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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

► Enhancing accuracy with a convolution filter: What works and why!
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): J. Docampo-Sánchez, G.B. Jacobs, X. Li, J.K. Ryan

► Construction and application of several new symmetrical flux limiters for hyperbolic conservation law
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Shujiang Tang, Mingjun Li

► Validation of volume-of-fluid OpenFOAM® isoAdvector solvers using single bubble benchmarks
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Lionel Gamet, Marco Scala, Johan Roenby, Henning Scheufler, Jean-Lou Pierson

► A flux split based finite-difference two-stage boundary variation diminishing scheme with application to the Euler equations
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Yucang Ruan, Xinting Zhang, Baolin Tian, Zhiwei He

► An L2-norm regularized incremental-stencil WENO scheme for compressible flows
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Yujie Zhu, Xiangyu Hu

► Temperature effects on the noise source mechanisms in a realistic subsonic dual-stream jet
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Romain Biolchini, Guillaume Daviller, Christophe Bailly, Guillaume Bodard

► A fully interior penalty discontinuous Galerkin method for variable density groundwater flow problems
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Ali Raeisi Isa-Abadi, Vincent Fontaine, Hamid-Reza Ghafouri, Anis Younes, Marwan Fahs

► Evaluation of the generalized bernoulli trial-transient adaptive subcell (GBT-TAS) collision scheme in treating rarefied gas flows
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Ahmad Shoja-Sani, Ehsan Roohi, Stefan Stefanov

► Turbulence modelling analysis in a corner separation flow
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Jean-François Monier, Feng Gao, Jérôme Boudet, Liang Shao

► A non-penetration FEM-MPM contact algorithm for complex fluid-structure interaction problems
    

Publication date: 15 December 2020

Source: Computers & Fluids, Volume 213

Author(s): Yan Song, Yan Liu, Xiong Zhang

International Journal of Computational Fluid Dynamics top

► Numerical Simulation of Water Spray Generated by Aircraft Multi-Wheels
  24 Nov, 2020
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► Application of SPH to Single and Multiphase Geophysical, Biophysical and Industrial Fluid Flows
  17 Nov, 2020
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► Accuracy Improvements for Single Precision Implementations of the SPH Method
  23 Oct, 2020
Volume 34, Issue 10, December 2020, Page 774-787
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► On Recirculation Region Length of Suddenly Expanded Supersonic Flows, Using CFD and Fuzzy Logic
  14 Oct, 2020
Volume 34, Issue 10, December 2020, Page 757-773
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► An Efficient Low-Dissipation Hybrid Central/WENO Scheme for Compressible Flows
  24 Sep, 2020
Volume 34, Issue 10, December 2020, Page 705-730
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► An Acoustic and Shock Wave Capturing Compact High-Order Gas-Kinetic Scheme with Spectral-Like Resolution
  23 Sep, 2020
Volume 34, Issue 10, December 2020, Page 731-756
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► Multiphase SPH Modelling of Supercooled Large Droplets Freezing on Aircraft Surfaces
  23 Sep, 2020
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► SPH Modelling of Dam-break Floods, with Damage Assessment to Electrical Substations
    2 Sep, 2020
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► Recent Features and Industrial Applications of the Hybrid SPH-FE Method
  11 Aug, 2020
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► An efficient algorithm for weakly compressible flows in spherical geometries
  25 Nov, 2020
An efficient algorithm for weakly compressible flows in spherical geometries

In this article, we present a direction splitting method, combined with a nonlinear iteration, for the compressible Navier‐Stokes equations in spherical coordinates. The aim of this work was to develop a method that would work efficiently in the limit of very small to vanishing Mach numbers, and we demonstrate here, using a numerical example, that the method shows good convergence and stability at Mach numbers in the range [10−2, 10−6]. The algorithm is particularly suitable for a massive parallel implementation, and we show some results demonstrating its excellent weak scalability.


Abstract

In this article, we present a direction splitting method, combined with a nonlinear iteration, for the compressible Navier‐Stokes equations in spherical coordinates. The method is aimed at solving the equations on the sphere, and can be used for a regional geophysical simulations as well as simulations on the entire sphere. The aim of this work was to develop a method that would work efficiently in the limit of very small to vanishing Mach numbers, and we demonstrate here, using a numerical example, that the method shows good convergence and stability at Mach numbers in the range [10−2, 10−6]. We also demonstrate the effect of some of the parameters of the model on the solution, on a common geophysical test case of a rising thermal bubble. The algorithm is particularly suitable for a massive parallel implementation, and we show below some results demonstrating its excellent weak scalability.

