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Annual Review of Fluid Mechanics top

► Fluid Dynamics of Polar Vortices on Earth, Mars, and Titan
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 265-289, January 2023.
► Particle Rafts and Armored Droplets
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 459-480, January 2023.
► Sharp Interface Methods for Simulation and Analysis of Free Surface Flows with Singularities: Breakup and Coalescence
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 707-747, January 2023.
► A Perspective on the State of Aerospace Computational Fluid Dynamics Technology
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 431-457, January 2023.
► 3D Lagrangian Particle Tracking in Fluid Mechanics
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 511-540, January 2023.
► Evaporation of Sessile Droplets
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 481-509, January 2023.
► Gas-Liquid Foam Dynamics: From Structural Elements to Continuum Descriptions
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 323-350, January 2023.
► Transition to Turbulence in Pipe Flow
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 575-602, January 2023.
► Icebergs Melting
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 377-402, January 2023.
► Elasto-Inertial Turbulence
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 675-705, January 2023.

Computers & Fluids top

► Quantifying uncertainties in direct numerical simulations of a turbulent channel flow
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Joseph O’Connor, Sylvain Laizet, Andrew Wynn, Wouter Edeling, Peter V. Coveney

► High-Order Implicit Large Eddy Simulation using Entropically Damped Artificial Compressibility
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Brian C. Vermeire

► High-order moving immersed boundary and its application to a resolved CFD-DEM model
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Lucka Barbeau, Shahab Golshan, Jieyao Deng, Stéphane Étienne, Cédric Béguin, Bruno Blais

► Adjoint-based aerodynamic shape optimization with hybridized discontinuous Galerkin methods
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Joachim Balis, Frederik Jacobs, Georg May

► A compressible multiphase Mass-of-Fluid model for the simulation of laser-based manufacturing processes
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Constantin Zenz, Michele Buttazzoni, Tobias Florian, Katherine Elizabeth Crespo Armijos, Rodrigo Gómez Vázquez, Gerhard Liedl, Andreas Otto

► High fidelity simulation of dense vapours with thermodynamic consistent modelling
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): A.P.S. Wheeler

► Well-balanced kinetic schemes for two-phase flows
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Jin Bao, Zhaoli Guo

► Generalization to a wider class of entropy split methods for compressible ideal MHD
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Björn Sjögreen, H.C. Yee

► <em>A priori</em> test of Large-Eddy Simulation models for the Sub-Grid Scale turbulent stress tensor in perfect and transcritical compressible real gas Homogeneous Isotropic Turbulence
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Alexis Giauque, Corentin Giguet, Aurélien Vadrot, Christophe Corre

► Explicit strong stability preserving second derivative multistep methods for the Euler and Navier–Stokes equations
    

Publication date: 15 January 2024

Source: Computers & Fluids, Volume 268

Author(s): Xueyu Qin, Jian Yu, Zhenhua Jiang, Lintao Huang, Chao Yan

International Journal of Computational Fluid Dynamics top

► Investigation of Low and High-Speed Fluid Dynamics Problems Using Physics-Informed Neural Network
    1 Dec, 2023
Volume 37, Issue 2, February 2023, Page 149-166
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► A Unified Grid Approach Using Hamiltonian Paths for Computing Aerodynamic Flows
    1 Dec, 2023
Volume 37, Issue 2, February 2023, Page 122-148
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► Prediction of Flow Field Over Airfoils Based on Transformer Neural Network
    1 Dec, 2023
Volume 37, Issue 2, February 2023, Page 167-180
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► A Shock Sensor Based on Image Segmentation with Application to a Hybrid Central/WENO Scheme
    1 Dec, 2023
Volume 37, Issue 2, February 2023, Page 83-121
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► Computational fluid–structure interaction framework for passive feathering and cambering in flapping insect wings
  29 Nov, 2023
Computational fluid–structure interaction framework for passive feathering and cambering in flapping insect wings

We developed a novel computational framework that consists of a fully parallelized solution method using algebraic splitting and semi-implicit scheme for monolithic FSI equation systems, the parallel CG method, and a pixel model wing combined with explicit node-positioning. Using the proposed framework, we found that feathering and cambering motions in flapping insect wings will be given based on the equilibrium between wing's elastic and aerodynamic forces, and these motions are enhanced by the wing's inertial force.


