Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Philipp Spelten, Dominik Wilde, Mario Christopher Bedrunka, Dirk Reith, Holger Foysi

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Amritpal Singh, Neeraj Kumar, Abdellah Hadjadj, Mostafa Safdari Shadloo

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Wasilij Barsukow, Praveen Chandrashekar, Christian Klingenberg, Lisa Lechner

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Jang Min Park

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Thomas S. Chyczewski, David A. Boger, Norman F. Foster

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Matteo Savino, Alessia Ferrari, Renato Vacondio, Paolo Mignosa

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Wenzhuo Xu, Madhav Karthikeyakannan, Christopher McComb, Noelia Grande Gutiérrez

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Anas Jnini, Flavio Vella, Marius Zeinhofer

Publication date: 15 March 2026

Source: Computers & Fluids, Volume 307

Author(s): Quentin Chauleur, Radu Chicireanu, Guillaume Dujardin, Jean-Claude Garreau, Adam Rançon

Publication date: Available online 5 February 2026

Source: Computers & Fluids

Author(s): Jing Zhang, Chris W. Letchford, Luca Patruno, Onkar Sahni, Daniel C. Lander

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Volume 39, Issue 2, February 2025, Page 102-131
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Volume 39, Issue 2, February 2025, Page 83-101
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Volume 39, Issue 2, February 2025, Page 69-82
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A gravity-driven granular flow model with Mohr–Coulomb friction is investigated. The exact Riemann solution is also established through classical and nonclassical symmetries of the Lie group. The derived solutions are discussed and validated in the context of weak discontinuity waves evolution and non-group invariants solutions through the method of conservation laws.

ABSTRACT

This study investigates a granular flow model on an inclined plane characterized by a constant friction coefficient to describe gravity-driven debris avalanches. We derive diverse sets of group-invariant solutions by employing both classical and nonclassical symmetries of the Lie group. Among these, solutions to 1 and 2 rarefaction waves corresponding to the Riemann problem are highlighted. The significance of nonclassical symmetries is explicitly demonstrated in our analysis. Essential conservation laws are derived using the multiplier method and the system's nonlinear self-adjointness. Additionally, we discuss the obtained solutions in the context of the evolution of weak discontinuity waves, and we derive new non-group invariant solutions using the method of conservation laws.

International Journal for Numerical Methods in Fluids, Volume 98, Issue 3, March 2026.

The gas–liquid–solid three-phase flow of deep-sea hydrates is one of the main factors that induce the failure of mining risers, and a comprehensive understanding of its flow characteristics can effectively improve the safety of mining risers. Based on this, a simulation experimental platform for gas–liquid–solid three-phase flow in deep-sea hydrate horizontal wells was developed using the principle of similarity, which can simulate gas–liquid–solid three-phase flow under different particle sizes, flow velocities, and gas–liquid–solid ratios. On this basis, the control variable and orthogonal analysis methods were used to determine the multiphase flow patterns in the vertical and horizontal sections under different solid particle sizes, gas–liquid–solid ratios, and internal flow velocities, and to track the axial and radial velocities of key particles and bubbles inside the riser. The experimental results show that when the gas–liquid–solid ratio is different, the axial velocity of particles decreases with the increase of gas phase ratio in both vertical and horizontal sections, while the axial velocity and radial velocity of bubbles in the vertical section fluctuate greatly when the gas phase content is high. When the particle size is different, the radial velocity variation of particles in the vertical and horizontal sections is larger, and the axial velocity variation is smoother compared to the radial velocity. The radial velocity variation of bubbles is most significant in the vertical section, and more stable in other sections. When the flow velocity is different, whether it is vertical or horizontal, axial velocity or radial velocity, particles and bubbles tend to stabilize at lower flow velocities. The range analysis shows that the particle size has the most significant impact on the axial velocity and radial velocity of particles and bubbles in the horizontal section, while the flow velocity has the greatest impact on the axial velocity of bubbles in the horizontal section. The particle size still has the greatest impact on the axial velocity and radial velocity of vertical particles and bubbles, while the flow velocity has a significant impact on the radial velocity of vertical particles. The research results can effectively guide the optimization of commercial exploitation parameters for deep-sea hydrates and other operating conditions containing gas–liquid–solid three-phase flow.