► An improved r‐factor algorithm for total variational diminishing (TVD) schemes on two‐dimension non‐uniform unstructured grids
  25 Nov, 2020
An improved r‐factor algorithm for total variational diminishing (TVD) schemes on two‐dimension non‐uniform unstructured grids

An improved r‐factor algorithm for TVD schemes is proposed to extend the TVD schemes to non‐uniform unstructured grids. The computational results show that the equation L UC /L CD  = (L Cf /L fD ) is appropriate to determine the further upwind node U, comparing with Hou's method. After that, the inverse‐distance weighting average method is superior to the Gauss theory method to estimate the value of node U. Furthermore, the deviations between the value of cell centroids and of their corresponding auxiliary points need to be exploited. The numerical studies indicate that monotonous behavior and higher accuracy can be obtained by using the new algorithm, in comparison to Hou's r‐factor algorithm. Convection of a double‐step profile using Superbee scheme with coarse grids: (A) ϕ profile at y = 0.5, (B) ϕ profile at x = 0.75. Convection of a sinusoidal profile using Superbee scheme with coarse grids: (C) ϕ profile at y = 0.5, (D) ϕ profile at x = 0.75.


Abstract

For advection simulation, an improved r‐factor algorithm for total variational diminishing (TVD) schemes is proposed to extend the TVD schemes to non‐uniform unstructured grids. In the new algorithm, the further upwind node (called node U) location is modified based on the size differences of the related grids, that is, the formula of the relationship between two length ratios, L UC /L CD  = L Cf /L fD , should be maintained to reveal the meaning of r‐factor correctly on non‐uniform unstructured grids. After that, the inverse‐distance weighting average method rather than the Gauss theory method is adopted to estimate the value of node U. Furthermore, the deviations between the value of cell centroids and of their corresponding auxiliary points are exploited to complete the algorithm. The new algorithm is utilized in two pure convection cases, including a double‐step profile and a sinusoidal profile. The algorithm was compared against the Hou's r‐factor algorithm by using Superbee and Van Leer limiters on two‐dimension non‐uniform unstructured grids. The results indicate that a monotonous behavior and a higher accuracy result can be obtained by using the new algorithm.

► A new corner boundary condition for the discrete unified gas kinetic scheme
  24 Nov, 2020
A new corner boundary condition for the discrete unified gas kinetic scheme

We proposes a new corner boundary condition for the DUGKS, which is deduced strictly in theory and available to satisfy conservation relations. The new corner boundary condition is validated by three numerical tests: the flow past a square cylinder (external flow), lid‐driven cavity flow (internal flow) and flow past the AUV (non right angle corner). The results show that the convergence efficiency and accuracy of the DUGKS are improved by the new method.


Abstract

The implementation of the boundary condition at the corner points is very important. The discontinuities at the corner points propagate in the computational domain and have a great impact on the surrounding points and the global solution in the evolution process, resulting in the poor precision or the unphysical oscillatory behavior. However, it had been a largely under explored domain in the discrete unified gas kinetic scheme (DUGKS) methods. In the last few years, the DUGKS is proposed as a mesoscopic finite volume method with great development potential. In order to improve accuracy and efficiency, this paper proposes a new corner boundary condition for the DUGKS, which is deduced strictly in theory and available to satisfy conservation relations. The new corner boundary condition is validated by three numerical tests: the flow past a square cylinder (external flow), lid‐driven cavity flow (internal flow) and flow past the AUV (nonright angle corner). The results show that the convergence efficiency and accuracy of the DUGKS are improved by the new corner boundary condition.

► Unconditionally energy stable, splitting schemes for magnetohydrodynamic equations
  22 Nov, 2020
Unconditionally energy stable, splitting schemes for magnetohydrodynamic equations

In this article, we propose first‐order and second‐order linear, unconditionally energy stable, splitting schemes for solving the magnetohydrodynamics (MHD) system. We transform a double saddle points problem into a set of elliptic type problems to solve the MHD system. We further prove that time semi‐discrete schemes and fully discrete schemes are unconditionally energy stable.