Abstract

In flapping insect wings, veins support flexible wing membranes such that the wings form feathering and cambering motions passively from large elastic deformations. These motions are essentially important in unsteady aerodynamics of insect flapping flight. Hence, the underlying mechanism of this phenomenon is an important issue in studies on insect flight. Systematic parametric studies on strong coupling between a model wing describing these elastic deformations and the surrounding fluid, which is a direct formulation of this phenomenon, will be effective for solving this issue. The purpose of this study is to develop a robust numerical framework for these systematic parametric studies. The proposed framework consists of two novel numerical methods: (1) A fully parallelized solution method using both algebraic splitting and semi-implicit scheme for monolithic fluid–structure interaction (FSI) equation systems, which is numerically stable for a wide range of properties such as solid-to-fluid mass ratios and large body motions, and large elastic deformations. (2) A structural mechanics model for insect flapping wings using pixel modeling (pixel model wing), which is combined with explicit node-positioning to reduce computational costs significantly in controlling fluid meshes. The validity of the proposed framework is demonstrated for some benchmark problems and a dynamically scaled model incorporating actual insect data. Finally, from a parametric study for the pixel model wing flapped in fluid with a wide range of solid-to-fluid mass ratios, we find a FSI mechanism of feathering and cambering motions in flapping insect wings.

► Layered shallow water equations: Spatiotemporally varying layer ratios with specific adaptation to wet/dry interfaces
  23 Nov, 2023
Layered shallow water equations: Spatiotemporally varying layer ratios with specific adaptation to wet/dry interfaces

A hydrostatic multilayered SWE system with spatiotemporally varying layer ratios is proposed in the current study. Periodical variation of layer ratios compounded with the spatial variability of the same. The existence of layer bounds as limits of integration only or a vertical conforming grid.


Abstract

The study of multilayered shallow water equations has developed from a consideration of immiscible layers as a vertical mesh to the layer bounds as imaginary extremes for vertical integration of the flow equations. In the current work, a quasi three-dimensional flow model has been developed with the consideration of the spatiotemporal flexibility/variability of the pervious vertical discretization/layer ratios. Thus, in principle, vertical layering offers a nonuniform grid with a temporal variation. The system of equations thus formulated comprises a conservative part and the appended source/sink terms. These source/sink terms pertain to the inter-layer interactions such as mass/momenta transfer and interfacial stress, which have been treated in a novel implicit form alongwith the subgrid-scale eddy-viscosity for interlayer shear. They are integrated into the system through different physical considerations so as to arrive at a well-balanced numerical scheme in a regular finite volume grid. The model has been validated through the standard test-cases highlighting the conservation properties and the model's adaptability to uniform and nonuniform vertical meshes alongwith the spatiotemporal transitions of layer ratios, with a specific interest in limiting cases of wet/dry fronts. The increase in layer ratios tends to nearly replicate the full-scale model results in experimental scenarios at a lesser computational overhead.

► Flow resistance co‐efficient of meandering river in alluvial plain and its prediction using artificial neural network
  21 Nov, 2023
Flow resistance co-efficient of meandering river in alluvial plain and its prediction using artificial neural network

Flow resistance co-efficient of meandering river in alluvial plain and its prediction using artificial neural network.


Abstract

A proper estimation of flow resistance coefficient of river is essential for precise simulations of river hydraulics. In addition to the cross-sectional geometry and hydraulic parameters, the alignment of the channel affects the flow resistance coefficient in case of meandering rivers. In the present study, a rigorous field study of 131 km along the Barak River was conducted to assess the influence of meandering on the flow resistance coefficient. The values of flow resistance co-efficient were calculated using Chezy and Manning's equations with measured field data and the values from both are compared. However, the variation in the flow resistance co-efficient along the channel calculated from Manning's equation is significantly less as it does not consider the undulation and meandering. Using these field data, an artificial neural network (ANN) model has been developed to predict the cross-sectional averaged flow resistance for meandering river. The model considered the influence of relative curvature, depth of flow, bed particle size, Froude number and Reynolds number including water temperature for accurate predictions of flow resistance coefficient. The ANN model was tested and validated using 237 field data sample. The values of the statistical parameters indicate a very good fit to the training dataset with coefficient of determination (R 2) = 0.9566 for training and good fit for testing with R 2 = 0.8131. The developed ANN model has been compared with other model with the same data set to check its applicability.

