CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home >

CFD Journal Feeds

Annual Review of Fluid Mechanics top

► Mixing by Oceanic Lee Waves
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 173-201, January 2021.
► Statistics of Extreme Events in Fluid Flows and Waves
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 85-111, January 2021.
► Layering, Instabilities, and Mixing in Turbulent Stratified Flows
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 113-145, January 2021.
► Statistical Properties of Subgrid-Scale Turbulence Models
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 255-286, January 2021.
► Leonardo da Vinci and Fluid Mechanics
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 1-25, January 2021.
► Introduction
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page v-v, January 2021.
► Turbulence Processes Within Turbidity Currents
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 59-83, January 2021.
► Levitation and Self-Organization of Droplets
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 203-225, January 2021.
► Exact Coherent States and the Nonlinear Dynamics of Wall-Bounded Turbulent Flows
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 227-253, January 2021.
► The Fluid Dynamics of Disease Transmission
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 473-508, January 2021.

Computers & Fluids top

► A hybrid first-order and WENO scheme for the high-resolution and computationally efficient modeling of pollutant transport
    

Publication date: Available online 1 May 2021

Source: Computers & Fluids

Author(s): Yuying HU, Peng HU, Wei Li, Weihong LIAO

► A review on Deep Reinforcement Learning for Fluid Mechanics
    

Publication date: Available online 1 May 2021

Source: Computers & Fluids

Author(s): Paul Garnier, Jonathan Viquerat, Jean Rabault, Aurélien Larcher, Alexander Kuhnle, Elie Hachem

► Data-driven RANS closures for three-dimensional flows around bluff bodies
    

Publication date: Available online 4 May 2021

Source: Computers & Fluids

Author(s): Jasper P. Huijing, Richard P. Dwight, Martin Schmelzer

► On the coupling of wall-model immersed boundary conditions and curvilinear body-fitted grids for the simulation of complex geometries
    

Publication date: Available online 4 May 2021

Source: Computers & Fluids

Author(s): Lucas Manueco, Pierre-Elie Weiss, Sébastien Deck

► Investigation of correlation between vorticity, Q, <math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg" class="math"><msub><mi>λ</mi><mrow><mi>c</mi><mi>i</mi></mrow></msub></math>, <math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg" class="math"><msub><mi>λ</mi><mn>2</mn></msub></math>, <math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg" class="math"><mstyle mathvariant="normal"><mi>Δ</mi></mstyle></math> and Liutex
    

Publication date: Available online 6 May 2021

Source: Computers & Fluids

Author(s): Yifei Yu, Pushpa Shrestha, Oscar Alvarez, Charles Nottage, Chaoqun Liu

► Role of Non-Zero Bulk Viscosity in Three-Dimensional Rayleigh-Taylor Instability: Beyond Stokes’ Hypothesis
    

Publication date: Available online 4 May 2021

Source: Computers & Fluids

Author(s): Aditi Sengupta, Roshan J. Samuel, Prasannabalaji Sundaram, Tapan K. Sengupta

► Relevance of two- and three-dimensional disturbance field explained with linear stability analysis of Orr-Sommerfeld equation by compound matrix method
    

Publication date: Available online 27 April 2021

Source: Computers & Fluids

Author(s): Raj Maddipati, Tapan K. Sengupta, Prasannabalaji Sundaram

► A symmetry-preserving second-order time-accurate PISO-based method
    

Publication date: Available online 27 April 2021

Source: Computers & Fluids

Author(s): E.M.J. Komen, J.A. Hopman, E.M.A. Frederix, F.X. Trias, R.W.C.P. Verstappen

► A geometrically accurate low-diffusive conservative interface capturing method suitable for multimaterial flows
    

Publication date: Available online 1 March 2021

Source: Computers & Fluids

Author(s): Florian De Vuyst, Christophe Fochesato, Vincent Mahy, Renaud Motte, Mathieu Peybernes

► A turbulent eddy-viscosity surrogate modeling framework for Reynolds-Averaged Navier-Stokes simulations
    

Publication date: Available online 2 November 2020

Source: Computers & Fluids

Author(s): Romit Maulik, Himanshu Sharma, Saumil Patel, Bethany Lusch, Elise Jennings

International Journal of Computational Fluid Dynamics top

► Modelling Airborne Transmission and Ventilation Impacts of a COVID-19 Outbreak in a Restaurant in Guangzhou, China
    7 Apr, 2021
.
► A Case Study on Pathogen Transport, Deposition, Evaporation and Transmission: Linking High-Fidelity Computational Fluid Dynamics Simulations to Probability of Infection
    1 Apr, 2021
.
► An Optimised Five-Point-Stencil Weighted Compact Nonlinear Scheme for Hyperbolic Conservation Laws
  29 Mar, 2021
.
► SPH Simulation for Short Fibre Recycling Using Water Jet Alignment
  26 Feb, 2021
.
► Assessing Numerical Aspects of Transitional Flow Simulations Using the RANS Equations
  15 Jan, 2021
.
► Vorticity Confinement Applied to Accurate Prediction of Convection of Wing Tip Vortices and Induced Drag
  16 Dec, 2020
.
► Numerical Simulation of Water Spray Generated by Aircraft Multi-Wheels
  24 Nov, 2020
.
► Application of SPH to Single and Multiphase Geophysical, Biophysical and Industrial Fluid Flows
  17 Nov, 2020
.
► Accuracy Improvements for Single Precision Implementations of the SPH Method
  23 Oct, 2020
Volume 34, Issue 10, December 2020, Page 774-787
.
► On Recirculation Region Length of Suddenly Expanded Supersonic Flows, Using CFD and Fuzzy Logic
  14 Oct, 2020
Volume 34, Issue 10, December 2020, Page 757-773
.

