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Annual Review of Fluid Mechanics top

► Dynamic Mode Decomposition and Its Variants
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 225-254, January 2022.
► FLEET Velocimetry for Aerodynamics
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 525-553, January 2022.
► Fluid Dynamics of Axial Turbomachinery: Blade- and Stage-Level Simulations and Models
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 255-285, January 2022.
► Continuum and Molecular Dynamics Studies of the Hydrodynamics of Colloids Straddling a Fluid Interface
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 495-523, January 2022.
► Flow and Drop Transport Along Liquid-Infused Surfaces
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 83-104, January 2022.
► Flood Inundation Prediction
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 287-315, January 2022.
► Physics and Modeling of Large Flow Disturbances: Discrete Gust Encounters for Modern Air Vehicles
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 469-493, January 2022.
► The Influence of Boundaries on Gravity Currents and Thin Films: Drainage, Confinement, Convergence, and Deformation Effects
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 27-56, January 2022.
► Vortex Reconnection and Turbulence Cascade
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 317-347, January 2022.
► Drop Impact Dynamics: Impact Force and Stress Distributions
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 57-81, January 2022.

Computers & Fluids top

► A particle resolved simulation approach for studying shock interactions with moving, colliding solid particles
    

Publication date: Available online 20 September 2022

Source: Computers & Fluids

Author(s): Y. Mehta, R.J. Goetsch, O.V. Vasilyev, J.D. Regele

► Two pressure boundary conditions for multi-component multiphase flow simulations using the pseudo-potential lattice Boltzmann model
    

Publication date: Available online 20 September 2022

Source: Computers & Fluids

Author(s): Zhicheng Wang, Muzammil Soomro, Cheng Peng, Luis F. Ayala, Orlando M. Ayala

► An ensemble Synthetic Eddy Method for accurate treatment of inhomogeneous turbulence
    

Publication date: Available online 22 September 2022

Source: Computers & Fluids

Author(s): Kyle A. Schau, Chelsea Johnson, Julia Muller, Joseph C. Oefelein

► On the use of high order central difference schemes for differential equation based wall distance computations
    

Publication date: Available online 22 September 2022

Source: Computers & Fluids

Author(s): Hemanth Chandra Vamsi Kakumani, Nagabhushana Rao Vadlamani, Paul Gary Tucker

► Three-dimensional study of double droplets impact on a wettability-patterned surface
    

Publication date: Available online 22 September 2022

Source: Computers & Fluids

Author(s): Jiangxu Huang, Lei Wang, Kun He

► A novel high-order solver for simulation of incompressible flows using the flux reconstruction method and lattice Boltzmann flux solver
    

Publication date: Available online 17 September 2022

Source: Computers & Fluids

Author(s): Chao Ma, Jie Wu, Liming Yang

► Assessment of a Point-Cloud Volume-of-Fluid method with sharp interface advection
    

Publication date: Available online 17 September 2022

Source: Computers & Fluids

Author(s): Rodrigo L.F. Castello Branco, Bruno B.M. Kassar, João N.E. Carneiro, Angela O. Nieckele

► Adjoint sensitivity analysis method based on lattice Boltzmann equation for flow-induced sound problems
    

Publication date: Available online 3 September 2022

Source: Computers & Fluids

Author(s): Kazuya Kusano

► Tracking and analysis of interfaces and flow structures in multiphase flows
    

Publication date: Available online 13 September 2022

Source: Computers & Fluids

Author(s): A. Bußmann, J. Buchmeier, M.S. Dodd, S. Adami, I. Bermejo-Moreno

► Skeleton-stabilized divergence-conforming B-spline discretizations for incompressible flow problems of high Reynolds number
    

Publication date: Available online 16 September 2022

Source: Computers & Fluids

Author(s): Guoxiang Grayson Tong, David Kamensky, John A. Evans

International Journal of Computational Fluid Dynamics top

► An Immersed Boundary Method Based on Parallel Adaptive Cartesian Grids for High Reynolds Number Turbulent Flow
  16 Aug, 2022
Volume 36, Issue 4, May 2022, Page 319-341
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► Parallel Domain Decomposition of a FEM-based Tool for Numerical Modelling Mineral Slurry-like Flows
  12 Aug, 2022
Volume 36, Issue 4, May 2022, Page 342-360
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► SPH Modelling of a Dike Failure with Detection of the Landslide Sliding Surface and Damage Scenarios for an Electricity Pylon
  11 Aug, 2022
Volume 36, Issue 4, May 2022, Page 265-293
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► An Investigation of Uncertainty Propagation in Non-equilibrium Flows
    2 Aug, 2022
Volume 36, Issue 4, May 2022, Page 294-318
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► A Conceptual Alternative Machine Learning-Based Method for Mesh Sensitivities Calculation in a Turbomachinery Blades Optimisation Framework
  28 Mar, 2022
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► A correction for Reynolds‐averaged‐Navier‐Stokes turbulence model under the effect of shock waves in hypersonic flows
  27 Sep, 2022

Abstract

Studies on the unphysical increase of turbulent quantities for RANS simulation induced by shock waves in hypersonic flows are carried out. Numerical experiments on the hypersonic flow over a blunt body reveal that the phenomenon of unphysical increase of turbulent quantities across the detached shock wave is induced by the strain-rate-based production terms of the k-w and k-w SST turbulence models, which leads to the over-prediction of aerothermal prediction. While this phenomenon does not occur for Spalart-Allmaras (S-A) turbulence model because of its vorticity-based production term. In order to eliminate this unphysical phenomenon, and to maintain the accuracy of the original models for boundary layer and separation flows, a new correction method for the k-w and k-w SST models is proposed: by comparing the orders of magnitude between the strain-rate-based and vorticity-based production terms, the vorticity-based production term is used near the shock waves, while the original strain-rate-based production term is still used in other regions. Finally, the correction method is applied to turbulence and transition flows over blunt bodies, and the numerical results show that the correction method effectively eliminates the unphysical increase of turbulent quantities across shock waves and improves the accuracy of aerothermal and transition onset location prediction.