ABSTRACT

To address the challenges of multiphase flow inside the mining riser of deep-sea natural gas hydrates, a three-dimensional simulation model of gas–liquid–solid three-phase flow in the mining riser of a horizontal well is established for deep-sea hydrate. A test system is developed for gas–liquid–solid three-phase flow in the hydrate mining riser using a similar principle. The experimental results are compared with numerical simulations, and the comparison accuracy is over 90.8%. The accuracy and effectiveness of the theoretical model are verified. Based on this, the effect of particle size, gas–liquid–solid ratio, and injection flow velocity on the multiphase flow transport characteristics and flow field-riser wall collision force are investigated. The results indicate that as the particle size increases, the overall gas and liquid phase velocities do not change significantly. In the radial direction, the velocities increase from near the wall to the center of the riser. However, the solid-phase velocity decreases with increasing particle size, while the gas-phase volume fraction decreases. In contrast, the liquid-phase volume fraction increases, the solid-phase concentration decreases, and the collision force on the riser wall becomes stronger. As the gas phase proportion increases, the velocity of the gas and liquid phases also increases, with the radial direction increasing from near the wall toward the center of the riser. The velocity of the solid phase decreases as the proportion of the gas phase increases. There is no clear trend in the volume fractions of the gas and liquid phases, but the concentration of the solid phase increases with the gas phase volume fraction, also increasing the collision force. In the actual mining project, a higher flow velocity should be selected, which can not only improve the transportation efficiency, but also effectively prevent the wear of the mining riser caused by the collision of particles on the riser wall. The research results can effectively guide the safe extraction of deep-sea natural gas hydrates.

A new Galerkin-type meshfree method is developed for incompressible Navier-Stokes equations by integrating the key strengths of SL method and EFG method. Given that the SL method exhibits unconditionally stable characteristics for convection terms, the Galerkin method offers an optimal approximation for diffusion terms, the fractional step algorithm decouples velocity and pressure variables, and the meshfree feature streamlines the implementation of the SL method, the proposed method becomes an efficient approach for solving the incompressible Navier-Stokes equations.

ABSTRACT

A new Galerkin-type meshfree method is developed for solving incompressible Navier-Stokes equations by integrating the key strengths of the semi-Lagrangian (SL) method and the element-free Galerkin (EFG) method. This integration not only effectively resolves the convection-dominance problem but also fully preserves the meshfree property of the EFG method. In the absence of grid constraints, the operations of backward tracing and interpolation in the SL method can be executed more conveniently. To achieve both good stability and accuracy, the SL method is employed to handle the convection terms, while the EFG method is utilized for the diffusion terms. To decouple the velocity and pressure, a novel fractional step algorithm is derived within the SL framework. This algorithm circumvents the Ladyzhenskaya-Babuška-Brezzi (LBB) constraint and permits the utilization of equal-order velocity-pressure interpolation. Given that the SL method exhibits unconditionally stable characteristics for convection terms, the Galerkin method offers an optimal approximation for diffusion terms, the fractional step algorithm decouples velocity and pressure variables, and the meshfree feature streamlines the implementation of the SL method, the proposed method is anticipated to be an efficient approach for solving the incompressible Navier-Stokes equations. Numerical examples with available analytical solutions are solved to show the accuracy, stability, and convergence behavior of the proposed method. The results demonstrate that the new method exhibits superior stability compared to the EFG method, and it reaches a first-order convergence rate in temporal direction and second-order convergence rate in spatial direction under first-order discretization. After that, numerical tests on the square-cavity-driven flow and the doubly periodic shear layer flow further validate the accuracy and stability of the proposed method.

