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Annual Review of Fluid Mechanics top

► Advances in Bioconvection
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 449-476, January 2020.
► Aeroacoustics of Silent Owl Flight
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 395-420, January 2020.
► Convective Phenomena in Mushy Layers
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 93-119, January 2020.
► Acoustic Tweezers for Particle and Fluid Micromanipulation
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 205-234, January 2020.
► Subglacial Plumes
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 145-169, January 2020.
► Capillarity in Soft Porous Solids
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 263-284, January 2020.
► Modeling Turbulent Flows in Porous Media
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 171-203, January 2020.
► Particles, Drops, and Bubbles Moving Across Sharp Interfaces and Stratified Layers
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 61-91, January 2020.
► Shear Thickening of Concentrated Suspensions: Recent Developments and Relation to Other Phenomena
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 121-144, January 2020.
► Electroconvection Near Electrochemical Interfaces: Experiments, Modeling, and Computation
    7 Jan, 2020
Annual Review of Fluid Mechanics, Volume 52, Issue 1, Page 509-529, January 2020.

Computers & Fluids top

► Numerical Investigation on the Salient Features of Flow over Standard Notchback Configurations using Scale Resolving Simulations
    

Publication date: Available online 8 July 2020

Source: Computers & Fluids

Author(s): K.K. Chode, H. Viswanathan, K. Chow

► A Preconditioned Lattice Boltzmann Flux Solver for Steady Flows on Unstructured Hexahedral Grids
    

Publication date: Available online 7 July 2020

Source: Computers & Fluids

Author(s): Brendan Walsh, Fergal J. Boyle

► Leveling Out Interface Temperature for Conjugate Heat Transfer Problems
    

Publication date: Available online 5 July 2020

Source: Computers & Fluids

Author(s): Gregorio Gerardo Spinelli, Bayram Celik

► Development and application of ANN model for property prediction of supercritical kerosene
    

Publication date: Available online 4 July 2020

Source: Computers & Fluids

Author(s): Bo Li, Yachao Lee, Wei Yao, Yang Lu, Xuejun Fan

► Direct numerical simulations of turbulent channel flows with mesh-refinement lattice Boltzmann methods on GPU cluster
    

Publication date: Available online 8 July 2020

Source: Computers & Fluids

Author(s): Chung-Ming Wu, You-Sheng Zhou, Chao-An Lin

► A new convected level-set method for gas bubble dynamics
    

Publication date: Available online 9 July 2020

Source: Computers & Fluids

Author(s): Malú Grave, José J. Camata, Alvaro L.G.A. Coutinho

► Using the SMG scheme to study the Rayleigh-Taylor instability growth in solids
    

Publication date: 15 August 2020

Source: Computers & Fluids, Volume 208

Author(s): Gabi Luttwak

► Explicit and implicit error inhibiting schemes with post-processing
    

Publication date: 15 August 2020

Source: Computers & Fluids, Volume 208

Author(s): Adi Ditkowski, Sigal Gottlieb, Zachary J. Grant

► An interface-aware sub-scale dynamics multi-material cell model for solids with void closure and opening at all speeds
    

Publication date: 15 August 2020

Source: Computers & Fluids, Volume 208

Author(s): Matej Klima, Andrew Barlow, Milan Kucharik, Mikhail Shashkov

► A supervised neural network for drag prediction of arbitrary 2D shapes in laminar flows at low Reynolds number
    

Publication date: Available online 10 July 2020

Source: Computers & Fluids

Author(s): Jonathan Viquerat, Elie Hachem

International Journal of Computational Fluid Dynamics top

► A Priori Sub-grid Modelling Using Artificial Neural Networks
  13 Jul, 2020
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► A GPU-accelerated Simulator of Turbulent Reacting Flows
  10 Jul, 2020
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► Prediction of Flow Separation and Side-loads in Rocket Nozzle Using Large-eddy Simulation
    7 Jul, 2020
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► Parallel Multiphysics Coupling: Algorithmic and Computational Performances
    2 Jul, 2020
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► MPI Parallelisation of 3D Multiphase Smoothed Particle Hydrodynamics
  30 Jun, 2020
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► A ‘R-to-H’ Mesh Adaptation Approach for Moving Immersed Complex Geometries Using Parallel Computers
  25 Jun, 2020
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► Coupled CFD/MBD Method for a Tilt Tri-rotor UAV in Conversion of Flight Modes
  16 Jun, 2020
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► Simple Fault-tolerant Computing for Field Solvers
    9 Jun, 2020
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► EdgeCFD: a parallel residual-based variational multiscale code for multiphysics
  27 May, 2020
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► Multi-objective optimisation of drag and lift coefficients of a car integrated with canards
  20 May, 2020
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International Journal for Numerical Methods in Fluids top

► Tetrahedral adaptive mesh refinement for two‐phase flows using conservative level‐set method
  12 Jul, 2020

Summary

In this paper, we describe a parallel adaptive mesh refinement strategy for two‐phase flows using tetrahedral meshes. The proposed methodology consists of combining a conservative level‐set method with tetrahedral adaptive meshes within a finite volume framework. Our adaptive algorithm applies a cell‐based refinement technique and adapts the mesh according to physics‐based refinement criteria defined by the two‐phase application. The new adapted tetrahedral mesh is obtained from mesh manipulations of an input mesh: operations of refinement and coarsening until a maximum level of refinement is achieved. For the refinement method of tetrahedral elements, geometrical characteristics are taking into consideration to preserve the shape quality of the subdivided elements. The present method is used for the simulation of two‐phase flows, with surface tension, to show the capability and accuracy of 3D adapted tetrahedral grids to bring new numerical research in this context. Finally, the applicability of this approach is shown in the study of the gravity‐driven motion of a single bubble/droplet in a quiescent viscous liquid on regular and complex domains.