► Enhancing physics informed neural networks for solving Navier–Stokes equations
  21 Nov, 2023
Enhancing physics informed neural networks for solving Navier–Stokes equations

We propose two enhanced approaches of physics informed neural networks (PINN) for solving the challenging Navier–Stokes equation (NSE). The first approach improves the model by approximating the velocity components and integrating a pressure-based loss function. The second approach directly approximates the NSE solution without assumptions, significantly reducing training duration while maintaining high accuracy. We successfully apply this approach to solve the three-dimensional NSE, demonstrating the advantages of our models in terms of trainability, flexibility, and efficiency.


Summary

Fluid mechanics is a critical field in both engineering and science. Understanding the behavior of fluids requires solving the Navier–Stokes equation (NSE). However, the NSE is a complex partial differential equation that can be challenging to solve, and classical numerical methods can be computationally expensive. In this paper, we propose enhancing physics-informed neural networks (PINNs) by modifying the residual loss functions and incorporating new computational deep learning techniques. We present two enhanced models for solving the NSE. The first model involves developing the classical PINN for solving the NSE, based on a stream function approach to the velocity components. We have added the pressure training loss function to this model and integrated the new computational training techniques. Furthermore, we propose a second, more flexible model that directly approximates the solution of the NSE without making any assumptions. This model significantly reduces the training duration while maintaining high accuracy. Moreover, we have successfully applied this model to solve the three-dimensional NSE. The results demonstrate the effectiveness of our approaches, offering several advantages, including high trainability, flexibility, and efficiency.

► A fast and accurate method for transport and dispersion of phosphogypsum in coastal zones: Application to Jorf Lasfar
  14 Nov, 2023
A fast and accurate method for transport and dispersion of phosphogypsum in coastal zones: Application to Jorf Lasfar

Fast and highly accurate numerical solutions for transport and dispersion of phosphogypsum in the coastal zone of Jorf Lasfar on the Atlantic Ocean at Morocco can be achieved by using a multilevel adaptive semi-Lagrangian finite element method. A class of enrichment techniques has been used to discretize the governing equations and the obtained results are assessed for different release scenarios.


Summary

We present a numerical method for modelling and simulation of transport and dispersion of phosphogypsum in the Jorf Lasfar coastal zone located on the Atlantic Ocean at Morocco. The governing equations consist of the well-established barotropic ocean model including the barometric pressure, friction terms, Coriolis and wind stresses. To model transport and dispersion of phosphogypsum we consider an advection-diffusion equation with an anisotropic dispersion tensor and source terms. As a numerical solver, we propose a novel multilevel adaptive semi-Lagrangian finite element method. The proposed method combines the modified method of characteristics to deal with convection terms, the finite element discretization to handle complex geometries, a projection-based algorithm to solve the Stokes problem, and an adaptive L2$$ {\mathrm{L}}^2 $$-projection using quadrature rules to improve the efficiency and accuracy of the method. Numerical results are presented to demonstrate the high resolution of the proposed method and to confirm its capability to provide accurate and efficient simulations for transport and dispersion of phosphogypsum in the Jorf Lasfar coastal zone.

► An immersed boundary method‐discrete unified gas kinetic scheme simulation of particle‐laden turbulent channel flow on a nonuniform orthogonal mesh
  14 Nov, 2023
An immersed boundary method-discrete unified gas kinetic scheme simulation of particle-laden turbulent channel flow on a nonuniform orthogonal mesh

Application of a nonuniform mesh IBM can significantly reduce the mesh requirement for particle-laden turbulent channel flow simulation while being capable of better resolving the near-wall small-scale flow dynamics. Nonuniform mesh simulations, while with a reduction of the CPU time by a factor of more than 7, are shown to provide comparable results as those based on a uniform mesh. The effects of finite-size solid particles on the mean flow velocity, Reynolds stress, and root-mean-squared velocity fluctuations are presented, to demonstrate various aspects of turbulence modulation by solid particles.