This study investigates heat and mass transfer in electrically conducting Casson ternary hybrid nanofluid flows (Al2O3–Cu–TiO2/blood$$ {\mathrm{Al}}_2{\mathrm{O}}_3\hbox{--} \mathrm{Cu}\hbox{--} {\mathrm{TiO}}_2/\mathrm{blood} $$) under variable thermal conductivity, Joule heating,viscous dissipation, and nonlinear thermal radiation. Using the Sixth-Order Runge–Kutta method in MATLAB, the results show that stronger magnetic fields, higher Casson parameters, and Darcy–Forchheimer effects reduce velocity, while curvature and Stefan blowing enhance heat transfer. Increased nanoparticle concentration improves thermal conductivity, leading to higher Nusselt numbers, making these fluids valuable for engineering applications like heat exchangers and electronic cooling.

ABSTRACT

In this study, the heat and mass transfer rates in electrically conducting Casson ternary hybrid nanofluid flows (Al2O3−Cu−TiO2/blood$$ A{l}_2{O}_3- Cu- Ti{O}_2/\mathrm{blood} $$) were investigated, considering various factors such as variable thermal conductivity, Joule heating, viscous dissipation, chemical reactions, Darcy–Forchheimer flow, and nonlinear thermal radiation. The use of ternary hybrid nanofluids, combining aluminum oxide, copper nanoparticles, and titanium oxide in blood, can significantly improve thermal conductivity and heat transfer efficiency, making them useful in engineering fields such as heat exchangers, aerospace, renewable energy, and electronic cooling. The study focuses on the effects of nonlinear thermal radiation, viscous dissipation, Joule heating, Soret number, chemical reactions, Darcy–Forchheimer effect, and curvature on the flow of Casson fluid over a stretching cylinder. The partial differential equations governing the system are transformed into ordinary differential equations using a similarity variable and solved using the Sixth-Order Runge–Kutta (RK6) method in MATLAB, validated against previous studies for accuracy. The analysis includes the impact of physical parameters on velocity, temperature, and concentration profiles, as well as skin friction coefficient, local Nusselt number, and Sherwood number. A higher Casson parameter leads to an increased yield stress, resulting in greater resistance and a reduction in the velocity distribution. Variable thermal conductivity, nonlinear thermal radiation, Eckert number, and nanoparticle volume fraction improve heat transfer. Higher nanoparticle concentrations increase thermal conductivity, leading to improved heat transfer and higher Nusselt numbers.

We present a numerical framework based on the Cahn-Hilliard-Navier-Stokes (CHNS) model to simulate biphasic flow in confined environments. After deriving the mathematical model, we develop the weak form of the system of PDEs using a pedagogical approach to enable its implementation in FEniCS. The model is validated against experimental data from the literature and subsequently applied to a microfluidic experiment conducted by the authors. All data and code related to this work are available on GitHub.

ABSTRACT

This study presents a numerical framework for modeling two-phase flow in confined environments, focusing on the interplay between capillary and viscous forces. The model integrates the Cahn-Hilliard and Navier-Stokes (CH-NS) equations, utilizing a diffuse-interface approach to capture interfacial dynamics without the limitations of sharp-interface models. Implemented in the finite element platform FEniCS, the framework incorporates Dirichlet boundary conditions to model a fully non-wetting phase. The validation of the proposed model is achieved through two applications: The retraction of an oil droplet from a capillary tube and the drainage of water-wet microfluidic chips. Numerical results align with experimental data, demonstrating the framework's ability to replicate interfacial behaviors, including capillary-driven dynamics and fingering phenomena. This work provides a versatile computational tool for studying immiscible fluid flow, offering potential for advancements in fundamental research on microfluidics, enhanced oil recovery, and remediation of contaminated soil.