► Explicit Two‐Step Runge‐Kutta Methods for Computational Fluid Dynamics Solvers
  12 Jul, 2020

Abstract

Computational fluid dynamics (CFD) has emerged as a successful tool for industry applications and basic science during the last decades. However, accurate solutions involving vortex propagation, and separated and turbulent flows, are still associated with high computing costs. In particular, large eddy simulations (LES) of complex geometries, such as a complete automobile, require several days on thousands of cores in order to obtain solutions with statistically relevant information. With an increase in the number of available cores, the number of degrees of freedom (DOF) per core can be reduced accordingly. When the number of DOF percore is below a certain threshold the total simulation time is not bounded by floating point operations (FLOPS), but by the time spend on communication between cores. To overcome this impediment we have identified and tested a class of twostep Runge‐Kutta (TSRK) methods of high order with low number of stages, for time discretization of differential systems resulting from space discretization of weakly compressible Navier‐Stokes equations. These methods have not been used before in CFD simulations. The advantage of using these methods is reduction in communication times between cores. The numerical experiments indicate that the gains in computational performance of this new class of TSRK methods, as compared with classical Runge‐Kutta (RK) methods or low storage Runge‐Kutta (LSRK) schemes, are of the order of 25~, with no loss in accuracy.

► Accelerating CFD solver computation time with reduced‐order modeling in a multigrid environment
  12 Jul, 2020

Abstract

Reduced‐order models are more and more considered for use in aerodynamic applications. Benefits of these methods can be expected for optimization problems or predicting aerodynamic loads for the entire flight envelope. For these applications it is often possible to perform computations for various parameter combinations before any ROM evaluations are needed. The order reduction of the CFD solutions in this paper is done using Proper Orthogonal Decomposition. Coupled with an interpolation method predictions for unknown parameter combinations can be made. The CFD solutions are computed using a Discontinuous Galerkin finite element method combined with a nonlinear multigrid scheme. The nonlinear multigrid solver algorithms depend on a good initial guess of the flow field to be computed. Typically, the initial guess on a fine mesh is obtained by solving the problem on a agglomerated mesh. Alternatively, an initial guess could be obtained from a ROM prediction, if available. The main objective is to identify the benefits of using a ROM in a higher‐order multigrid environment. Integral as well as distributed surface quantities of the ROM predictions originating from several multigrid levels will be compared to fully converged flow solutions on the top level of the multigrid algorithm. Furthermore, initializing the flow solver with predictions from several multigrid levels will be analyzed in comparison to full multigrid computations as well as to a scenario where already converged solutions for similar parameter combinations are used as initial flow solutions on the top multigrid level.

► Implicit linearisation scheme for non‐standard two‐phase flow in porous media
  12 Jul, 2020

Abstract

In this paper, we consider a non‐local (in time) two‐phase flow model. The nonlocality is introduced through the wettability alteration induced dynamic capillary pressure function. We present a monotone fixed‐point iterative linearization scheme for the resulting non‐standard model. The scheme treats the dynamic capillary pressure functions semi‐implicitly and introduces an L ‐scheme type1,2 stabilization term in the pressure as well as the transport equations. We prove the convergence of the proposed scheme theoretically under physically acceptable assumptions, and verify the theoretical analysis with numerical simulations. The scheme is implemented and tested for a variety of reservoir heterogeneities in addition to the dynamic change of the capillary pressure function. The proposed scheme satisfies the predefined stopping criterion within a few number of iterations.We also compared the performance of the proposed scheme against the iterative IMplicit Pressure Explicit Saturation scheme.

► Multiphase SPH Modeling of Forced Liquid Sloshing
    8 Jul, 2020

Abstract

In the present paper, an improved multiphase weakly compressible smoothed particle hydrodynamics model for balancing the accuracy and stability of the long‐term simulations is proposed to model the forced liquid sloshing in a tank. The governing equations of the multiphase flow are discretized by considering the density discontinuity over the interface. To suppress the pressure oscillation, a previous density correction term suitable only for single phase problems is modified and incorporated into the discrete continuity equation to suit multiphase problems. The modified density re‐initialization algorithm is implemented to calculate the pressure of the boundary particles, and the coupled dynamic solid boundary treatment is employed to determine the rigid wall condition. For convenience, a numerical probe algorithm is also proposed to accurately measure the wave height. The present model exhibits a better numerical stability than the previous multiphase smoothed particle hydrodynamics model, and its results well confirm with the experimental data of the forced sloshing of liquid excited by swaying or rolling.

► Development of an improved divergence‐free‐condition compensated coupled framework to solve flow problems with time‐varying geometries
    6 Jul, 2020
Development of an improved divergence‐free‐condition compensated coupled framework to solve flow problems with time‐varying geometries

A coupled version of the improved divergence‐free‐condition compensated method is proposed to simulate time‐varying geometries by direct forcing immersed boundary method. The proposed quasi‐multi‐moment framework due to the fact that the momentum equations are discretized by both cell‐centered and cell‐face velocity. A semi‐implicit iterative method is proposed for calculating the direct forcing terms. Treatments for suppressing spurious force oscillations, calculating drag/lift forces, and evaluating velocity and pressure for freshly cells will also be addressed.


Summary

In this article a coupled version of the improved divergence‐free‐condition compensated method will be proposed to simulate time‐varying geometries by direct forcing immersed boundary method. The proposed method can be seen as a quasi‐multi‐moment framework due to the fact that the momentum equations are discretized by both cell‐centered and cell‐face velocity. For simulating time‐varying geometries, a semi‐implicit iterative method is proposed for calculating the direct forcing terms. Treatments for suppressing spurious force oscillations, calculating drag/lift forces, and evaluating velocity and pressure for freshly cells will also be addressed. In order to show the applicability and accuracy, analytical as well as benchmark problems will be investigated by the present framework and compared with other numerical and experimental results.