International Journal for Numerical Methods in Fluids top

► A scalable parallel unstructured finite volume lattice Boltzmann method for three‐dimensional incompressible flow simulations
    7 May, 2021
A scalable parallel unstructured finite volume lattice Boltzmann method for three‐dimensional incompressible flow simulations

We introduce a parallel cell‐centered finite volume lattice Boltzmann method on 3D unstructured grids for incompressible flow simulations. The proposed method is validated by three benchmark problems and the results agree well with the published data. The parallel algorithm scales well on a supercomputer with a large number of processors.


Abstract

The standard lattice Boltzmann method, which employs certain regular lattices coupled with discrete velocities as the computational grid, is limited in its flexibility to simulate flows in irregular geometries. To simulate large‐scale complex flows, we present a cell‐centered finite volume lattice Boltzmann method for incompressible flows on three‐dimensional (3D) unstructured grids and its corresponding parallel algorithm. The advective fluxes are calculated by the low‐diffusion Roe scheme, and the gradients of the particle distribution functions are computed with a least squares method. The presented scheme is validated by three benchmark flows: (a) a 3D Poiseuille flow, (b) cubic cavity flows with Reynolds numbers Re = 100 and 400, and (c) flows past a sphere with Re = 50, 100, 150, 200, and 250. Some parallel performance results are presented to show the scalability of the method, which reveal that the proposed parallel algorithm has considerable scalability and that the parallel efficiency is higher than 87% on 3840 processor cores. It can be seen that the presented parallel solver has significant potential for the accurate simulation of flows in complex 3D geometries.

► Phase‐field formulation of a fictitious domain method for particulate flows interacting with complex and evolving geometries
    4 May, 2021
Phase‐field formulation of a fictitious domain method for particulate flows interacting with complex and evolving geometries

We introduce a phase‐field formulation of an existing fictitious domain method and a corresponding collision model in order to simulate rigid particulate flows interacting with arbitrary solid structures. The phase‐field framework enables the consideration of both the particulate flow and the structure on a fix Cartesian grid. It also enables the consideration of phase transition within the boundary structure, for example, due to grain growth, making it change its shape during runtime.


Abstract

A distributed Lagrange multiplier/fictitious domain method in a phase‐field formulation for the simulation of rigid bodies in incompressible fluid flow is presented. The phase‐field method yields an implicit representation of geometries and thus rigid body particulate flows within arbitrary geometries can be simulated based on a fixed Cartesian grid. Therefore, a phase‐field based collision model is introduced in order to address contact of particles with arbitrary solid structures as boundaries. In addition, grain growth within the boundary geometry can be considered leading to changes in its shape during the simulation. The method is validated on benchmark problems and a convergence study is performed. Multiple numerical experiments are carried out in order to show the methods' capability to simulate problems with differently shaped rigid bodies and particulate flows involving complex boundary geometries like foam structures.

► Variationally derived closure models for large eddy simulation of incompressible turbulent flows
    4 May, 2021
Variationally derived closure models for large eddy simulation of incompressible turbulent flows

A variationally approach for deriving residual‐based closure models for incompressible Navier–Stokes equations is presented. Variational consistency of the model lends itself to rigorous linearization that results in quadratic rate of convergence of the method in the nonlinear solution strategy. The method is shown to work for a family of linear and quadratic hexahedral and tetrahedral elements as well as composite discretizations that are comprised of hexahedral and tetrahedral elements in the same computational domain.


Abstract

We present a variationally consistent method for deriving residual‐based closure models for incompressible Navier–Stokes equations. The method is based on the fine‐scale variational structure facilitated by the variational multiscale framework where fine scales are driven by the residuals of the Euler–Lagrange equations of the resolved scales in the balance of momentum and conservation of mass equations. A bubble‐functions based approach is applied directly to the fine‐scale variational equation to derive analytical expressions for the closure model. Variational consistency of the model lends itself to rigorous linearization that results in quadratic rate of convergence of the method in the iterative solution strategy for the nonlinear equations. The method is shown to work for a family of linear and quadratic hexahedral and tetrahedral elements as well as composite discretizations that are comprised of hexahedral and tetrahedral elements in the same computational domain. Numerical tests with the proposed model are presented for various classes of turbulent flow problems to show its generality and range of applicability. The test cases investigated include Taylor–Green vortex stretching, statistically stationary wall‐bounded channel flows, and modeling the effects of the geometry of the leading edge of the plate on the instability of the boundary layer that leads to flow separation and flow reversal over flat plates of finite thickness.

► Lattice Boltzmann method for the simulation of White–Metzner fluid flows
    3 May, 2021
Lattice Boltzmann method for the simulation of White–Metzner fluid flows

A coupled lattice Boltzmann method with two types of distribution functions is used to simulate non‐Newtonian White‐Metzner fluid flows inside a lid driven square cavity. A very detailed investigation of the effect of each parameter on the behavior of the WM fluid flows is presented. The method proves to be good in simulating WM fluids and encounter the instabilities that arise in such flows.


Abstract

The simulation of non‐Newtonian fluids is a challenging task from theoretical and numerical points of view. Different numerical methods has been used to study this class of fluids. In this article, a novel numerical scheme based on lattice Boltzmann method is presented for the simulation of White–Metzner (WM) fluid flows, where two types of distribution functions are defined for the evolution of momentum and stress, respectively. We study the accuracy, and the influence of different parameters on the flow on different benchmarks: we validate our model first for a two‐dimensional planar channel, and then we investigate in details the behavior of WM fluid flows in a square lid driven cavity. In the numerical results, we give a very detailed investigation to elaborate the effect over a large range of each parameter on the flow field.

► Front capturing by level set method for the reactive Euler equations
  28 Apr, 2021

Abstract

In this paper, we present a front capturing method for compressible reactive flows, where a shock tracking technique is applied with level set method. In stiff reaction problems, the difficulty arises when the time scale of the source term is significantly shorter than the time scale of the homogeneous conservation law. The spurious numerical phenomenon may occur due to the smeared out shock profiles. In order to overcome the difficulty, the detonation front is captured sharply by level set technique, and a modified finite volume scheme is developed at the computational cells cut by detonation front. The mass, momentum and energy transitions are occurred on shock front for chemical reactive flows. In our method, the interface exchanges are calculated by considering a Riemann problem. Unlike the standard level set/ghost fluid method, our method can maintain the conservation on the shock discontinuity. One‐ and two‐dimensional numerical examples including stable and unstable detonation problems are illustrated to verify the good reliability and robustness of our method.