Abstract

In this article, we propose first‐order and second‐order linear, unconditionally energy stable, splitting schemes for solving the magnetohydrodynamics (MHD) system. These schemes are based on the projection method for Navier–Stokes equations and implicit–explicit treatments for nonlinear coupling terms. We transform a double saddle points problem into a set of elliptic type problems to solve the MHD system. Our schemes are efficient, easy to implement, and stable. We further prove that time semidiscrete schemes and fully discrete schemes are unconditionally energy stable. Various numerical experiments, including Hartmann flow and lid‐driven cavity problems, are implemented to demonstrate the stability and the accuracy of our schemes.

► Fluid‐fluid interactions in pseudopotential lattice Boltzmann models: Effects of model schemes and fluid properties
  22 Nov, 2020

This paper demonstrates the characteristics of interparticle forces and forcing schemes in pseudopotential lattice Boltzmann simulations. The Yuan‐Schaefer (YS), multipseudopotential interaction (MPI) and piecewise linear methods are examined as techniques of equation of state (EOS) inclusion in pseudopotential models. It is suggested here that it is important to have an understanding of the interparticle forces generated by the models in order to obtain good quality results. Poor choice of parameters can lead to generation of unphysical interactions. The piecewise linear method is found to perform well and to decouple parameters. It decouples the density ratio from the surface tension and from the collision operator relaxation rates. It is proposed that the decoupling occurs due to generation of lower values of high‐order error terms in the interfacial region by the piecewise linear EOS. In general, the multiple‐relaxation‐time (MRT) collision operator should be combined with the Huang‐Wu forcing scheme for simulating high values of surface tension and with the Li‐Luo method for simulating low values of surface tension. It is found that reducing kinematic viscosity is more detrimental to the stability of the simulations than increasing the density ratio. Introducing a kinematic viscosity ratio between the phases practically eliminates the influence of density ratio on spurious velocities. The factors affecting stability of dynamic simulations are examined. It is found that they have the following hierarchy from the greatest impact to the least: kinematic viscosity ratio between the phases; bulk viscosity; method of EOS inclusion and reduced temperature/ density ratio.

► Accuracy aspects of conventional discretization methods for scalar transport with nonzero divergence velocity field arising from the energy balance equation
  18 Nov, 2020
Accuracy aspects of conventional discretization methods for scalar transport with nonzero divergence velocity field arising from the energy balance equation

We analyze the accuracy properties of both first‐order upwind finite difference and vertex‐centered finite volume approximations for linear transport problems with nonzero divergence velocity field. Such upwind schemes are routinely used in established spectral wave models. We conclude that the finite difference scheme offers superior convergence rates over the finite volume scheme for nonsmooth velocities. Theoretical results are supported by numerical computations, whereas practical consequences for the simulation of wave shoaling and refraction over shoals are demonstrated.


Abstract

We are concerned with the numerical solution of a linear transport problem with nonzero divergence velocity field that originates from the spectral energy balance equation describing the evolution of wind waves and swells in coastal seas. The discretization error of the commonly used first‐order upwind finite difference and first‐order vertex‐centered upwind finite volume schemes in one space dimension is analyzed. Smoothness of nondivergent velocity field plays a crucial role in this. No such analysis has been attempted to date for such problems. The two schemes studied differ in the manner in which they treat the scalar flux numerically. The finite difference variant is shock captured, whereas the vertex‐centered finite volume approximation employs an arithmetic mean of the velocity and appears not to be flux conservative. The methods are subsequently extended to two dimensions on triangular meshes. Numerical experiments are provided to verify the convergence analysis. The main finding is that the finite difference scheme displays optimal rates of convergence and offers higher accuracy over the finite volume scheme, regardless the regularity of the velocity field. The latter scheme notably yields convergence rates of 0.5 and 0 in L 2‐norm and L ‐norm, respectively, when the velocity field is not smooth. A test case illustrating wave shoaling and refraction over submerged shoals is also presented and demonstrates the practical importance of flux conservation.

► A conservative flux‐splitting method for steady shock capturing
  16 Nov, 2020

Summary

A novel flux‐splitting method that can preserve the flux conservation properties for the steady shock waves is proposed in present paper. The proposed method is based on the total variation diminishing (TVD) scheme, and the dissipative flux term is constructed by the flux directly instead of conservative variables. Similar transformations are imposed on the anti‐dissipation term, which is formulated by utilizing the limiter function. Numerical experiments were performed on the one, quasi one and two dimensional shock wave problems. The computed results show that, the steady shock waves can be accurately captured within two grid points by the proposed conservative flux‐splitting (CFS) method, and numerical oscillations are successfully eliminated near the flow discontinuities. The conservation and accuracy of present method are demonstrated by comparisons with the TVD, flux vector splitting (FVS) and flux difference splitting (FDS) methods.