► A deforming‐mesh finite‐element approach applied to the large‐translation and free‐surface scenario of fused deposition modeling
  26 Sep, 2022

Abstract

A numerical study of the fused deposition modeling (FDM) process using a boundary-conforming free-surface finite element approach is performed. Due to the complexity of the FDM process, among all of its parts, we focus on the deposition and spreading of an individual filament. The polymer behavior, i.e., the shear rate dependent and temperature-dependent viscosity, is included by the Cross-WLF viscosity model. The moving domain is addressed by the virtual region mesh update method, which, in the present paper, is extended to free-surface problems. The particularity of dividing the mesh domain into an activated and a deactivated domain makes it possible to handle large translatory mesh deformation. In this work, we make use of the level of detail offered by a boundary-conforming approach regarding both topology accuracy and the imposition of boundary conditions in order to study the deposition of a single filament at a small scale. Parameters with a direct impact on the mechanical properties of the final object can be straightforwardly computed by a boundary-conforming approach, for instance, the cross-section, the contact area, the temperature distribution, and the heat fluxes over the surfaces. The presented approach is validated by a two-dimensional benchmark test case before the numerical results of the three-dimensional simulation of the filament deposition are shown.

► Construction of hybrid 1D‐0D networks for efficient and accurate blood flow simulations
  22 Sep, 2022
Construction of hybrid 1D-0D networks for efficient and accurate blood flow simulations

A set of coupling equations to appropriately couple nonlinear 1D and lumped-parameter (0D) models for blood flow in compliant vessels is defined. Then, a methodology for the high-order numerical coupling between 1D and 0D vessels through hybrid junctions is proposed. Finally, an effective methodology to construct hybrid 1D-0D networks of vessels is developed, where different a-priori model selection criteria are explored, focusing on obtaining the best possible trade-off between computational cost of the simulations and accuracy of the computed solutions for the hybrid network with respect to the reference 1D network.


Abstract

The one-dimensional (1D) modeling of blood flow in complex networks of vessels and cardiovascular models can result in computationally expensive simulations. The complexity of such networks has significantly increased in the last years, in terms of both enhanced anatomical detail and modeling of physiological mechanisms and mechanical characteristics. To address such issue, the main goal of this work is to present a novel methodology to construct hybrid networks of coupled 1D and 0D vessels and to perform computationally efficient and accurate blood flow simulations in such networks. Departing from both the 1D and lumped-parameter (0D) nonlinear models for blood flow, we propose high-order numerical coupling strategies to solve the 1D, 0D, and hybrid coupling of vessels at junctions. To effectively construct hybrid networks, we explore different a-priori model selection criteria focusing in obtaining the best possible trade-off between computational cost of the simulations and accuracy of the computed solutions for the hybrid network with respect to the 1D network. The achievement of the expected order of accuracy is verified in several test cases. The novel methodology is applied to two different arterial networks, the 37-artery network and the reduced ADAN56 model, where, in order to identify the best performing a-priori model selection criteria, the quantitative assessment of CPU times and errors and the qualitative comparison between results are carried out and discussed.

► Solution to the Riemann problem for drift‐flux model with modified Chaplygin two‐phase flows
  21 Sep, 2022
Solution to the Riemann problem for drift-flux model with modified Chaplygin two-phase flows

Riemann problem for compressible no-slip drift-flux model with modified Chaplygin two-phase flows is investigated. The exact Riemann solution is also established through the characteristic analysis of compressible gas-liquid flow. The compete wave structure is validated numerically through test cases selected form the open literature showing the main two-phase flow variables.


Abstract

In this paper, we concern about the Riemann problem for compressible no-slip drift-flux model which represents a system of quasi-linear partial differential equations derived by averaging the mass and momentum conservation laws with modified Chaplygin two-phase flows. We obtain the exact solution of Riemann problem by elaborately analyzing characteristic fields and discuss the elementary waves namely, shock wave, rarefaction wave and contact discontinuity wave. By employing the equality of pressure and velocity across the middle characteristic field, two nonlinear algebraic equations with two unknowns as gas density ahead and behind the middle wave are formed. The Newton–Raphson method of two variables is applied to find the unknowns with a series of initial data from the literature. Finally, the exact solution for the physical quantities such as gas density, liquid density, velocity, and pressure are illustrated graphically.

► Modelling interactions between waves and diffused interfaces
  21 Sep, 2022
Modelling interactions between waves and diffused interfaces

When simulating multiphase compressible flows using the diffuse-interface methods, important issues arise during the interaction of waves with diffused interfaces. A solution is to use a pressure-disequilibrium model with finite, instead of infinite, pressure-relaxation rate. A new numerical method to solve this model is proposed and solutions of the new modelling are examined and compared to literature for different problems and in particular for the study of a shock on a water–air interface.