► Reduced order models for the incompressible Navier‐Stokes equations on collocated grids using a ‘discretize‐then‐project’ approach
  28 Apr, 2021

Summary

A novel reduced order model (ROM) for incompressible flows is developed by performing a Galerkin projection based on a fully (space and time) discrete full order model (FOM) formulation. This ‘discretize‐then‐project’ approach requires no pressure stabilization technique (even though the pressure term is present in the ROM) nor a boundary control technique (to impose the boundary conditions at the ROM level). These are two main advantages compared to existing approaches. The fully discrete FOM is obtained by a finite volume discretization of the incompressible Navier‐Stokes equations on a collocated grid, with a forward Euler time discretization. Two variants of the time discretization method, the inconsistent and consistent flux method, have been investigated. The latter leads to divergence‐free velocity fields, also on the ROM level, whereas the velocity fields are only approximately divergence‐free in the former method. For both methods, accurate results have been obtained for test cases with different types of boundary conditions: a lid‐driven cavity and an open‐cavity (with an inlet and outlet). The ROM obtained with the consistent flux method, having divergence‐free velocity fields, is slightly more accurate but also slightly more expensive to solve compared to the inconsistent flux method. The speedup ratio of the ROM and FOM computation times is the highest for the open cavity test case with the inconsistent flux method.

► A stable discontinuous Galerkin method based on high‐order dual splitting scheme without additional stabilization term for incompressible flows
  27 Apr, 2021
A stable discontinuous Galerkin method based on high‐order dual splitting scheme without additional stabilization term for incompressible flows

The DG formulation of the velocity divergence term and the pressure gradient term is required in order to improve the stability of the scheme at small time steps. Moreover, an appropriate penalty term for pressure Poisson equation still provides inf‐sup stabilization for equal‐order polynomials. The combination of these methods is a relatively simple and effective way to improve the stability and robustness of the solver for incompressible flows with high‐Reynolds numbers.


Abstract

In this work, we consider equal‐order discontinuous Galerkin (DG) solver for incompressible Navier–Stokes equations based on high‐order dual splitting scheme. In order to stay stable, the time step size of this method has been reported that is strictly limited. The upper bound of time step size is restricted by Courant–Friedrichs–Lewy (CFL) condition (Hesthaven and Warburton, 2007) and lower bound is required to be larger than the critical value which depends on Reynolds number and spatial resolution (Ferrer et al., 2014). For high‐Reynolds‐number flow problems, if the spatial resolution is low, the critical value may be larger than CFL condition, then instability will occur for any time step size. Therefore, sufficiently high spatial resolution is indispensable in order to maintain stability, which increases the computational cost. To overcome these difficulties and develop a robust solver for high‐Reynolds‐number flow problem, it is necessary to further study the instability problem at small time steps. We numerically investigate the effect of the pressure gradient term in projection step and the velocity divergence term in pressure Poisson equation on the stability for small time step size, respectively, and conclude that the DG formulation of the pressure gradient term has a more significant effect on the stability of the scheme than that of the velocity divergence term. Integration by parts of these terms is essential in order to improve the stability of the scheme. Based on this discretization format, an appropriate penalty parameter for pressure Poisson equation is utilized so as to provide the scheme with an inf‐sup stabilization. Moreover, the lid‐driven cavity flow is considered to verify that this numerical algorithm enhances the stability without additional stabilization term at small time step size and high‐Reynolds number for equal‐order polynomial approximations.

► Imposing C0 and C1 continuity constraints during CAD‐based adjoint optimization
  27 Apr, 2021
Imposing C0 and C1 continuity constraints during CAD‐based adjoint optimization

This article presents a method for performing CAD‐based aerodynamic shape optimization by manipulating the standard geometry. The method addresses the continuity issues that emerge during the update of the boundary representation (BRep) of the shape by imposing up to C1 constraints between neighboring patches and defines a parameterization scheme which inherently satisfies them. The proposed method is demonstrated in an academic case and in industrial‐like cases such as the S‐section of a duct and the tail surface of a car.


Abstract

This article presents a method for performing adjoint‐based aerodynamic shape optimization by manipulating the standard CAD geometry of the shape to be optimized. A standard CAD file gives access to the boundary representation (BRep) of the shape and consequently its boundary surfaces which are usually trimmed patches. This is a sensible choice as the open format of such files is a requirement for the computation of the shape derivatives. The method addresses the continuity issues that emerge during the update of the shape by imposing up to C 1 constraints between different CAD patches. A parameterization scheme based on NURBS surfaces is then defined, in which the aforementioned constraints are inherently satisfied. The proposed method is firstly demonstrated in a simple geometric case and then in the full‐scale optimization of industrial‐like cases such as the S‐section of a cooling duct and the tail surface of a passenger car.

► Computational aeroacoustic modeling using hybrid Reynolds averaged Navier–Stokes/large‐eddy simulations methods with modified acoustic analogies
  26 Apr, 2021
Computational aeroacoustic modeling using hybrid Reynolds averaged Navier–Stokes/large‐eddy simulations methods with modified acoustic analogies

We develop and implement a hybrid Reynolds averaged Navier–Stokes/large‐eddy simulations improved delayed detached eddy simulation model for flow and noise predictions of tandem cylinder experiment. The aeroacoustic simulations were conducted via modified acoustic analogies, namely the Curle's and Ffowcs‐Williams and Hawkings analogies. The results from the implemented computational methodologies were in good agreement with experimental acoustic power spectral density data as measured at NASA's Quiet Flow Facility.