► Improved third‐order weighted essentially nonoscillatory schemes with new smoothness indicators
    4 Jul, 2020
Improved third‐order weighted essentially nonoscillatory schemes with new smoothness indicators

In this article, we present two improved third‐order weighted essentially nonoscillatory (WENO) schemes for recovering their design‐order near first‐order critical points. The schemes are constructed in the framework of third‐order WENO‐Z scheme. Two new global smoothness indicators, τ L 3 and τ L 4, are devised by a nonlinear combination of local smoothness indicators (IS k ) and reference values (IS G ) based on Lagrangian interpolation polynomial. Numerical results indicate the presented schemes provide less dissipation and higher resolution than the original WENO3‐JS and subsequent WENO3‐N scheme.


Summary

In this article, we present two improved third‐order weighted essentially nonoscillatory (WENO) schemes for recovering their design‐order near first‐order critical points. The schemes are constructed in the framework of third‐order WENO‐Z scheme. Two new global smoothness indicators, τ L 3 and τ L 4, are devised by a nonlinear combination of local smoothness indicators (IS k ) and reference values (IS G ) based on Lagrangian interpolation polynomial. The performances of the proposed schemes are evaluated on several numerical tests governed by one‐dimensional linear advection equation or one‐ and two‐dimensional Euler equations. Numerical results indicate that the presented schemes provide less dissipation and higher resolution than the original WENO3‐JS and subsequent WENO3‐N scheme.

► Issue Information
    2 Jul, 2020

No abstract is available for this article.

► An improved diffuse interface method for three‐dimensional multiphase flows with complex interface deformation
    2 Jul, 2020
An improved diffuse interface method for three‐dimensional multiphase flows with complex interface deformation

1. An improved DI method with interface compression is proposed and validated.

2. Interface dispersion is suppressed effectively and properly by using the magnitude of local vorticity as the interface compression factor.

3. Tiny interfacial structure can be captured accurately in flow field.


Summary

An improved diffuse interface (DI) method is proposed for accurately capturing complex interface deformation in simulations of three‐dimensional (3D) multiphase flows. In original DI methods, the unphysical phenomenon of interface thickening or blurring can affect the accuracy of numerical simulations, especially for flows with large density ratio and high Reynolds number. To remove this drawback, in this article, an interface‐compression term is introduced into the Cahn‐Hilliard equation to suppress the interface dispersion. The additional term only takes effect in the interface region and works normal to the interface. The difference of the current method from the previous work is that the compression rate can be adjusted synchronously according to the magnitude of local vorticity, which is strongly correlated to the interface dispersion and changes with the computational time and interface position. Numerical validations of the proposed method are implemented by simulating problems of Laplace law, Rayleigh‐Taylor instability, bubble rising in a channel, and binary droplet collision. The obtained results agree well with the analytical solutions and published data. The numerical results show that the phenomenon of interface dispersion is suppressed effectively and the tiny interfacial structures in flow field can be captured accurately.

► Gas‐kinetic unified algorithm for plane external force‐driven flows covering all flow regimes by modeling of Boltzmann equation
    2 Jul, 2020
Gas‐kinetic unified algorithm for plane external force‐driven flows covering all flow regimes by modeling of Boltzmann equation

1. The gas‐kinetic unified algorithm is developed for the plane external force‐driven flows covering the rarefied free‐molecule flow to the continuum flow regimes by the computable modeling of the Boltzmann equation. 2. The non‐equilibrium flow phenomena including the bimodal temperature, nonconstant pressure, flow velocity and heat flux profiles in the external force‐driven Poiseuille flows are revealed and analyzed by the current algorithm. 3. For the lid‐driven cavity flow under gravitational field, with an increase in rarefaction of Knudsen numbers Kn = 5 × 10−5 ∼ 10, both expansion cooling and gravity play greater roles in determining the heat transfer characteristics from the continuum to the free‐molecule flow regimes.


Summary

The nonequilibrium steady gas flows under the external forces are essentially associated with some extremely complicated nonlinear dynamics, due to the acceleration or deceleration effects of the external forces on the gas molecules by the velocity distribution function. In this article, the gas‐kinetic unified algorithm (GKUA) for rarefied transition to continuum flows under external forces is developed by solving the unified Boltzmann model equation. The computable modeling of the Boltzmann equation with the external force terms is presented at the first time by introducing the gas molecular collision relaxing parameter and the local equilibrium distribution function integrated in the unified expression with the flow state controlling parameter, including the macroscopic flow variables, the gas viscosity transport coefficient, the thermodynamic effect, the molecular power law, and molecular models, covering a full spectrum of flow regimes. The conservative discrete velocity ordinate (DVO) method is utilized to transform the governing equation into the hyperbolic conservation forms at each of the DVO points. The corresponding numerical schemes are constructed, especially the forward‐backward MacCormack predictor‐corrector method for the convection term in the molecular velocity space, which is unlike the original type. Some typical numerical examples are conducted to test the present new algorithm. The results obtained by the relevant direct simulation Monte Carlo method, Euler/Navier‐Stokes solver, unified gas‐kinetic scheme, and moment methods are compared with the numerical analysis solutions of the present GKUA, which are in good agreement, demonstrating the high accuracy of the present algorithm. Besides, some anomalous features in these flows are observed and analyzed in detail. The numerical experience indicates that the present GKUA can provide potential applications for the simulations of the nonequilibrium external‐force driven flows, such as the gravity, the electric force, and the Lorentz force fields covering all flow regimes.