► A continuous finite element framework for the pressure Poisson equation allowing non‐Newtonian and compressible flow behavior
  16 Nov, 2020
A continuous finite element framework for the pressure Poisson equation allowing non‐Newtonian and compressible flow behavior

For the frequent task of computing pressure from given flow velocities, we devise the first variational formulation for the pressure Poisson equation that fully accounts for viscous effects, including non‐Newtonian behavior, and also allows for compressibility — while still enabling the use of standard Lagrangian finite element basis functions for all quantities of interest. Various numerical examples are provided to showcase the potential of this novel approach.


Abstract

Computing pressure fields from given flow velocities is a task frequently arising in engineering, biomedical, and scientific computing applications. The so‐called pressure Poisson equation (PPE) derived from the balance of linear momentum provides an attractive framework for such a task. However, the PPE increases the regularity requirements on the pressure and velocity spaces, thereby imposing theoretical and practical challenges for its application. In order to stay within a Lagrangian finite element framework, it is common practice to completely neglect the influence of viscosity and compressibility when computing the pressure, which limits the practical applicability of the pressure Poisson method. In this context, we present a mixed finite element framework which enables the use of this popular technique with generalized Newtonian fluids and compressible flows, while allowing standard finite element spaces to be employed for the unknowns and the given data. This is attained through the use of appropriate vector calculus identities and simple projections of certain flow quantities. In the compressible case, the mixed formulation also includes an additional equation for retrieving the density field from the given velocities so that the pressure can be accurately determined. The potential of this new approach is showcased through numerical examples.

► Robust low‐dissipative scheme for curvilinear grids
  15 Nov, 2020

Abstract

This work describes a detailed mathematical procedure in relation to a novel third‐order WENO scheme for the inviscid term of a system of nonlinear equations in the generalized grid system. The scheme developed minimizes the linear and nonlinear sources of dissipation error associated with the classical fifth‐order WENO scheme. The former is minimized by optimizing the resolving efficiency of the scheme whereas the latter is minimized by fixing the accuracy at the second‐order critical point via re‐defining the nonlinear weights. Moreover, the spectral property of second‐order viscous derivative, approximated by the single and double applications of the standard fourth‐order central finite difference scheme, is presented. The two‐dimensional Euler and Navier‐Stokes equations in the generalized grids are mainly pursued. For the robustness in terms of capturing discontinuous and smooth structures, particularly two problems, which are difficult to handle in Cartesian grids, are chosen for discussion. The first one deals with a supersonic shock hitting the circular cylinder and generating all the possible flow inconsistencies. The other one deals with a subsonic flow over a circular cylinder at the incompressible limit. The numerical results are found to be in good agreement with the experimental data.

► The Moving Discontinuous Galerkin Finite Element Method with Interface Condition Enforcement for Compressible Viscous Flows
  15 Nov, 2020

Abstract

The moving discontinuous Galerkin finite element method with interface condition enforcement (MDG‐ICE) is applied to the case of viscous flows. This method uses a weak formulation that separately enforces the conservation law, constitutive law, and the corresponding interface conditions in order to provide the means to detect interfaces or under‐resolved flow features. To satisfy the resulting overdetermined weak formulation, the discrete domain geometry is introduced as a variable, so that the method implicitly fits a priori unknown interfaces and moves the grid to resolve sharp, but smooth, gradients, achieving a formof anisotropic curvilinear r‐adaptivity. This approach avoids introducing low‐order errors that arise using shock capturing, artificial dissipation, or limiting. The utility of this approach is demonstrated with its application to a series of test problems culminating with the compressible Navier‐Stokes solution to a Mach 5 viscous bow shock for a Reynolds number of 105 in two‐dimensional space. Time accurate solutions of unsteady problems are obtained via a space‐time formulation, in which the unsteady problem is formulated as a higher dimensional steady space‐time problem. The method is shown to accurately resolve and transport viscous structures without relying on numerical dissipation for stabilization.