Abstract

This study employs a numerical approach to investigate noise mechanisms in tandem cylinders. The tandem cylinder configuration is an excellent model for aircraft landing gear. The study is therefore primarily motivated by the need to understand the aircraft landing gear as a primary contributor to airframe noise during approach and landing. Fluctuations in the flow properties induced by turbulent flow around the tandem cylinders are computed and analyzed. A hybrid improved delayed detached eddy simulation turbulence model is employed to compute the boundary layer flow as well as to compute the fluctuations in the flow properties. The numerical methodologies for the turbulent flow computations are implemented on the OpenFOAM software platform. The study also considers the impact of neglecting the volume sources and employs two modified versions of Curle's and Ffowcs‐Williams and Hawkings analogies. The results are in good agreement with published numerical and experimental data in the existing literature.

► On the Stabilization of Steady Continuous Adjoint Solvers in the Presence of Unsteadiness, in Shape Optimization
  25 Apr, 2021

Abstract

Adjoint‐based shape optimization using unsteady solvers is costly and/or memory demanding. When mild unsteadiness is present or the flow in/around the optimized shape is not expected to be time‐varying, steady primal and adjoint solvers can be used instead. However, in such a case, convergence difficulties caused by flow unsteadiness must properly be treated. In this article, the steady primal and the corresponding (continuous) adjoint solvers are both stabilized by implementing the Recursive Projection Method (RPM). This is carried out in the adjointOptimisation library of OpenFOAM, developed and made publicly available by the group of authors. Upon completion of the optimization using steady solvers, unsteady reevaluations of the optimized solutions confirm a reduction in the time‐averaged objective function. In complex cases, in which the RPM may not necessarily ensure convergence of the adjoint solver on its own, the controlled damping of the adjoint transposed‐convection (ATC) term is additionally implemented. This is demonstrated in the shape optimization of a motorbike fairing where averaged primal fields over a number of iterations of the steady flow solver are used for the solution of the adjoint equations. Cases in which the RPM is, on its own, sufficient in ensuring convergence of the adjoint solver are additionally studied by using a controlled ATC damping, to assess its impact on the computed sensitivity derivatives. Comparisons show that controlled/mild ATC damping is harmless and greatly contributes to robustness.

Journal of Computational Physics top

► A fully Eulerian solver for the simulation of multiphase flows with solid bodies: Application to surface gravity waves
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Francesco De Vita, Filippo De Lillo, Roberto Verzicco, Miguel Onorato

► Sparse approximate matrix-matrix multiplication for density matrix purification with error control
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Anton G. Artemov, Emanuel H. Rubensson

► Efficient and energy stable scheme for the hydrodynamically coupled three components Cahn-Hilliard phase-field model using the stabilized-Invariant Energy Quadratization (S-IEQ) Approach
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Xiaofeng Yang

► Convergence of stochastic-extended Lagrangian molecular dynamics method for polarizable force field simulation
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Dong An, Sara Y. Cheng, Teresa Head-Gordon, Lin Lin, Jianfeng Lu

► An arbitrary high-order Spectral Difference method for the induction equation
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Maria Han Veiga, David A. Velasco-Romero, Quentin Wenger, Romain Teyssier

► Active training of physics-informed neural networks to aggregate and interpolate parametric solutions to the Navier-Stokes equations
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Christopher J. Arthurs, Andrew P. King

► An ADI-Yee's scheme for Maxwell's equations with discontinuous coefficients
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Shaozhong Deng, Zhilin Li, Kejia Pan

► Multi-fidelity Bayesian neural networks: Algorithms and applications
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Xuhui Meng, Hessam Babaee, George Em Karniadakis

► An order-adaptive compact approximation Taylor method for systems of conservation laws
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): H. Carrillo, E. Macca, C. Parés, G. Russo, D. Zorío

► On generalized residual network for deep learning of unknown dynamical systems
    

Publication date: 1 August 2021

Source: Journal of Computational Physics, Volume 438

Author(s): Zhen Chen, Dongbin Xiu

Journal of Turbulence top

► Transition from axisymmetric to three-dimensional turbulence
    7 May, 2021
.
► Special issue on rotating turbulence
    4 May, 2021
Volume 22, Issue 4-5, April - May 2021, Page 231-231
.
► A study of the influence of coflow on flame dynamics in impinging jet diffusion flames
  26 Apr, 2021
.
► Sidewall controlling large-scale flow structure and reversal in turbulent Rayleigh-Bénard convection
  26 Apr, 2021
.
► Spatial-scaling method and modified large eddy simulation to examine rough-wall turbulence
  26 Apr, 2021
.
► Rotating turbulence
  17 Mar, 2021
Volume 22, Issue 4-5, April - May 2021, Page 232-241
.
► Tornado-like vortices in the quasi-cyclostrophic regime of Coriolis-centrifugal convection
  15 Mar, 2021
Volume 22, Issue 4-5, April - May 2021, Page 297-324
.
► Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification
    4 Mar, 2021
.
► Impulse response of turbulent flow in smooth and riblet-walled channels to a sudden velocity increase
  16 Feb, 2021
.
► The geostrophic regime of rapidly rotating turbulent convection
  26 Jan, 2021
Volume 22, Issue 4-5, April - May 2021, Page 267-296
.