Journal of Computational Physics top

► L-Sweeps: A scalable, parallel preconditioner for the high-frequency Helmholtz equation
    

Publication date: Available online 10 July 2020

Source: Journal of Computational Physics

Author(s): Matthias Taus, Leonardo Zepeda-Núñez, Russell J. Hewett, Laurent Demanet

► Compressible multiphase particle-in-cell method (CMP-PIC) for full pattern flows of gas-particle system
    

Publication date: 1 October 2020

Source: Journal of Computational Physics, Volume 418

Author(s): Baolin Tian, Junsheng Zeng, Baoqing Meng, Qian Chen, Xiaohu Guo, Kun Xue

► Structure-preserving and efficient numerical methods for ion transport
    

Publication date: 1 October 2020

Source: Journal of Computational Physics, Volume 418

Author(s): Jie Ding, Zhongming Wang, Shenggao Zhou

► Energy and quadratic invariants preserving (EQUIP) multi-symplectic methods for Hamiltonian wave equations
    

Publication date: 1 October 2020

Source: Journal of Computational Physics, Volume 418

Author(s): Chuchu Chen, Jialin Hong, Chol Sim, Kwang Sonwu

► Corrigendum to “Coupled optoelectronic simulation and optimization of thin-film photovoltaic solar cells” [J. Comput. Phys. 407 (2020) 109242]
    

Publication date: Available online 9 July 2020

Source: Journal of Computational Physics

Author(s): Tom H. Anderson, Benjamin J. Civiletti, Peter B. Monk, Akhlesh Lakhtakia

► Pass-efficient methods for compression of high-dimensional turbulent flow data
    

Publication date: Available online 9 July 2020

Source: Journal of Computational Physics

Author(s): Alec M. Dunton, Lluís Jofre, Gianluca Iaccarino, Alireza Doostan

► Simple and reliable boundary detection for meshfree particle methods using interval analysis
    

Publication date: Available online 9 July 2020

Source: Journal of Computational Physics

Author(s): Marcos Sandim, Afonso Paiva, Luiz Henrique de Figueiredo

► Estimation of distributions via multilevel Monte Carlo with stratified sampling
    

Publication date: Available online 9 July 2020

Source: Journal of Computational Physics

Author(s): Søren Taverniers, Daniel M. Tartakovsky

► A novel dual-stage adaptive Kriging method for profust reliability analysis
    

Publication date: Available online 9 July 2020

Source: Journal of Computational Physics

Author(s): Kaixuan Feng, Zhenzhou Lu, Lu Wang

► GENE-3D: A global gyrokinetic turbulence code for stellarators
    

Publication date: Available online 9 July 2020

Source: Journal of Computational Physics

Author(s): M. Maurer, A. Bañón Navarro, T. Dannert, M. Restelli, F. Hindenlang, T. Görler, D. Told, D. Jarema, G. Merlo, F. Jenko

Journal of Turbulence top

► Comparison between temporal and spatial direct numerical simulations for bypass transition flows
  13 Jul, 2020
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► Transitional and turbulent flows in rectangular ducts: budgets and projection in principal mean strain axes
  19 Jun, 2020
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► Numerical study of passive forcing on the secondary instability of a laminar planar free shear layer
    1 Jun, 2020
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► A perspective on machine learning in turbulent flows
  24 Apr, 2020
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► Staircase scaling of short-time energy transfer in turbulence
  21 Apr, 2020
Volume 21, Issue 4, April 2020, Page 234-242
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► Three-dimensional inert and reactive shock-interface interactions: statistical flow characterisations
  21 Apr, 2020
Volume 21, Issue 4, April 2020, Page 209-233
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► Length scales in turbulent free shear flows
  15 Apr, 2020
Volume 21, Issue 4, April 2020, Page 243-257
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► Monte Carlo science
  19 Mar, 2020
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► Neural network models for the anisotropic Reynolds stress tensor in turbulent channel flow
  24 Dec, 2019
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Physics of Fluids top

► Statistical and modal analysis of surface pressure fluctuations in tornado-like vortices
  10 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
Surface pressure measurement is a general tool for evaluating wind flow qualitatively and quantitatively. Due to its complex temporal and spatial features, modal analysis is an interesting tool to be used for interpretation and discussion. The most common technique for modal representation is proper orthogonal decomposition (POD), also referred to as principal component analysis. However, it is believed that POD sometimes fails to extract meaningful features of the pressure field. To remove the non-physical POD modes and provide a closer physical description of the pressure field, an advanced method independent component analysis (ICA) is applied. Furthermore, these two methods are generalized in the frequency domain, called dynamic POD and dynamic ICA, to provide the temporal evolutions of coherent structures over the spatial domain. Modal analysis is used to isolate the different coherent structures in tornado-like vortices, e.g., wandering, vortex breakdown, and two-cell structure, and find the spectral characteristic of each phenomenon. Moreover, a comparison of modal analysis between the current paper and the previous paper on the velocity field {see Karami et al., [“Coherent structures in tornado-like vortices,” Phys. Fluids 31, 085118 (2019)]} presents new insight into the pressure–velocity correlation of the POD modes.
► Rheological behavior of a wormlike micelle and an amphiphilic polymer combination for enhanced oil recovery
  10 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
Amphiphilic polymers have been widely studied and applied in oil fields as effective enhanced oil recovery (EOR) agents. However, the viscosity of their aqueous solution is low at low concentration. In order to improve their poor viscosity-thickening ability at low concentrations, a combined system was used by mixing a zwitterionic surfactant (erucyl dimethyl amidopropyl betaine, EDAB) and an amphiphilic polymer (APC16) at the mass ratio of 2:1. The rheological properties and aggregate microstructure of the EDAB–APC16 combination system were investigated by rheometry, scanning electron microscopy, transmission electron microscopy, and fluorescence spectroscopy, and the EOR efficiency was measured using core flood tests. The results showed that EDAB can significantly increase the viscosity of the APC16 solution, even though the polymer concentration is lower than its critical aggregation concentration. In the EDAB–APC16 combination system, the wormlike micelles of EDAB can synergistically interact with APC16 through hydrophobic association and electrostatic attraction to achieve a thickening effect. Furthermore, the effects of temperature, pH, and the inorganic salts on the viscoelasticity of the EDAB–APC16 system were investigated. An optimized EDAB–APC16 system at 1500 mg/l [pH = 7.5 and c (NaCl) = 5000 mg/l] was selected to be the oil displacing agent, which achieved an EOR of 18.4% after the initial water flood. The polymer–surfactant composite system under development exhibited great potential as an effective chemical EOR agent.
► An investigation on a supercritical aerofoil with a wavy leading edge in a transonic flow
  10 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
A compressible large-eddy simulation is performed to study the effects of a wavy leading edge (WLE) applied on a supercritical aerofoil in a transonic flow at Re∞ = 5.0 × 105 and M∞ = 0.7. The wavelength and peak-to-trough amplitude of the WLE used in this study are 5% and 2.5%, respectively, of the mean aerofoil chord. The primary aim of this study is to understand the aerodynamic characteristics of the modified aerofoil over a range of incidence angles. For this reason, a slow heaving motion is imposed where the geometric angle of attack is gradually increased from αg = 2° to 7° without a significant dynamic (added mass) effect, i.e., a quasi-linear range. The new transonic flow study shows significantly different findings (with some similar features) to the previous low-speed flow studies. It is observed in the quasi-linear range that the modified aerofoil achieves a performance improvement at low and moderate angles because of a drag reduction in the leading edge region and downstream of the laminar–turbulent (L–T) transition point. The leading edge (LE) analysis shows that the maximum pressure coefficient remains equal to that of the baseline case only at the trough and peak sections. The relative decrease in pressure at the LE results in the drag reduction. The transonic flow at the LE is analyzed in further detail to show a reversed flow region at the trough and its influence on the boundary layer development over the aerofoil. In addition, the spanwise variation of the boundary layer characteristics over the modified aerofoil is evaluated and analyzed. One of the most notable findings in this paper is that the flow at the trough becomes supersonic even at low angles of attack, and this results in an enhanced LE flow acceleration spread across the span, which seems facilitated by using a short WLE wavelength. This flow behavior is qualitatively explained by using an analogy between a channeling effect and a convergent–divergent nozzle in a transonic flow. Another notable observation is that there is an upstream movement of the laminar–turbulent transition point seemingly related to the flow distortion around the WLE. Interestingly, the flow distortion introduces a three dimensionality into the laminar boundary layer, but it keeps the flow laminar, so the benefits of the laminar supercritical aerofoil are not lost. These LE phenomena have a major impact on the shock structure at high incidence angles where the more energetic laminar boundary layer changes the shock structure over the modified aerofoil. This can be crucial to control the shock buffet phenomenon.
► A numerical investigation of conjugate thermal boundary layers in a differentially heated partitioned cavity filled with different fluids
  10 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
In this study, the instability mechanisms in the conjugate thermal boundary layers (TBLs) adjacent to a partition in a differentially heated dual-chamber cavity are investigated numerically. The two chambers of the cavity are filled with air (Pr = 0.71) and water (Pr = 8.58), and the partition is assumed to have a zero thickness. The effects of the aspect ratios of both chambers (Aair and Awater) and the overall temperature difference (ΔT) on the interactions between the conjugate air- and water-side TBLs are extensively investigated. It is found that Aair has a more significant impact on the instabilities of the TBLs than Awater. For a relatively small Aair (e.g., 10/3) and a relatively large Aair (e.g., 10), the water-side TBL resonates with the air-side TBL at the frequencies of the corner flow instabilities in the air chamber. For medium Aair (e.g., 4, 5, and 20/3), the air-side TBL becomes weakly turbulent, causing the water-side TBL to become chaotic. Furthermore, if ΔT is reduced, the air-side TBL becomes quasi-periodic, limiting the turbulence growth on the water side. A stability map illustrating the major instability mechanisms controlling the interactions between the conjugate TBLs is presented on the (Aair, Ra) domain for Awater = 5.
► Propulsive performance and flow-field characteristics of a jellyfish-like ornithopter with asymmetric pitching motion
    9 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
Direct force and time-resolved two-dimensional particle image velocimetry measurements were performed on a jellyfish-like ornithopter model, which consists of two anti-phase flapping wings in a side-by-side arrangement. The focus is to study the effect of the time asymmetric pitching motion on the propulsive performance of this kind of ornithopter in a hovering state. It was shown that the fast downstroke and slow upstroke pattern is superior to symmetric back and forth pitching. Namely, more thrust and less fluctuations in the side force can be achieved. In order to provide explanations for this observation, various analyzing techniques, including vortex identification and tracking, spectral analysis, velocity triple decomposition, and reduced-order representation, were taken for a systematical characterization of the flow field in the wake. The spatiotemporal evolution of leading-edge vortices shedding from the wingtip during the downstroke and upstroke stages, as well as their mutual interaction, was found to be one of the key factors to account for the role of time asymmetric pitching on the alternation of thrust generation. Moreover, the delay of the transition of the wake to a turbulent state was observed in the scenario of fast downstroke. This is expected to be beneficial for the improvement of the hovering stability of the ornithopter.
► Wave interactions in neutrally stable shear layers: Regular and singular modes, and non-modal growth
    9 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
A recent letter [J. R. Carpenter and A. Guha, “Instability of a smooth shear layer through wave interactions,” Phys. Fluids 31, 081701 (2019)] compared the neutral modes of a smooth two-dimensional shear profile without an inflection point to the modes of its corresponding piecewise-linear profile. The regular mode in the smooth profile was identified as the one least sensitive to the numerical resolution, while the singular modes displayed high sensitivity. Here, we provide a physical interpretation using a wave interaction approach for understanding the structure and behavior of both the regular and singular modes. The regular modes are the interfacial Rossby waves located at the concentrated mean vorticity gradient of the shear profile. In contrast, the singular modes result from a one way phase-locking interaction between singular vorticity disturbances, passively advected by the mean flow at different levels of the profile, and the interfacial Rossby waves. We show that this one way interaction can also lead to a sustained non-modal growth of the interfacial Rossby waves that cannot be captured by standard eigenvalue analysis.
► Highly dilute gas flows through a non-isothermal planar microchannel
    9 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
This paper reports theoretical and numerical investigations on free molecular gas flows through microchannels. Both diffusely and specularly reflective channel surfaces are considered. Gas kinetic methods are adopted to develop the analytical solutions for surface and flowfield properties. The crucial steps include constructing the velocity distribution functions (VDFs) for points at the plate surfaces and inside flowfield and then completing the integration over the related velocity phases. For diffusely reflective surfaces, the VDFs are related to the densities and temperatures at the two exits and the plate, respectively. For surfaces with specular reflections, the VDFs at the plate surface and inside the flowfield are identical and are independent of the surface temperature ratio and the geometric aspect ratio. Based on the VDFs and velocity phases, surface property coefficients (e.g., Cp, Cf, and Cq) and flowfield properties (e.g., density, velocity components, and temperature) are obtained. For the diffusely reflective surface scenario, the mass flow rate can be approximated and the results include four non-dimensional parameters: the aspect ratio, the density ratio, and two temperature ratios. For the specularly reflective surface scenario, the surface and flowfield properties are uniform everywhere, and the channel aspect ratio and plate temperatures do not have any influence. Particle simulations with the direct simulation Monte Carlo method are performed, and essentially identical results validate the theoretical work. This work is heuristic and can be used to investigate less rarefied microchannel gaseous flows, for example, aid experimental measurement design for thermal transpiration flows.
► Bifurcation analysis for a two-dimensional peristaltic driven flow of power-law fluid in asymmetric channel
    9 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
In the present article, the bifurcations of equilibrium points and their streamlined patterns for the peristaltic transport of shear-thinning and shear-thickening fluids through an asymmetric channel are studied by incorporating a power-law model. An exact solution in the wave frame of reference is obtained under the vanishing Reynolds number and long wavelength approximations. A system of non-linear autonomous differential equations is developed to locate the equilibrium points in the flow. The qualitative nature of equilibrium points and their bifurcations are investigated through the dynamical system method. There exist three distinct flow conditions (backward flow, trapping, and augmented flow). It is observed that the shifting of these flow phenomena corresponds to bifurcations where non-hyperbolic degenerate points are conceived. The impacts of various embedded parameters on flow phenomena and their bifurcations are demonstrated through graphical representations. It is found that the trapping phenomenon manifests at a high flow rate for shear-thinning fluids. That is, the backward flow region shrinks for large values of the power-law index. Trapping in mechanical devices can be diminished by enlarging the phase difference of channel walls, while an opposite trend is observed for amplitude ratios. The obtained results are concluded through global bifurcation diagrams. At the end, findings of this analysis are verified by making a comparison with the existing literature.
► Kinetic energy balance in turbulent particle-laden channel flow
    9 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
The present study investigates the influence of particle additives on the transfer, conversion, and dissipation of kinetic energy (KE) of a turbulent gas–solid channel flow. We derived the equations of KE, mean-flow KE, and turbulent KE (TKE) of the particle-laden flow and further performed two-way coupled direct numerical simulations of channel flow laden with four-million particles with Stokes number St = 30 (corresponding to a mass loading ratio of around one) with an Eulerian–Lagrangian approach. We found that, in the unladen flow, more than half of the input energy is directly dissipated in the mean flow, whereas the rest is converted to maintain the turbulence. By contrast, in the laden flow, both mean dissipation and energy supply are comparable with the unladen flow. However, the turbulence production is greatly reduced in the particle-laden flow. Another sink term due to the presence of the particle–fluid interactions corresponds to the rest loss of the total energy supply. The results reveal the particle-induced redistribution of mean KE, which is transferred from the mean flow to particles in the channel core, whereas the flow gains energy from particles in the near-wall region. In total, there is a loss of the mean-flow energy due to the presence of the inertial particles. Regarding TKE balance, the particles, gaining energy from the mean flow, transfer the energy to the fluid across the channel, which contributes around one third of the TKE source. The present results provide a general picture of KE balance of a particle-laden channel flow.
► Surface tension driven flow of blood in a rectangular microfluidic channel: Effect of erythrocyte aggregation
    9 Jul, 2020
Physics of Fluids, Volume 32, Issue 7, July 2020.
Microfluidic platforms have increasingly been explored for in vitro blood diagnostics and for studying complex microvascular processes. The perfusion of blood in such devices is typically achieved through pressure-driven setups. Surface tension driven blood flow provides an alternative flow delivery option, and various studies in the literature have examined the behavior of blood flow in such fluidic devices. In such flows, the influence of red blood cell (RBC) aggregation, the phenomenon majorly responsible for the non-Newtonian nature of blood, requires particular attention. In the present work, we examine differences in the surface tension driven flow of aggregating and non-aggregating RBC and Newtonian suspensions, in a rectangular microchannel. The velocity fields were obtained using micro-PIV techniques. The analytical solution for blood velocity in the channel is developed utilizing the power law model for blood viscosity. The results showed that RBC aggregation has an impact at the late stages of the flow, observed mainly in the bluntness of the velocity profiles. At the initial stages of the flow, the shearing conditions are found moderately elevated, preventing intense RBC aggregate formation. As the flow decelerates in the channel, RBC aggregation increases, affecting the flow characteristics.

Theoretical and Computational Fluid Dynamics top

► Phase-field modeling and computer simulation of the coffee-ring effect
  12 Jul, 2020

Abstract

In this study, we propose a novel computational model for simulating the coffee-ring phenomenon. The proposed method is based on a phase-field model and Monte Carlo simulation. We use the Allen–Cahn equation with a pinning boundary condition to model a drying droplet. The coffee particles inside the droplet move according to a random walk function with a truncated standard normal distribution under gravitational force. We perform both two-dimensional and three-dimensional computational experiments to demonstrate the accurate simulation of the coffee-ring phenomenon by the proposed model.

► Effects of finite ion size on transport of neutral solute across porous wall of a nanotube
    9 Jul, 2020

Abstract

Effect of finite ion size on the transport of a neutral solute across the porous wall of a nanotube is presented in this study. Modified Poisson–Boltzmann equation without the Debye–Huckel approximation is used to determine the potential distribution within the tube. Power law fluid is selected for the study, as its rheology resembles closely to the real-life physiological fluids. The flow within the tube is actuated by the combined effects of pressure and electroosmotic forces. Steady-state solute balance equation is solved by the similarity technique in order to track the solute transport across the tube. The effects of ionic radius, ionic concentration, and flow behavioral index on the length-averaged Sherwood number, permeate flux, and permeate concentration are analyzed. This study will be extremely helpful in predicting the transport characteristics of a neutral solute in real physiological systems and also to fine-tune the performance of microfluidic devices having porous wall.

► On dispersion of solute in steady flow through a channel with absorption boundary: an application to sewage dispersion
    8 Jul, 2020

Abstract

The paper describes the longitudinal dispersion of passive tracer materials released into an incompressible viscous fluid, flowing through a channel with walls having first-order reaction. Its model is based on a steady advection–diffusion equation with Dirichlet’s and mixed boundary conditions, and whose solution represents the concentration of the tracers in different downstream stations. For imposing the boundary conditions properly, artanh transformation is used to convert the infinite solution space to a finite one. A finite difference implicit scheme is used to solve the advection–diffusion equation in the computational region, and an inverse transformation is employed for the solution in the physical region. It is shown how the mixing of the tracer molecule influenced by the shear flow and due to the action of the absorption parameter at both the walls of the channel. For convection-dominated flow, uniform mesh is failed to capture the layer phenomena along the different downstream stations and a piecewise uniform mesh; namely, Shishkin mesh is used. The results are compared with existing experimental and numerical data available in the literature, and we have achieved an excellent agreement with them. The study plays a significant role to understand the basic mechanisms of sewage dispersion.

► Data-driven modeling of the chaotic thermal convection in an annular thermosyphon
    1 Jul, 2020

Abstract

Identifying accurate and yet interpretable low-order models from data has gained a renewed interest over the past decade. In the present work, we illustrate how the combined use of dimensionality reduction and sparse system identification techniques allows us to obtain an accurate model of the chaotic thermal convection in a two-dimensional annular thermosyphon. Taking as guidelines the derivation of the Lorenz system, the chaotic thermal convection dynamics simulated using a high-fidelity computational fluid dynamics solver are first embedded into a low-dimensional space using dynamic mode decomposition. After having reviewed the physical properties the reduced-order model should exhibit, the latter is identified using SINDy, an increasingly popular and flexible framework for the identification of nonlinear continuous-time dynamical systems from data. The identified model closely resembles the canonical Lorenz system, having the same structure and exhibiting the same physical properties. It moreover accurately predicts a bifurcation of the high-dimensional system (corresponding to the onset of steady convection cells) occurring at a much lower Rayleigh number than the one considered in this study.

► Toward particle-resolved accuracy in Euler–Lagrange simulations of multiphase flow using machine learning and pairwise interaction extended point-particle (PIEP) approximation
  30 Jun, 2020

Abstract

This study presents two different machine learning approaches for the modeling of hydrodynamic force on particles in a particle-laden multiphase flow. Results from particle-resolved direct numerical simulations (PR-DNS) of flow over a random array of stationary particles for eight combinations of particle Reynolds number ( \({\mathrm {Re}}\) ) and volume fraction ( \(\phi \) ) are used in the development of the models. The first approach follows a two-step process. In the first flow prediction step, the perturbation flow due to a particle is obtained as an axisymmetric superposable wake using linear regression. In the second force prediction step, the force on a particle is evaluated in terms of the perturbation flow induced by all its neighbors using the generalized Faxén form of the force expression. In the second approach, the force data on all the particles from the PR-DNS simulations are used to develop an artificial neural network (ANN) model for direct prediction of force on a particle. Due to the unavoidable limitation on the number of fully resolved particles in the PR-DNS simulations, direct force prediction with the ANN model tends to over-fit the data and performs poorly in the prediction of test data. In contrast, due to the millions of grid points used in the PR-DNS simulations, accurate flow prediction is possible, which then allows accurate prediction of particle force. This hybridization of multiphase physics and machine learning is particularly important, since it blends the strength of each, and the resulting pairwise interaction extended point-particle model cannot be developed by either physics or machine learning alone.

► Actuator and sensor placement for closed-loop control of convective instabilities
  24 Jun, 2020

Abstract

This work deals with the characterization of the closed-loop control performance aiming at the delay of transition. We focus on convective wavepackets, typical of the initial stages of transition to turbulence, starting with the linearized Kuramoto–Sivashinsky equation as a model problem representative of the transitional 2D boundary layer; its simplified structure and reduced order provide a manageable framework for the study of fundamental concepts involving the control of linear wavepackets. The characterization is then extended to the 2D Blasius boundary layer. The objective of this study is to explore how the sensor–actuator placement affects the optimal control problem, formulated using linear quadratic Gaussian (LQG) regulators. This is carried out by evaluating errors of the optimal estimator at positions where control gains are significant, through a proposed metric, labelled as \(\gamma \). Results show, in quantitative manner, why some choices of sensor–actuator placement are more effective than others for flow control: good (respectively, bad) closed-loop performance is obtained when estimation errors are low (respectively, high) in the regions with significant gains in the full-state-feedback problem. Unsatisfactory performance is further understood as dominant estimation error modes that overlap spatially with control gains, which shows directions for improvement of a given set-up by moving sensors or actuators. The proposed metric and analysis explain most trends in closed-loop performance as a function of sensor and actuator position, obtained for the model problem and for the 2D Blasius boundary layer. The spatial characterization of the \(\gamma \)-metric provides thus a valuable and intuitive tool for the problem of sensor–actuator placement, targeting here transition delay but possibly extending to other amplifier-type flows.

► Mixing in three-dimensional cavity by moving cavity walls
  18 Jun, 2020

Abstract

The mixing in three-dimensional enclosures is investigated numerically using flow in cubical cavity as a geometrically simple model of various natural and engineering flows. The mixing rate is evaluated for up to the value of Reynolds number \(\hbox {Re}=2000\) for several representative scenarios of moving cavity walls: perpendicular motion of the parallel cavity walls, motion of a wall in its plane along its diagonal, motion of two perpendicular walls outward the common edge, and the parallel cavity walls in motion either in parallel directions or in opposite directions. The mixing rates are compared to the well-known benchmark case in which one cavity wall moves along its edge. The intensity of mixing for the considered cases was evaluated for (i) mixing in developing cavity flow initially at rest, which is started by the impulsive motion of cavity wall(s), and (ii) mixing in the developed cavity flow. For both cases, the initial interface of the two mixing fluids is a horizontal plane located at the middle of the cavity. The mixing rates are ranked from fastest to slowest for twenty time units of flow mixing. The pure convection mixing is modeled as a limit case to reveal convective mechanism of mixing. Mixing of fluids with different densities is modeled to show the advantage in terms of mixing rate of genuinely 3D cases. Grid convergence study and comparison with published numerical solutions for 3D and 2D cavity flows are presented. The effects of three-dimensionality of cavity flow on the mixing rate are discussed.

► Filtration of micropolar liquid through a membrane composed of spherical cells with porous layer
    1 Jun, 2020

Abstract

This paper considers membranes of globular structure in the framework of the cell model technique. The flow of a micropolar fluid through a spherical cell consisting of a solid core, porous layer and liquid envelope is modeled using coupled micropolar and Brinkman-type equations. The solution is obtained in analytical form. Boundary value problems with different conditions on the hypothetical cell surface are considered and compared. The hydrodynamic permeability of the membrane is investigated as a function of micropolar and porous media characteristics.

► A systematic study of blockage in three-dimensional branching networks with an application to model human bronchial tree
    1 Jun, 2020

Abstract

A major aim of the present study is to understand and thoroughly document the fluid dynamics in three-dimensional branching networks when an intermediate branch is partially or completely obstructed. Altogether, 26 different three-dimensional networks each comprising six generations of branches (involving 63 straight portions and 31 bifurcation modules) are constructed and appropriately meshed to conduct a systematic study of the effects of varying the locations of a blockage of a given relative extent and varying the extent of a blockage at a fixed location. The side-by-side consideration of two branching configurations (in-plane and \(90^{\circ }\) out-of-plane) gives a quantitative assessment of the dependence of flow alteration due to blockage on the three-dimensional arrangement of the same individual branches. A blockage in any branch affects the flow in both downstream and upstream branches. The presence of a blockage can make three-dimensional asymmetric alteration to the flow field, even when the blockage itself is geometrically symmetric. The overall mass flow rate entering the network is found to remain nearly unaltered if a blockage is shifted within the same generation but is progressively reduced if the blockage is shifted to upstream generations. A blockage anywhere in the network increases the degree of mass flow asymmetry \(\delta _{\mathrm{G}n} \) in any generation. The order of magnitude disparity in \(\delta _{\mathrm{G}n} \) between the in-plane and out-of-plane configurations, characteristic of unobstructed networks, can be significantly reduced in the presence of a single blockage. The present three-dimensional computations show that the effects of blockage on the mass flow distribution in a large network are complex, often non-intuitive and sometimes dramatic, and cannot be captured by any simple one-dimensional model.

► Compound droplet dynamics of a tumor cell squeezing through conical microfilters
    1 Jun, 2020

Abstract

Circulating tumor cells (CTCs) are regarded as important biomarkers for early cancer detection and treatment. Decades of research have made progress in CTC detection using deformability-based microfilters; however, developing a high-throughput CTC microfilter remains a challenging task due to the lack of the essential understanding of microscopic multiphase flow. To design and optimize a CTC microfilter, in-depth studies of the dynamics of a CTC squeezing through a confined constriction are necessary. In this study, numerical simulation was employed. Utilizing the octree-based Adaptive-Mesh-Refinement algorithm, a CTC was modeled as a compound Newtonian droplet moving through a microfilter with non-uniform cross sections. The immiscible interface was tracked by the volume-of-fluid method with the surface tension accounted for using the continuum surface force method. Pressure signature, shear stress and instantaneous cell velocity during the passing process through a conical microfilter were investigated in great detail in order to understand the fluid dynamics affecting the cell squeezing process. Then, the crucial design parameters including pore angles and operating flow rates were analyzed. The shear stress and critical pressure under different flow rates were investigated as well. Results reveal that the deformation-induced surface tension pressure of the cell nucleus is the dominant component of the critical pressure. Additionally, the maximum instantaneous cell velocity, shear stress and pressure all occur at the same critical stage, as the nucleus passes through the exit of the microfilter channel. Our study provides insights into the dynamics of a compound droplet squeezing through a conical-shaped microfilter and offers constructive guidance for the design and optimization of high-throughput CTC microfilters.


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