Abstract

When simulating multiphase compressible flows using the diffuse-interface methods, the test cases presented in the literature to validate the modellings with regard to interface problems are always textbook cases: interfaces are sharp and the simulations therefore easily converge to the exact solutions. In real problems, it is rather different because the waves encounter moving interfaces which consequently have already undergone the effects of numerical diffusion. Numerical solutions resulting from the interactions of waves with diffused interfaces have never been precisely investigated and for good reasons, the results obtained are extremely dependent on the model used. Precisely, well-posed models present similar and important issues when such an interaction occurs, coming from the appearance of a wave-trapping phenomenon. To circumvent those issues, we propose to use a thermodynamically-consistent pressure-disequilibrium model with finite, instead of infinite, pressure-relaxation rate to overcome the difficulties inherent in the computation of these interactions. Because the original method to solve this model only enables infinite relaxation, we propose a new numerical method allowing infinite as well as finite relaxation rates. Solutions of the new modeling are examined and compared to literature, in particular we propose the study of a shock on a water–air interface, but also for problems of helium–air and water–air shock tubes, spherical and nonspherical bubble collapses.

► An all‐at‐once algebraic multigrid method for finite element discretizations of Stokes problem
  19 Sep, 2022
An all-at-once algebraic multigrid method for finite element discretizations of Stokes problem

The transform-then-solve approach is extended to standard finite element discretizations of the Stokes problem. The extension exploits two complementary strategies to limit the impact of the complexity increase due to the transformation. The comparison with a state-of-the-art block diagonal preconditioner demonstrates that the transform-then-solve approach is always competitive while being significantly more robust for problems with open flow boundary condition and-or variable viscosity.


Abstract

We consider numerical solution of finite element discretizations of the Stokes problem. We focus on the transform-then-solve approach, which amounts to first apply a specific algebraic transformation to the linear system of equations arising from the discretization, and then solve the transformed system with an algebraic multigrid method. The approach has recently been applied to finite difference discretizations of the Stokes problem with constant viscosity, and has recommended itself as a robust and competitive solution method. In this work, we examine the extension of the approach to standard finite element discretizations of the Stokes problem, including problems with variable viscosity. The extension relies, on one hand, on the use of the successive over-relaxation method as a multigrid smoother for some finite element schemes. On the other hand, we present strategies that allow us to limit the complexity increase induced by the transformation. Numerical experiments show that for stationary problems our method is competitive compared to a reference solver based on a block diagonal preconditioner and MINRES, and suggest that the transform-then-solve approach is also more robust. In particular, for problems with variable viscosity, the transform-then-solve approach demonstrates significant speed-up with respect to the block diagonal preconditioner. The method is also particularly robust for time-dependent problems whatever the time step size.

► A semi‐implicit finite volume scheme for a simplified hydrostatic model for fluid‐structure interaction
  17 Sep, 2022
A semi-implicit finite volume scheme for a simplified hydrostatic model for fluid-structure interaction

In this article, a novel mass and momentum conservative semi-implicit finite volume scheme is developed for the coupled solution of hydrostatic shallow water flow and the movement of one or more floating rigid bodies. The coupling is achieved via a nonlinear volume function in the mass conservation equation that depends on the coordinates of the floating objects, whose dynamics is computed by solving a set of ordinary differential equations for their six degrees of freedom. The proposed algorithm may be useful for hydraulic engineering, such as for the simulation of ships moving in inland waterways and coastal regions.


Abstract

Simulating fluid-structure interaction problems usually requires a considerable computational effort. In this article, a novel semi-implicit finite volume scheme is developed for the coupled solution of free surface shallow water flow and the movement of one or more floating rigid structures. The model is well-suited for geophysical flows, as it is based on the hydrostatic pressure assumption and the shallow water equations. The coupling is achieved via a nonlinear volume function in the mass conservation equation that depends on the coordinates of the floating structures. Furthermore, the nonlinear volume function allows for the simultaneous existence of wet, dry and pressurized cells in the computational domain. The resulting mildly nonlinear pressure system is solved using a nested Newton method. The accuracy of the volume computation is improved by using a subgrid, and time accuracy is increased via the application of the theta method. Additionally, mass is always conserved to machine precision. At each time step, the volume function is updated in each cell according to the position of the floating objects, whose dynamics is computed by solving a set of ordinary differential equations for their six degrees of freedom. The simulated moving objects may for example represent ships, and the forces considered here are simply gravity and the hydrostatic pressure on the hull. For a set of test cases, the model has been applied and compared with available exact solutions to verify the correctness and accuracy of the proposed algorithm. The model is able to treat fluid-structure interaction in the context of hydrostatic geophysical free surface flows in an efficient and flexible way, and the employed nested Newton method rapidly converges to a solution. The proposed algorithm may be useful for hydraulic engineering, such as for the simulation of ships moving in inland waterways and coastal regions.

► An immersed boundary vector potential‐vorticity meshless solver of the incompressible Navier–Stokes equation
  17 Sep, 2022
An immersed boundary vector potential-vorticity meshless solver of the incompressible Navier–Stokes equation

We present a new strong-form meshless solver combined with the boundary condition-enforced immersed boundary method for the numerical solution of the nonstationary, incompressible, viscous Navier–Stokes equations in their stream function-vorticity (in 2D) and vector potential-vorticity (in 3D) formulation. We use a Cartesian grid to discretize the spatial domain. We apply explicit time integration to update the transient vorticity equations. Spatial derivatives of the unknown field functions are computed using the discretization-corrected particle strength exchange method.