Physics of Fluids top

► Modeling of dusty gas flows due to plume impingement on a lunar surface
    7 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
A novel, two-way coupled, dusty-gas flow model has been developed in the direct simulation Monte Carlo (DSMC) framework and employed for the dust-dispersion study on lunar surface. In this model, the gas–gas collisions are modeled probabilistically, whereas, grain–grain interactions are computed deterministically. Most importantly, the gas–grain interactions are modeled in a two-way coupled manner through the consideration of momentum and energy exchange between the two phases. The proposed model is validated against the two-phase theoretical relations for a zero-dimensional simulation. The computational model is used to study the dust dispersion problem due to plume impingement on lunar surface. The influence of particle diameter and hovering altitudes on gas and grain phases, and dust transportation are analyzed in the modified DSMC framework. Furthermore, the sensitivity of the two-way coupled gas–grain interaction model is discussed in relation to the one-way coupled model.
► Robust and unstable axisymmetric vortices, including neutral vortices, of a new two-dimensional vortex family
    7 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
Solutions of robust axisymmetric neutral vortices, that is, vortices with zero amount of vorticity, in two-dimensional (2D) Euler flows with distributed vorticity are obtained. These solutions are particular linear combinations of vorticity layer-modes, which are defined as truncated, shifted, and conveniently normalized Bessel functions of order-0, each one occupying a circular layer defined by a zero of the Bessel function of order-1. It is found that some linear combinations of these modes have a vanishing net amount of vorticity and remain axysimmetrically robust to small amplitude vorticity perturbations. These neutral vortices are quiescent and remain steady in the presence of similar vortices. Other linear combinations of these vorticity layer-modes give rise to unstable neutral vortices that develop into neutral tripoles, pentapoles, etc. It is found numerically that the robustness of these neutral vortices is related to the spiralization and axisymmetrization of the initially growing vorticity disturbances as are advected by a convex azimuthal velocity distribution beyond its first inflection point. In particular, it is found that two co-rotating neutral tripoles attract due to the phase synchronization of their respective octupolar potential flow but repel when touched due to vorticity exchange. This interaction mechanism makes possible equilibrium states for sets of a large number of neutral tripoles. Other linear combinations of these vorticity layer-modes give rise to non-neutral shielded vortices which interact and may form coherent vortex structures as pairs of co-rotating shielded vortices sharing their outermost vorticity layer or counter-rotating shielded vortices translating with uniform speed as vortex dipoles.
► A prediction model for vertical turbulence momentum flux above infinite wind farms
    7 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
Large wind farms can significantly change the vertical layered structures and some of the statistical characteristics of the atmospheric boundary layer (ABL). The vertical turbulence momentum flux (VTMF) above a wind farm, which quantifies the vertical transport of the ABL, is important to meteorological simulation and power absorption of the wind farm. However, we still lack a fast prediction model for the VTMF. To this end, a suite of large-eddy simulations (LESs) is performed for infinite wind farms with various turbine positionings. We show that, in the outer layer above a wind farm, the VTMF normalized by the wind farm's equivalent frictional velocity exhibits a linear relationship with height, which agrees well with the linear law for the canonical rough wall. In contrast, in both the wake layer and the inner layer, the VTMF is significantly dependent on the turbine positionings. Consequently, a prediction model for the VTMF in the outer layer of the ABL is proposed only using the mean velocity in the inner layer of the ABL (below the wind rotors). The kinetic energy transport downward to wind farms is also calculated using the proposed model.
► Manipulation of toroidal-spiral particles internal structure by fluid flow
    7 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
We report on the precise manipulation of the fine structures of toroidal-spiral particles (TSPs) generated by a self-assembly process of droplet sedimentation at low Reynolds numbers in a miscible bulk solution followed by solidification. The biocompatible polymeric TSP can serve as a device for drug delivery and in vivo therapeutic cell expansion, activation, and delivery, for which highly tunable and reproducible structures are essential to design dosages and release kinetics. TSP formation can be divided into two stages: initial infusion of the drop vs its subsequent sedimentation, deformation, and entrainment of the surrounding bulk solution. The infusion rate affects the drop shape and tail length. These two features represent crucial initial conditions for subsequent shape evolution, which determines the overall morphology of the TSP and fine structure of the internal channel. Our computer simulations of drop dynamics add a new capability to the swarm-of-Stokeslets technique: unequal viscosities of the drop and bulk phases (i.e., non-unit viscosity ratio). During sedimentation, the density difference between the droplet and the bulk solution played a more pronounced role than the viscosity ratio, which was revealed both by experimental observations and numerical simulations. Understanding the fundamental hydrodynamics and developing a flow map will ultimately aid in the design of TSPs with tunable empty channels toward drug delivery and cell encapsulation.
► Boltzmann's colloidal transport in porous media with velocity-dependent capture probability
    7 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
Mathematical modeling of suspension-colloidal-nano transport in porous media at different scales has long been a fascinating topic of fluid mechanics. In this study, we discuss the multi-pore scale, where Boltzmann's approach of distributed velocities is valid, and average (homogenize) the micro-scale equation up to the core scale. The focus is on the filtration function (particle capture probability per unity trajectory length) that highly depends on the carrier fluid velocity. We develop a modified form of the Boltzmann equation for micro-scale particle capture and diffusion. An equivalent sink term is introduced into the kinetic equation instead of non-zero initial data, resulting in the solution of an operator equation in the Fourier space and an exact homogenization. The upper scale transport equation is obtained in closed form. The upscaled model contains the dimensionless delay number and large-scale dispersion and filtration coefficients. The explicit formulas for the large-scale model coefficients are derived in terms of the micro-scale parameters for any arbitrary velocity-dependent filtration function. We focus on three micro-scale models for the velocity-dependent particle capture rate corresponding to various retention mechanisms, i.e., straining, attachment, and inertial capture. The explicit formulas for large-scale transport coefficients reveal their typical dependencies of velocity and the micro-scale parameters. Treatment of several laboratory tests reveals close match with the modeling-based predictions.
► Growth of barchan dunes of bidispersed granular mixtures
    6 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
