larger diameters require moderate turbulence levels to prevent breakup, so that vortex shedding may be insignificant. [Display omitted] • Solids with diameter above 10% of the turbulence integral length scale increases turbulence. • Bubbles and emulsion droplets may not induce turbulence enhancement by vortex shedding. • Model imitations on coupling between the particles and the turbulence were removed. • Explicit algebraic forms for homogeneous turbulent shear flow were derived. • Improved fits to measured turbulence modulation for varying diameter were achieved. [ABSTRACT FROM AUTHOR]

By applying state-of-the-art, high-fidelity methods for compressible multiphase flows, this work features a parametric study of a turbulent liquid jet atomizing under the influence of standing acoustic waves with five different amplitude values. We perform these simulations in a semi-periodic domain to isolate interactions between acoustic and atomization processes and to emulate a pure liquid fuel spray placed at the pressure node of a resonant acoustic field. Characterization of the flow includes measurements of the mean dimensions, droplet number, and surface area of the liquid distribution, alongside qualitative depictions of the interface evolution, breakup events, and hydrodynamic fields. As acoustic forcing is provided to the flow, the added energy provokes atomization when shear would otherwise be insufficient for breakup. The magnitude of the forcing determines the rate of droplet production, which accelerates as the evolving liquid surface modifies the induced motion of the gas phase. At the nodal plane, the acoustic field produces an organizing effect on the liquid structures. • Parametric study of acoustically enhanced atomization with varying amplitude. • Semi-periodic flow configuration provides simplified canonical test case. • Acoustic forcing activates and accelerates atomization with time. • Ligament breakup and bag breakup contribute to droplet creation. • Levitating droplet validation case and mesh-coarsening study confirm accuracy. [ABSTRACT FROM AUTHOR]

An in-house Navier–Stokes multiphase VOF (Volume-Of-Fluid) solver is used to study sloshing phenomena in a partially filled rectangular tank subjected to vertical sinusoidal excitation, with the main goal of characterizing the effects of vertical acceleration and excitation frequency on the flow dynamics. The solver is validated through comparison with analytical solutions and with available experiments of vertical sloshing. We find that the computed forces acting on the tank walls are well predicted, both in two- and three-dimensional numerical simulations. The flow dynamics is found to be significantly affected by the forcing parameters, and to exhibit more chaotic and three-dimensional nature in cases with strong acceleration and low forcing frequency. As a consequence, certain properties such as the energy dissipation and the mixing efficiency of the system are poorly predicted by two-dimensional simulations in that range of parameters, necessitating the use of more expensive three-dimensional simulations. The time history of the sloshing force and instantaneous flow visualizations are used to analyze the effects of liquid impacting on the walls on energy exchanges between the fluid and the tank. Finally, the evolution of mixing efficiency and its influence on the energy losses are discussed. • A multiphase Navier–Stokes solver is used to predict vertical sloshing. • Excellent agreement of computed forces with recent experiments. • The flow dynamics is strongly affected from tank acceleration and forcing frequency. • Initial filling level of liquid and mixing efficiency have an impact on energy losses. [ABSTRACT FROM AUTHOR]

• A new optical technique is investigated that recovers the true thickness of thin-film flows. • The technique relies on an adaptation of PLIF (S-PLIF) by modulating the laser-light intensity. • S-PLIF is not vulnerable to errors due to light reflection or circumferential film non-uniformity. • Simultaneous S-PLIF and PLIF measurements are performed and the resulting data compared. • The simultaneous application S-PLIF and PIV for velocity information is also demonstrated. Annular flows are employed in numerous engineering and industrial processes relating to the chemical, oil and gas, solar and nuclear energy industries. Yet, the reliable and detailed (i.e., time- and space-resolved) measurement of film thickness in these flows continues to elude us, as the moving and wavy interface renders the application of optical diagnostics, such as planar laser-induced fluorescence (PLIF), particularly challenging. In this research article, we present a novel adaptation of PLIF, which we refer to as structured PLIF (S-PLIF), which can suppress the errors in PLIF-derived film thickness measurements that arise due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on the periodic modulation of the laser-light intensity and angled illumination of the examined region of the flow, in order to generate fluorescence images with alternating bright and dark regions at an angle to the channel wall and film interface. An image-processing methodology capable of recovering the location of the true gas-liquid interface from such images is presented, and the application of S-PLIF is demonstrated in liquid films in a vertical pipe over the Reynolds number range R e L ≈ 150 − 1500. The results from this technique are compared to simultaneously recovered, "conventional" (uncorrected) PLIF data, as well as data from other techniques over the same range of conditions, demonstrating the efficacy of S-PLIF. A comparison amongst S-PLIF data obtained with the observation angle between the laser-sheet plane and the camera's observation axis set to β = 70 ∘ and 90 ∘ is also performed, showing that the employment of β = 70 ∘ is highly advantageous in avoiding distortions caused by optical interactions of the emitted fluorescence at the free surface of circumferentially non-uniform films. The instantaneous and average film-thickness uncertainties of S-PLIF are estimated to be below 10% and 5%, respectively, when measuring smooth films; an improvement over the other optical measurement techniques considered in this work. Finally, the application of S-PLIF is demonstrated in the presence of a gas-shear flow with gas entrainment in the liquid, and simultaneously with particle image velocimetry (PIV). [ABSTRACT FROM AUTHOR]