Abstract

We present a strong form meshless solver for numerical solution of the nonstationary, incompressible, viscous Navier–Stokes equations in two (2D) and three dimensions (3D). We solve the flow equations in their stream function-vorticity (in 2D) and vector potential-vorticity (in 3D) formulation, by extending to 3D flows the boundary condition-enforced immersed boundary method, originally introduced in the literature for 2D problems. We use a Cartesian grid, uniform or locally refined, to discretize the spatial domain. We apply an explicit time integration scheme to update the transient vorticity equations, and we solve the Poisson type equation for the stream function or vector potential field using the meshless point collocation method. Spatial derivatives of the unknown field functions are computed using the discretization-corrected particle strength exchange method. We verify the accuracy of the proposed numerical scheme through commonly used benchmark and example problems. Excellent agreement with the data from the literature was achieved. The proposed method was shown to be very efficient, having relatively large critical time steps.

► Optimization of the numerical treatment of the Darcy–Forchheimer flow of Ree–Eyring fluid with chemical reaction by using artificial neural networks
  17 Sep, 2022
Optimization of the numerical treatment of the Darcy–Forchheimer flow of Ree–Eyring fluid with chemical reaction by using artificial neural networks

The findings obtained as a result of the study showed that artificial neural networks are an ideal tool that can be used to model Darcy–Forchheimer Ree–Eyring fluid flow towards a permeable stretch layer with activation energy and a convective boundary condition.


Abstract

In this study, Darcy Forchheimer flow paradigm, which is a useful paradigm in fields such as petroleum engineering where high flow velocity effects are common, has been analyzed with artificial intelligence approach. In this context, first of all, Darcy–Forchheimer flow of Ree–Eyring fluid along a permeable stretching surface with convective boundary conditions has been examined and heat and mass transfer mechanisms have been investigated by including the effect of chemical process, heat generation/absorption, and activation energy. Cattaneo–Christov heat flux model has been used to analyze heat transfer properties. Within the scope of optimizing Darcy–Forchheimer flow of Ree–Eyring fluid; three different artificial neural network models have been developed to predict Nusselt number, Sherwood number, and skin friction coefficient values. The developed artificial neural network model has been able to predict Nusselt number, Sherwood number, and skin friction coefficient values with high accuracy. The findings obtained as a result of the study showed that artificial neural networks are an ideal tool that can be used to model Darcy–Forchheimer Ree–Eyring fluid flow towards a permeable stretch layer with activation energy and a convective boundary condition.

► Numerical modeling of flow past a volumeless and thin rigid body using direct forcing immersed boundary method
  14 Sep, 2022
Numerical modeling of flow past a volumeless and thin rigid body using direct forcing immersed boundary method

With the exception of thin rigid bodies, the DFIB approach successfully models FSI problems. Modeling thin rigid structures is a significant challenge when employing the DFIB approach to generate FSI solutions. A VOS-based algorithm with DFIB was developed to model thin and volumeless rigid bodies.


Abstract

The new capability has been added as the numerical method for modeling volumeless and thin rigid bodies to the direct forcing immersed boundary (DFIB) method. The DFIB approach is based on adding a virtual force to the Navier–Stokes equations of incompressible flow to account for the interaction between the fluid and structures. The volume of a solid function (VOS) identifies the stationary or moving solid structures in a given fluid domain. A new VOS-based algorithm was developed to identify thin, rigid structure boundary points in fluid flow and ensure that the fluid cannot cross through the boundary of a thin rigid structure while moving or stationary. The DFIB method was first validated in a three-dimensional (3D) turbulent flow over a circular cylinder. The large-eddy simulation simulated the turbulent flow scales. The proposed algorithm was tested using a 3D turbulent flow past a stationary and rotating Savonius wind turbine that functions as a thin, rigid body. The validation results showed that the selected DFIB approach, combined with the novel algorithm, could simulate a thin, volumeless, rigid structure that is stationary and rotating in incompressible turbulent flows. The current method is also applicable for two-way fluid-structure interaction problems.

Journal of Computational Physics top

► An efficient finite element iterative method for solving a nonuniform size modified Poisson-Boltzmann ion channel model
    

Publication date: 1 December 2022

Source: Journal of Computational Physics, Volume 470

Author(s): Dexuan Xie

► FBSDE based neural network algorithms for high-dimensional quasilinear parabolic PDEs
    

Publication date: 1 December 2022

Source: Journal of Computational Physics, Volume 470

Author(s): Wenzhong Zhang, Wei Cai

► Parallel fully coupled methods for bound-preserving solution of subsurface flow and transport in porous media
    

Publication date: 1 December 2022

Source: Journal of Computational Physics, Volume 470

Author(s): Tianpei Cheng, Haijian Yang, Shuyu Sun

► A first order hyperbolic reformulation of the Navier-Stokes-Korteweg system based on the GPR model and an augmented Lagrangian approach
    

Publication date: 1 December 2022

Source: Journal of Computational Physics, Volume 470

Author(s): Firas Dhaouadi, Michael Dumbser

► An Eulerian finite-volume approach of fluid-structure interaction problems on quadtree meshes
    

Publication date: Available online 27 September 2022

Source: Journal of Computational Physics

Author(s): Michel Bergmann, Antoine Fondanèche, Angelo Iollo

► A phase-field method for elastic mechanics with large deformation
    

Publication date: Available online 26 September 2022

Source: Journal of Computational Physics

Author(s): Jiacheng Xu, Dan Hu, Han Zhou

► Numerical approximation of the square phase-field crystal dynamics on the three-dimensional objects
    