Abstract

Particle-resolved simulations of turbulent particle-laden flows provide a powerful research tool to explore detailed flow physics at all scales. However, efficient particle-resolved simulations for wall-bounded turbulent particle-laden flows remain a challenging task. In this article, we develop a simulation approach for a turbulent channel flow laden with finite-size particles on a nonuniform mesh by combining the discrete unified gas kinetic scheme (DUGKS) and the immersed boundary method (IBM). The standard discrete delta function was modified according to reproducible kernel particle method to take into account mesh non-uniformity and correctly conserve force moments. Simulation results based on uniform and nonuniform meshes are compared to validate and examine the accuracy of the nonuniform mesh DUGKS-IBM. Finally, the computational performance of the nonuniform mesh DUGKS-IBM is discussed.

► An improved slip‐wall model for large eddy simulation and its implementation in the local domain‐free discretization method
  11 Nov, 2023
An improved slip-wall model for large eddy simulation and its implementation in the local domain-free discretization method

Complement is given to the physics-based interpretation of a Robin-type wall closure and then the slip-wall model is improved by redefining the slip length. The improved wall model is implemented in the local domain-free discretization method for large eddy simulation of high-Re turbulent flows. The predicted results agree well with the referenced experimental data and numerical results.


Abstract

In this paper, a slip-wall model for large eddy simulation (LES) is improved and implemented in an immersed boundary method named the local domain-free discretization (DFD) method. Considering that the matching location may be in the viscous sublayer, the physics-based interpretation of a Robin-type wall closure is complemented. Then, the slip-wall wall model is improved, in which the slip length is redefined and the Robin boundary condition is imposed at the solid wall. The improved slip-wall model is implemented in the local DFD method to evaluated the tangential velocity at an exterior dependent node, and then the requirement on high resolution of boundary layers can be alleviated. The non-equilibrium effects are accounted for by adding an explicit correction to the wall shear stress. In order to validate the present wall-modeled LES/DFD method, a series of turbulent channel flows at various Reynolds numbers, the flow over periodic hills and the flows over a NACA 4412 airfoil at a high Reynolds number are simulated. The predicted results agree well with the referenced experimental data and numerical results. Especially, the results of the separated flow over the airfoil at a near-stall condition demonstrate the performance of the present wall-modeled LES/DFD method for complex flows.

► A two‐dimensional multimaterial ALE method for compressible flows using coupled volume of fluid and level set interface reconstruction
    3 Nov, 2023
A two-dimensional multimaterial ALE method for compressible flows using coupled volume of fluid and level set interface reconstruction

A 2D multimaterial ALE method for simulating compressible flows is presented in which a novel coupled volume of fluid and level set (VOSET) interface reconstruction method is developed for interface capturing. The novel VOSET method improves the accuracy and fidelity in interface reconstruction procedure, especially in under-resolved regions.


Abstract

In this work, we present a two-dimensional multimaterial arbitrary Lagrangian–Eulerian (ALE) method for simulating compressible flows in which a novel coupled volume of fluid and level set interface reconstruction (VOSET) method is developed for interface capturing. The VOSET method combines the merits of both the volume of fluid method and the level set method by using a geometrical iterative operation. Compared to the original VOSET method, the novel VOSET method proposed in this work further improves the accuracy and fidelity in interface reconstruction procedure, especially in under-resolved regions. Several typical two-dimensional numerical experiments are presented to investigate the effectiveness of the proposed VOSET method and its performance when coupling with the multimaterial ALE solver. Numerical results demonstrate its good capability in capturing material interfaces during the simulation of compressible two-material flows.

► A coupled SPH‐EBG numerical model for deformations of MCF7 cancer cell in a microchannel flow
    3 Nov, 2023
A coupled SPH-EBG numerical model for deformations of MCF7 cancer cell in a microchannel flow

This work presents a workflow using coupled particle methods to relate cell deformation to cell properties. Numerical results were validated with microscopic experimental data of MCF7 cancerous cell. Key findings include: (i) Stress and deformation are generally correlated, but with dependence on cell shape. (ii) Cell deformation is sensitive to flow profile, especially wall shear stress. (iii) Low cell stiffness and high flow blockage can aggravate cell deformation.


Summary

Properties of a cell can determine its deformations, which can aggravate cancer metastasis. In laboratory, microfluidic technology has been adopted to study cell deformations. However, quantifying the effects of cell deformations has remained difficult. To this end, this paper presents a two-dimensional particle-based model that can capture flow-induced cell deformations in a microchannel. The numerical model is validated with an experimental dataset for MCF7 cell. The simulations show that cell deformations are dominantly attributed to flow acceleration. Stress analyses, conducted by inputting the simulated cell deformations as boundary conditions, show that the maximum normal stresses correspond well to high deformations. Shear stress is in general proportional to the cell's distance from a wall. The simulations also suggest a deformed cell shape that apparently may reduce the average normal stresses. This study highlights the potential of the numerical model to relate the measurable cell deformations to the more elusive cell properties.