• Three-dimensional multiphase model combining LES equations for a multi-component-species dense fluid and kinetic theory-based model for granular medium are coupled. • Five studies undertaken: effects of (1) surface evenness; (2) jet Mach number; (3) particle diameter; (4) jet composition; (5) inter-granular stress coefficient. • The new ephemeral phenomenon of crater-in-crater is observed and its causes are identified. • The fluid-jet composition does not influence the crater characteristics, as those are shown to depend on the fluid density, but compositional effects lead to effective uphill diffusion of some species. • Variation of the mean and root mean square of solid volume fraction and particle momentum in the radial and vertical directions are quantified. The dynamics of cratering caused by a multi-component supersonic jet due to its interaction with the granular soil on the surface of Titan is investigated using a two-phase model and numerical simulations. Both fluid and particles are mathematically described in an Eulerian framework. The fluid is modeled using multi-species Large Eddy Simulation accounting for the details of diffusion among species, and the solid phase is described by a Kinetic-Theory-based model. The general framework is that of a recent model that successfully predicted specific details of Mars/Earth cratering observed by the Mars Science Laboratory (Balakrishnan and Bellan, International Journal of Multiphase Flow, 99, 1–29, 2018), however the distinctive differences between the present and previous model allows new physics to be uncovered that is peculiar to cratering in the presence of a dense atmosphere. Unsteady, three-dimensional simulations are performed to elucidate the morphological features of the crater and the dynamics of its evolution with time. Parametric variations of the initial conditions are conducted to understand the effect of the surface evenness, jet Mach number, particle size in the granular bed, jet composition, and the inter-granular stress coefficient. The new observation of crater-in-a-crater formation at early times is discussed and explained; this phenomenon is transient and the subsequent consumption of the inner crater by the surrounding crater walls is explained in detail. The jet exhibits complex morphological features revealed by vorticity and helicity analysis. The primary-crater walls display ripples along their surface that are traced to the interaction with the dense jet fluid which, having reached the crater bottom, changes direction to exit the crater using the spaces unoccupied by the jet and rubs the crater walls. Detailed analyses of the particle volume fraction and the particle momentum revealed the detailed morphology of the craters and the ejections from the craters, and related them to the initial conditions. A larger jet Mach number results in increased erosion due to the higher momentum content of the jet. A smaller increase in the jet momentum obtained by increasing the jet-fluid molar mass jet results in a crater that is more angled at the top, this feature being due to the increased jet expansion rate. The inter-granular stress coefficient does not affect the large-scale features of the crater, but does affect the peak compaction values. Species diffusion is investigated and it is found that both regular and uphill diffusion occur, the latter being primarily concentrated in the vicinity of the jet and the depths of the crater. The presented detailed formulation and numerical methodology for investigating supersonic jet-induced cratering on granular soil are sufficiently robust to accommodate plume-induced cratering on a variety of planetary bodies having an atmosphere. [ABSTRACT FROM AUTHOR]

• A novel regularization procedure for deterministic Lagrangian point-particle source terms based on the resolution of a nonlinear diffusion equation which allows to locally adapt the regularization length scale while naturally ensuring parallel efficiency. • Corrections of the particle's self-induced velocity compatible with the aforementioned novel regularization procedure based on theoretical results of (Maxey and Patel, International Journal of Multiphase Flow, 2001). • Demonstration of the compatibility with finite Reynolds number corrections of the particle's self-induced velocity proposed by (Balachandar et al., Journal of Comutational Physics, 2019). • A discussion of the interplay between regularization and correction of the particle's self-induced velocity as a compromise between stability and accuracy in the point-particle context. The current article presents a regularization procedure of the Lagrangian point-force approach commonly used to account for the perturbation of a fluid phase by a dispersed particle phase. The regularization procedure is based on a nonlinear diffusion equation to naturally ensure parallel efficiency when the regularization length scale extends over several grid cells. The diffusion coefficient thus becomes a function of the particle source term gradient and expressions allowing to approximately adjust the regularization length scale according to the local particle to mesh size ratio are proposed, so that mesh refinement or polydisperse sprays may be handled. Elementary numerical test cases confirm the convergence of the present procedure under mesh refinement and its ability to locally adapt the regularization length scale. Furthermore, the chosen regularization length scale allows to match the leading order term of the perturbation flow field set by the particle beyond approximately two particle diameters in the Stokes regime. When applying the presented source term regularization procedure, the terminal velocity of a particle settling under gravity in the Stokes regime becomes relatively insensitive to mesh refinement. However, errors with respect to the theoretical settling velocity remain substantial and removal of the particle's self induced velocity appears necessary to recover the undisturbed fluid velocity at the particle location and correctly evaluate the drag force. As the current regularization procedure yields source terms that are close to Gaussian, an analytic expression from the literature is used to estimate the particle's self induced velocity. When combining source term regularization and removal of the particle's self induced velocity, good results are obtained for the terminal settling speed in the Stokes regime. Results obtained for horizontally separated particle pairs settling under gravity in the Stokes regime show equally good agreement with theoretical results. Because analytic expressions for the particle's self-induced velocity are no longer available at finite particle Reynolds numbers, correlations recently proposed in the literature are used to obtain correct settling velocities beyond the Stokes regime. [ABSTRACT FROM AUTHOR]

• A phase fraction-bounded numerical method for two-fluid multiphase model is developed. • Simulations using the phase-bounded method are compared to unbounded methods. • Effects of interfacial/continuous phase pressure equality are studied. An understanding of the hydrodynamics of multiphase processes is essential for their design and operation. Multiphase computational fluid dynamics (CFD) simulations enable researchers to gain insight which is inaccessible experimentally. The model frequently used to simulate these processes is the two-fluid (Euler–Euler) model where fluids are treated as inter-penetrating continua. It is formulated for the multiphase flow regime where one phase is dispersed within another and enables simulation on experimentally relevant scales. Phase fractions are used to describe the composition of the mixture and are bounded quantities. Consequently, numerical solution methods used in simulations must preserve boundedness for accuracy and physical fidelity. In this work, a numerical method for the two-fluid model is developed in which phase fraction constraints are imposed through the use of an nonlinear variational inequality solver which implicitly imposes inequality constraints. The numerical method is verified and compared to an established explicit numerical method. [ABSTRACT FROM AUTHOR]

• This work presents a consistent and robust interfacial lattice Boltzmann flux solver (ILBFS) to simulate multiphase flows. • The present method employs the Chapman–Enskog analysis to reconstruct the convective and diffusive terms of the macroscopic governing equation. • The proposed ILBFS only applies LBE locally at each cell interface, and macroscopic variables at cell centers are given by directly solving the governing partial differential equations. • The physical boundary conditions in ILBFS are not necessary to transform to the LB distribution functions. • The obtained numerical results are in good agreement with the analytical solutions or the published results in the literature. In this paper, a robust interfacial lattice Boltzmann flux solver (ILBFS) is presented to simulate multiphase flows, which is able to deal with large density contrasts. Different from the conventional finite-volume multiphase flow solvers, which calculate the fluxes at each cell interface based on macroscopic variables, the present method employs the Chapman–Enskog analysis to re-construct the convective and diffusive terms of the macroscopic governing equation, and uses the local equilibrium solutions of the lattice Boltzmann equation (LBE) for calculating the fluxes through the cell interface. As the phase information of multiphase flows is naturally brought by the LBE from both sides of the interface, the ILBFS simulating interfacial phenomena is more physical and precise. In addition, different from the conventional LB model for simulating the multiphase flows, the proposed ILBFS only applies LBE locally at each cell interface. Macroscopic variables at cell centers are given by directly solving the governing partial differential equations. Moreover, the physical boundary conditions in the present solver can be directly implemented by using the macroscopic variables, and are not necessary to transform to the LB distribution functions. Hence, the current solver requires less memory resources for data storage as only the equilibrium distribution function related to the conservative variables is involved. To validate the present solver, several benchmark cases, including the Laplace law, two stationary bubbles without collision, Rayleigh–Taylor instability, bubble rising under buoyancy and droplet splashing on a thin film at density radio of 1000, are studied. The obtained numerical results are in good agreement with the analytical solutions or the published results in the literature. [ABSTRACT FROM AUTHOR]