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Annual Review of Fluid Mechanics top

► Advances in Modeling Dense Granular Media
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 215-240, January 2024.
► Turbulent Drag Reduction by Streamwise Traveling Waves of Wall-Normal Forcing
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 69-90, January 2024.
► Fluid-Elastic Interactions Near Contact at Low Reynolds Number
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 491-519, January 2024.
► The Fluid Mechanics of Female Reproduction: A Review of the Biofluid Mechanics of Pregnancy and Delivery
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 171-188, January 2024.
► Statistical Models for the Dynamics of Heavy Particles in Turbulence
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 189-213, January 2024.
► Multiscale Velocity Gradients in Turbulence
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 463-490, January 2024.
► Deformation and Breakup of Bubbles and Drops in Turbulence
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 319-347, January 2024.
► Fluid Dynamics of Squirmers and Ciliated Microorganisms
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 119-145, January 2024.
► Building Ventilation: The Consequences for Personal Exposure
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 405-434, January 2024.
► Interfacial Dynamics Pioneer Stephen H. Davis (1939–2021)
  19 Jan, 2024
Annual Review of Fluid Mechanics, Volume 56, Issue 1, Page 1-20, January 2024.

Computers & Fluids top

► Well-balanced finite difference WENO-AO scheme for rotating shallow water equations with Coriolis force
    

Publication date: 15 April 2024

Source: Computers & Fluids, Volume 273

Author(s): Nan Zhang

► The harmonic linearized Navier–Stokes equations for transition prediction in three-dimensional flows
    

Publication date: 15 April 2024

Source: Computers & Fluids, Volume 273

Author(s): Pedro Paredes, Meelan Choudhari, Mark H. Carpenter, Fei Li

► Data-driven approach for modeling Reynolds stress tensor with invariance preservation
    

Publication date: Available online 20 February 2024

Source: Computers & Fluids

Author(s): Xuepeng Fu, Shixiao Fu, Chang Liu, Mengmeng Zhang, Qihan Hu

► A generating absorbing boundary condition for simulating wave interaction with maritime structures in current or at forward speed
    

Publication date: Available online 16 February 2024

Source: Computers & Fluids

Author(s): X. Chang, P.R. Wellens

► An all-Mach consistent numerical scheme for simulation of compressible multi-component fluids including surface tension, cavitation, turbulence modeling and interface sharpening on compact stencils
    

Publication date: Available online 6 February 2024

Source: Computers & Fluids

Author(s): Yu Jiao, Steffen J. Schmidt, Nikolaus A. Adams

► Output-based mesh adaptation for high-speed flows
    

Publication date: Available online 8 February 2024

Source: Computers & Fluids

Author(s): James G. Coder, Benjamin L.S. Couchman, Marshall C. Galbraith, Steven R. Allmaras, Nick Wyman

► Meshfree one-fluid modeling of liquid–vapor phase transitions
    

Publication date: 15 April 2024

Source: Computers & Fluids, Volume 273

Author(s): Pratik Suchde, Heinrich Kraus, Benjamin Bock-Marbach, Jörg Kuhnert

► A review of Hyperloop aerodynamics
    

Publication date: 15 April 2024

Source: Computers & Fluids, Volume 273

Author(s): Alex J. Lang, David P. Connolly, Gregory de Boer, Shahrokh Shahpar, Benjamin Hinchliffe, Carl A. Gilkeson

► An efficient thermal lattice Boltzmann method for simulating three-dimensional liquid–vapor phase change
    

Publication date: 15 April 2024

Source: Computers & Fluids, Volume 273

Author(s): Jiangxu Huang, Lei Wang, Xuguang Yang

► Eulerian finite volume method using Lagrangian markers with reference map for incompressible fluid–structure interaction problems
    

Publication date: 30 April 2024

Source: Computers & Fluids, Volume 274

Author(s): Koji Nishiguchi, Tokimasa Shimada, Christian Peco, Keito Kondo, Shigenobu Okazawa, Makoto Tsubokura

International Journal of Computational Fluid Dynamics top

► An Accurate and Robust Line-Hybrid Method for Hypersonic Heating Predictions
  22 Jan, 2024
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► High-Order WENO-based Semi-Implicit Projection Method for Incompressible Turbulent Flows: Development, Accuracy, and Reynolds Number Effects
    5 Jan, 2024
Volume 37, Issue 4, May 2023, Page 251-278
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► Numerical Investigation of Gasper Air Jet Dynamics in an Aircraft Cabin
    3 Jan, 2024
Volume 37, Issue 4, May 2023, Page 298-315
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► Multi-Viscosity Physics-Informed Neural Networks for Generating Ultra High Resolution Flow Field Data
  22 Dec, 2023
Volume 37, Issue 4, May 2023, Page 279-297
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► Identification of Oil and Gas Two-Phase Flow Patterns in Aero-Engine Bearing Chambers Based on Kriging Method
    5 Dec, 2023
Volume 37, Issue 4, May 2023, Page 316-332
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► A third‐order entropy condition scheme for hyperbolic conservation laws
  16 Feb, 2024

Abstract

Following the solution formula method given in Dong et al. (High order discontinuities decomposition entropy condition schemes for Euler equations. CFD J. 2002;10(4): 448–457), this article studies a type of one-step fully-discrete scheme, and constructs a third-order scheme which is written into a compact form via a new limiter. The highlights of this study and advantages of new third-order scheme are as follows: ① We proposed a very simple new methodology of constructing one-step, consistent high-order and non-oscillation schemes that do not rely on Runge–Kutta method; ② We systematically studied new scheme's theoretical problems about entropy conditions, error analysis, and non-oscillation conditions; ③ The new scheme achieves exact solution in linear cases and performing better in nonlinear cases when CFL → 1; ④ The new scheme is third order but high resolution with excellent shock-capturing capacity which is comparable to fifth order WENO scheme; ⑤ CPU time of new scheme is only a quarter of WENO5 + RK3 under same computing condition; ⑥ For engineering applications, the new scheme is extended to multi-dimensional Euler equations under curvilinear coordinates. Numerical experiments contain 1D scalar equation, 1D,2D,3D Euler equations. Accuracy tests are carried out using 1D linear scalar equation, 1D Burgers equation and 2D Euler equations and two sonic point tests are carried out to show the effect of entropy condition linearization. All tests are compared with results of WENO5 and finally indicate EC3 is cheaper in computational expense.