► Issue Information
    3 Nov, 2023
International Journal for Numerical Methods in Fluids, Volume 95, Issue 12, December 2023.

Journal of Computational Physics top

► Non-intrusive data-driven reduced-order modeling for time-dependent parametrized problems
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Junming Duan, Jan S. Hesthaven

► A cell-centred Eulerian volume-of-fluid method for compressible multi-material flows
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Timothy R. Law, Philip T. Barton

► Multiscale sampling for the inverse modeling of partial differential equations
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Alsadig Ali, Abdullah Al-Mamun, Felipe Pereira, Arunasalam Rahunanthan

► Two-step multi-resolution reconstruction-based compact gas-kinetic scheme on tetrahedral mesh
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Xing Ji, Fengxiang Zhao, Wei Shyy, Kun Xu

► Unified gas-kinetic wave-particle methods VII: Diatomic gas with rotational and vibrational nonequilibrium
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Yufeng Wei, Yajun Zhu, Kun Xu

► A high-order residual-based viscosity finite element method for incompressible variable density flow
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Lukas Lundgren, Murtazo Nazarov

► Higher-continuity s-version of finite element method with B-spline functions
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Nozomi Magome, Naoki Morita, Shigeki Kaneko, Naoto Mitsume

► A new type of non-polynomial based TENO scheme for hyperbolic conservation laws
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Tian Liang, Lin Fu

► A hybrid shifted Laplacian multigrid and domain decomposition preconditioner for the elastic Helmholtz equations
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Eran Treister, Rachel Yovel

► On the coupling between direct numerical simulation of nucleate boiling and a micro-region model at the contact line
    

Publication date: 15 January 2024

Source: Journal of Computational Physics, Volume 497

Author(s): Loric Torres, Annafederica Urbano, Catherine Colin, Sébastien Tanguy

Journal of Turbulence top

► Advanced detached-eddy simulation of the MD 30P-30N three-element airfoil
    6 Nov, 2023
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► Fractional and tempered fractional models for Reynolds-averaged Navier–Stokes equations
  31 Oct, 2023
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► Wall heat flux in supersonic turbulent expansion flow with shock impingement
  13 Oct, 2023
Volume 24, Issue 9-10, September - October 2023, Page 445-473
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► A surrogate non-intrusive reduced order model of quasi-geostrophic turbulence dynamics based on a combination of LSTM and different approaches of DMD
  13 Oct, 2023
Volume 24, Issue 9-10, September - October 2023, Page 474-505
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► Evolution of turbulent mixing driven by implosion in spherical geometry
  18 Jul, 2023
Volume 24, Issue 9-10, September - October 2023, Page 419-444
.

Physics of Fluids top

► A sharp interface immersed edge-based smoothed finite element method with extended fictitious domain scheme
  17 Apr, 2023
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.
► Enhanced and reduced solute transport and flow strength in salt finger convection in porous media
  17 Apr, 2023
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.
► On the instability of the magnetohydrodynamic pipe flow subject to a transverse magnetic field
  17 Apr, 2023
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.
► The turbulence development at its initial stage: A scenario based on the idea of vortices decay
  17 Apr, 2023
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.
► Interstage difference and deterministic decomposition of internal unsteady flow in a five-stage centrifugal pump as turbine
  17 Apr, 2023
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.
► Effect of gravity on phase transition for liquid–gas simulations
  17 Apr, 2023
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.
► Entrapment and mobilization dynamics during the flow of viscoelastic fluids in natural porous media: A micro-scale experimental investigation
  17 Apr, 2023
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.
► Impact of wettability on interface deformation and droplet breakup in microcapillaries
  17 Apr, 2023
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.
► Drag increase and turbulence augmentation in two-way coupled particle-laden wall-bounded flows
  17 Apr, 2023
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.
► Partial and complete wetting of thin films with dynamic contact angle
  17 Apr, 2023
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.