Publication date: Available online 26 September 2022

Source: Journal of Computational Physics

Author(s): Junxiang Yang, Junseok Kim

► Hermite spectral method for multi-species Boltzmann equation
    

Publication date: Available online 26 September 2022

Source: Journal of Computational Physics

Author(s): Ruo Li, Yixiao Lu, Yanli Wang, Haoxuan Xu

► An efficient IMEX-DG solver for the compressible Navier-Stokes equations for non-ideal gases
    

Publication date: Available online 26 September 2022

Source: Journal of Computational Physics

Author(s): Giuseppe Orlando, Paolo Francesco Barbante, Luca Bonaventura

► Space-time formulation, discretization, and computational performance studies for phase-field fracture optimal control problems
    

Publication date: 1 December 2022

Source: Journal of Computational Physics, Volume 470

Author(s): D. Khimin, M.C. Steinbach, T. Wick

Journal of Turbulence top

► Drag reduction using velocity control in Taylor–Couette flows
    6 Aug, 2022
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► Drag reduction by a superhydrophobic surface with longitudinal grooves: the effects of the rib surface curvature
  14 Jul, 2022
Volume 23, Issue 8, August 2022, Page 405-432
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► Questions on the effects of roughness and its analysis in non-equilibrium flows
    8 Jul, 2022
Volume 23, Issue 8, August 2022, Page 454-466
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► Turbulence modulation by finite-size particles of different diameters and particle–fluid density ratios in homogeneous isotropic turbulence
    6 Jul, 2022
Volume 23, Issue 8, August 2022, Page 433-453
.