Journal of Computational Physics top

► An integral equation method for the simulation of doubly-periodic suspensions of rigid bodies in a shearing viscous flow
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s): Jun Wang, Ehssan Nazockdast, Alex Barnett

► Elastic wave propagation in anisotropic solids using energy-stable finite differences with weakly enforced boundary and interface conditions
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s): Martin Almquist, Eric M. Dunham

► QTT-isogeometric solver in two dimensions
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s): L. Markeeva, I. Tsybulin, I. Oseledets

► A Characteristic Mapping method for the two-dimensional incompressible Euler equations
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s): Xi-Yuan Yin, Olivier Mercier, Badal Yadav, Kai Schneider, Jean-Christophe Nave

► A weighted Shifted Boundary Method for free surface flow problems
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s): Oriol Colomés, Alex Main, Léo Nouveau, Guglielmo Scovazzi

► Intrinsic finite element method for advection-diffusion-reaction equations on surfaces
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s): Elena Bachini, Matthew W. Farthing, Mario Putti

► Editorial Board
    

Publication date: 1 January 2021

Source: Journal of Computational Physics, Volume 424

Author(s):

► Efficient smoothed particle radiation hydrodynamics II: adiation hydrodynamics
    

Publication date: Available online 25 November 2020

Source: Journal of Computational Physics

Author(s): Brody R. Bassett, J. Michael Owen, Thomas A. Brunner

► Efficient smoothed particle radiation hydrodynamics I: Thermal radiative transfer
    

Publication date: Available online 25 November 2020

Source: Journal of Computational Physics

Author(s): Brody R. Bassett, J. Michael Owen, Thomas A. Brunner

► Enriched Galerkin discretization for modeling poroelasticity and permeability alteration in heterogeneous porous media
    

Publication date: Available online 25 November 2020

Source: Journal of Computational Physics

Author(s): T. Kadeethum, H.M. Nick, S. Lee, F. Ballarin

Journal of Turbulence top

► Aero-optical suppression for supersonic turbulent boundary layer
  24 Nov, 2020
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► A scaling law for the required transition zone depth in hybrid LES-DNS
  20 Nov, 2020
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► Effects of CFJ flow control on aerodynamic performance of symmetric NACA airfoils
  18 Nov, 2020
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► Direct numerical simulation study on the mechanisms of the magnetic field influencing the turbulence in compressible magnetohydrodynamic flow
  16 Nov, 2020
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► A note on fitting a generalised Moody diagram for wall modelled large-eddy simulations
  30 Oct, 2020
Volume 21, Issue 11, November 2020, Page 650-673
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► Observing production and growth of Tollmien–Schlichting waves in subsonic flat plate boundary layer via exciters-free high fidelity numerical simulation
  12 Oct, 2020
Volume 21, Issue 11, November 2020, Page 632-649
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► Calibration and evaluation of a spatial scaling method for the near-wall turbulent flow of viscoelastic fluids
  21 Sep, 2020
Volume 21, Issue 11, November 2020, Page 607-631
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Physics of Fluids top

► Nonlinear transport of rarefied Couette flows from low speed to high speed
  25 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
The nonlinear transport properties and macroscopic flow features of rarefied plane Couette flows from low speed to high speed for a monatomic gas are investigated in detail using the direct simulation Monte Carlo (DSMC) method. The effective viscosity and thermal conductivity are directly computed from the DSMC results according to the linear constitutive relations. The detailed structure of the Knudsen layer (KL) and the functional dependence of the effective transport coefficients on local Knudsen numbers in the whole system are presented and compared with existing theoretical models. The results show that the effective viscosity and thermal conductivity distributions in the KL for different Mach number flows can be recast into the same profile (i.e., isothermal scaling function) in terms of a scaled wall distance [math], though the local flow is nonisothermal. For all cases, the shear-stress Knudsen number distributions across the channel show a well opposite trend to the effective transport coefficient profiles. The functional dependence between them in the bulk region always coincides with the normal solution that is derived from the Boltzmann model equations for unbounded shear flows, while that in the KL for low-speed cases shows a large difference with the normal solution. As the Mach number increases, the DSMC data in the KL can also agree approximately with the normal solution at a large shear-stress Knudsen number. These results can be very useful for developing phenomenological models to describe a wall-bounded rarefied shear flow, showing a good prospect in both microflow and high-altitude applications.
► A combined numerical and experimental study to elucidate primary breakup dynamics in liquid metal droplet-on-demand printing
  25 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
Droplet-on-demand liquid metal jetting is emerging as a powerful technology for the additive manufacturing of metallic parts. The success of this method hinges on overcoming several technological challenges. The principal one among these challenges is the controlled repeatable ejection of single uniform droplets. Due to the high density and surface tension of liquid metals, the droplet ejection process occurs near the minimal extremes of the printability phase diagram, defined by acceptable ranges for the Weber (We) and Ohnesorge (Oh) numbers. In this work, we experimentally demonstrate the satellite-free ejection of pneumatically actuated molten tin droplets in this extreme corner of printability and use a combination of high-speed video analysis and volume-of-fluid modeling to elucidate the droplet dynamics. While the simulations at low Oh and We can correctly describe several aspects of the breakup process, such as an increasing tail and pinch-point near the nozzle, no single parameter set can completely capture the droplet shape at breakup. Instead, the experimental droplet dynamics appear to include features from both high and low Oh breakup. This disagreement is ascribed to the incomplete description of the droplet ejection process including wetting and exit effects near the nozzle opening and surface effects such as transient cooling and oxide formation.
► The role of inertia in the rupture of ultrathin liquid films
  24 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
Theory and numerical simulations of the Navier–Stokes equations are used to unravel the influence of inertia on the dewetting dynamics of an ultrathin film of Newtonian liquid deposited on a solid substrate. A classification of the self-similar film thinning regimes at finite Ohnesorge numbers is provided, unifying previous findings. We reveal that, for Ohnesorge numbers smaller than one, the structure of the rupture singularity close to the molecular scales is controlled by a balance between liquid inertia and van der Waals forces, leading to a self-similar asymptotic regime with hmin ∝ τ2/5 as τ → 0, where hmin is the minimum film thickness and τ is the time remaining before rupture. The flow exhibits a three-region structure comprising an irrotational core delimited by a pair of boundary layers at the wall and at the free surface. A potential-flow description of the irrotational core is provided, which is matched with the vortical layers, allowing us to present a complete parameter-free asymptotic description of inertia-dominated film rupture.
► The perspective of fluid flow behavior of respiratory droplets and aerosols through the facemasks in context of SARS-CoV-2
  24 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
In the unfortunate event of the current ongoing pandemic COVID-19, where vaccination development is still in the trial phase, several preventive control measures such as social distancing, hand-hygiene, and personal protective equipment have been recommended by health professionals and organizations. Among them, the safe wearing of facemasks has played a vital role in reducing the likelihood and severity of infectious respiratory disease transmission. The reported research in facemasks has covered many of their material types, fabrication techniques, mechanism characterization, and application aspects. However, in more recent times, the focus has shifted toward the theoretical investigations of fluid flow mechanisms involved in the virus-laden particles’ prevention by using facemasks. This exciting research domain aims to address the complex fluid transport that led to designing a facemask with a better performance. This Review discusses the recent updates on fluid flow dynamics through the facemasks. Key design aspects such as thermal comfort and flow resistance are discussed. Furthermore, the recent progress in the investigations on the efficacy of facemasks for the prevention of COVID-19 spread and the impact of wearing facemasks is presented.
► How coronavirus survives for days on surfaces
  24 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
Our previous study [R. Bhardwaj and A. Agrawal, “Likelihood of survival of coronavirus in a respiratory droplet deposited on a solid surface,” Phys. Fluids 32, 061704 (2020)] showed that the drying time of typical respiratory droplets is on the order of seconds, while the survival time of the coronavirus on different surfaces was reported to be on the order of hours in recent experiments. We attribute the long survival time of the coronavirus on a surface to the slow evaporation of a thin nanometer liquid film remaining after the evaporation of the bulk droplet. Accordingly, we employ a computational model for a thin film in which the evaporating mass rate is a function of disjoining and Laplace pressures inside the film. The model shows a strong dependence on the initial thickness of the film and suggests that the drying time of this nanometric film is on the order of hours, consistent with the survival time of the coronavirus on a surface, seen in published experiments. We briefly examine the change in the drying time as a function of the contact angle and type of surface. The computed time-varying film thickness or volume qualitatively agrees with the measured decay of the coronavirus titer on different surfaces. The present work provides insights on why coronavirus survival is on the order of hours or days on a solid surface under ambient conditions.
► Effect of density of a sphere launched vertically in water on the water-surface behavior and sphere motion in air
  23 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
Submerged solid spheres with specific gravities relative to water ranging from 1.36 to 7.93 were launched vertically upward toward the free surface of calm water. The motion of each sphere and the behavior of the water surface were investigated from the time the sphere passed through the calm water surface until it attained its maximum displacement position. The energy lost in the interaction between the sphere and the water surface (i.e., the interfacial containing energy Eo) was estimated from energy conservation. A larger Eo at the maximum displacement position of the sphere led to a larger increase in the height and width of the interfacial water sheet where the upper side of the sphere intersected with the free surface of calm water. This result corresponded to the result obtained by changing the submergence depth, as reported by Takamure and Uchiyama [“Air–water interface dynamics and energy transition in air of a sphere passed vertically upward through the interface,” Exp. Therm. Fluid Sci. 118, 110167 (2020)]. This aspect suggests that the characteristics of the interfacial water sheet are the dominant parameters influencing Eo. The presented findings can facilitate the determination of parameters to model the water exit problem.
► On the modeling of scalar mixing timescale in filtered density function simulation of turbulent premixed flames
  23 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
A new closure of the scalar mixing timescale is formulated to enhance the predictability of large eddy simulation (LES)/filtered density function (FDF) simulations for turbulent premixed flames. Specifically, the new model integrates a dynamic closure for turbulence-induced mixing with a closure for reaction-enhanced mixing, such that the model explicitly accounts for the subgrid mixing due to turbulence and reaction. The model adaptively adjusts the relative contribution from these two aspects according to the local state of combustion and requires no tuning for the mixing rate parameter (CM). To evaluate the model performance, LES/FDF simulations are carried out for the Sydney piloted premixed jet burner flames PM1-50 and PM1-150. Compared with the constant CM model with the baseline CM = 2, the proposed model notably improved the prediction of the overall combustion progress of both flames. The relative importance of the reaction-enhanced mixing in comparison with the turbulence-induced mixing is further investigated. For flame PM1-50, the reaction-enhanced mixing has a prominent impact throughout the combustion progress, resulting in a large variation in CM in the progress variable space. This illustrates the advantage of the proposed model for the flame close to the flamelet regime. For flame PM1-150, the variation in CM during the combustion progress is relatively small owing to the relatively weak reaction-enhanced mixing compared to PM1-50. However, this desired CM is much larger than the order of unity. Therefore, the proposed model also has its advantage for the flame close to the broken-reaction zones regime.
► Liquid-curtain-based strategy to restrain plume during flushing
  23 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
How to prevent the flushing-induced plume without changing people’s daily habits? Enlightened by thoughts of redesigning the restroom, this article provides a redesigned toilet using liquid-curtain-based strategy and verifies its advantages from the computational fluid dynamics. Two favorable effects are spotted: (1) the liquid curtain can suppress the upward virus particles (only 1% viruses can be lifted out of the toilet) and (2) the flow distribution caused by the liquid curtain can deliver virus particles into the sewage efficiently.
► Leidenfrost drop impact on inclined superheated substrates
  20 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
In real applications, drops always impact on solid walls with various inclinations. For the oblique impact of a Leidenfrost drop, which has a vapor layer under its bottom surface to prevent its direct contact with the superheated substrate, the drop can nearly frictionlessly slide along the substrate accompanied by spreading and retracting. To individually study these processes, we experimentally observe the impact of ethanol drops on superheated inclined substrates using high-speed imaging from two different views synchronously. We first study the dynamic Leidenfrost temperature, which mainly depends on the normal Weber number We⊥. Then, the substrate temperature is set to be high enough to study the Leidenfrost drop behavior. During the spreading process, drops are always kept uniform, and the maximum spreading factor Dm/D0 follows a power-law dependence on the large normal Weber number We⊥ as [math] for We⊥ ≥ 30. During the retracting process, drops with low impact velocities become non-uniform due to the gravity effect. For the sliding process, the residence time of all studied drops is nearly a constant, which is not affected by the inclination and the We number. The frictionless vapor layer resulting in the dimensionless sliding distance L/D0 follows a power-law dependence on the parallel Weber number We|| as [math]. Without direct contact with the substrate, the behaviors of drops can be separately determined by We⊥ and We||. When the impact velocity is too high, the drop fragments into many tiny droplets, which is called the splashing phenomenon. The critical splashing criterion is found to be [math] 120 or [math] 5300 in the current parameter regime.
► A study on bubble nuclei population dynamics under reduced pressure
  20 Nov, 2020
Physics of Fluids, Volume 32, Issue 11, November 2020.
The existence of cavitation nuclei is one of the necessary conditions for liquid cavitation. Bubble nucleus is the most basic cavitation nucleus, and bubble nuclei size distribution is a parameter describing the population of gas nuclei. To study the dynamics of bubble nuclei population after artificial seeding under reduced pressure, a decompression chamber was built, combined with the artificial seeding system and the acoustic nuclei measurement system. After the nuclei seeding, the experimental study of the nuclei population dynamics with pressure and time was carried out. It is found that as the pressure decreases, the number density of larger size nuclei decreases, while the number density of smaller size nuclei increases. In the measured size range, the maximum value of number density of the nuclei size distribution increases. In addition, based on the theory of bubble dynamics, the growth process of the nucleus under reduced pressure is calculated and analyzed, which can realize the preliminary prediction of the nuclei population dynamics under reduced pressure.

Theoretical and Computational Fluid Dynamics top

► Effects of finite ion size on transport of neutral solute across porous wall of a nanotube
    1 Dec, 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.

► A priori tests of eddy viscosity models in square duct flow
    1 Dec, 2020

Abstract

We carry out a priori tests of linear and nonlinear eddy viscosity models using direct numerical simulation (DNS) data of square duct flow up to friction Reynolds number \({\text {Re}}_\tau =1055\) . We focus on the ability of eddy viscosity models to reproduce the anisotropic Reynolds stress tensor components \(a_{ij}\) responsible for turbulent secondary flows, namely the normal stress \(a_{22}\) and the secondary shear stress \(a_{23}\) . A priori tests on constitutive relations for \(a_{ij}\) are performed using the tensor polynomial expansion of Pope (J Fluid Mech 72:331–340, 1975), whereby one tensor base corresponds to the linear eddy viscosity hypothesis and five bases return exact representation of \(a_{ij}\) . We show that the bases subset has an important effect on the accuracy of the stresses and the best results are obtained when using tensor bases which contain both the strain rate and the rotation rate. Models performance is quantified using the mean correlation coefficient with respect to DNS data \({\widetilde{C}}_{ij}\) , which shows that the linear eddy viscosity hypothesis always returns very accurate values of the primary shear stress \(a_{12}\) ( \({\widetilde{C}}_{12}>0.99\) ), whereas two bases are sufficient to achieve good accuracy of the normal stress and secondary shear stress ( \({\widetilde{C}}_{22}=0.911\) , \({\widetilde{C}}_{23}=0.743\) ). Unfortunately, RANS models rely on additional assumptions and a priori analysis carried out on popular models, including k \(\varepsilon \) and \(v^2\) f, reveals that none of them achieves ideal accuracy. The only model based on Pope’s expansion which approaches ideal performance is the quadratic correction of Spalart (Int J Heat Fluid Flow 21:252–263, 2000), which has similar accuracy to models using four or more tensor bases. Nevertheless, the best results are obtained when using the linear correction to the \(v^2\) f model developed by Pecnik and Iaccarino (AIAA Paper 2008-3852, 2008), although this is not built on the canonical tensor polynomial as the other models.

► On dispersion of solute in steady flow through a channel with absorption boundary: an application to sewage dispersion
    1 Dec, 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.

► Phase-field modeling and computer simulation of the coffee-ring effect
    1 Dec, 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.

► Instability of natural convection in a laterally heated cube with perfectly conducting horizontal boundaries
    1 Dec, 2020

Abstract

Oscillatory instability of buoyancy convection in a laterally heated cube with perfectly thermally conducting horizontal boundaries is studied. The effect of the spanwise boundaries on the oscillatory instability onset is examined. The problem is treated by Krylov-subspace-iteration-based Newton and Arnoldi methods. The Krylov basis vectors are calculated by a novel approach that involves the SIMPLE iteration and a projection onto a space of functions satisfying all linearized and homogeneous boundary conditions. The finite volume grid is gradually refined from \(100^{3}\) to \(256^{3}\) finite volumes. A self-sustaining oscillatory process responsible for the instability onset is revealed, visualized and explained.

► Mixing in three-dimensional cavity by moving cavity walls
    1 Dec, 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.

► Actuator and sensor placement for closed-loop control of convective instabilities
    1 Dec, 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.

► Under-resolved and large eddy simulations of a decaying Taylor–Green vortex with the cumulant lattice Boltzmann method
  19 Nov, 2020

Abstract

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

► A calculus for flows in periodic domains
  17 Nov, 2020

Abstract

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

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

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

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

► On the boundary conditions in the Stokesian flows
  10 Nov, 2020

Abstract

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


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