Theoretical and Computational Fluid Dynamics top

► Inverted stochastic lattice Boltzmann-Lagrangian model for identifying indoor particulate pollutant sources
    1 Dec, 2023

Abstract

This paper studies the inverted stochastic lattice Boltzmann-Lagrangian approach for identifying indoor particulate pollutant sources. The dynamics of the fluid (indoor air) as well as the transport of the particles in the Eulerian description are solved using the lattice Boltzmann method. The particles regard as rigid bodies, and the data interactions between lattice fluid and particle movement are implemented by calculating for interaction force and void fraction. Particle-wall collision process is based on the softball model which describes the dynamic characteristics of particles in microscopic state. The results are shown that the particle forward and inverted drifting paths and its mechanisms are investigated clearly than previous methods. Indoor particulate pollutant sources can exactly identify with this approach. This research can offer theoretical relevance to the modeling of multi-phase particle fluid.

Graphical abstract

► Shock stand-off distances over sharp wedges for thermally non-equilibrium dissociating nitrogen flows
    1 Dec, 2023

Abstract

In this study, shock stand-off distances for thermally and chemically non-equilibrium flows of nitrogen over wedges are computationally investigated via a hypersonic computational fluid dynamics solver, hyperReactingFoam by spanning a parameter space that consists of ranges of Mach number, 4–10, specific heat ratio, 1.40–1.61 and wedge angles, 60 \(^\circ \) –90 \(^\circ \) . Then, the space is reduced into the parameters of inverse density ratio across the shock and dimensionless wedge angle which will be used as variables for quadratic functions that represent shock stand-off distances. Besides the functions of shock stand-off distances, detached shock profiles of computationally modeled flows are represented by parabolic equations. The flows are observed to be chemically frozen for Mach number ranges of 4–5 regardless of the specific heat ratio value of the nitrogen mixture. Our results show that the shock stand-off distance decreases as Mach number is increased from 4 to 7, if the wedge angle and free-stream specific heat ratio are kept the same. On the other hand, if Mach number is increased beyond 7, the shock stand-off distance starts to extend due to the dissociation of nitrogen molecules behind the shock wave. At Mach 10, nitrogen completely dissociates over 90 \(^\circ \) wedge for all specific heat ratios considered in the present study. Increased leading edge angle of the wedge or specific heat ratio of free-stream yields longer shock stand-off distance.

Graphic abstract

► An Eulerian–Eulerian–Lagrangian modeling of two-phase combustion
    1 Dec, 2023

Abstract

In simulating two-phase combustion, most Reynolds-averaged Navier–Stokes (RANS) simulation and large-eddy simulation (LES) used Eulerian–Lagrangian (E–L) modeling (Eulerian treatment of gas phase and Lagrangian treatment of particles/droplets) which needs much more computational time than the Eulerian–Eulerian (E–E) or two-fluid modeling. However, in the E–E modeling, the problem of how to reduce the computation time for poly-dispersed particles is encountered . To solve this problem, the present author proposed an Eulerian–Eulerian–Lagrangian (E–E–L) modeling of two-phase combustion for both RANS modeling and LES. The E–E–L modeling is an Eulerian treatment of gas phase and a combined Eulerian–Lagrangian treatment of particles/droplets, in which the particle velocity and concentration are solved by Eulerian modeling, and particle temperature and mass change due to reaction are solved by Lagrangian modeling. In this paper, a review is given for an E–E–L modeling of coal combustion, its application in RANS simulation and its possible application in LES. For E–E–L LES, an energy equation model of two-phase sub-grid scale (SGS) stresses accounting for the interaction between two-phase SGS stresses is suggested, and a second-order moment SGS (SOM-SGS) turbulence-chemistry model is adopted to simulate gas-phase reaction in two-phase combustion. These SGS models were separately assessed by comparison with experiments.

Graphic abstract

► Faster flicker of buoyant diffusion flames by weakly rotatory flows
    1 Dec, 2023

Abstract

Flickering buoyant diffusion methane flames in weakly rotatory flows were computationally and theoretically investigated. The prominent computational finding is that the flicker frequency nonlinearly increases with the nondimensional rotational intensity R (up to 0.24), which is proportional to the nondimensional circumferential circulation. This finding is consistent with the previous experimental observations that rotatory flows enhance flame flicker to a certain extent. Based on the vortex-dynamical understanding of flickering flames that the flame flicker is caused by the periodic shedding of buoyancy-induced toroidal vortices, a scaling theory is formulated for flickering buoyant diffusion flames in weakly rotatory flows. The theory predicts that the increase of flicker frequency f obeys the scaling relation \(\left( f-f_{0} \right) \propto R^{2}\) , which agrees very well with the present computational results. In physics, the external rotatory flow enhances the radial pressure gradient around the flame, and the significant baroclinic effect \(\mathrm {\nabla }p\times \mathrm {\nabla }\rho \) contributes an additional source for the growth of toroidal vortices so that their periodic shedding is faster.