Physics of Fluids top

► Response to “Comment on ‘Linear stability of a rotating channel flow subjected to a static magnetic field’ [Phys. Fluids 34, 054116 (2022)]”
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
► Flow control using single dielectric barrier discharge plasma actuator for flow over airfoil
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
The single dielectric barrier discharge (SDBD) plasma actuator has been developed in the present work for high-accuracy, high-performance computing of flow control applications. The present physics-based SDBD model is a significant improvement over the one developed by Bagade et al., [“Frequency-dependent capacitance–based plasma model for direct simulation of Navier–Stokes equation,” AIAA J. 55, 180–194 (2017)], which was used for planar geometry using sequential computation. Based on the physics of SDBD operation, phase-averaged fully developed body force over an ac cycle is computed and stored, which is reused. Thus, the intensive body force computations are bypassed in the new model, and the body force due to the SDBD plasma actuator is incorporated in the compressible Navier–Stokes equation that is solved in a body-fitted curvilinear coordinates. Here, the modified SDBD model enables performing large-scale simulations for the aerodynamic flow control at low speed and transonic flow past airfoils used in unmanned aerial vehicles and executive jets. The flow control by SDBD plasma actuation is finally compared with other forms of flow control strategies.
► Influence of the properties of the plate surface on the oscillations of the cramped drop
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
We consider free and forced oscillations of a clamped liquid drop. The drop is surrounded by an incompressible fluid of a different density. In equilibrium, the drop has the form of a circular cylinder bounded axially by parallel solid planes, and the contact angle is right. These plates have different surface (chemical, mechanical, and geometrical) properties. The solution is represented as a Fourier series in eigenfunctions of the Laplace operator. The resulting system of complex equations for unknown amplitudes was solved numerically. The fundamental frequency of free oscillations can vanish in a certain interval of values of the Hocking parameter. The length of this interval depends on the aspect ratio of the drop. Frequencies of other eigenmodes of the drop decrease monotonically with increasing Hocking parameters.
► Comment on “Linear stability of a rotating channel flow subjected to a static magnetic field” [Phys. Fluids 34, 054116 (2022)]
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
Recently, Sengupta and Ghosh [“Linear stability of a rotating channel flow subjected to a static magnetic field,” Phys. Fluids 34, 054116 (2022)] published a linear stability analysis of a pressure-driven channel flow, which is subject to rotation around a spanwise axis and a uniform magnetic field applied in the same direction. Unfortunately, the formulation of the magnetohydrodynamic part of the problem contains an elementary error, which makes the obtained results unphysical. The error is due to unfounded omission of the electric potential contribution in the induced electric current which, thus, does not satisfy the charge conservation.
► Statistical properties of three-dimensional Hall magnetohydrodynamics turbulence
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
The three-dimensional (3D) Hall magnetohydrodynamics (HMHD) equations are often used to study turbulence in the solar wind. Some earlier studies have investigated the statistical properties of 3D HMHD turbulence by using simple shell models or pseudospectral direct numerical simulations (DNSs) of the 3D HMHD equations; these DNSs have been restricted to modest spatial resolutions and have covered a limited parameter range. To explore the dependence of 3D HMHD turbulence on the Reynolds number Re and the ion-inertial scale di, we have carried out detailed pseudospectral DNSs of the 3D HMHD equations and their counterparts for 3D MHD (di = 0). We present several statistical properties of 3D HMHD turbulence, which we compare with 3D MHD turbulence by calculating (a) the temporal evolution of the energy-dissipation rates and the energy; (b) the wave-number dependence of fluid and magnetic spectra; (c) the probability distribution functions of the cosines of the angles between various pairs of vectors, such as the velocity and the magnetic field; and (d) various measures of the intermittency in 3D HMHD and 3D MHD turbulence.
► Numerical study of the thermocapillary instability in a thin liquid–air film
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
It is well known that thermal field would cause instability on a two-fluid interface due to the Marangoni effect. This phenomenon is also referred to as thermocapillary flow. A thin liquid/air film experiences thermocapillary instability when it is confined between hot and cold plates. The periodic micro/nano-patterns can generally be observed in the film. Therefore, the thermocapillary instability can be used to fabricate micro/nano-polymeric structures. The present paper proposes a fully nonlinear numerical model based on the phase field method to study the dynamic process of thermocapillary instability. Interfacial evolution and nonlinear effects of the thin liquid film are especially investigated. The impact of the key parameters, e.g., the Marangoni and Reynolds numbers, on the stability are also explored. In addition to the single-mode analysis, the thermocapillary instability is also studied in the multi-mode condition. The conventional single-mode approach facilitates the stability analysis of different wavelengths, while the multi-mode method describes the experiments in a more practical way.
► A mass-conserving and volume-preserving lattice Boltzmann method with dynamic grid refinement for immiscible ternary flows
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
In this paper, a lattice Boltzmann model with dynamic grid refinement is proposed for immiscible ternary flows, which is capable of conserving the total mass and preserving the volume of each phase. The application of interpolation schemes in adaptive mesh refinement (AMR) techniques results in the violation of the total mass of the fluids system within the lattice Boltzmann method (LBM) framework. In the present model, a source term with two free parameters is introduced into the interface capturing equation, which can be determined by the mass conservation and the volume preservation properties. A piecewise constant function has been deliberately incorporated into the source term in order to avoid the appearance of an unphysical fluid at the interface of other two fluids. Based on a block-structured AMR method, the governing equations for phase-field variables and flow hydrodynamic properties are solved by the finite-difference multiple-relaxation-time (MRT) LBM. Simulations of several typical problems are performed in order to evaluate the accuracy and applicability of the proposed model. The numerical results demonstrate that the present model can conserve both mass and volume at the same time as well as reduce numerical dispersion in the bulk region.
► Numerical simulation analysis of symmetric impact of two droplets on a liquid film
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
Analyzing the impact of a droplet on a liquid film is a typical free-surface problem. However, a few studies have investigated the impact of multiple droplets, and in most cases, the droplet impact direction is assumed to be vertically downward. The effect of the impact angle on the characteristic parameters of the spray remains unknown. In this study, the coupled level set and volume of fluid method is used to analyze the development of the gas–liquid interface when two droplets impact a liquid film, and the grid of the gas–liquid interface is refined using the grid adaptive refinement method. The accuracy of numerical simulation has also been verified. The numerical simulation results showed significant differences in the liquid splash morphology corresponding to the different impact modes. The inward impact of the two droplets promotes splashing, whereas the outward impact inhibits it. The liquid film thickness primarily affects the local splash morphology. The droplet spacing affects the spreading diameter and jet angle. The cylindrical jet formed by the symmetrical inward impact of the droplets is unstable, and the rupture mechanism of the cylindrical jet is elucidated for different impact modes. These results provide reference for the application of droplets impingement.
► Steady flow of power-law fluids past a slotted circular cylinder at low Reynolds number
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
Steady laminar flow past a slotted circular cylinder was investigated for non-Newtonian power-law fluids at the low Reynolds number (Re) range ([math]). Flow simulation was carried out for shear-thinning fluids with their power-law indices (n) varying from 0.2 to 1 (n = 0.2, 0.4, 0.6, 0.8, and 1). The normal (case A) and the slotted (case B) circular cylindrical geometries were considered, where the slit was placed between the front and the base pressure stagnation points. A finite volume method was used to calculate the flow field. The flow characteristics, such as flow separation angles, wake size, coefficients of pressure (Cp), and drag (CD), were studied for different Re and n values. For all n values, the slotted cylinder effectively delayed the flow separation. It showed much better pressure recovery than the normal cylinder due to the interaction between the self-bleed from the slit exit to the cylinder wake. The vorticity of this bleed influenced the wake's vorticity, and an increase of 3%–26.4% in higher maximum surface vorticity was reported for the slotted cylinder. An increase of 0.7%–6.5% in the bubble length was observed for the normal cylinder due to early flow separation. An enhanced pressure recovery across the slotted cylinder resulted in a significant drop in the pressure drag with 0.2%–4.56% reduction in the overall drag coefficient.
► Aero-thermal numerical characterization of blunt fin-induced shock wave–boundary layer interaction and its control through leading-edge cooling injection
  27 Sep, 2022
Physics of Fluids, Volume 34, Issue 9, September 2022.
This study represents a novel evaluation of active flow control to alleviate the aerothermal penalties created by the blunt fin-induced shock wave–boundary layer interaction. The manuscript analyzes the effect of flow injection on a blunt fin-induced shock wave–boundary layer interaction via computational fluid dynamics simulations with various degrees of resolution. The impact on the mean flow topology and wall variables was investigated utilizing Reynolds-averaged Navier–Stokes simulations. Detached-eddy simulations revealed the low-frequency shock motion, shock wave–boundary layer, and horseshoe vortex interaction. The test article was exposed to two different incoming boundary layer thicknesses; the thicker boundary layer led to the appearance of larger turbulent scales. The Detached-eddy simulations revealed the time history of the shock wave–boundary layer interaction, focusing on the inception and development of the recirculated flow regions. Ultimately, spectral proper orthogonal decomposition was employed to identify the structures associated with the low-frequency shock motion caused by the shock wave–boundary layer interaction.