Barchans are dunes of crescentic shape found on Earth, Mars, and other celestial bodies, growing usually on polydisperse granular beds. In this Letter, we investigate experimentally the growth of subaqueous barchans consisting of bidisperse grains. We found that the grain distribution within the dune changes with the employed pair, and that a transient stripe appears on the dune surface. We propose that observed patterns result from the competition between fluid entrainment and easiness of rolling for each grain type, and that grains segregate with a diffusion-like mechanism. Our results provide new insights into barchan structures found in other environments.
► An efficient deep learning framework to reconstruct the flow field sequences of the supersonic cascade channel
    6 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
Accurate and comprehensive flow field reconstruction is essential for promptly monitoring the flow state of the supersonic cascade. This paper proposes a novel data-driven method for reconstructing the slices of the two-dimensional (2D) pressure field in three-dimensional (3D) flow of the supersonic cascade by using deep neural networks. Considering the complicated spatial effects of 2D pressure field slices, the architecture embeds the convolution into the long short-term memory (LSTM) network to realize the purpose of using the upstream pressure to reconstruct downstream pressure. Numerical simulations of the supersonic cascade under different back pressures are performed to establish the database capturing the complex relationship between the upstream and downstream flow. The pressure of different upstream slices can be used as a spatial-dependent sequence as the input of the model to reconstruct the pressure of different downstream slices. A deep neural network including special convolutional LSTM layers and convolutional layers is designed. The trained model is then tested under different back pressures. The reconstruction results are in good agreement with the computational fluid dynamics, especially for the identification of shock wave position changes and the recognition of complex curved shock waves in 3D flow with high accuracy. Moreover, analyzing the frequency distribution of reconstructed pressure at different positions can clearly distinguish the flow separated zone, which will further improve the accuracy of the state monitoring. Specifically, it is of great significance for identifying the stall of the flow field promptly.
► Statistical properties of a model of a turbulent patch arising from a breaking internal wave
    6 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
The turbulent patch arising from internal gravity wave breaking is investigated with direct numerical simulation of a stably stratified flow over a two-dimensional hill. The turbulent patch is distinguished from the non-turbulent wave region with potential vorticity. The turbulent patch is highly intermittent, and its location fluctuates with space and time. The buoyancy Reynolds number slowly decays with time in the turbulent patch and the mixing efficiency stays around 0.2. The turbulent patch is separated from the non-turbulent wave region by a turbulent/non-turbulent interfacial (TNTI) layer, whose thickness is about five times the Kolmogorov scale. The kinetic energy dissipation rate also sharply decreases from the turbulent to the wave region while the potential energy dissipation rate has a large peak within the TNTI layer. Both shear and stable stratification are strong in the upper area of the turbulent patch. On the other hand, the lower area has a small mean density gradient, i.e., weak stratification, which is related to the strong intermittency of the turbulent patch in the lower area. Furthermore, weak stratification in the lower area results in a low gradient Richardson number, which is below the critical value for the shear instability, and the roller vortex appears. The outer edge of the turbulent patch aligns with the perimeter of the roller vortex, and the vortex affects the spatial distribution of the turbulent patch.
► Mass-balance and locality versus accuracy with the new boundary and interface-conjugate approaches in advection-diffusion lattice Boltzmann method
    6 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
We introduce two new approaches, called A-LSOB and N-MR, for boundary and interface-conjugate conditions on flat or curved surface shapes in the advection-diffusion lattice Boltzmann method (LBM). The Local Second-Order, single-node A-LSOB enhances the existing Dirichlet and Neumann normal boundary treatments with respect to locality, accuracy, and Péclet parametrization. The normal-multi-reflection (N-MR) improves the directional flux schemes via a local release of their nonphysical tangential constraints. The A-LSOB and N-MR restore all first- and second-order derivatives from the nodal non-equilibrium solution, and they are conditioned to be exact on a piece-wise parabolic profile in a uniform arbitrary-oriented tangential velocity field. Additionally, the most compact and accurate single-node parabolic schemes for diffusion and flow in grid-inclined pipes are introduced. In simulations, the global mass-conservation solvability condition of the steady-state, two-relaxation-time (S-TRT) formulation is adjusted with either (i) a uniform mass-source or (ii) a corrective surface-flux. We conclude that (i) the surface-flux counterbalance is more accurate than the bulk one, (ii) the A-LSOB Dirichlet schemes are more accurate than the directional ones in the high Péclet regime, (iii) the directional Neumann advective-diffusive flux scheme shows the best conservation properties and then the best performance both in the tangential no-slip and interface-perpendicular flow, and (iv) the directional non-equilibrium diffusive flux extrapolation is the least conserving and accurate. The error Péclet dependency, Neumann invariance over an additive constant, and truncation isotropy guide this analysis. Our methodology extends from the d2q9 isotropic S-TRT to 3D anisotropic matrix collisions, Robin boundary condition, and the transient LBM.
► Numerical analysis of two-phase flow in heterogeneous porous media during pre-flush stage of matrix acidizing: Optimization by response surface methodology
    5 May, 2021
Physics of Fluids, Volume 33, Issue 5, May 2021.
Oil trapping behavior during the pre-flush stage is critically important to evaluate the effectiveness of matrix acidizing for the oil well stimulation. In this study, the visco-capillary behavior of the two-phase flow in the pore-scale is analyzed to investigate the influence of wetting properties for a natural rock sample. A two-dimensional model, based on Cahn–Hilliard phase-field and Navier–Stokes equations, was established and solved using the finite element method. A stability phase diagram for log capillary number (Ca)–log viscosity ratio (M) was constructed and then compared with the reported experimental works. The maximum and minimum ranges of capillary number and viscosity ratio to identify both viscous and capillary fingering regions were found to be Log M ≈ −2.5, Log Ca ≈ −5, and Log M ≈ −0.5, Log Ca ≈ −5, respectively. However, the most stable displacement region was found to be located at Log M ≈ 0.5 and Log Ca ≈ −2. Furthermore, the impact of four independent variables, including pore volume of injection (1 < PV < 5), capillary number (−6 < Log Ca < 0), viscosity ratio (−5 < Log M < 2), and contact angle ([math]), on recovery factor (RF) was investigated using central composite design of response surface methodology. For the chosen range of independent variables, the optimum conditions for the immiscible two-phase flow (e.g., RF > 0.95) occurred at Log M > 0, −4.5 < Log Ca < −2, PV > 1, θ > π/6 condition. It is worth mentioning that for Log M< 0, the optimum condition occurred at Log M ≈ 0, Log Ca ≈ −3.5, PV ≈ 4, and θ ≈ π/6.