Graphical abstract

► Network-theoretic modeling of fluid–structure interactions
    1 Dec, 2023

Abstract

The coupling interactions between deformable structures and unsteady fluid flows occur across a wide range of spatial and temporal scales in many engineering applications. These fluid–structure interactions (FSI) pose significant challenges in accurately predicting flow physics. In the present work, two multi-layer network approaches are proposed that characterize the interactions between the fluid and structural layers for an incompressible laminar flow over a two-dimensional compliant flat plate at a 35 \(^{\circ }\) angle of attack. In the first approach, the network nodes are formed by wake vortices and bound vortexlets, and the edges of the network are defined by the induced velocity between these elements. In the second approach, coherent structures (fluid modes), contributing to the kinetic energy of the flow, and structural modes, contributing to the kinetic energy of the compliant structure, constitute the network nodes. The energy transfers between the modes are extracted using a perturbation approach. Furthermore, the network structure of the FSI system is simplified using the community detection algorithm in the vortical approach and by selecting dominant modes in the modal approach. Network measures are used to reveal the temporal behavior of the individual nodes within the simplified FSI system. Predictive models are then built using both data-driven and physics-based methods. Overall, this work sets the foundation for network-theoretic reduced-order modeling of fluid–structure interactions, generalizable to other multi-physics systems.

Graphical abstract

► Application of the lattice Boltzmann method to the study of ultrasound propagation and acoustic streaming in three-dimensional cavities: advantages and limitations
    1 Dec, 2023

Abstract

The paper presents a three-dimensional numerical study of the acoustic streaming induced by the dissipation of ultrasounds during their propagation in the air. The waves are generated by a circular acoustic source positioned at the center of the left wall of a parallelepipedic cavity. The simulations are performed with the lattice Boltzmann method associated with the D3Q19 multiple relaxation time model. A validation of this model is first performed by comparing the numerical and analytical acoustic intensities along the central axis of the acoustic source. The main objective of this study is to use two different methods to calculate the acoustic streaming flow. The first method is the direct calculation of the mean velocity fields as the mean values of the instantaneous velocities. The second method is an indirect technique, which first calculates the acoustic streaming force and then injects this force into the numerical code to produce the streaming. A comparison between the results obtained by the two methods was carried out and a good agreement was found between them. These different investigations, rather new in three-dimensional configurations, have allowed us to discuss the advantages and limitations of the lattice Boltzmann approach to simulate real situations of wave propagation and acoustic streaming.

Graphical abstract

► Investigation of Stokes flow in a grooved channel using the spectral method
    1 Nov, 2023

Abstract

Pressure-driven Newtonian fluid flow between grooved and flat surfaces is analysed with no-slip boundary conditions at walls. The effect of corrugation on the fluid flow is investigated using the mesh-free spectral method. The primary aim of the present work is to develop an asymptotic/semi-analytical theory for confined transverse flows to bridge the gap between the limits of thin and thick channels. The secondary aim is to calculate permeability with reference to the effect of wall corrugation (roughness) without the restriction of pattern amplitude. We performed mathematical modelling and evaluated the analytical solution for hydraulic permeability with respect to the flat channel. The Pad \(\acute{e}\) approximate is employed to improve the solution accuracy of an asymptotic model. The results elucidate that permeability always follows a decreasing trend with increasing pattern amplitude using the spectral approach at the long-wave and short-wave limits. The prediction of the spectral model is more accurate than the asymptotic-based model by Stroock et al. (Anal Chem 74(20):5306, 2002) and Pad \(\acute{e}\) approximate, regardless of the grooved depth and wavelength of the channel. The finite-element-based numerical simulation is also used to understand the usefulness of theoretical models. A very low computational time is required using the mesh-free spectral model as compared to the numerical study. The agreement between the present model and the fully resolved numerical results is gratifying. Regarding numerical values, we calculated the relative error for different theoretical models such as an asymptotic model, Pad \(\acute{e}\) approximate, and a mesh-free spectral model. The spectral model always predicts the maximum relative error as less than \(3 \%\) , regardless of the large pattern amplitude and wavelength. In addition, the results of the molecular dynamic (MD) simulations by Guo et al. (Phys Rev Fluids 1(7):074102, 2016) and the theoretical model by Wang (Phys Fluids 15(5):1121, 2003) are found to be quantitatively compatible with the predictions of effective slip length from the spectral model in the thick channel limit.