Theoretical and Computational Fluid Dynamics top

► Effect of Reynolds number and airfoil thickness on the leading-edge suction in unsteady flows
    1 Oct, 2022

Abstract

Determining the behavior of the leading-edge suction force, represented non-dimensionally by the leading-edge suction parameter (LESP), can reliably help indicate the state of flow over the airfoil and therefore the force and moment characteristics. The current work aims at studying the variations in the LESP, forces, and pitching moment with freestream Reynolds number and airfoil thickness in unsteady flows. Computational data for the NACA 0012, 0015, and 0018 airfoils undergoing a baseline pitching motion over a range of freestream Reynolds number conditions are analyzed. The critical LESP, which is the instantaneous value of LESP at leading-edge vortex initiation, is observed to first decrease and subsequently increase with Reynolds number. This behavior can be correlated to the rate at which leading-edge flow curvature increases with Reynolds number. Thicker airfoils are observed to sustain a larger amount of suction force prior to breakdown and ensuing leading-edge vortex (LEV) shedding. Lift, drag, and moment are found to be dependent on thickness and Reynolds number prior to LEV shedding due to differences in the boundary layer characteristics, but independent after suction breakdown due to the similarity in LEV dynamics. These findings serve to support the development of a more generalized definition of a suction-force parameter that is independent of flow conditions and airfoil geometry.

Graphical abstract

► Acoustic receptivity of high-speed boundary layers on a flat plate at angles of attack
    1 Oct, 2022

Abstract

Direct numerical simulation and theoretical analysis of acoustic receptivity are performed for the boundary layer on a flat plate in Mach 6 flow at various angles of attack (AoA). Slow or fast acoustic wave passes through: a bow shock at AoA \(=-5^{\circ }\) , a weak shock induced by the viscous–inviscid interaction at AoA \(=0^{\circ }\) or an expansion fan emanating from the plate leading edge at AoA \(=5^{\circ }\) . The study is focused on cases where the integral amplification of unstable mode S (or Mack second mode) is sufficiently large \((N\approx 8.4)\) to be relevant to transition in low-disturbance environments. It is shown that excitation of dominant modes F and S occurs in a small vicinity of the plate leading edge. The initial disturbance propagates further downstream in accord with the two-mode approximation model accounting for the mean-flow nonparallel effects and the intermodal exchange mechanism. This computationally economical model can be useful for predictions of the second mode dominated transition onset using the physics-based amplitude method.

Graphic abstract

► A specific slip length model for the Maxwell slip boundary conditions in the Navier–Stokes solution of flow around a microparticle in the no-slip and slip flow regimes
    1 Oct, 2022

Abstract

In the case of microscopic particles, the momentum exchange between the particle and the gas flow starts to deviate from the standard macroscopic particle case, i.e. the no-slip case, with slip flow occurring in the case of low to moderate particle Knudsen numbers. In order to derive new drag force models that are valid also in the slip flow regime for the case of non-spherical particles of arbitrary shapes using computational fluid dynamics, the no-slip conditions at the particle surface have to be modified in order to account for the velocity slip at the surface, mostly in the form of the Maxwell’s slip model. To allow a continuous transition in the boundary condition at the wall from the no-slip case to the slip cases for various Knudsen (Kn) number value flow regimes, a novel specific slip length model for the use with the Maxwell boundary conditions is proposed. The model is derived based on the data from the published experimental studies on spherical microparticle drag force correlations and Cunningham-based slip correction factors at standard conditions and uses a detailed CFD study on microparticle fluid dynamics to determine the correct values of the specific slip length at selected Kn number conditions. The obtained data on specific slip length are correlated using a polynomial function, resulting in the specific slip length model for the no-slip and slip flow regimes that can be applied to arbitrary convex particle shapes.

Graphic abstract

► An integral equation for solitary surface gravity waves of finite amplitude
    1 Oct, 2022

Abstract

The problem of a solitary surface gravity wave in a flow of an inviscid incompressible fluid in a channel of constant depth is considered. The problem is solved in two-dimensional formulation. The wave moves at a constant speed. In a coordinate system moving along with the wave, the flow is stationary. Its mathematical model is reduced to a boundary value problem for a strip in the complex potential plane. This is converted to a boundary value problem for a half-plane by conformal mapping. The solution is obtained using a Cauchy-type integral for the density of which a nonlinear integral equation is derived. Its solution is found with the Galerkin method and the Newton–Raphson technique. The calculated results are compared with the experimental data and the calculations by other researchers. The lower limit of the speed of a solitary wave is found. The advantage of the proposed method is the simplicity of the resulting integral equation, which makes it possible to effectively apply numerical methods of solution.

Graphic abstract

► Linear stability analysis of compressible vortex flows considering viscous effects
    1 Oct, 2022

Abstract

This study investigates the stability of compressible swirling wake flows including the viscous effects using linear stability theory. A spatial stability analysis is performed to evaluate the influence of the axial velocity deficit and circulation as well as the Reynolds number and Mach number as the main parameters that affect the instability. The growth rates of the unstable modes at several azimuthal wavenumbers are compared. The maximum growth rates and their dependency with respect to each parameter are analyzed. It is confirmed that the instability monotonically increases as the axial velocity deficit increases. For small axial velocity deficit, characteristics that are different from the results reported using inviscid analysis are identified and analyzed. Additionally, a decrease in instability is observed as the viscous and compressibility effects become stronger. In terms of circulation, it is confirmed that there is a certain region of circulation that exhibits maximum instability. The stability analysis is expected to serve as a part of a useful methodology for preliminary design and parametric study for engineering problems such as vortex generators in high-speed flows, owing to both efficiency and accuracy.