Theoretical and Computational Fluid Dynamics top

► The implications of two outlet boundary conditions on blood flow simulations in normal aorta of pediatric subjects
    9 Apr, 2021

Abstract

The blood flow in a normal aorta is simulated directly based on patient-specific data for boundary conditions. This study provides insight into the implications of using two different outlet boundary conditions in accurate modeling of the blood flow based on using pressure or flow rate outlet boundary conditions. Both boundary conditions at the outlet provided similar blood flow characteristics through the main aortic pathway with a reasonable accuracy of 4% relative to the laboratory measurements. Compared to the pressure outlet boundary condition, however, specifying the flow rate at the outlet underestimates pressure in the aortic arch and the flow rates through the outlets of the aortic arch. Additionally, using the flow rate boundary condition leads to an overestimation of peak local Reynolds Number in the aortic arch, and an inaccurate prediction of transition to turbulence.

► Intermittent statistics of the 0-mode pressure fluctuations in the near field of Mach 0.9 circular jets at low and high Reynolds numbers
    1 Apr, 2021

Abstract

The present paper reports an investigation of the statistical properties of pressure fluctuations in the near field of subsonic compressible jets. The data-base analyzed has been obtained numerically by DNS and LES of two single-stream circular jets, having diameter-based Reynolds numbers of 3125 and 100,000 and Mach number 0.9, respectively, initially laminar and highly disturbed. Pressure fluctuations are extracted from several virtual probes positioned in the near field of the jets and covering a region from 0 to 20 diameters in the axial direction and from 0.5 to 3 diameters in the radial. An azimuthal decomposition of the pressure fluctuations is performed, and the statistical analysis is applied to the axisymmetric 0-mode component and compared to the results obtained from the full original signals. The intermittent behavior is investigated by the estimation of standard statistical indicators, such as probability distribution functions and flatness factor, as well as through conditional statistics based on the application of the wavelet transform. It is shown that downstream of the potential core, intermittency estimated through the traditional indicators is relevant even at the lowest Re for the full signals, whereas it is apparently not significant for the 0-mode component. The wavelet analysis provides an estimation of intermittency scale-by-scale and allows for the calculation of a frequency-dependent FF. This approach reveals that the 0-mode component has a relevant degree of intermittency around the frequencies associated with the Kelvin–Helmholtz instability. The statistics of the intermittent events, in terms of their temporal appearance and energy content, are shown to be weakly sensitive to the jet Reynolds number and the universal behavior can be reproduced by simple stochastic models.

► On the boundary conditions in the Stokesian flows
    1 Apr, 2021

Abstract

The problem of the boundary condition setting is considered for creeping flows over cylindrical and spherical obstacles. The interaction of Newtonian and micropolar liquid with the solid surface is discussed in the context of the Stokes paradox and the cell model technique. Mathematical and mechanical aspects of various types of boundary conditions at the hypothetical liquid surface are considered in the framework of the spherical cell model used for the simulation of membrane flows. New properties of the flow pattern in a spherical cell are found, and their independence of the boundary conditions is rigorously proved. The criteria of the boundary conditions equivalence are derived in terms of the membrane porosity and hydrodynamic permeability.

► The impact of steady streaming and conditional turbulence on gas transport during high-frequency ventilation
    1 Apr, 2021

Abstract

High-frequency ventilation is a type of mechanical ventilation therapy applied on patients with damaged or delicate lungs. However, the transport of oxygen down, and carbon dioxide up, the airway is governed by subtle transport processes which hitherto have been difficult to quantify. We investigate one of these mechanisms in detail, nonlinear mean streaming, and the impact of the onset of turbulence on this streaming, via direct numerical simulations of a model 1:2 bifurcating pipe. This geometry is investigated as a minimal unit of the fractal structure of the airway. We first quantify the amount of gas recirculated via mean streaming by measuring the recirculating flux in both the upper and lower branches of the bifurcation. For conditions modeling the trachea-to-bronchi bifurcation of an infant, we find the recirculating flux is of the order of 3–5% of the peak flux . We also show that for conditions modeling the upper generations, the mean recirculation regions extend a significant distance away from the bifurcation, certainly far enough to recirculate gas between generations. We show that this mean streaming flow is driven by the formation of longitudinal vortices in the flow leaving the bifurcation. Second, we show that conditional turbulence arises in the upper generations of the airway. This turbulence appears only in the flow leaving the bifurcation, and at a point in the cycle centered around the maximum instantaneous flow rate. We hypothesize that its appearance is due to an instability of the longitudinal-vortices structure.

► Morphological evolution and spreading flow of hollow oil droplet impact on a heated wall
    1 Apr, 2021

Abstract

A coupled level set and volume–of–fraction method is applied to investigate hollow oil droplet impacts on heated walls. Results show that given the increase in impact velocity, three evolutionary processes of spreading, transition, and central jet occur after the hollow oil droplet impact on a heated wall. The variation in the spreading length of hollow oil droplets is similar in different evolutionary processes, but the variation in the center height of hollow oil droplets is relatively different. The wall heat flux and the position of the maximum heat flux increase with impact velocity. In addition, the wall temperature influences the flow and heat transfer characteristics of the hollow oil droplet impingement. Considering the viscosity–temperature characteristics of the lubricating oil, the spreading length of the hollow oil droplet increases with the wall temperature, but the central height of the hollow oil droplet is unaffected by the wall surface temperature. The wall heat flux and the position of the maximum heat flux also increase with the impact velocity. Pressure and velocity distribution indicate that the bubble rupture at the central jet originates from the combined effect of inertial force and surface tension. The results of this study provide a basis for an improved understanding of the flow and heat transfer characteristics of hollow oil droplet impact on a heated wall and serve as a theoretical reference for investigating the effect of bubbles on oil–gas lubrication processes.