This study examines buoyancy-driven magneto-convection within an anisotropic porous cavity incorporating internal heat generation/absorption. The top and bottom boundaries are subjected to sinusoidal heat fluxes, whereas the vertical walls are thermally insulated. The flow and heat transfer behavior are numerically analyzed using the Darcy-Brinkman extended model, implemented via FVM and the semi-implicit method for pressure-linked equations (SIMPLE) algorithm. The influence of key parameters, including the periodicity parameter, permeability ratio, thermal conductivity ratio, Hartmann number, internal heat generation/absorption, and orientation angle on the flow structure and heat transfer efficiency within the system is analyzed. The findings show that the cavity exhibits a multicellular convective pattern, where the anisotropic permeability tilt induces sinusoidal flow features near the thermally active walls. In contrast, a strong magnetic field (Ha=100)$$ \left( Ha=100\right) $$suppresses the flow circulation and higher levels of internal heat generation/absorption (Q=5,−5)$$ \left(Q=5,-5\right) $$lead to reduced heat transfer efficiency. The study is conducted for a steady, two-dimensional, Darcy-Brinkman model. This study could be beneficial for the solar collector designs, thermal management systems, setups for room ventilation, and electronic cooling applications.

ABSTRACT

This study examines buoyancy-driven magneto-convection within an anisotropic porous cavity incorporating internal heat generation/absorption. The top and bottom boundaries are subjected to sinusoidal heat fluxes, whereas the vertical walls are thermally insulated. The flow and heat transfer behavior are numerically analyzed using the Darcy-Brinkman extended model, implemented via FVM and the semi-implicit method for pressure-linked equations (SIMPLE) algorithm. The influence of key parameters, including the periodicity parameter, permeability ratio, thermal conductivity ratio, Hartmann number, internal heat generation/absorption, and orientation angle on the flow structure and heat transfer efficiency within the system is analyzed. The findings show that the cavity exhibits a multicellular convective pattern, where the anisotropic permeability tilt induces sinusoidal flow features near the thermally active walls. In contrast, a strong magnetic field (Ha=100$$ Ha=100 $$) suppresses the flow circulation, and higher levels of internal heat generation/absorption (Q=5,−5)$$ \left(Q=5,-5\right) $$ lead to reduced heat transfer efficiency. The study is conducted for a steady, two-dimensional, Darcy-Brinkman model. This study could be beneficial for the solar collector designs, thermal management systems, setups for room ventilation, and electronic cooling applications.

The Lagrange multipliers approach and Knudsen-number-dependent relaxation time is introduced in the lattice Boltzmann community. Large adaptive stencils in the velocity discretization scheme are introduced to reduce the perturbations in macroscopic characteristics. Tandem, vertical, and staggered arrangements of the cylinders are studied for validation. New insights into the separation of the bow shock wave inside the channel are presented.

ABSTRACT

The present study is focused on several numerical experiments on high-speed compressible fluid flows inside a long horizontal multi-cylinder positioned channel using a double distribution function based extended lattice Boltzmann (LB) approach. Initially, an algorithm of the lattice Boltzmann approach to simulate the compressible flows is developed and stabilized by introducing the Lagrange multipliers approach to calculate the equilibrium distribution function, Knudsen-number-dependent relaxation time, and large adaptive stencils in the velocity discretization scheme. Subsequently, the algorithm/code is validated by comparison of the present results against the existing benchmark results. The LB simulations are carried out at the supersonic state for two different Mach numbers, 1.5 and 1.7. The channel is enclosed from the top and bottom sides, with surfaces having symmetric boundary conditions. At the inlet and outlet, Dirichlet and Neumann boundary conditions are employed, respectively, for density, velocity, temperature, and pressure. Three different studies based on the configuration of the multi-cylinders are carried out. Inside the channel, the multi-cylinders are either positioned in a tandem manner, vertical (side-by-side) manner, or staggered manner with varying angles of incidences. Various physical parameters like the coefficient of pressure, drag and lift coefficient, temperature flow field, and so forth, are computed and reported throughout the study.

Comparison of pressure at 10 seconds. Left–reference data, right–model predictions.