Graphical abstract

► Numerical study on hydrodynamics of two types of unsteady bubbles in shear-thinning liquids
    1 Oct, 2022

Abstract

A volume of fluid method combined with an adaptive grid method was used to study the influence of Galilei (Ga) and Eötvös (Eo) numbers and characteristic parameters (such as rheological index (n) and characteristic time ( \(\lambda \) )) of shear-thinning liquids on the hydrodynamics of two types of unsteady bubbles. One is the bubble with central breakup behaviors, of which the rise trajectory is a straight line and the shape is symmetrical; however, the shape and centroid velocity cannot reach a steady state. Bubble shape becomes annular after radial expansion, and the centroid velocity has two peaks. The other is the unsteady bubble, of which the rise trajectory is zigzag, but both the shape and rise velocity cannot reach a steady state. The shape of this unstable bubble is flat, which causes periodic vortex shedding at the tail of a bubble. Thus, bubble rise velocity cannot reach a steady state. When the influence of viscous force is relatively weak and Eo is in the range of 50–55, a bubble shows central breakup behaviors. When Eo is low (Eo \(<10\) ), effective Morton numbers (Mo \(^{\mathrm{eff}}\) ) decrease to the magnitude of \(10^{-7}\) and effective Reynolds numbers meet the condition of Re \(^{\mathrm{eff}}\ge 125.2\) , a bubble shows the second type of unsteady characteristics.

Graphic abstract

► Spectral proper orthogonal decomposition using multitaper estimates
    1 Oct, 2022

Abstract

The use of multitaper estimates for spectral proper orthogonal decomposition (SPOD) is explored. Multitaper and multitaper-Welch estimators that use discrete prolate spheroidal sequences (DPSS) as orthogonal data windows are compared to the standard SPOD algorithm that exclusively relies on weighted overlapped segment averaging, or Welch’s method, to estimate the cross-spectral density matrix. Two sets of turbulent flow data, one experimental and the other numerical, are used to discuss the choice of resolution bandwidth and the bias-variance tradeoff. Multitaper-Welch estimators that combine both approaches by applying orthogonal tapers to overlapping segments allow for flexible control of resolution, variance, and bias. At additional computational cost but for the same data, multitaper-Welch estimators provide lower variance estimates at fixed frequency resolution or higher frequency resolution at similar variance compared to the standard algorithm.

Graphic abstract

► Axisymmetric flow structure of thin liquid film under radial temperature difference
    1 Oct, 2022

Abstract

Motivated by recent advances in the development of the numerical calculation of fine flow in liquid film, the thermocapillary convection in thin liquid film (1mm) due to temperature difference is studied in this paper. To describe the formation of the thermocapillary convection on gas-liquid interface, a two-phase system was designed, in which the momentum and energy interact directly through the free surface. The finite volume method is used to solve the N-S equation in gas phase and liquid phase, respectively, and the velocity and temperature information are exchanged on the free surface in each time step. The results show that a thermocapillary wave appears in the liquid film when the temperature difference exceeds a certain value. The temperature and velocity fluctuations on the free surface show a radiation shape. The flow field structure is completely symmetrical in the basic state, but it is axisymmetric in the case of oscillation state. The propagation direction of thermocapillary wave is affected by many factors (ambient temperature or inner wall rotation). The wave propagation direction is consistent with the rotation direction when the inner wall rotates. When the angular velocity of inner wall rotation is 8 rad/s, the wave number of thermocapillary wave will be reduced to 3, which is independent of the rotation direction.

Graphical abstract

► Numerical tripping of high-speed turbulent boundary layers
  11 Aug, 2022

Abstract

The influence of turbulence inflow generation on direct numerical simulations (DNS) of high-speed turbulent boundary layers at Mach numbers of 2 and 5.84 is investigated. Two main classes of inflow conditions are considered, based on the recycling/rescaling (RR) and the digital filtering (DF) approach, along with suitably modified versions. A series of DNS using very long streamwise domains is first carried out to provide reliable data for the subsequent investigation. A set of diagnostic parameters is then selected to verify achievement of an equilibrium state, and correlation laws for those quantities are obtained based on benchmark cases. Simulations using shorter domains, with extent comparable with that used in the current literature, are then carried out and compared with the benchmark data. Significant deviations from equilibrium conditions are found, to a different extent for the various flow properties, and depending on the inflow turbulence seeding. We find that the RR method yields superior performance in the evaluation of the inner-scaled wall pressure fluctuations and the turbulent shear stress. DF methods instead yield quicker adjustment and better accuracy in the prediction of wall friction and of the streamwise Reynolds stress in supersonic cases. Unrealistically high values of the wall pressure variance are obtained by the baseline DF method, while the proposed DF alternatives recover a closer agreement with respect to the benchmark. The hypersonic test case highlights that similar distribution of wall friction and heat transfer are obtained by both RR and DF baseline methods.

Graphical abstract

► On short-wave instability of the stratified Kolmogorov flow
    1 Aug, 2022

Abstract

The problem of the linear stability of the stratified Kolmogorov flow driven by a sinusoidal in space force in a viscous and diffusive Boussinesq fluid is re-visited using the Floquet theory, Galerkin approximations and the method of (generalized) continued fractions. Numerical and analytical arguments are provided in favor of a conjecture that an ideal stratified Kolmogorov flow is prone to short-wave instability for Richardson numbers markedly greater than the critical Richardson number Ri  \(=\)  ¼ that appears in the Miles–Howard theorem. The short-wave instability of the stratified Kolmogorov flow is conjectured to be due to a resonance amplification of the Doppler-shifted internal gravity wave modes, in the presence of critical levels of the main flow that are ignored in the proof of the Miles–Howard theorem, but it is emphasized that the complete resolution of the above paradox is a task for future research.

Graphical abstract


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