► A parallel grad‐div stabilized finite element algorithm for the Navier–Stokes equations with a nonlinear damping term
  15 Feb, 2024
A parallel grad-div stabilized finite element algorithm for the Navier–Stokes equations with a nonlinear damping term

A parallel grad-div stabilized finite element algorithm based on fully overlapping domain decomposition is proposed for the Navier–Stokes equations with damping. The algorithm calculates a local solution in a subdomain on a global composite mesh that is locally refined around the subdomain, making it simple to carry out on the basis of available sequential solvers. Effectiveness of the algorithm is verified by theoretical analysis and numerical experiments.


Abstract

In this work, we propose a parallel grad-div stabilized finite element algorithm for the Navier–Stokes equations attached with a nonlinear damping term, using a fully overlapping domain decomposition approach. In the proposed algorithm, we calculate a local solution in a defined subdomain on a global composite mesh which is fine around the defined subdomain and coarse in other regions. The algorithm is simple to carry out on the basis of available sequential solvers. By a local a priori estimate of the finite element solution, we deduce error bounds of the approximations from our presented algorithm. We perform also some numerical experiments to verify the effectiveness of the proposed algorithm.

► Employment of an efficient particle tracking algorithm based on barycentric coordinates in hybrid finite‐volume/probability‐density‐function Monte Carlo methods
  14 Feb, 2024
Employment of an efficient particle tracking algorithm based on barycentric coordinates in hybrid finite-volume/probability-density-function Monte Carlo methods

The present study shows that the number of particle time-steps required to reach the statistically steady-state condition is at least one-sixth less than the previously developed algorithms. This indicates that the current hybrid algorithm requires much less computational work and time to converge to solution. Moreover, the implementation of the present extended method can highly improve its capabilities in numerical prediction of turbulent flows in very complex geometries.


Abstract

One main concern of this work is to develop an efficient particle-tracking-managing algorithm in the framework of a hybrid pressure-based finite-volume/probability-density-function (FV/PDF) Monte-Carlo (MC) solution algorithm to extend the application of FV/PDF MC methods to absolutely incompressible flows and speedup the convergence rate of solving the fluctuating velocity-turbulent frequency joint PDF equation in turbulent flow simulations. Contrary to the density-based algorithms, the pressure-based algorithms have stable convergence rates even in zero-Mach number flows. As another contribution, literature shows that the past developed methods mostly used mesh searching techniques to attribute particles to cells at the beginning of each tracking time-step. Also, they had to calculate the linear basis functions at every time-step to estimate the particle mean fields and interpolate the data. These calculations would be computationally very expensive, time-consuming, and inefficient in computational domains with arbitrary-shaped 3D meshes. As known, the barycentric tracking is a continuous particle tracking method, which provides more efficiency in case of handling 3D domains with general mesh shapes. The barycentric tracking eliminates any mesh searching technique and readily provides the convenient linear basis functions. So, this work benefits from these advantages and tracks the particles based on their barycentric coordinates.  It leads to less computational work and a better efficiency for the present method. A bluff-body turbulent flow case is examined to validate the present FV/PDF MC method. From the accuracy perspective, it is shown that the results of the present algorithm are in great agreement with experimental data and available numerical solutions. The present study shows that the number of particle time-steps required to reach the statistically steady-state condition is at least one-sixth less than the previously developed algorithms. This also approves a faster convergence rate for the present hybrid pressure-based algorithm.

► A novel stabilized nodal integration formulation using particle finite element method for incompressible flow analysis
  13 Feb, 2024
A novel stabilized nodal integration formulation using particle finite element method for incompressible flow analysis

This study presents a stabilized PFEM formulation to simulate an incompressible fluid with free-surface flow. Comparisons results demonstrate the strong ability of the proposed stabilized PFEM to solve incompressible free-surface flow with high accuracy and promising application prospects.


Abstract

In simulations using the particle finite element method (PFEM) with node-based strain smoothing technique (NS-PFEM) to simulate the incompressible flow, spatial and temporal instabilities have been identified as crucial problems. Accordingly, this study presents a stabilized NS-PFEM-FIC formulation to simulate an incompressible fluid with free-surface flow. In the proposed approach, (1) stabilization is achieved by implementing the gradient strain field in place of the constant strain field over the smoothing domains, handling spatial and temporal instabilities in direct nodal integration; (2) the finite increment calculus (FIC) stabilization terms are added using nodal integration, and a three-step fractional step method is adopted to update pressures and velocities; and (3) a novel slip boundary with the predictor–corrector algorithm is developed to deal with the interaction between the free-surface flow with rigid walls, avoiding the pressure concentration induced by standard no-slip condition. The proposed stabilized NS-PFEM-FIC is validated via several classical numerical cases (hydrostatic test, water jet impinging, water dam break, and water dam break on a rigid obstacle). Comparisons of all simulations to the experimental results and other numerical solutions reveal good agreement, demonstrating the strong ability of the proposed stabilized NS-PFEM-FIC to solve incompressible free-surface flow with high accuracy and promising application prospects.

► Two‐ and three‐dimensional multiphase mesh‐free particle modeling of transitional landslide with μ(I) rheology
  13 Feb, 2024
Two- and three-dimensional multiphase mesh-free particle modeling of transitional landslide with μ(I) rheology

In the present study, describes the rheological transition from dry to wet sedimentary materials as a transition from shear thinning behavior to shear thickness behavior. This transition can clearly have a significant effect on saturated materials. In this study, a transition period is defined which the sliding materials are saturated. Although this period is constant, for each particle based on the time it reaches the initial water level, it is separately initialized.


Abstract

Landslides, which are the sources of most catastrophic natural disasters, can be subaerial (dry), submerged (underwater), or semi-submerged (transitional). Semi-submerged or transitional landslides occur when a subaerial landslide enters water and turns to submerged condition. Predicting the behavior of such a highly dynamic multi-phase granular flow system is challenging, mainly due to the water entry effects, such as wave impact and partial saturation (and resulted cohesion). The mesh-free particle methods, such as the moving particle semi-implicit (MPS) method, have proven their capabilities for the simulation of the highly dynamic multiphase systems. This study develops and evaluates a numerical model, based on the MPS particle method in combination with the μ(I) rheological model, to simulate the morphodynamic of the granular mass in semi-submerged landslides in two and three dimensions. An algorithm is developed to consider partial saturation (and resulting cohesion) during the water entry. Comparing the numerical results with the experimental measurements shows the ability of the proposed model to accurately reproduce the morphological evolution of the granular mass, especially at the moment of water entry.

► Comment on the paper “an explicit‐implicit numerical scheme for time fractional boundary layer flows, International Journal for Numerical Methods in Fluids, 2022, 94:920–940”
  12 Feb, 2024
International Journal for Numerical Methods in Fluids, EarlyView.
► Moving least‐squares aided finite element method: A powerful means to predict flow fields in the presence of a solid part
  12 Feb, 2024
Moving least-squares aided finite element method: A powerful means to predict flow fields in the presence of a solid part

Many physical and industrial problems comprise one or more moving parts, which change the flow domain during a process, such as lubrication and mixing. For such problems, we have suggested a new method, which uses a fixed background mesh to avoid the time-consuming task of boundary-fitted grid generation. The new technique, which is based on the finite element method, uses the moving least-squares interpolation functions, where the solid and fluid meet each other in the flow domain.


Abstract

With the assistance of the moving least-squares (MLS) interpolation functions, a two-dimensional finite element code is developed to consider the effects of a stationary or moving solid body in a flow domain. At the same time, the mesh or grid is independent of the shape of the solid body. We achieve this goal in two steps. In the first step, we use MLS interpolants to enhance the pressure (P) and velocity (V) shape functions. By this means, we capture different discontinuities in a flow domain. In our previous publications, we have named this technique the PVMLS method (pressure and velocity shape functions enhanced by the MLS interpolants) and described it thoroughly. In the second step, we modify the PVMLS method (the M-PVMLS method) to consider the effect of a solid part(s) in a flow domain. To evaluate the new method's performance, we compare the results of the M-PVMLS method with a finite element code that uses boundary-fitted meshes.

► High‐order gas kinetic flux solver for viscous compressible flow simulations
    9 Feb, 2024
High-order gas kinetic flux solver for viscous compressible flow simulations

A weighted essentially non-oscillatory-based gas kinetic flux solver for viscous compressible flow is proposed. The proposed C-GKFS could improve the accuracy because the high-order WENO scheme is adopted. The current method can use fewer grids to achieve more details of viscous compressible flows.


Abstract

Although the gas kinetic schemes (GKS) have emerged as one of the powerful tools for simulating compressible flows, they exhibit several shortcomings. Since the local solution of continuous Boltzmann equation with the Maxwellian distribution function is used to calculate the numerical fluxes at the cell interface, the flux expression in GKS is usually more complicated. In this paper, a high-order simplified gas kinetic flux solver (GKFS) is presented for simulating two-dimensional compressible flows. Circular function-based GKFS (C-GKFS), which simplifies the Maxwellian distribution function into the circular function, combined with an improved weighted essentially non-oscillatory (WENO-Z) scheme is applied to capture more details of the flow fields with fewer grids. As a result, a simple high-order accurate C-GKFS is obtained, which improves the computing efficiency and reduce its complexity to facilitate the practical application of engineering. A series of benchmark-test problems are simulated and good agreement can be obtained compared with the references, which demonstrate that the high-order C-GKFS can achieve the desired accuracy.

► A sharp immersed method for electrohydrodynamic flows accompanied by charge evaporation
    9 Feb, 2024
A sharp immersed method for electrohydrodynamic flows accompanied by charge evaporation

An adaptive sharp immersed method is proposed to simulate electrohydrodynamic flows accompanied by ion evaporation. A splitting error-free iterative projection algorithm is used to solve the Navier–Stokes equations, and a robust iterative algorithm is used to address the surface charge transport. Our simulations captured the protrusion structure caused by charge evaporation and showed that charge evaporation can suppress the sharp development of Taylor cones at the ends of the drops.


Abstract

This article presents a sharp immersed method for simulating electrohydrodynamic (EHD) flows that involve charge evaporation. This well-known multi-scale, multi-physics problem is widely used in various fields, including industry and medicine. The method adopts a fully sharp model, where surface tension and Maxwell stress are treated as surface forces and free charges are concentrated on the zero thickness liquid-vacuum interface. Incorporating charge evaporation imposes strict restrictions on the time-step, as the rate of evaporation sharply increases with surface evolution. To overcome this challenge, an iterative algorithm that couples the electric field and surface charge density is proposed to obtain accurate results, even with significantly large time-steps. To mitigate the numerical residuals near the interface, which may introduce parasitic flows and cause numerical instability, an immersed interface method-based iterative projection method for the Navier–Stokes equations is proposed, in which a traction boundary condition involving multiple surface forces is imposed on the sharp interface. Numerical experiments were carried out, and the results show that the method is splitting-error-free and stable. The sharp immersed method is applied to simulate the electric-induced deformation of an ionic liquid drop with charge evaporation. The results indicate that charge evaporation can suppress the sharp development of Taylor cones at the ends of the drops. These findings have significant implications for the design and optimization of EHD systems in various applications.

► Implicit coupling methods for nonlinear interactions between a large‐deformable hyperelastic solid and a viscous acoustic fluid of infinite extent
    7 Feb, 2024
Implicit coupling methods for nonlinear interactions between a large-deformable hyperelastic solid and a viscous acoustic fluid of infinite extent

A hybrid explicit/implicit method is developed to accommodate the nonlinear vibro-acoustic interaction. Theoretical formulations for stability analysis of the implicit methods is proposed and verified numerically. The methods are applied to investigate the nonlinear dynamic behaviors of a coupled hyperelastic elliptical ring and infinite acoustic fluid system. An interesting nonlinear phenomenon of 4:2:1 internal resonance is simulated and discussed.


Abstract

This paper addresses the challenges in studying the interaction between high-intensity sound waves and large-deformable hyperelastic solids, which are characterized by nonlinearities of the hyperelastic material, the finite-amplitude acoustic wave, and the large-deformable fluid–solid interface. An implicit coupling method is proposed for predicting nonlinear structural-acoustic responses of the large-deformable hyperelastic solid submerged in a compressible viscous fluid of infinite extent. An arbitrary Lagrangian–Eulerian (ALE) formulation based on an unsplit complex-frequency-shifted perfectly matched layer method is developed for long-time simulation of the nonlinear acoustic wave propagation without exhibiting long-time instabilities. The solid and acoustic fluid domains are discretized using the finite element method, and two different options of staggered implicit coupling procedures for nonlinear structural-acoustic interactions are developed. Theoretical formulations for stability analysis of the implicit methods are provided. The accuracy, robustness, and convergence properties of the proposed methods are evaluated by a benchmark problem, that is, a hyperelastic rod interacting with finite-amplitude acoustic waves. The numerical results substantiate that the present methods are able to provide long-time steady-state solutions for a nonlinear coupled hyperelastic solid and viscous acoustic fluid system without numerical constraints of small time step sizes and long-time instabilities. The methods are applied to investigate nonlinear dynamic behaviors of coupled hyperelastic elliptical ring and acoustic fluid systems. Physical insights into 2:1 and 4:2:1 internal resonances of the hyperelastic elliptical ring and period-doubling bifurcations of the structural and acoustic responses of the system are provided.

Journal of Computational Physics top

► Generalized finite difference method on unknown manifolds
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Shixiao Willing Jiang, Rongji Li, Qile Yan, John Harlim

► Helmholtz decomposition based windowed Green function methods for elastic scattering problems on a half-space
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Tao Yin, Lu Zhang, Xiaopeng Zhu

► Modeling and simulation of the cavitation phenomenon in turbopumps
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Joris Cazé, Fabien Petitpas, Eric Daniel, Matthieu Queguineur, Sébastien Le Martelot

► Efficient and fail-safe quantum algorithm for the transport equation
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Merel A. Schalkers, Matthias Möller

► Weak collocation regression method: Fast reveal hidden stochastic dynamics from high-dimensional aggregate data
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Liwei Lu, Zhijun Zeng, Yan Jiang, Yi Zhu, Pipi Hu

► A non-conforming-in-space numerical framework for realistic cardiac electrophysiological outputs
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Elena Zappon, Andrea Manzoni, Alfio Quarteroni

► A comprehensive framework for robust hybrid RANS/LES simulations of wall-bounded flows in LBM
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): J. Husson, M. Terracol, S. Deck, T. Le Garrec

► Mesh-free hydrodynamic stability
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Tianyi Chu, Oliver T. Schmidt

► Octree-based hierarchical sampling optimization for the volumetric super-resolution of scientific data
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Xinjie Wang, Maoquan Sun, Yundong Guo, Chunxin Yuan, Xiang Sun, Zhiqiang Wei, Xiaogang Jin

► Gaussian process regression and conditional Karhunen-Loève models for data assimilation in inverse problems
    

Publication date: 1 April 2024

Source: Journal of Computational Physics, Volume 502

Author(s): Yu-Hong Yeung, David A. Barajas-Solano, Alexandre M. Tartakovsky

Journal of Turbulence top

► Variational calculus in hybrid turbulence transport models with passive scalar
  25 Jan, 2024
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► Quantifying instantaneous flow reversal of tracer particles in subsonic, transonic and supersonic flows past a circular cylinder
  12 Jan, 2024
Volume 24, Issue 11-12, November - December 2023, Page 613-653
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► Exciting turbulence in an elongated domain
    9 Jan, 2024
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► Evaluating anisotropic minimum dissipation, sigma and modulated gradient subgrid-scale models in large-eddy simulation of compressible mixing layers
  25 Dec, 2023
Volume 24, Issue 11-12, November - December 2023, Page 654-685
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► Circulation in turbulent flow through a contraction
  20 Dec, 2023
Volume 24, Issue 11-12, November - December 2023, Page 577-612
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► Advanced detached-eddy simulation of the MD 30P-30N three-element airfoil
    6 Nov, 2023
Volume 24, Issue 11-12, November - December 2023, Page 554-576
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► Fractional and tempered fractional models for Reynolds-averaged Navier–Stokes equations
  31 Oct, 2023
Volume 24, Issue 11-12, November - December 2023, Page 507-553
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Physics of Fluids top

► A sharp interface immersed edge-based smoothed finite element method with extended fictitious domain scheme
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
This paper proposes a versatile and robust immersed edge-based smoothed finite element method with the mass conservation algorithm (IESFEM/Mass) to solve partitioned fluid–structure interaction (FSI). A gradient smoothing technique was used to solve the system governing equations, which can improve the calculated capability of the linear triangular elements in two phases. Based on the quadratic sharp interface representation of immersed boundary, an extended fictitious domain constructed by a least squares method approximately corrected the residual flux error. The compatibility for boundary conditions on moving interfaces was satisfied, thus eliminating spurious oscillations. The results from all numerical examples were consistent with those from the existing experiments and published numerical solutions. Furthermore, the present divergence-free vector field had a faster-converged rate in the flow velocity, pressure, and FSI force. Even if in distorted meshes, the proposed algorithm maintained a stable accuracy improvement. The aerodynamics of one- and two-winged flapping motions in insect flight has been investigated through the IESFEM/Mass. It can be seen that the wing–wake interaction mechanism is a vital factor affecting the lift. The applicability of the present method in the biological FSI scenario was also well-demonstrated.
► Enhanced and reduced solute transport and flow strength in salt finger convection in porous media
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
We report a pore-scale numerical study of salt finger convection in porous media, with a focus on the influence of the porosity in the non-Darcy regime, which has received little attention in previous research. The numerical model is based on the lattice Boltzmann method with a multiple-relaxation-time scheme and employs an immersed boundary method to describe the fluid–solid interaction. The simulations are conducted in a two-dimensional, horizontally periodic domain with an aspect ratio of 4, and the porosity [math] is varied from 0.7 to 1, while the solute Rayleigh number [math] ranges from [math] to [math]. Our results show that, for all explored [math], solute transport first enhances unexpectedly with decreasing [math] and then decreases when [math] is smaller than a [math]-dependent value. On the other hand, while the flow strength decreases significantly as [math] decreases at low [math], it varies weakly with decreasing [math] at high [math] and even increases counterintuitively for some porosities at moderate [math]. Detailed analysis of the salinity and velocity fields reveals that the fingered structures are blocked by the porous structure and can even be destroyed when their widths are larger than the pore scale, but become more ordered and coherent with the presence of porous media. This combination of opposing effects explains the complex porosity dependencies of solute transport and flow strength. The influence of porous structure arrangement is also examined, with stronger effects observed for smaller [math] and higher [math]. These findings have important implications for passive control of mass/solute transport in engineering applications.
► On the instability of the magnetohydrodynamic pipe flow subject to a transverse magnetic field
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
The linear stability of a fully developed liquid–metal magnetohydrodynamic pipe flow subject to a transverse magnetic field is studied numerically. Because of the lack of axial symmetry in the mean velocity profile, we need to perform a BiGlobal stability analysis. For that purpose, we develop a two-dimensional complex eigenvalue solver relying on a Chebyshev–Fourier collocation method in physical space. By performing an extensive parametric study, we show that in contrast to the Hagen–Poiseuille flow known to be linearly stable for all Reynolds numbers, the magnetohydrodynamic pipe flow with transverse magnetic field is unstable to three-dimensional disturbances at sufficiently high values of the Hartmann number and wall conductance ratio. The instability observed in this regime is attributed to the presence of velocity overspeed in the so-called Roberts layers and the corresponding inflection points in the mean velocity profile. The nature and characteristics of the most unstable modes are investigated, and we show that they vary significantly depending on the wall conductance ratio. A major result of this paper is that the global critical Reynolds number for the magnetohydrodynamic pipe flow with transverse magnetic field is Re = 45 230, and it occurs for a perfectly conducting pipe wall and the Hartmann number Ha = 19.7.
► The turbulence development at its initial stage: A scenario based on the idea of vortices decay
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
In this paper, a model of the development of a quantum turbulence in its initial stage is proposed. The origin of the turbulence in the suggested model is the decay of vortex loops with an internal structure. We consider the initial stage of this process, before an equilibrium state is established. As result of our study, the density matrix of developing turbulent flow is calculated. The quantization scheme of the classical vortex rings system is based on the approach proposed by the author earlier.
► Interstage difference and deterministic decomposition of internal unsteady flow in a five-stage centrifugal pump as turbine
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
A five-stage centrifugal pump is utilized to investigate the interstage flow characteristics of the multistage centrifugal pump as turbine (PAT). The simulation results of performance are verified by comparing with the experimental results. Owing to the distinct structural attributes, significant differences in flow occur between the first stage and the other stages of the multistage PAT. To enhance the understanding of these disparities and explore their repercussions, this study focuses on analyzing the flow within the impellers in the first and second stages by a deterministic analysis. The main conclusions are as follows: The discrepancies in the inflow conditions are the major reason for the dissimilarities in the flow of impellers between stages. The impact loss generated by the misalignment between the positive guide vane outlet angle and the impeller inlet angle leads to flow deviation between impeller passages and affects the internal flow pattern. The unsteadiness under low flow rates is mostly produced by the spatial gradient of the blade-to-blade nonuniformities, which is relevant to the relative position between blades and the positive guide vanes. At high flow rates, especially in the second-stage impeller, the pure unsteady term is the primary cause of flow unsteadiness as a result of the flow separation induced by interactions between the blades and the positive guide vanes. This study can provide some references for the practical operation and performance optimization of the multistage PATs in the future.
► Effect of gravity on phase transition for liquid–gas simulations
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
Direct simulations of phase-change and phase-ordering phenomena are becoming more common. Recently, qualitative simulations of boiling phenomena have been undertaken by a large number of research groups. One seldom discussed limitation is that large values of gravitational forcing are required to simulate the detachment and rise of bubbles formed at the bottom surface. The forces are typically so large that neglecting the effects of varying pressure in the system becomes questionable. In this paper, we examine the effect of large pressure variations induced by gravity using pseudopotential lattice Boltzmann simulations. These pressure variations lead to height dependent conditions for phase coexistence and nucleation of either gas or liquid domains. Because these effects have not previously been studied in the context of these simulation methods, we focus here on the phase stability in a one-dimensional system, rather than the additional complexity of bubble or droplet dynamics. Even in this simple case, we find that the different forms of gravitational forces employed in the literature lead to qualitatively different phenomena, leading to the conclusion that the effects of gravity induced pressure variations on phase-change phenomena should be very carefully considered when trying to advance boiling and cavitation as well as liquefaction simulations to become quantitative tools.
► Entrapment and mobilization dynamics during the flow of viscoelastic fluids in natural porous media: A micro-scale experimental investigation
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
Capillary desaturation process was investigated as a function of wetting phase rheological signatures during the injection of Newtonian and non-Newtonian fluids. Two sets of two-phase imbibition flow experiments were conducted on a water-wet sandstone core sample using brine and viscoelastic polymer solutions. During the experiments, a high-resolution micro-computed tomography scanner was employed to directly map pore-level fluid occupancies within the pore space. The results of the experiments revealed that at a given capillary number, the viscoelastic polymer was more efficient than the brine in recovering the non-wetting oil phase. At low capillary numbers, this is attributed to the improved accessibility of the viscoelastic polymer solution to the entrance of pore elements, which suppressed snap-off events and allowed more piston-like and cooperative pore-body filling events to contribute to oil displacement. For intermediate capillary numbers, the onset of elastic turbulence caused substantial desaturation, while at high capillary numbers, the superimposed effects of higher viscous and elastic forces further improved the mobilization of the trapped oil ganglia by the viscoelastic polymer. In the waterflood, however, the mobilization of oil globules was the governing recovery mechanism, and the desaturation process commenced only when the capillary number reached a threshold value. These observations were corroborated with the pore-level fluid occupancy maps produced for the brine and viscoelastic polymer solutions during the experiments. Furthermore, at the intermediate and high capillary numbers, the force balance and pore-fluid occupancies suggested different flow regimes for the non-Newtonian viscoelastic polymer. These regions are categorized in this study as elastic-capillary- and viscoelastic-dominated flow regimes, different from viscous-capillary flow conditions that are dominant during the flow of Newtonian fluids. Moreover, we have identified novel previously unreported pore-scale displacement events that take place during the flow of viscoelastic fluids in a natural heterogeneous porous medium. These events, including coalescence, fragmentation, and re-entrapment of oil ganglia, occurred before the threshold of oil mobilization was reached under the elastic-capillary-dominated flow regime. In addition, we present evidence for lubrication effects at the pore level due to the elastic properties of the polymer solution. Furthermore, a comparison of capillary desaturation curves generated for the Newtonian brine and non-Newtonian viscoelastic polymer revealed that the desaturation process was more significant for the viscoelastic polymer than for the brine. Finally, the analysis of trapped oil clusters showed that the ganglion size distribution depends on both the capillary number and the rheological properties of fluids.
► Impact of wettability on interface deformation and droplet breakup in microcapillaries
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
The objective of this research paper is to relate the influence of dynamic wetting in a liquid/liquid/solid system to the breakup of emulsion droplets in capillaries. Therefore, modeling and simulation of liquid/liquid flow through a capillary constriction have been performed with varying dynamic contact angles from highly hydrophilic to highly hydrophobic. Advanced advection schemes with geometric interface reconstruction (isoAdvector) are incorporated for high interface advection accuracy. A sharp surface tension force model is used to reduce spurious currents originating from the numerical treatment and geometric reconstruction of the surface curvature at the interface. Stress singularities from the boundary condition at the three-phase contact line are removed by applying a Navier-slip boundary condition. The simulation results illustrate the strong dependency of the wettability and the contact line and interface deformation.
► Drag increase and turbulence augmentation in two-way coupled particle-laden wall-bounded flows
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
The exact regularized point particle method is used to characterize the turbulence modulation in two-way momentum-coupled direct numerical simulations of a turbulent pipe flow. The turbulence modification is parametrized by the particle Stokes number, the mass loading, and the particle-to-fluid density ratio. The data show that in the wide region of the parameter space addressed in the present paper, the overall friction drag is either increased or unaltered by the particles with respect to the uncoupled case. In the cases where the wall friction is enhanced, the fluid velocity fluctuations show a substantial modification in the viscous sub-layer and in the buffer layer. These effects are associated with a modified turbulent momentum flux toward the wall. The particles suppress the turbulent fluctuations in the buffer region and concurrently provide extra stress in the viscous sub-layer. The sum of the turbulent stress and the extra stress is larger than the Newtonian turbulent stress, thus explaining the drag increase. The non-trivial turbulence/particles interaction turns out in a clear alteration of the near-wall flow structures. The streamwise velocity streaks lose their spatial coherence when two-way coupling effects are predominant. This is associated with a shift of the streamwise vortices toward the center of the pipe and with the concurrent presence of small-scale and relatively more intense vortical structures near the wall.
► Partial and complete wetting of thin films with dynamic contact angle
  17 Apr, 2023
Physics of Fluids, Volume 35, Issue 4, April 2023.
The wetting of thin films depends critically on the sign of the spreading coefficient [math]. We discuss the cases S < 0, S = 0, and S > 0 for transient models with contact line dissipation and find that the use of a dynamic contact angle solves problems for S > 0 that models might otherwise have. For initial data with a non-zero slope and S > 0, we show that there exists a finite time [math] at which the contact angle of the thin film goes to zero. Then, a molecular precursor emerges from the thin film and moves outward at a constant velocity.

Theoretical and Computational Fluid Dynamics top

► Wavy ground effects on the stability of cylinder wakes
  21 Feb, 2024

Abstract

The stability of the flow past a circular cylinder in the presence of a wavy ground is investigated numerically in this paper. The wavy ground consists of two complete waves with a wavelength of 4D and an amplitude of 0.5D, where D is the cylinder diameter. The vertical distance between the cylinder and the ground is varied, and four different cases are considered. The stability analysis shows that the critical Reynolds number increases for cases close to the ground when compared to the flow past a cylinder away from the ground. The maximum critical Reynolds number is obtained when the cylinder is located in front of the waves. The wavy ground adds layers of clockwise (negative) vorticity due to flow separation from the wave peak, to the oscillating Kármán vortex. This negative vorticity from the wave peak also cancels part of the positive (counterclockwise) vorticity shed from the bottom half of the cylinder. In addition, the negative vorticity from the wave peak strengthens the clockwise (negative) vorticity shed from the top half of the cylinder. These interactions combined with the ground effect skewed the flow away from the ground. The base flow is skewed upward for all the near-ground cases. However, this skew is larger when the cylinder is located over the wavy ground. The vortex shedding frequency is also altered due to the presence of the waves. The main eigenmode found for plain flow past a cylinder appears to become suppressed for cases closer to the ground. Limited particle image velocimetry experiments are reported which corroborate the finding from the stability analysis.

Graphical abstract

► Simulation of the unsteady vortical flow of freely falling plates
  14 Feb, 2024

Abstract

An inviscid vortex shedding model is numerically extended to simulate falling flat plates. The body and vortices separated from the edge of the body are described by vortex sheets. The vortex shedding model has computational limitations when the angle of incidence is small and the free vortex sheet approaches the body closely. These problems are overcome by using numerical procedures such as a method for a near-singular integral and the suppression of vortex shedding at the plate edge. The model is applied to a falling plate of flow regimes of various Froude numbers. For \(\text {Fr}=0.5\) , the plate develops large-scale side-to-side oscillations. In the case of \(\text {Fr}=1\) , the plate motion is a combination of side-to-side oscillations and tumbling and is identified as a chaotic type. For \(\text {Fr}=1.5\) , the plate develops to autorotating motion. Comparisons with previous experimental results show good agreement for the falling pattern. The dependence of change in the vortex structure on the Froude number and its relation with the plate motion is also examined.

Graphical abstract

► Linear stability analysis of surface waves of liquid jet injected in transverse gas flow with different angles
  12 Feb, 2024

Abstract

A theoretical and experimental study was conducted to investigate the effect of injection angle on surface waves. Linear stability theory was utilized to obtain the analytical relation. In the experimental study, high-speed photography and shadowgraph techniques were used. Image processing codes were developed to extract information from photos. The results obtained from the theoretical relation were validated with the experimental results at different injection angles. In addition, at the injection angle of 90 \({^\circ }\) , the theoretical results were evaluated with the experimental results of other researchers. This evaluation showed that the theory results were in good agreement with the experimental data. The proper orthogonal decomposition (POD) and the power spectra density (PSD) analysis were also used to investigate the effect of the injection angle on the flow structures. The results obtained from the linear stability were used to determine the maximum waves’ growth rate, and a relation was presented for the breakup length of the liquid jet at different injection angles. The breakup length results were compared with theory and published experimental data. The presented relation is more consistent with experimental data than other theories due to considering the nature of waves. The results showed that the instability of the liquid jet is influenced by three forces: inertial, surface tension, and aerodynamic. Therefore, Rayleigh–Taylor, Kelvin–Helmholtz, Rayleigh–Plateau, and azimuthal instabilities occur in the process. Decreasing the injection angle changes the nature of waves and shifts from Rayleigh–Taylor to Kelvin–Helmholtz. That reduces the wavelength and increases the growth rate of the waves. Axial waves have a significant impact on the physics of the waves and influence parameters. If axial waves are not formed, the growth rate of the waves is independent of the injection angle. An increase in the gas Weber number causes a change in the type of dominant waves and a greater instability of the liquid jet. In contrast, an increase in the liquid Weber number causes an enhancement in the resistance of the liquid jet against the transverse flow without changing the type of the dominant waves. Decreasing the density ratio reduces the effect of Rayleigh–Taylor waves and strengthens the Kelvin–Helmholtz waves. It causes two trends to be observed for the growth rate of waves at low spray angles, while one trend occurs at high spray angles.

Graphical abstract

► Fluid flow past a freely moving body in a straight or distorted channel
  25 Jan, 2024

Abstract

The focus here is on a thin solid body passing through a channel flow and interacting with the flow. Unsteady two-dimensional interactive properties from modelling, analysis and computation are presented along with comparisons. These include the effects of a finite dilation or constriction, as the body travels through, and the effects of a continuing expansion of the vessel. Finite-time clashing of the body with the channel walls is investigated as well as the means to avoid clashing. Sustained oscillations are found to be possible. Wake properties behind the body are obtained, and broad agreement in trends between full-system and reduced-system responses is found for increased body mass.

Graphical abstract

► Free surface wave interaction with a submerged body using a DtN boundary condition
  19 Jan, 2024

Abstract

Recently, Rim (Ocean Engng 239:711, 2021; J Ocean Engng Mar Energy 9:41-51, 2023 ) suggested an exact DtN artificial boundary condition to study the three-dimensional wave diffraction by stationary bodies. This paper is concerned with three-dimensional linear interaction between a submerged oscillating body with arbitrary shape and the regular water wave with finite depth. An exact Dirichlet-to-Neumann (DtN) boundary condition on a virtual cylindrical surface is derived, where the virtual surface is chosen so as to enclose the body and extract an interior subdomain with finite volume from the horizontally unbounded water domain. The DtN boundary condition is then applied to solve the interaction between the body and the linear wave in the interior subdomain by using boundary integral equation. Based on verification of the present model for a submerged vertical cylinder, the model is extended to the case of a submerged chamfer box with fillet radius in order to study 6-DoF oscillatory motion of the body under the free surface wave.

Graphical abstract

► Theory and simulation of shock waves freely propagating through monoatomic non-Boltzmann gas
  18 Jan, 2024

Abstract

The effect of non-Boltzmann energy distributions on the free propagation of shock waves through a monoatomic gas is investigated via theory and simulation. First, the non-Boltzmann heat capacity ratio \(\gamma \) , as a key property for describing shock waves, is derived from first principles via microcanonical integration. Second, atomistic molecular dynamics simulations resembling a shock tube setup are used to test the theory. The presented theory provides heat capacity ratios ranging from the well-known \(\gamma = 5/3\) for Boltzmann energy-distributed gas to \(\gamma \rightarrow 1\) for delta energy-distributed gas. The molecular dynamics simulations of Boltzmann and non-Boltzmann driven gases suggest that the shock wave propagates about 9% slower through the non-Boltzmann driven gas, while the contact wave appears to be about 4% faster if it trails non-Boltzmann driven gas. The observed slowdown of the shock wave through applying a non-Boltzmann energy distribution was found to be consistent with the classical shock wave equations when applying the non-Boltzmann heat capacity ratio. These fundamental findings provide insights into the behavior of non-Boltzmann gases and might help to improve the understanding of gas dynamical phenomena.

Graphical abstract

► Stability of supersonic boundary layer over an unswept wing with a parabolic airfoil
  28 Dec, 2023

Abstract

Under the low-noise Mach 3 flight conditions for a supersonic passenger aircraft having unswept wings with a thin parabolic airfoil, laminar-turbulent transition is due to amplification of the first mode. Stability of a local self-similar boundary layer over such a wing is investigated both using the \(e^{N}\) method in the framework of linear stability theory and direct numerical simulation (DNS). It is found that the instability amplitude should reach a maximum over the entire spectral range above the profiles of 2.5% and thicker. The locus of maximum appears at the trailing edge and moves to the leading edge as the profile becomes thicker, while the maximum amplitude decreases. The theoretical findings are supported by DNS of the linear wave packets propagating in the boundary layer. Significance of these results to the design of laminar supersonic wings is discussed.

Graphical abstract

► An adjoint-based methodology for calculating manufacturing tolerances for natural laminar flow airfoils susceptible to smooth surface waviness
  28 Dec, 2023

Abstract

An adjoint-based method is presented for determining manufacturing tolerances for aerodynamic surfaces with natural laminar flow subjected to wavy excrescences. The growth of convective unstable disturbances is computed by solving Euler, boundary layer, and parabolized stability equations. The gradient of the kinetic energy of disturbances in the boundary layer (E) with respect to surface grid points is calculated by solving adjoints of the governing equations. The accuracy of approximations of \(\Delta E\) , using gradients obtained from adjoint, is investigated for several waviness heights. It is also shown how second-order derivatives increase the accuracy of approximations of \(\Delta E\) when surface deformations are large. Then, for specific flight conditions, using the steepest ascent and the sequential least squares programming methodologies, the waviness profile with minimum \(L2-\) norm that causes a specific increase in the maximum value of N- factor, \(\Delta N\) , is found. Finally, numerical tests are performed using the NLF(2)-0415 airfoil to specify tolerance levels for \(\Delta {N}\) up to 2.0 for different flight conditions. Most simulations are carried out for a Mach number and angle of attack equal to 0.5 and \(1.25^{\circ }\) , respectively, and with Reynolds numbers between \(9\times 10^6\) and \(15\times 10^6\) and for waviness profiles with different ranges of wavelengths. Finally, some additional studies are presented for different angles of attack and Mach numbers to show their effects on the computed tolerances.

Graphic abstract

► Inviscid modeling of unsteady morphing airfoils using a discrete-vortex method
    7 Dec, 2023

Abstract

A low-order physics-based model to simulate the unsteady flow response to airfoils undergoing large-amplitude variations of the camber is presented in this paper. Potential-flow theory adapted for unsteady airfoils and numerical methods using discrete-vortex elements are combined to obtain rapid predictions of flow behavior and force evolution. To elude the inherent restriction of thin-airfoil theory to small flow disturbances, a time-varying chord line is proposed in this work over which to satisfy the appropriate boundary condition, enabling large deformations of the camber line to be modeled. Computational fluid dynamics simulations are performed to assess the accuracy of the low-order model for a wide range of dynamic trailing-edge flap deflections. By allowing the chord line to rotate with trailing-edge deflections, aerodynamic loads predictions are greatly enhanced as compared to the classical approach where the chord line is fixed. This is especially evident for large-amplitude deformations.

Graphical abstract

► Faster flicker of buoyant diffusion flames by weakly rotatory flows
    1 Dec, 2023

Abstract

Flickering buoyant diffusion methane flames in weakly rotatory flows were computationally and theoretically investigated. The prominent computational finding is that the flicker frequency nonlinearly increases with the nondimensional rotational intensity R (up to 0.24), which is proportional to the nondimensional circumferential circulation. This finding is consistent with the previous experimental observations that rotatory flows enhance flame flicker to a certain extent. Based on the vortex-dynamical understanding of flickering flames that the flame flicker is caused by the periodic shedding of buoyancy-induced toroidal vortices, a scaling theory is formulated for flickering buoyant diffusion flames in weakly rotatory flows. The theory predicts that the increase of flicker frequency f obeys the scaling relation \(\left( f-f_{0} \right) \propto R^{2}\) , which agrees very well with the present computational results. In physics, the external rotatory flow enhances the radial pressure gradient around the flame, and the significant baroclinic effect \(\mathrm {\nabla }p\times \mathrm {\nabla }\rho \) contributes an additional source for the growth of toroidal vortices so that their periodic shedding is faster.

Graphical abstract


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