Graphical abstract

► Aerodynamic and aeroacoustic performance of a pitching foil with trailing edge serrations at a high Reynolds number
    9 Oct, 2023

Abstract

The aerodynamic and aeroacoustic performance of a low-aspect-ratio ( \(\hbox {AR}=0.2\) ) pitching foil during dynamic stall are investigated numerically with focus on the effects of trailing edge serrations. A hybrid method coupling an immersed boundary method for incompressible flows with the Ffowcs Williams–Hawkings acoustic analogy is employed. Large eddy simulation and turbulent boundary layer equation wall model are also employed to capture the turbulent effects. A modified NACA0012 foil with a rectangular trailing edge flap attached to the trailing edge (baseline case) undergoing pitching motion is considered. Trailing edge serrations are applied to the trailing edge flap and their effects on the aerodynamic and aeroacoustic performance of the oscillating airfoil are considered by varying the wave amplitude ( \(2h^*= 0.05, 0.1\) , and 0.2) at a Reynolds number of 100,000 and a Mach number of 0.05. It is found that the reduction of the sound pressure level at the dimensionless frequency band \(St_{b}\in [1.25,4]\) can be over 4 dB with the presence of the trailing edge serrations ( \(2h^*=0.1\) ), while the aerodynamic performance and its fluctuations are not significantly altered except a reduction around 10% in the negative moment coefficient and it fluctuations. This is due to the reduction of the average spanwise coherence function and the average surface pressure with respect to that of the baseline case, suggesting the reduction of the spanwise coherence and the noise source may result in the noise reduction. Analysis of the topology of the near wake coherent structure for \(2h^*=0.1\) reveals that the alignment of the streamwise-oriented vortex with the serration edge may reduce the surface pressure fluctuation.

Graphical abstract

► GPU computing of yield stress fluid flows in narrow gaps
    1 Oct, 2023

Abstract

We present a Graphic Processing Units (GPU) implementation of non-Newtonian Hele-Shaw flow that models the displacement of Herschel-Bulkley fluids along narrow eccentric annuli. This flow is characteristic of many long-thin flows that require extensive calculation due to an inherent nonlinearity in the constitutive law. A common method of dealing with such flows is via an augmented Lagrangian algorithm, which is often painfully slow. Here we show that such algorithms, although involving slow iterations, can often be accelerated via parallel implementation on GPUs. Indeed, such algorithms explicitly solve the nonlinear aspects only locally on each mesh cell (or node), which makes them ideal candidates for GPUs. Combined with other advances, the optimized GPU implementation takes \(\approx 2.5\%\) of the time of the original algorithm.

Graphical abstract

► Highly conservative Allen–Cahn-type multi-phase-field model and evaluation of its accuracy
    1 Oct, 2023

Abstract

In the engineering field, it is necessary to construct a numerical model that can reproduce multiphase flows containing three or more phases with high accuracy. In our previous study, by extending the conservative Allen–Cahn (CAC) model, which is computationally considerably more efficient than the conventional Cahn–Hilliard (CH) model, to the multiphase flow problem with three or more phases, we developed the conservative Allen–Cahn type multi-phase-field (CAC–MPF) model. In this study, we newly construct the improved CAC–MPF model by modifying the Lagrange multiplier term of the previous CAC–MPF model to a conservative form. The accuracy of the improved CAC–MPF model is evaluated through a comparison of five models: three CAC–MPF models and two CH–MPF models. The results indicate that the improved CAC–MPF model can accurately and efficiently perform simulations of multiphase flows with three or more phases while maintaining the same level of volume conservation as the CH model. We expect that the improved CAC–MPF model will be applied to various engineering problems with multiphase flows with high accuracy.

Graphic abstract


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