► Breakup a droplet passing through an obstacle in an orthogonal cross-section microchannel
    1 Apr, 2021

Abstract

In this work, the breakup of a droplet passing through an obstacle in an orthogonal cross section is numerically investigated. The relevant boundary data of the velocity field is numerically computed by solving the depth-averaged Brinkman equation via a self-consistent integral equation using the boundary element method. To study the dependence of the droplet breakup on the obstacle shape, two different shapes of obstacle, circular and elliptical, are considered in the present work. We investigate the effect of obstacle size, obstacle position, and capillary number on the breakup treatment of the droplet. Numerical results indicate that the critical capillary number depends on the obstacle shape, obstacle position and droplet size. In the elliptical obstacle, in addition, the results also show that the area ratio of daughter droplets depends on the capillary number. Results show that the area ratio of daughter droplets depends on the capillary number, obstacle shape, and obstacle position. Our results is in a good agreement with the previous studies.

► A calculus for flows in periodic domains
    1 Apr, 2021

Abstract

Purpose: We present a constructive procedure for the calculation of 2-D potential flows in periodic domains with multiple boundaries per period window.

Methods: The solution requires two steps: (i) a conformal mapping from a canonical circular domain to the physical target domain, and (ii) the construction of the complex potential inside the circular domain. All singly periodic domains may be classified into three distinct types: unbounded in two directions, unbounded in one direction, and bounded. In each case, we use conformal mappings to relate the target periodic domain to a canonical circular domain with an appropriate branch structure.

Results: We then present solutions for a range of potential flow phenomena including flow singularities, moving boundaries, uniform flows, straining flows and circulatory flows.

Conclusion: By using the transcendental Schottky-Klein prime function, the ensuing solutions are valid for an arbitrary number of obstacles per period window. Moreover, our solutions are exact and do not require any asymptotic approximations.

► Under-resolved and large eddy simulations of a decaying Taylor–Green vortex with the cumulant lattice Boltzmann method
    1 Apr, 2021

Abstract

We present a comprehensive analysis of the cumulant lattice Boltzmann model with the three-dimensional Taylor–Green vortex benchmark at Reynolds number 1600. The cumulant model is investigated in several different variants, using regularization, fourth-order convergent diffusion and fourth-order convergent advection with and without limiters. In addition, a cumulant model combined with a WALE sub-grid scale model is being evaluated. The turbulence model is found to filter out the high wave number contributions from the energy spectrum and the enstrophy, while the non-filtered cumulant methods show good correspondence to spectral simulations even for the high wave numbers. The application of the WALE turbulence model appears to be counter productive for the Taylor–Green vortex at a Reynolds number of 1600. At much higher Reynolds numbers ( \({\hbox {Re}}=160{,}000\) ) a deviation from the ideal Kolmogorov theory can be observed in the absence of an explicit turbulence model. Cumulant models with fourth-order convergent diffusion show much better results than single relaxation time methods.

► Global transition dynamics of flow in a lid-driven cubical cavity
  19 Mar, 2021

Abstract

The dynamics of a fully three-dimensional lid-driven cubical cavity (3D-LDC) flow at several post-critical conditions, i.e., beyond the first bifurcation, are elucidated using both linear and nonlinear analyses. When the Reynolds number is increased beyond the critical value, symmetry breaks down intermittently with subsequent gradual growth in spanwise inhomogeneity. This results in crossflow as well as pronounced secondary flow due to enhanced imbalance between centrifugal and pressure forces. Thus, while a stable solution is obtained at \(\hbox {Re}=1900\) (Reynolds number based on lid velocity and cavity side length), nonlinear analysis identifies intermittent and nearly saturated regimes at \(\hbox {Re}=2100\) and \(\hbox {Re}=3000\) , respectively. These changes in the regime are examined by considering five basic states at different Reynolds numbers starting from \(\hbox {Re}=1900\) . At the lowest Reynolds number, linear analysis yields only symmetric modes, characterized by Taylor–Görtler-like (TGL) vortices. However, at \(\hbox {Re}=2100\) , the intermittent breakdown of symmetry results in the emergence of an antisymmetric low-frequency mode apart from primary high-frequency mode. The frequencies of both these modes are numerically close to those obtained from corresponding nonlinear simulations. When the Reynolds number is increased even further, the TGL structures drift under the influence of the crossflow to occupy the previously structureless region near the wall. The frequency of each mode decreases with increase in \(\hbox {Re}\) ; between 1900 and 3000, the frequency of the primary mode changes by more than 20%. Furthermore, the spatial support of each mode becomes larger within the cavity. Both primary and secondary modes are increasingly destabilized with \(\hbox {Re}\) ; however, the secondary antisymmetric mode is destabilized at a higher rate. The current study thus provides a comprehensive picture of the overall dynamics of 3D-LDC flows in pre- and post-bifurcation regimes in an extended \(\hbox {Re}\) range not considered hitherto.

► The three-dimensional instabilities and destruction of the viscous Hill’s vortex
  11 Mar, 2021

Abstract

Hill’s vortex is a three-dimensional vortex structure form-preserving solution of the Euler equations (Hill in Philos Trans R Soc Lond A 185:213–245, 1894). For small amplitude axisymmetric disturbances on the external surface, the linear stability analysis by Moffat and Moore (J Fluid Mech 87:749–760, 1978) predicted the formation of a tail. Successive linear and nonlinear investigations confirmed this fact and in addition observed that the shape of the tail was linked to number of small amplitude azimuthal disturbances of the surface. In this paper, the Navier–Stokes equations are solved, at high Reynolds number, by imposing large amplitude axisymmetric and three-dimensional disturbances on the surface of the vortex. The axisymmetric disturbances are convected in the rear side, are dumped and form an axisymmetric wave increasing at the same rate as that in the linear stability analysis. The azimuthal disturbances produce a hierarchy of structures inside the vortex, and in a short-time evolution, the shape of the vortex is maintained. For a long-time evolution, direct numerical simulations show that Hill’s vortex for azimuthal disturbances loses its original form for the formation of a wide range of energy containing scales characteristic of three-dimensional flows. Although a true turbulent state has not been reached, the DNS of this simple vortex structure shows the passage from a vortex dominated to a turbulent state.


return

Layout Settings:

Entries per feed:
Display dates:
Width of titles:
Width of content: