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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

► An exact non-equilibrium extrapolation scheme for pressure and velocity boundary conditions with large gradients in the lattice Boltzmann method
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Long Ju, Baochao Shan, Zhou Yang, Zhaoli Guo

► Droplet spreading dynamics on hydrophobic textured surfaces: A lattice Boltzmann study
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Eslam Ezzatneshan, Aliasghar Khosroabadi

► On shear layer atomization within closed channels: Numerical simulations of a cough-replicating experiment
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): César Pairetti, Raphaël Villiers, Stéphane Zaleski

► High resolution central scheme using a new upwind slope limiter for hyperbolic conservation laws
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Zhenyu Cai, Decai Li, Yang Hu, Mingjun Li, Xiangshen Meng

► SLAU2-HLLD numerical flux with wiggle-sensor for stable low mach Magnetohydrodynamics simulations
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Tomohiro Mamashita, Keiichi Kitamura, Takashi Minoshima

► Dynamics of interfacial gravity–capillary waves in three-dimensional fluids of great depth
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Haiyan Li, Zhan Wang

► Vorticity-based polynomial adaptation for moving and deforming domains
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Ramin Ghoreishi, Brian C. Vermeire

► A volume-of-fluid reconstruction based interface sharpening algorithm for a reduced equation model of two-material compressible flow
    

Publication date: 15 December 2021

Source: Computers & Fluids, Volume 231

Author(s): Murray Cutforth, Philip T. Barton, Nikos Nikiforakis

► Application of projection methods to simulating mass transport in reverse osmosis systems
    

Publication date: Available online 13 October 2021

Source: Computers & Fluids

Author(s): Jacob Johnston, Jincheng Lou, Nils Tilton

► Numerical estimation of multiple leakage positions of a marine pollutant using the adjoint marginal sensitivity method
    

Publication date: Available online 13 October 2021

Source: Computers & Fluids

Author(s): Shunsuke Kanao, Toru Sato

International Journal of Computational Fluid Dynamics top

► SPH Viscous Flow Around a Circular Cylinder: Impact of Viscous Formulation and Background Pressure
  11 Oct, 2021
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► Constrained Direct Optimisation of the Geometrical Parameters of a Twisted-Tape for Laminar Flow in a Circular Tube
    5 Oct, 2021
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► Source Term-Based Turbulent Flow Simulation on GPU with Link-Wise Artificial Compressibility Method
    4 Oct, 2021
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► Plasma Sheath Modelling for Computational Aerothermodynamics and Magnetohydrodynamics
  22 Sep, 2021
Volume 35, Issue 5, June 2021
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► Advances in the Numerical Simulation of Hypersonic Flows
  22 Sep, 2021
Volume 35, Issue 5, June 2021
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► Perturbation Analysis of Nonlinear Stages in Hypersonic Transition
  22 Sep, 2021
Volume 35, Issue 5, June 2021
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► On an Axisymmetric Direct Simulation Monte Carlo Method
  22 Sep, 2021
Volume 35, Issue 5, June 2021
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► Development and Optimisation of a DNS Solver Using Open-source Library for High-performance Computing
  14 Sep, 2021
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► A Low-Communication-Overhead Parallel DNS Method for the 3D Incompressible Wall Turbulence
    8 Sep, 2021
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► Gas-Kinetic Scheme Coupled with Turbulent Kinetic Energy Equation for Computing Hypersonic Turbulent and Transitional Flows
  13 Jul, 2021
Volume 35, Issue 5, June 2021
.

International Journal for Numerical Methods in Fluids top

► Cell‐based Hybrid Adaptive Mesh Refinement Algorithm for Immersed Boundary Method
  21 Oct, 2021

Abstract

This paper proposes a hybrid adaptive mesh refinement (AMR) algorithm for the immersed boundary method (IBM) combination, and the AMR code is split into mesh and physics codes to optimize each part individually. The uniform parent grid solver is used for AMR grids by constructing hanging cells for the high-order extension. The restrictive spatial refinement in a fully threaded tree (FTT) data structure is explored, and a simplified stencil search algorithm for hanging cells construction is introduced, including current and following child AMR level cells, if any. The proposed AMR method was applied to IBM, which offers flexibility in treating complex geometries in the Cartesian grid, leading to algorithmic simplicity and computational efficiency. Local near immersed boundary refinement is proposed to avoid complex and computationally expensive IBM and AMR algorithms near the solid bodies. Finally, a high-order flux scheme extension at AMR level transition cells and the proposed method's applicability for steady and transient flows are demonstrated. Besides state and flux variables storage, the proposed hybrid AMR method's additional cost is 4.5 words/cell instead of 3.5 words/cell for 2D and 3.75 words/cell instead of 2.75 words/cell for 3D compared to the conventional FTT-based AMR method. By comparing the uniform grid, the AMR final grid distribution shows that the optimal cell distribution based on flow physics reduces the cells required by more than 40% to resolve the complex flow features in less computational time. The enhanced performance and accuracy of the proposed AMR method in resolving different scale flow features are validated through benchmark problems.

► On the accuracy, robustness, and performance of high order interpolation schemes for the overset method on unstructured grids
  18 Oct, 2021
On the accuracy, robustness, and performance of high order interpolation schemes for the overset method on unstructured grids

For a second-order finite volume code, on diffusion dominant flows, interpolation schemes higher than second order are needed not to increase the total discretization errors. For convection dominated flows, the results suggest that second order schemes are sufficient to prevent overset schemes tainting discretization errors. Considering performance, the total overset-grid interpolation over-head (including donor search) may be less than 4% of the total run time for second-order schemes and 8% for third-order ones, therefore empowering higher-order schemes and more accurate solutions.


Abstract

A comprehensive study on interpolation schemes used in overset grid techniques is here presented. Based on a literature review, numerous schemes are implemented, and their robustness, accuracy, and performance are assessed. Two code verification exercises are performed for this purpose: a 2D analytical solution of a laminar Poiseuille steady flow; and an intricate manufactured solution of a turbulent flow case, characteristic of a boundary layer flow combined with an unsteady separation bubble. For both cases, the influence of grid layouts, grid refinement, and time-step is investigated. Local and global errors, convergence orders, and mass imbalance are quantified. In terms of computational performance, strong scalability, cpu timings, load imbalancing, and domain connectivity information (DCI) overhead are reported. The effect of the overset-grid interpolation schemes on the numerical performance of the solver, that is, number of nonlinear iterations, is also scrutinized. The results show that, for a second order finite volume code, once diffusion is dominant (low Reynolds number), interpolation schemes higher than second order, for example, least squares of degree 2, are needed not to increase the total discretization errors. For convection dominated flows (high Reynolds numbers), the results suggest that second order schemes, for example, nearest cell gradient, are sufficient to prevent overset grid schemes to taint the underlying discretization errors. In terms of performance, by single-process and parallel communication optimization, the total overset-grid overhead (with DCI done externally to the CFD code) may be less than 4% of the total run time for second-order schemes and 8% for third-order ones, therefore empowering higher-order schemes and more accurate solutions.

► High‐order implicit time‐stepping with high‐order central essentially‐non‐oscillatory methods for unsteady three‐dimensional computational fluid dynamics simulations
  15 Oct, 2021
High-order implicit time-stepping with high-order central essentially-non-oscillatory methods for unsteady three-dimensional computational fluid dynamics simulations

Parallel high-order implicit time-stepping methods coupled with a fourth-order central essentially-non-oscillatory (CENO) finite volume scheme are developed to handle stiff three-dimensional inviscid, viscous, and magnetohydrodynamic flows in a Newton–Krylov–Schwarz framework. A fourth-order accurate explicitly singly diagonally implicit Runge–Kutta (ES-DIRK4) method is shown to be robust and efficient in providing speedups of up to 150 compared with explicit RK methods for stiff large-scale problems. Backward differentiation formulas (BDF) and Rosenbrock-type methods, however, lack stability, robustness, or accuracy on fine grids.


Abstract

This article develops high-order implicit time-stepping methods combined with the fourth-order central essentially-non-oscillatory (CENO) scheme for stiff three-dimensional computational fluid dynamics problems having disparate characteristic time scales. Both aerodynamic and magnetohydrodynamic problems are considered on three-dimensional multiblock body-fitted grids with hexahedral cells. Several implicit time integration methods of third- and fourth-order accuracy are considered, including the multistep backward differentiation formulas (BDF4), multistage explicitly singly diagonally implicit Runge-Kutta (ESDIRK4), and Rosenbrock-type methods (ROS34POW2). The resulting nonlinear algebraic system of equations is solved via a preconditioned Jacobian-free inexact Newton–Krylov method with additive Schwarz preconditioning using block-based incomplete LU decomposition. The performance of the high-order implicit time-stepping methods on smooth and stiff problems is compared with a standard fourth-order explicit Runge-Kutta (RK4) method. It is shown that the Rosenbrock methods, despite their advantage of only requiring the solution of linear systems, have significant drawbacks in terms of robustness issues for highly nonlinear compressible flows. The implicit BDF4 and ESDIRK4 methods are found to be much more efficient than the explicit fourth-order RK4 method for a stiff resistive magnetohydrodynamic (MHD) problem discretized with the fourth-order CENO method. When applied to the problem of vortex shedding governed by the Navier–Stokes equations, an A-stable ESDIRK4 scheme proved to be the more robust and accurate implicit time-marching scheme and was able to offer significant speedup compared with the RK4 method. Initial results are also shown for high-order implicit time integration applied to two problems with discontinuities. The current study represents the first to achieve high-order implicit time integration for MHD, enabling large time steps and substantial speedups for stiff MHD problems with high-order accuracy, and it also represents the first to establish high-order implicit time integration for high-order CENO in space.

► An explicit finite volume scheme on staggered grids for the Euler equations: Unstructured meshes, stability analysis, and energy conservation
  14 Oct, 2021
An explicit finite volume scheme on staggered grids for the Euler equations: Unstructured meshes, stability analysis, and energy conservation

We set up a finite volume scheme for the Euler equations on staggered unstructured grids with numerical densities, energies, and velocities stored on different locations. This time-explicit method is strongly inspired, on the one hand, from the kinetic framework for the definition of the numerical fluxes, and, on the other hand, from the discrete duality finite volume (DDFV) framework. We exhibit stability conditions that guaranty the positivity of the discrete densities and internal energies; and, remarkably, while the scheme works on the internal energy equation, we can define a discrete total energy which satisfies a local conservation equation.


Abstract

We set up a numerical strategy for the simulation of the Euler equations, in the framework of finite volume staggered discretizations where numerical densities, energies, and velocities are stored on different locations. The main difficulty relies on the treatment of the total energy, which mixes quantities stored on different grids. The proposed method is strongly inspired, on the one hand, from the kinetic framework for the definition of the numerical fluxes, and, on the other hand, from the discrete duality finite volume (DDFV) framework, which has been designed for the simulation of elliptic equations on complex meshes. The time discretization is explicit and we exhibit stability conditions that guaranty the positivity of the discrete densities and internal energies. Moreover, while the scheme works on the internal energy equation, we can define a discrete total energy which satisfies a local conservation equation. We provide a set of numerical simulations to illustrate the behavior of the scheme.

► Development and assessment of an intrusive polynomial chaos expansion‐based continuous adjoint method for shape optimization under uncertainties
  12 Oct, 2021
Development and assessment of an intrusive polynomial chaos expansion-based continuous adjoint method for shape optimization under uncertainties

The intrusive Polynomial Chaos Expansion (iPCE) to the Navier–Stokes equations for laminar and incompressible flows is mathematically developed, along with its continuous adjoint counterpart, to support aerodynamic shape optimization under uncertainties. Two variants of the adjoint equations are developed, which prove to be analytically and numerically equivalent. The primal and adjoint iPCE solvers, developed in OpenFOAM, are validated against Monte Carlo and finite differences, respectively, and are, then, used for the shape optimization of two airfoils with uncertain farfield conditions.


Abstract

This article contributes to the development of methods for shape optimization under uncertainties, associated with the flow conditions, based on intrusive Polynomial Chaos Expansion (iPCE) and continuous adjoint. The iPCE to the Navier–Stokes equations for laminar flows of incompressible fluids is developed to compute statistical moments of the Quantity of Interest which are, then, compared with those obtained through the Monte Carlo method. The optimization is carried out using a continuous adjoint-enabled, gradient-based loop. Two different formulations for the continuous adjoint to the iPCE PDEs are derived, programmed, and verified. Intrusive PCE methods for the computation of the statistical moments require mathematical development, derivation of a new system of governing equations and their numerical solution. The development is presented for a chaos order of two and two uncertain variables and can be used as a guide to those willing to extend this development to a different set of uncertain variables or chaos order. The developed method and software, programmed in OpenFOAM, is applied to two optimization problems pertaining to the flow around isolated airfoils with uncertain farfield conditions.

► Multiscale evaluation method of the drag effect on shallow water flow through coastal forests based on 3D numerical simulations
  11 Oct, 2021
Multiscale evaluation method of the drag effect on shallow water flow through coastal forests based on 3D numerical
 simulations

A method for determining the drag parameter in the 2D shallow water (SW) equation for flows through a coastal forest by conducting 3D numerical simulations (3D NSs) is presented. Following the theory of multiscale modeling, we prepare a local test domain that contains a sufficient number of trees to constitute part of a coastal forest. With various inflow conditions, 3D NSs are conducted for constructing the response surfaces that surrogates the drag parameter in the global SW flow simulations.


Abstract

This study presents a method for determining the drag parameter in the 2D shallow water (SW) equation for flows through a coastal forest by conducting a series of 3D numerical simulations (3D NSs). Following the theory of multiscale modeling, an evaluation method procedure is proposed. We first prepare a local test domain that contains a sufficient number of trees to constitute part of a coastal forest. Then, 3D NSs are conducted in this test domain with various inflow conditions. Based on the corresponding results, the momentum losses over the test domain are converted into the drag parameter of the global SW equation. A response surface of the drag parameter is constructed as a function of the flow conditions. The stabilized finite element method is employed for both the local and the global NSs, and the phase-field method is utilized to represent 3D free surfaces. Comparisons between the 2D SW calculation results and the 3D NS results are also performed to verify the validity of the proposed method.

► Lattice Boltzmann modeling of the capillary rise of non‐Newtonian power‐law fluids
    7 Oct, 2021
Lattice Boltzmann modeling of the capillary rise of non-Newtonian power-law fluids

We proposed a lattice Boltzmann (LB) model to study the capillary rise phenomena of two-phase immiscible non-Newtonian power-law fluids. The present LB model is more stable for the problems with large density difference. The capillary rising process under the influence of gravity, viscosity, contact angle, power-law index and displaced fluids are analyzed in detail.


Abstract

In this work, a lattice Boltzmann (LB) model is proposed to study the capillary rise phenomena of two-phase immiscible non-Newtonian power-law fluids. The main advantage of the LB model is that it not only allows us to solve the problems of two-phase flow with a large density difference, but also can overcome the numerical instability caused by the change of relaxation time in the collision operator. In this model, two LB evolution equations are used to solve the conservative Allen–Cahn equation for capturing phase interface and incompressible Navier–Stokes equations for non-Newtonian power-law fluid dynamics. Some benchmark examples, including a droplet spreading on a smooth wall and two-phase power-law fluid flows between parallel plates, are used to test present LB model, and the results show that the present model is efficient and accurate in the study of two-phase power-law fluid flows. Furthermore, we pay attention to the phenomena of capillary rise of non-Newtonian fluids, and carry out a parametric study on the effects of the gravity, viscosity, contact angle, power-law index, and displaced fluids. The results show that the factors mentioned above have some significant influences on the rising process of the absorbed fluids.

► An iterative method to compute conformal mappings and their inverses in the context of water waves over topographies
    6 Oct, 2021
An iterative method to compute conformal mappings and their inverses in the context of water waves over topographies

In this article we present a numerical approach to compute conformal mappings that requires lower computational time than the Schwarz–Christoffel mapping. The method can be easily extended to solve time-dependent water wave problems with large topographic obstacles. Besides, we present an alternative approach to compute the inverse conformal mapping.


Abstract

An iterative numerical method to compute the conformal mapping in the context of propagating water waves over uneven topographies is investigated. The map flattens the fluid domain onto a canonical strip in which computations are performed. The accuracy of the method is tested by using the MATLAB Schwarz–Christoffel toolbox mapping as a benchmark. Besides, we give a numerical alternative to compute the inverse of the conformal map.

► Economically high‐order unstructured‐grid methods: Clarification and efficient FSR schemes
    6 Oct, 2021
Economically high-order unstructured-grid methods: Clarification and efficient FSR schemes

We clarify reconstruction-based discretization schemes for unstructured grids and discuss their economically high-order versions that achieve high-order accuracy under certain conditions at little extra cost. The clarification leads to the most economical approaches: the flux-and-solution- reconstruction (FSR) approach with an extended κ-scheme and cheap flux reconstruction formulas, achieving up to 5th-order accuracy (6th-order with zero dissipation) when a grid is regular.


Abstract

In this article, we clarify reconstruction-based discretization schemes for unstructured grids and discuss their economically high-order versions, which can achieve high-order accuracy under certain conditions at little extra cost. The clarification leads to one of the most economical approaches: the flux-and-solution-reconstruction (FSR) approach, where highly economical schemes can be constructed based on an extended κ-scheme combined with economical flux reconstruction formulas, achieving up to fifth-order accuracy (sixth-order with zero dissipation) when a grid is regular. Various economical FSR schemes are presented and their formal orders of accuracy are verified by numerical experiments.

► An efficient and simplified Gay‐Lussac approach in secondary variables form for the non‐Boussinesq simulation of free convection problems
    6 Oct, 2021
An efficient and simplified Gay-Lussac approach in secondary variables form for the non-Boussinesq simulation of free convection problems

An efficient form of the Gay-Lussac approach in the context of the secondary variables, that is, vorticity stream-function is presented for buoyancy driven flows in which the density variations are extended to the advection and convection terms of the momentum and energy equations, respectively. In the proposed formulation, the density ratio is removed from the continuity equation under the traditional Gay-Lussac approach. It is shown that, the proposed simplified Gay-Lussac approach gives identical results to the traditional one with a reduced computational cost. The figures show the difference for different parameters between the traditional and simplified Gay-Lussac approaches in a steady state at Ra = 105 that ranges within 1% of the considered parameters values.


Abstract

The Gay-Lussac (GL) approach is an incompressible-based strategy for non-Boussinesq treatment of the governing equations for free convection problems that is established based on extending the density variations beyond the gravity term. Such a strategy leads to emerging the GL parameter as a non-Boussinesq prefactor of different terms in the governing equations. In this article, the GL approach is expressed/discussed in terms of the secondary variables, that is, vorticity and stream-function, for the first time and a simplified version of this approach is proposed by removing density variations from the continuity equation. The difference of results under the simplified and traditional GL approach ranges within a maximum of 1% for different parameters. The lower computational cost of numerical solution of governing equations in the secondary variables formula and the corresponding convergence rate is scrutinized for the simplified GL approach showing around 25% lower computational cost. The performance of this approach is evaluated at high relative temperature differences against the low Mach number scheme and the Boussinesq approximations. In this respect, natural convection in an annulus cavity is numerically simulated using a CVFEM solver under the aforementioned approximations up to Rayleigh number Ra=105 at Prandtl number Pr=1 and high relative temperature differences (ϵ=0.15 and 0.3). The largest deviations found for either the simplified GL or Boussinesq methods from the low Mach number scheme solution are less than 20% for velocity magnitude, 14% for stream function, 6% for vorticity, and 5% for temperature. Results under the three approximations are also analyzed in terms of the skin friction and local and average Nusselt number, indicating that the Gay-Lussac approach requires some revisions to act more accurately than the classical Boussinesq approximation at high relative temperature differences in natural convection problems, especially within the convection dominated regime.

Journal of Computational Physics top

► Face-based Volume-of-Fluid interface positioning in arbitrary polyhedra
    

Publication date: Available online 25 October 2021

Source: Journal of Computational Physics

Author(s): Johannes Kromer, Dieter Bothe

► A new symmetric linearly implicit exponential integrator preserving polynomial invariants or Lyapunov functions for conservative or dissipative systems
    

Publication date: Available online 25 October 2021

Source: Journal of Computational Physics

Author(s): Lu Li

► Hamilton-Green Solver for the Forward and Adjoint Problems in Photoacoustic Tomography
    

Publication date: Available online 25 October 2021

Source: Journal of Computational Physics

Author(s): Francesc Rul⋅lan, Marta M. Betcke

► A consistent and conservative Phase-Field model for thermo-gas-liquid-solid flows including liquid-solid phase change
    

Publication date: Available online 25 October 2021

Source: Journal of Computational Physics

Author(s): Ziyang Huang, Guang Lin, Arezoo M. Ardekani

► The exact Riemann solver for the shallow water equations with a discontinuous bottom
    

Publication date: Available online 25 October 2021

Source: Journal of Computational Physics

Author(s): Andrey I. Aleksyuk, Maxim A. Malakhov, Vitaly V. Belikov

► Data-driven eigensolution analysis based on a spatio-temporal Koopman decomposition, with applications to high-order methods
    

Publication date: Available online 22 October 2021

Source: Journal of Computational Physics

Author(s): Jiaqing Kou, Soledad Le Clainche, Esteban Ferrer

► Thermodynamically consistent physics-informed neural networks for hyperbolic systems
    

Publication date: Available online 20 October 2021

Source: Journal of Computational Physics

Author(s): Ravi G. Patel, Indu Manickam, Nathaniel A. Trask, Mitchell A. Wood, Myoungkyu Lee, Ignacio Tomas, Eric C. Cyr

► A new FV scheme and fast cell-centered multigrid solver for 3D anisotropic diffusion equations with discontinuous coefficients
    

Publication date: Available online 22 October 2021

Source: Journal of Computational Physics

Author(s): Kejia Pan, Xiaoxin Wu, Hongling Hu, Yunlong Yu, Zhilin Li

► High-order numerical solutions to the shallow-water equations on the rotated cubed-sphere grid
    

Publication date: Available online 22 October 2021

Source: Journal of Computational Physics

Author(s): Stéphane Gaudreault, Martin Charron, Valentin Dallerit, Mayya Tokman

► An HLLC-type approximate Riemann solver for two-dimensional elastic-perfectly plastic model
    

Publication date: 1 January 2022

Source: Journal of Computational Physics, Volume 448

Author(s): Xiao Li, Jiayin Zhai, Zhijun Shen

Journal of Turbulence top

► A multispecies turbulence model for the mixing and de-mixing of miscible fluids
  15 Oct, 2021
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► Large-eddy simulations of the flow on an aerofoil with leading-edge imperfections
  12 Sep, 2021
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► Characteristics of wall-shear stress fluctuations in shock wave and turbulent boundary layer interaction
  12 Sep, 2021
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► A double-averaged Navier-Stokes k – ω turbulence model for wall flows over rough surfaces with heat transfer
  12 Sep, 2021
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► Turbulent drag reduction over liquid-infused textured surfaces: effect of the interface dynamics
  12 Sep, 2021
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► A simplified model for drag evaluation of a streamlined body with leading-edge damage
  12 Sep, 2021
Volume 22, Issue 10, October 2021, Page 656-679
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► Metric inertia for eddy densities of nonlocal matter-space
  23 Aug, 2021
Volume 22, Issue 10, October 2021, Page 623-639
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► Evolution of the invariants of the velocity gradient tensor in the developing region of a round jet using tomographic PIV
  27 Jul, 2021
Volume 22, Issue 10, October 2021, Page 640-655
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► Numerical investigation of the effect of rotation on non-premixed hydrogen combustion in developing turbulent mixing layers
  29 Jun, 2021
Volume 22, Issue 10, October 2021, Page 597-622
.

Physics of Fluids top

► Effects of rotation on vorticity dynamics on a sphere with discrete exterior calculus
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
We investigate incompressible, inviscid vorticity dynamics on a rotating unit sphere using a discrete exterior calculus scheme. For a prescribed initial vorticity distribution, we vary the rotation rate of the sphere from zero [non-rotating case, which corresponds to infinite Rossby number (Ro)] to 320 (which corresponds to Ro = [math]) and investigate the evolution with time of the vorticity field. For the non-rotating case, the vortices evolve into thin filaments due to so-called forward/direct enstrophy cascade. The energy cascades to the larger scales due to the inverse energy cascade, and at late times, an oscillating quadrupolar vortical field emerges. Rotation diminishes the forward cascade of enstrophy (and hence the inverse cascade of energy) and tends to align the vortical structures in the azimuthal/zonal direction. Our investigation reveals that, for the initial vorticity field comprising intermediate-wavenumber spherical harmonics, the zonalization of the vortical structures is not monotonic with ever decreasing Rossby numbers, and the structures revert back to a non-zonal state below a certain Rossby number. On the other hand, for the initial vorticity field comprising intermediate to large-wavenumber spherical harmonics, the zonalization is monotonic with decreasing Rossby number. Although rotation diminishes the forward cascade of enstrophy, it does not completely cease/arrest the cascade for the parameter values employed in the present work.
► Dispersion tensor in a two-phase flow in a slit
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
Taylor's approach on the dispersion phenomenon is generalized for solute transport in a two-phase laminar flow of immiscible fluids in a slit. The reduced-order models for solute transport are derived using Reynolds decomposition and averaging techniques from which the exact analytical expressions for all elements of the dispersion tensor and the matrix of coefficients of the advection term are derived. It is shown that the dispersion tensor is generally not symmetric, and the asymmetry originates from the presence of an interface between the two fluids. We also discussed conditions at which the solute transport in a two-phase laminar flow in a slit lead to dispersion barrier, osmotic dispersion, and reverse dispersion. The results provide a thorough insight into modeling solute transport across an interface/film in two-phase stratified flows and find applications in the design and optimization of microfluidic devices where two fluids flow in laminar contact.
► Particle leakage through the exhalation valve on a face mask under flow conditions mimicking human breathing: A critical assessment
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
In today's era of active personal protections against airborne respiratory disease, general interest in the multiphase flow physics underlying face masks is greater than ever. The exhalation valves, installed on some masks to mitigate the breathing resistance, have also received more attention. However, the current certification protocol of evaluating airflow leakage only when suction pressure is applied is insufficient to capture practical aspects (particle penetration or leakage). Here, we experimentally measure two-phase flow across valve-type masks under conditions mimicking actual breathing. During exhalation, a high-speed jet through the valve accelerates the transmission of particles from inside while reasonable protection from external pollutants is achieved during inhalation, which supports the warnings from various public health officials. Based on the mechanism of particle penetration found here, we hope a novel design that both achieves high-efficiency shielding and facilitates easy breathing can be developed.
► Computational study on the transmission of the SARS-CoV-2 virus through aerosol in an elevator cabin: Effect of the ventilation system
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
Aerosol transmission is now well-established as a route in the spread of the SARS-CoV-2 virus. Factors influencing the transport of virus-laden particles in an elevator cabin are investigated computationally and include human respiratory events, locations of the infected person(s), and the ventilation system (ventilation mode, ventilation capacity, and vent schemes). “Breath,” “cough,” and “sneeze” are defined quantitatively by the fluid jet velocities and particle sizes. For natural ventilation, most particles exhaled by sneezing and coughing tend to deposit on surfaces quickly, but aerosol generated by breathing will remain suspended in the air longer. For forced ventilation, motions of particles under different ventilation capacities are compared. Larger particles otherwise deposited readily on solid surfaces may be slowed down by airflow. Air currents also accelerate the motions of smaller particles, facilitating the subsequent deposition of micrometer or sub-micrometer particles. Locations of the infected person(s) lead to different spreading scenarios due to the distinctive motions of the particles generated by the various respiratory events. Sneeze particles will likely contaminate the person in front of the infected passenger only. Cough particles will increase the risk of all the people around the injector. Breath particles tend to spread throughout the confined environment. An optimized vent scheme is introduced and can reduce particles suspended in the air by up to 80% as compared with commonly used schemes. The purification function of this vent model is robust to various positions of the infected passenger.
► Exponential shock wave in perfectly conducting self-gravitating rotational axi-symmetric dusty gas with magnetic field, radiative and conductive heat fluxes
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
In the present paper, we study the exponential shock propagation in a self-gravitating rotational axisymmetric perfectly conducting mixture of van der Waal gas and solid particles with magnetic field either axial or azimuthal and radiative and conductive heat fluxes. In our model, the solid particles are distributed continuously in the mixture and are chemically inert, and the equilibrium conditions for flow are preserved in the entire region of flow field behind shock wave. In a thick gray gas model case, the radiation is assumed to be of diffusion type. The Fourier's heat conduction law is used to express the heat conduction. The effects of the problem parameters variations are discussed. It is shown that the density of micro size solid particles to the gas initial density ratio or the gravitational parameter or the rotational parameter or the gas adiabatic index has effects to enhance the shock wave strength. Also, it is derived that an increase in the nonidealness of the gas, Alfvén Mach number, and the mass concentration of solid particles in the mixture have decaying effects on the strength of shock wave. It is shown that the shock wave is stronger when magnetic field is axial and weaker for azimuthal magnetic field.
► Forced convective flow and heat transfer past a blunt headed cylinder with corner modification
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
A numerical analysis is performed to elucidate the forced convective fluid flow and heat transfer characteristics past a blunt-headed cylinder. Simulations are carried out employing air (Pr = 0.71) as an operating fluid in a Reynolds numbers range [math]. The curvature ratio is varied from 0 to 0.5. The flow and heat transport features are elucidated in detail for different curvature ratios. Important flow parameters such as boundary layer thickness, vortex strength, wake width, drag, lift, skin friction coefficient, pressure coefficient, Strouhal number, and recirculation length are computed. Also, a primary stability analysis has been carried out using the Landau equation, whereas secondary stability analysis has been done implementing dynamic mode decomposition (DMD) to compute the critical Reynolds number at each curvature ratio. Calculating local and time-averaged values of Nusselt numbers, heat transfer characteristics are studied. An entropy generation analysis is done to investigate the effects of corner modification on the efficacy of thermofluid transport characteristics.
► Breakup regimes of double emulsion droplets in a microfluidic Y-junction
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
The droplet breakup technology can effectively increase the generation throughput and adjust the droplets size, which has an important impact on the performance of the double emulsion droplets in medical, chemical, and other applications. This work presents an experimental study on the breakup regimes of double emulsion droplets after their on-chip generation. Five distinct breakup regimes are categorized according to the breakup times and the existence of the coupling effect during breakup process. Evolutions of the neck widths and thinning rates of both inner droplets and outer droplets are provided to discuss the dynamics of different regimes as well as different stages. In particular, the influences of the coupling effect on the interfacial evolution, collapsing mechanism, force analysis, and breakup critical condition are confirmed by comparisons with the results of single emulsion droplets.
► On the analysis of magnetohydrodynamics and magnetic field-dependent viscosity effects on thermogravitational convection of hybrid nanofluid in an enclosure with curved walls
  25 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
This work deals with the impact of magnetic field-dependent viscosity and magnetohydrodynamics on the natural convection heat transfer of Ag–MgO (50%–50%) water hybrid nanoliquid in a complicated enclosure. The considered cavity with concave or convex horizontal boundaries has been differentially heated and cooled by multiple heat sources and heat sinks. The governing equations (Navier–Stokes equations) constituting stream function (ψ)-vorticity (ζ) formulation with energy equation are solved by adopting a compact finite difference scheme. Meanwhile, the flow domain that is influenced by several factors including Hartmann number ([math]), hybrid nanoparticles volume fraction ([math]), Rayleigh number ([math]), orientation angle of magnetic field ([math]), magnetic number ([math]), different cases (Case-I to III), and internal heat generation or absorption ([math]) is analyzed generously. Experimentally based correlations for thermal conductivity and dynamic viscosity have been used throughout the study. The outcomes show that the geometric parameters can be used as an excellent controller of the thermal performance inside the wavy chamber.
► Mesh-free peridynamic coupled simulation of impacting collapse of a granular column with various heights
  22 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
In this study, a coupled model of Peridynamics into the mesh-free method is extended to simulate the impacting collapse of a granular column with various suspended heights. Experiments on the impacting collapse were conducted to validate the numerical model. It is found that the simulated free surface profiles have a good agreement with the experimental measurements. The numerical model is also validated by simulating the granular column collapse. The simulated kinetic energy and potential energy agree well with the discrete element method (DEM) simulation results in the literature. In the validations, different numerical particle distances are implemented to discrete the fluid and a good numerical convergence is achieved for the numerical method. After model validation, the energy variations in the impacting collapse of the granular column with different suspended heights are analyzed, which include evolutions of the potential energy, horizontal kinetic energy, and vertical kinetic energy. In the analysis, the relative total energy dissipation rate for the flow with various suspended heights is calculated. The analysis shows that the suspended height in the impacting collapse of the granular column can affect the energy dissipation significantly.
► Rotational and reflectional equivariant convolutional neural network for data-limited applications: Multiphase flow demonstration
  22 Oct, 2021
Physics of Fluids, Volume 33, Issue 10, October 2021.
This article deals with approximating steady-state particle-resolved fluid flow around a fixed particle of interest under the influence of randomly distributed stationary particles in a dispersed multiphase setup using convolutional neural network (CNN). The considered problem involves rotational symmetry about the mean velocity (streamwise) direction. Thus, this work enforces this symmetry using SE(3)-equivariant, special Euclidean group of dimension 3, CNN architecture, which is translation and three-dimensional rotation equivariant. This study mainly explores the generalization capabilities and benefits of a SE(3)-equivariant network. Accurate synthetic flow fields for Reynolds number and particle volume fraction combinations spanning over a range of [86.22, 172.96] and [0.11, 0.45], respectively, are produced with careful application of symmetry-aware data-driven approach.

Theoretical and Computational Fluid Dynamics top

► Transient Stokes flow past a spherical droplet with a stagnant cap due to contaminated surfactant layer
    6 Oct, 2021

Abstract

In this paper, we consider a viscous droplet migrating in a viscous fluid of a different viscosity. Further, we assume that the surface of the droplet is partially contaminated with a stagnant layer of surfactant (surface active agent which reduces the interfacial tension). We analyze the effects of the following phenomena associated with the thermocapillary migration of a droplet in a transient Stokes flow. The first is the influence of surfactant cap for an arbitrary cap angle which is partially coated on the droplet surface for both high and low surface Péclet number cases. The second is the influence of the energy changes associated with stretching and shrinkage of the interfacial area elements, when the droplet is in motion. It can be noted that for the vanishing cap angle, both high and low surface Péclet number limits reduce to the case of a pure thermocapillary migration of a droplet in a transient Stokes flow. For a given ambient flow, the migration of the droplet is controlled by the magnitude of the ambient velocity and the surface tension variations due to temperature and surfactant concentration. In particular, these surface tension variations balance the tangential stress balance. Considering axisymmetric transient Stokes flow, we obtain analytical solutions in two limiting cases, namely low and high surface Péclet number. This work considers linear variation of interfacial tension on both thermal and surfactant gradients. The main contribution is pertaining to the capillary drift and the corresponding surfactant transport on the droplet for an axisymmetric hydrodynamic as well as thermal and surfactant fields. We have analyzed the level curves corresponding to stream function and temperature fields, i.e., streamlines and isotherms for various parameters in order to develop a realistic picture of the migration pattern and the influence of thermal fields. We observe that the streamlines in the vicinity of rear end of the droplet show asymmetry due to the surfactant accumulation at that region. Increasing cap angle breaks the symmetry of the induced stream. It is seen that increasing values of nondimensional parameter that accounts for the stretching and shrinkage of the droplet surface immobilizes the surface and offers retardation to the migrating droplet. The variation of migration velocity with time suggests a control mechanism for the migration of the drop under external/surface gradients and hence may serve as a useful tool in applications like targeted drug delivery systems.

► Evaluation of near-singular integrals with application to vortex sheet flow
    1 Oct, 2021

Abstract

This paper presents a method to evaluate the near-singular line integrals that solve elliptic boundary value problems in planar and axisymmetric geometries. The integrals are near-singular for target points not on, but near the boundary, and standard quadratures lose accuracy as the distance d to the boundary decreases. The method is based on Taylor series approximations of the integrands that capture the near-singular behaviour and can be integrated in closed form. It amounts to applying the trapezoid rule with meshsize h, and adding a correction for each of the basis functions in the Taylor series. The corrections are computed at a cost of \(O(n_w)\) per target point, where typically, \(n_w\) =10–40. Any desired order of accuracy can be achieved using the appropriate number of terms in the Taylor series expansions. Two explicit versions of order \(O(h^2)\) and \(O(h^3)\) are listed, with errors that decrease as \(d\rightarrow 0\) . The method is applied to compute planar potential flow past a plate and past two cylinders, as well as long-time vortex sheet separation in flow past an inclined plate. These flows illustrate the significant difficulties introduced by inaccurate evaluation of the near-singular integrals and their resolution by the proposed method. The corrected results converge at the analytically predicted rates.

► Correction to: Unsteady lifting-line theory and the influence of wake vorticity on aerodynamic loads
    1 Oct, 2021

A Correction to this paper has been published: 10.1007/s00162-021-00578-8

► Convolutional neural networks for fluid flow analysis: toward effective metamodeling and low dimensionalization
    1 Oct, 2021

Abstract

We focus on a convolutional neural network (CNN), which has recently been utilized for fluid flow analyses, from the perspective on the influence of various operations inside it by considering some canonical regression problems with fluid flow data. We consider two types of CNN-based fluid flow analyses: (1) CNN metamodeling and (2) CNN autoencoder. For the first type of CNN with additional scalar inputs, which is one of the common forms of CNN for fluid flow analysis, we investigate the influence of input placements in the CNN training pipeline. As an example, estimation of drag and lift coefficients of an inclined flat plate and two side-by-side cylinders in laminar flows is considered. For the example of flat plate wake, we use the chord Reynolds number \(\hbox {Re}_\mathrm{c}\) and the angle of attack \(\alpha \) as the additional scalar inputs to provide the information on the complexity of wake. For the wake interaction problem comprising flows over two side-by-side cylinders, the gap ratio and the diameter ratio are utilized as the additional inputs. We find that care should be taken for the placement of additional scalar inputs depending on the problem setting and the complexity of flows that users handle. We then discuss the influence of various parameters and operations on the CNN performance, with the utilization of autoencoder (AE). A two-dimensional decaying homogeneous isotropic turbulence is considered for the demonstration of AE. The results obtained through the AE highly rely on the decaying nature. Investigation on the influence of padding operation at a convolutional layer is also performed. The zero padding shows reasonable ability compared to other methods which account for the boundary conditions assumed in the numerical data. Moreover, the effect of the dimensional reduction/extension methods inside CNN is also examined. The CNN model is robust against the difference in dimension reduction operations, while it is sensitive to the dimensional extension methods. The findings of this paper will help us better design a CNN architecture for practical fluid flow analysis.

► Lift on Janus and stick spheres in laminar channel flow: a computational study
    1 Oct, 2021

Abstract

The instantaneous motion of a spherical particle in a channel flow is governed by the forces experienced by the particle. The magnitude and direction of the forces depend on the particle to channel size ratio, particle position, nature of the sphere surface (sticky/slippery), fluid properties and relative velocity between the fluid and the particle. In this work, we report the lift, the force component directed normal to the streamwise direction, on two classes of spheres, sticky and Janus, in a channel of square cross-section. The Janus spheres considered have both sticky and slippery hemispheres with the boundary between the two hemispheres parallel to the channel midplane. The effect of particle to channel size ratio, dimensionless particle position and particle Reynolds number on the lift are studied. The Janus sphere placed at the channel centerline is observed to experience the lift directed from the sticky to the slippery hemisphere. A correlation is proposed to predict the lift on the Janus sphere placed at the centerline of the channel. A sticky sphere positioned close to the channel wall experiences a significant lift directed away from it. For the Janus sphere placed at an off-center position two possibilities arise—slippery hemisphere facing the channel centerline (case A) or sticky hemisphere facing the channel centerline (case B). For case A, the lift is always directed away from the wall. For case B, the direction of lift depends on the particle position as well as particle Reynolds number. The moment coefficients for the sticky and Janus sphere are also presented.

► Model-based multi-sensor fusion for reconstructing wall-bounded turbulence
    1 Oct, 2021

Abstract

Wall-bounded turbulent flows can be challenging to measure within experiments due to the breadth of spatial and temporal scales inherent in such flows. Instrumentation capable of obtaining time-resolved data (e.g., hot-wire anemometers) tends to be restricted to spatially localized point measurements; likewise, instrumentation capable of achieving spatially resolved field measurements (e.g., particle image velocimetry) tends to lack the sampling rates needed to attain time resolution in many such flows. In this study, we propose to fuse measurements from multi-rate and multi-fidelity sensors with predictions from a physics-based model to reconstruct the spatiotemporal evolution of a wall-bounded turbulent flow. A “fast” filter is formulated to assimilate high-rate point measurements with estimates from a linear model derived from the Navier–Stokes equations. Additionally, a “slow” filter is used to update the reconstruction every time a new field measurement becomes available. By marching through the data both forward and backward in time, we are able to reconstruct the turbulent flow with greater spatiotemporal resolution than either sensing modality alone. We demonstrate the approach using direct numerical simulations of a turbulent channel flow from the Johns Hopkins Turbulence Database. A statistical analysis of the model-based multi-sensor fusion approach is also conducted.

► Compressible and nonisothermal viscoelastic flow between eccentrically rotating cylinders
    1 Oct, 2021

Abstract

A Taylor–Galerkin finite element time marching scheme is derived to numerically simulate the flow of a compressible and nonisothermal viscoelastic liquid between eccentrically rotating cylinders. Numerical approximations to the governing flow and constitutive equations are computed over a custom refined unstructured grid of piecewise linear Galerkin finite elements. An original extension to the DEVSS formulation for compressible fluids is introduced to stabilise solutions of the discrete problem. The predictions of two models: the extended White–Metzner and FENE-P-MP are presented. Comparisons between the torque and load bearing capacity predicted by both models are made over a range of viscoelastic parameters. The results highlight the significant and interacting effects of elasticity and compressibility on journal torque and resultant load, and the stability of the journal bearing system.

► Data-driven sensor placement for fluid flows
    1 Oct, 2021

Abstract

Optimal sensor placement for fluid flows is an important and challenging problem. In this study, we propose a completely data-driven and computationally efficient method for sensor placement. We use adjoint-based gradient descent to find the sensor location that minimizes the trace of an approximation of the estimation error covariance matrix. The proposed methodology can be used in conjunction with any reduced-order modeling technique that provides a linear approximation of the fluid dynamics. Moreover, the objective function can be augmented for different applications, which we illustrate by proposing a control-oriented objective function. We demonstrate the performance of our method for reconstruction and prediction of the complex linearized Ginzburg–Landau equation in the globally unstable regime. We also construct a low-dimensional observer-based feedback controller for the flow over an inclined flat plate that is able to suppress the wake vortex shedding in the presence of system and measurement noise.

► Unsteady lifting-line theory and the influence of wake vorticity on aerodynamic loads
    1 Oct, 2021

Abstract

Frequency-domain unsteady lifting-line theory (ULLT) provides a means by which the aerodynamics of oscillating wings may be studied at low computational cost without neglecting the interacting effects of aspect ratio and oscillation frequency. Renewed interest in the method has drawn attention to several uncertainties however. Firstly, to what extent is ULLT practically useful for rectangular wings, despite theoretical limitations? And secondly, to what extent is a complicated wake model needed in the outer solution for good accuracy? This paper aims to answer these questions by presenting a complete ULLT based on the work of Sclavounos, along with a novel ULLT that considers only the streamwise vorticity and a Prandtl-like pseudosteady ULLT. These are compared to Euler CFD for cases of rectangular wings at multiple aspect ratios and oscillation frequencies. The results of this work establish ULLT as a low computational cost model capable of accounting for interacting finite-wing and oscillation frequency effects and identify the aspect ratio and frequency regimes where the three ULLTs are most accurate. This research paves the way towards the construction of time-domain or numerical ULLTs which may be augmented to account for nonlinearities such as flow separation.

► Influence of separation structure on the dynamics of shock/turbulent-boundary-layer interactions
  14 Sep, 2021

Abstract

Shock/turbulent-boundary-layer interactions (STBLIs) are ubiquitous in high-speed flight and propulsion applications. Experimental and computational investigations of swept, three-dimensional (3-D) interactions, which exhibit quasi-conical mean-flow symmetry in the limit of infinite span, have demonstrated key differences in unsteadiness from their analogous, two-dimensional (2-D), spanwise-homogeneous counterparts. For swept interactions, represented by the swept–fin-on-plate and swept–compression–ramp-on-plate configurations, differences associated with the separated shear layers may be traced to the intermixing of 2-D (spanwise independent) and 3-D (spanwise dependent) scaling laws for the separated mean flow. This results in a broader spectrum of unsteadiness that includes relatively lower frequencies associated with the separated shear layers in 3-D interactions. However, lower frequency ranges associated with the global “breathing” of strongly separated 2-D interactions are significantly less prominent in these simple, swept 3-D interactions. A logical extension of 3-D interaction complexity is the compound interaction formed by the merging of two simple interactions. The first objective of this work is therefore to analyze the more complex picture of the dynamics of such interactions, by considering as an exemplar, wall-resolved simulations of the double-fin-on-plate configuration. We show that in the region of interaction merging, new flow scales, changes in separation topology, and the emergence of lower-frequency phenomena are observed, whereas the dynamics of the interaction near the fin leading edges are similar to those of the simple, swept interactions. The second objective is to evolve a unified understanding of the dynamics of STBLIs associated with complex configurations relevant to actual propulsion systems, which involve the coupling between multiple shock systems and multiple flow separation and attachment events. For this, we revisit the salient aspects of scaling phenomena in a manner that aids in assimilating the double-fin flow with simpler swept interactions. The emphasis is on the influence of the underlying structure of the separated flow on the dynamics. The distinct features of the compound interactions manifest in a centerline symmetry pattern that replaces the quasi-conical symmetry of simple interactions. The primary separation displays topological closure to reveal new length scales, associated unsteadiness bands, and secondary flow separation.


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