ABSTRACT

This paper examines the application of PINN models to solving a two-dimensional cylinder flow problem with limited data. Using data obtained by direct numerical simulation, a surrogate PINN model was developed and trained. The model utilizes the governing equations of fluid dynamics and heat transfer, enabling it to accurately predict flow parameters such as velocity components, pressure, and temperature. The direct computational flow model was numerically solved using the SIMPLE algorithm, which couples pressures and velocities. The results showed that the PINN model, which does not contain initial and boundary conditions from direct numerical simulation, is capable of reproducing complex dynamic processes such as the formation of a Kármán vortex street behind a cylinder. However, limitations were identified due to the lack of initial and boundary conditions, which led to increased errors at the boundaries of the computational domain. For example, from the data obtained using the PINN model, a very small absolute difference in error for the velocity and temperature components between the reference data and the predicted values can be noted. Thus, for the horizontal velocity component, the maximum relative error was no more than 2.5%. For the temperature component, the relative error was no more than 0.02%. However, the relative error for pressure was 60%–75%. The main reason for this large error is the lack of a reference pressure value or initial pressure conditions in the loss function. The results show that the PINN surrogate model with eight hidden layers of 200 neurons successfully copes with the task of modeling complex unsteady flow. The integration of physical laws made it possible to achieve relatively satisfactory accuracy using only 10,000 data points.

A hybrid RANS-LES turbulence model adapted for the Moving Particle Semi-implicit method is employed to investigate a turbulent free surface flow. A method based on the cell-linked list is proposed to speed up the nearest wall search for the turbulence model. Validation using lid-driven flow showed better convergence and improvements achieved by the turbulence model. The flows around a square cylinder near the surface with a Reynolds number of 25,000 were simulated and the influence of the cylinder submergence depths was investigated.

ABSTRACT

Engineering problems often comprise free-surface flows in turbulent regime. Lagrangian mesh-free particle-based methods are well suited for the simulation of flows involving complex free-surface deformation. However, the analysis of turbulent modeling for particle-based methods is relatively scarce in the literature. In this work, an analysis of a hybrid RANS-LES turbulence model adapted for the Moving Particle Semi-implicit (MPS) method is performed. In the turbulence model, a zero-equation RANS is applied near the wall boundaries and a standard Smagorinsky LES model is applied elsewhere. Given that the eddy viscosity of the turbulent modeling depends on the distance between the fluid and the nearest wall particle, the calculation of the fluid-wall particle distance may demand a high computational cost due to undefined topology among moving particles. In this way, a method based on the cell-linked list is proposed to improve the nearest wall search for the turbulence model. The implementation is verified through simulation of a lid-driven flow with Reynolds number between 10,000$$ \mathrm{10,000} $$ and 50,000$$ \mathrm{50,000} $$. The result shows that despite the overhead when the turbulence model is adopted, the time needed to reach steady state is shortened so that the overall computational costs are almost the same. In addition, the improvement due to the adoption of turbulence model is more evident for the highest Reynolds numbers. As an application, the flow around a submerged square cylinder near the surface with Reynolds number of 25,000$$ \mathrm{25,000} $$ is simulated. The influences of the cylinder submergence depths on the drag and lift coefficients are investigated for a range of depth-to-length ratios between 0.3$$ 0.3 $$ and 3.0$$ 3.0 $$. When the turbulence model is applied, a smoother convergence tendency is obtained as the resolution increases. Moreover, the flow around the square cylinder is better represented, resulting in more regular vortex shedding. Different flow behaviors were identified around the square cylinder as the submergence depth changes.

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): Tong Zhu, Bingqian Si, Lin Fu, Yanglong Lu

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): Longqiang Xu, Weishi Yin, Pinchao Meng, Zhengxuan Shen, Hongyu Liu

Publication date: Available online 7 February 2026

Source: Journal of Computational Physics

Author(s): Xiuping Wang, Huangxin Chen, Jisheng Kou, Shuyu Sun

Publication date: Available online 7 February 2026

Source: Journal of Computational Physics

Author(s): P. Gan, J. Li, F. Fang, X. Wu, J. Zhu, Z. Wang, M. Zhu, X. Zou

Publication date: Available online 6 February 2026

Source: Journal of Computational Physics

Author(s): Zhengshou Lai, Shuai Huang, Yong Kong, Shiwei Zhao, Jidong Zhao, Linchong Huang

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): Tom Hickling, Jonathan F. MacArt, Justin Sirignano, Den Waidmann

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): Jingfei Chen, Minxin Chen, Jingrun Chen

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): A. Colaïtis, S. Guisset, J. Breil

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): Jiachuan Zhang

Publication date: 1 May 2026

Source: Journal of Computational Physics, Volume 552

Author(s): Arnaud Colaïtis, Sébastien Guisset, Jérôme Breil

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Volume 26, Issue 10-11, October-November 2025, Page 354-381
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Volume 26, Issue 10-11, October-November 2025, Page 323-353
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Volume 26, Issue 10-11, October-November 2025, Page 305-322
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Physics of Fluids, Volume 35, Issue 4, April 2023.
This paper proposes a versatile and robust immersed edge-based smoothed finite element method with the mass conservation algorithm (IESFEM/Mass) to solve partitioned fluid–structure interaction (FSI). A gradient smoothing technique was used to solve the system governing equations, which can improve the calculated capability of the linear triangular elements in two phases. Based on the quadratic sharp interface representation of immersed boundary, an extended fictitious domain constructed by a least squares method approximately corrected the residual flux error. The compatibility for boundary conditions on moving interfaces was satisfied, thus eliminating spurious oscillations. The results from all numerical examples were consistent with those from the existing experiments and published numerical solutions. Furthermore, the present divergence-free vector field had a faster-converged rate in the flow velocity, pressure, and FSI force. Even if in distorted meshes, the proposed algorithm maintained a stable accuracy improvement. The aerodynamics of one- and two-winged flapping motions in insect flight has been investigated through the IESFEM/Mass. It can be seen that the wing–wake interaction mechanism is a vital factor affecting the lift. The applicability of the present method in the biological FSI scenario was also well-demonstrated.

Physics of Fluids, Volume 35, Issue 4, April 2023.
We report a pore-scale numerical study of salt finger convection in porous media, with a focus on the influence of the porosity in the non-Darcy regime, which has received little attention in previous research. The numerical model is based on the lattice Boltzmann method with a multiple-relaxation-time scheme and employs an immersed boundary method to describe the fluid–solid interaction. The simulations are conducted in a two-dimensional, horizontally periodic domain with an aspect ratio of 4, and the porosity [math] is varied from 0.7 to 1, while the solute Rayleigh number [math] ranges from [math] to [math]. Our results show that, for all explored [math], solute transport first enhances unexpectedly with decreasing [math] and then decreases when [math] is smaller than a [math]-dependent value. On the other hand, while the flow strength decreases significantly as [math] decreases at low [math], it varies weakly with decreasing [math] at high [math] and even increases counterintuitively for some porosities at moderate [math]. Detailed analysis of the salinity and velocity fields reveals that the fingered structures are blocked by the porous structure and can even be destroyed when their widths are larger than the pore scale, but become more ordered and coherent with the presence of porous media. This combination of opposing effects explains the complex porosity dependencies of solute transport and flow strength. The influence of porous structure arrangement is also examined, with stronger effects observed for smaller [math] and higher [math]. These findings have important implications for passive control of mass/solute transport in engineering applications.

Physics of Fluids, Volume 35, Issue 4, April 2023.
The linear stability of a fully developed liquid–metal magnetohydrodynamic pipe flow subject to a transverse magnetic field is studied numerically. Because of the lack of axial symmetry in the mean velocity profile, we need to perform a BiGlobal stability analysis. For that purpose, we develop a two-dimensional complex eigenvalue solver relying on a Chebyshev–Fourier collocation method in physical space. By performing an extensive parametric study, we show that in contrast to the Hagen–Poiseuille flow known to be linearly stable for all Reynolds numbers, the magnetohydrodynamic pipe flow with transverse magnetic field is unstable to three-dimensional disturbances at sufficiently high values of the Hartmann number and wall conductance ratio. The instability observed in this regime is attributed to the presence of velocity overspeed in the so-called Roberts layers and the corresponding inflection points in the mean velocity profile. The nature and characteristics of the most unstable modes are investigated, and we show that they vary significantly depending on the wall conductance ratio. A major result of this paper is that the global critical Reynolds number for the magnetohydrodynamic pipe flow with transverse magnetic field is Re = 45 230, and it occurs for a perfectly conducting pipe wall and the Hartmann number Ha = 19.7.

Physics of Fluids, Volume 35, Issue 4, April 2023.
In this paper, a model of the development of a quantum turbulence in its initial stage is proposed. The origin of the turbulence in the suggested model is the decay of vortex loops with an internal structure. We consider the initial stage of this process, before an equilibrium state is established. As result of our study, the density matrix of developing turbulent flow is calculated. The quantization scheme of the classical vortex rings system is based on the approach proposed by the author earlier.

Physics of Fluids, Volume 35, Issue 4, April 2023.
A five-stage centrifugal pump is utilized to investigate the interstage flow characteristics of the multistage centrifugal pump as turbine (PAT). The simulation results of performance are verified by comparing with the experimental results. Owing to the distinct structural attributes, significant differences in flow occur between the first stage and the other stages of the multistage PAT. To enhance the understanding of these disparities and explore their repercussions, this study focuses on analyzing the flow within the impellers in the first and second stages by a deterministic analysis. The main conclusions are as follows: The discrepancies in the inflow conditions are the major reason for the dissimilarities in the flow of impellers between stages. The impact loss generated by the misalignment between the positive guide vane outlet angle and the impeller inlet angle leads to flow deviation between impeller passages and affects the internal flow pattern. The unsteadiness under low flow rates is mostly produced by the spatial gradient of the blade-to-blade nonuniformities, which is relevant to the relative position between blades and the positive guide vanes. At high flow rates, especially in the second-stage impeller, the pure unsteady term is the primary cause of flow unsteadiness as a result of the flow separation induced by interactions between the blades and the positive guide vanes. This study can provide some references for the practical operation and performance optimization of the multistage PATs in the future.

Physics of Fluids, Volume 35, Issue 4, April 2023.
Direct simulations of phase-change and phase-ordering phenomena are becoming more common. Recently, qualitative simulations of boiling phenomena have been undertaken by a large number of research groups. One seldom discussed limitation is that large values of gravitational forcing are required to simulate the detachment and rise of bubbles formed at the bottom surface. The forces are typically so large that neglecting the effects of varying pressure in the system becomes questionable. In this paper, we examine the effect of large pressure variations induced by gravity using pseudopotential lattice Boltzmann simulations. These pressure variations lead to height dependent conditions for phase coexistence and nucleation of either gas or liquid domains. Because these effects have not previously been studied in the context of these simulation methods, we focus here on the phase stability in a one-dimensional system, rather than the additional complexity of bubble or droplet dynamics. Even in this simple case, we find that the different forms of gravitational forces employed in the literature lead to qualitatively different phenomena, leading to the conclusion that the effects of gravity induced pressure variations on phase-change phenomena should be very carefully considered when trying to advance boiling and cavitation as well as liquefaction simulations to become quantitative tools.

Physics of Fluids, Volume 35, Issue 4, April 2023.
Capillary desaturation process was investigated as a function of wetting phase rheological signatures during the injection of Newtonian and non-Newtonian fluids. Two sets of two-phase imbibition flow experiments were conducted on a water-wet sandstone core sample using brine and viscoelastic polymer solutions. During the experiments, a high-resolution micro-computed tomography scanner was employed to directly map pore-level fluid occupancies within the pore space. The results of the experiments revealed that at a given capillary number, the viscoelastic polymer was more efficient than the brine in recovering the non-wetting oil phase. At low capillary numbers, this is attributed to the improved accessibility of the viscoelastic polymer solution to the entrance of pore elements, which suppressed snap-off events and allowed more piston-like and cooperative pore-body filling events to contribute to oil displacement. For intermediate capillary numbers, the onset of elastic turbulence caused substantial desaturation, while at high capillary numbers, the superimposed effects of higher viscous and elastic forces further improved the mobilization of the trapped oil ganglia by the viscoelastic polymer. In the waterflood, however, the mobilization of oil globules was the governing recovery mechanism, and the desaturation process commenced only when the capillary number reached a threshold value. These observations were corroborated with the pore-level fluid occupancy maps produced for the brine and viscoelastic polymer solutions during the experiments. Furthermore, at the intermediate and high capillary numbers, the force balance and pore-fluid occupancies suggested different flow regimes for the non-Newtonian viscoelastic polymer. These regions are categorized in this study as elastic-capillary- and viscoelastic-dominated flow regimes, different from viscous-capillary flow conditions that are dominant during the flow of Newtonian fluids. Moreover, we have identified novel previously unreported pore-scale displacement events that take place during the flow of viscoelastic fluids in a natural heterogeneous porous medium. These events, including coalescence, fragmentation, and re-entrapment of oil ganglia, occurred before the threshold of oil mobilization was reached under the elastic-capillary-dominated flow regime. In addition, we present evidence for lubrication effects at the pore level due to the elastic properties of the polymer solution. Furthermore, a comparison of capillary desaturation curves generated for the Newtonian brine and non-Newtonian viscoelastic polymer revealed that the desaturation process was more significant for the viscoelastic polymer than for the brine. Finally, the analysis of trapped oil clusters showed that the ganglion size distribution depends on both the capillary number and the rheological properties of fluids.

Physics of Fluids, Volume 35, Issue 4, April 2023.
The objective of this research paper is to relate the influence of dynamic wetting in a liquid/liquid/solid system to the breakup of emulsion droplets in capillaries. Therefore, modeling and simulation of liquid/liquid flow through a capillary constriction have been performed with varying dynamic contact angles from highly hydrophilic to highly hydrophobic. Advanced advection schemes with geometric interface reconstruction (isoAdvector) are incorporated for high interface advection accuracy. A sharp surface tension force model is used to reduce spurious currents originating from the numerical treatment and geometric reconstruction of the surface curvature at the interface. Stress singularities from the boundary condition at the three-phase contact line are removed by applying a Navier-slip boundary condition. The simulation results illustrate the strong dependency of the wettability and the contact line and interface deformation.

Physics of Fluids, Volume 35, Issue 4, April 2023.
The exact regularized point particle method is used to characterize the turbulence modulation in two-way momentum-coupled direct numerical simulations of a turbulent pipe flow. The turbulence modification is parametrized by the particle Stokes number, the mass loading, and the particle-to-fluid density ratio. The data show that in the wide region of the parameter space addressed in the present paper, the overall friction drag is either increased or unaltered by the particles with respect to the uncoupled case. In the cases where the wall friction is enhanced, the fluid velocity fluctuations show a substantial modification in the viscous sub-layer and in the buffer layer. These effects are associated with a modified turbulent momentum flux toward the wall. The particles suppress the turbulent fluctuations in the buffer region and concurrently provide extra stress in the viscous sub-layer. The sum of the turbulent stress and the extra stress is larger than the Newtonian turbulent stress, thus explaining the drag increase. The non-trivial turbulence/particles interaction turns out in a clear alteration of the near-wall flow structures. The streamwise velocity streaks lose their spatial coherence when two-way coupling effects are predominant. This is associated with a shift of the streamwise vortices toward the center of the pipe and with the concurrent presence of small-scale and relatively more intense vortical structures near the wall.

Physics of Fluids, Volume 35, Issue 4, April 2023.
The wetting of thin films depends critically on the sign of the spreading coefficient [math]. We discuss the cases S < 0, S = 0, and S > 0 for transient models with contact line dissipation and find that the use of a dynamic contact angle solves problems for S > 0 that models might otherwise have. For initial data with a non-zero slope and S > 0, we show that there exists a finite time [math] at which the contact angle of the thin film goes to zero. Then, a molecular precursor emerges from the thin film and moves outward at a constant velocity.

Abstract Graphic Abstract