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Annual Review of Fluid Mechanics top

► Large-Scale Eddy-Mean Flow Interaction in the Earth's Extratropical Atmosphere
  19 Jan, 2024

Large-scale circulation of the atmosphere in the Earth's extratropics is dominated by eddies, eastward (westerly) zonal winds, and their interaction. Eddies not only bring about weather variabilities but also help maintain the average state of climate. In recent years, our understanding of how large-scale eddies and mean flows interact in the extratropical atmosphere has advanced significantly due to new dynamical constraints on finite-amplitude eddies and the related eddy-free reference state. This article reviews the theoretical foundations for finite-amplitude Rossby wave activity and related concepts. Theory is then applied to atmospheric data to elucidate how angular momentum is redistributed by the generation, transmission, and dissipation of Rossby waves and to reveal how an anomalously large wave event such as atmospheric blocking may arise from regional eddy-mean flow interaction.

► Interfacial Dynamics Pioneer Stephen H. Davis (1939–2021)
  19 Jan, 2024

Stephen H. Davis (1939–2021) was an applied mathematician, fluid dynamicist, and materials scientist who lead the field in his contributions to interfacial dynamics, thermal convection, thin films, and solidification for over 50 years. Here, we briefly review his personal and professional life and some of his most significant contributions to the field.

► Fluid Dynamics of Airtanker Firefighting
  19 Jan, 2024

Airtanker firefighting is the most spectacular tool used to fight wildland fires. However, it employs a rudimentary large-scale spraying technology operating at a high speed and a long distance from the target. This review gives an overview of the fluid dynamics processes that govern this practice, which are characterized by rich and varied physical phenomena. The liquid column penetration in the air, its large-scale fragmentation, and an intense surface atomization give shape to the rainfall produced by the airtanker and the deposition of the final product on the ground. The cloud dynamics is controlled by droplet breakup, evaporation, and wind dispersion. The process of liquid deposition onto the forest canopy is full of open questions of great interest for rainfall retention in vegetation. Of major importance, but still requiring investigation, is the role of the complex non-Newtonian viscoelastic and shear-thinning behavior of the retardant dropped to stop the fire propagation. The review describes the need for future research devoted to the subject.

► The Early Days and Rise of Turbulence Simulation
  19 Jan, 2024

This review highlights major developments and milestones during the early days of numerical simulation of turbulent flows and its use to increase our understanding of turbulence phenomena. The period covered starts with the first simulations of decaying homogeneous isotropic turbulence in 1971–1972 and ends about 25 years later. Some earlier history of the progress in weather prediction is included if relevant. Only direct simulation, in which all scales of turbulence are accounted for explicitly, and large-eddy simulation, in which the effect of the smaller scales is modeled, are discussed. The method by which all scales are modeled, Reynolds-averaged Navier–Stokes, is not covered.

► Multiscale Velocity Gradients in Turbulence
  19 Jan, 2024

Understanding and predicting turbulent flow phenomena remain a challenge for both theory and applications. The nonlinear and nonlocal character of small-scale turbulence can be comprehensively described in terms of the velocity gradients, which determine fundamental quantities like dissipation, enstrophy, and the small-scale topology of turbulence. The dynamical equation for the velocity gradient succinctly encapsulates the nonlinear physics of turbulence; it offers an intuitive description of a host of turbulence phenomena and enables establishing connections between turbulent dynamics, statistics, and flow structure. The consideration of filtered velocity gradients enriches this view to express the multiscale aspects of nonlinearity and flow structure in a formulation directly applicable to large-eddy simulations. Driven by theoretical advances together with growing computational and experimental capabilities, recent activities in this area have elucidated key aspects of turbulence physics and advanced modeling capabilities.

► Flows Over Rotating Disks and Cones
  19 Jan, 2024

Rotating-disk flows were first considered by von Kármán in a seminal paper in 1921, where boundary layers in general were discussed and, in two of the nine sections, results for the laminar and turbulent boundary layers over a rotating disk were presented. It was not until in 1955 that flow visualization discovered the existence of stationary cross-flow vortices on the disk prior to the transition to turbulence. The rotating disk can be seen as a special case of rotating cones, and recent research has shown that broad cones behave similarly to disks, whereas sharp cones are susceptible to a different type of instability. Here, we provide a review of the major developments since von Kármán's work from 100 years ago, regarding instability, transition, and turbulence in the boundary layers, and we include some analysis not previously published.

► Bubble Plumes in Nature
  19 Jan, 2024

Bubble plumes are ubiquitous in nature. Instances in the natural world include the release of methane and carbon dioxide from the seabed or the bottom of a lake and from a subsea oil well blowout. This review describes the dynamics of bubble plumes and their various spreading patterns in the surrounding environment. We explore how the motion of the plume is affected by the density stratification in the external environment, as well as by internal processes of dissolution of the bubbles and chemical reaction. We discuss several examples, such as natural disasters, global warming, and fishing techniques used by some whales and dolphins.

► Turbulent Drag Reduction by Streamwise Traveling Waves of Wall-Normal Forcing
  19 Jan, 2024

We review some fundamentals of turbulent drag reduction and the turbulent drag reduction techniques using streamwise traveling waves of blowing/suction from the wall and wall deformation. For both types of streamwise traveling wave controls, their significant drag reduction capabilities have been well confirmed by direct numerical simulation at relatively low Reynolds numbers. The drag reduction mechanisms by these streamwise traveling waves are considered to be the combination of direct effects due to pumping and indirect effects of the attenuation of velocity fluctuations due to reduced receptivity. Prediction of their drag reduction capabilities at higher Reynolds numbers and attempts at experimental validation are also intensively ongoing toward their practical implementation.

► Building Ventilation: The Consequences for Personal Exposure
  19 Jan, 2024

Ventilation is central to human civilization. Without it, the indoor environment rapidly becomes uncomfortable or dangerous, but too much ventilation can be expensive. We spend much of our time indoors, where we are exposed to pollutants and can be infected by airborne diseases. Ventilation removes pollution and bioaerosols from indoor sources but also brings in pollution from outdoors. To determine an appropriate level of ventilation and an appropriate way of providing it, one must understand that the needs for ventilation extend beyond simple thermal comfort; the quality of indoor air is at least as important. An effective ventilation system will remove unwanted contaminants, whether generated within the space by activities or by the simple act of breathing, and ensure that the ventilation system does not itself introduce or spread contaminants from elsewhere. This review explores how ventilation flows in buildings influence personal exposure to indoor pollutants and the spread of airborne diseases.

► Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?
  19 Jan, 2024

In the last ten years, advances in experimental techniques have enabled remarkable discoveries of how the dynamics of thin gas films can profoundly influence the behavior of liquid droplets. Drops impacting onto solids can skate on a film of air so that they bounce off solids. For drop–drop collisions, this effect, which prevents coalescence, has been long recognized. Notably, the precise physical mechanisms governing these phenomena have been a topic of intense debate, leading to a synergistic interplay of experimental, theoretical, and computational approaches. This review attempts to synthesize our knowledge of when and how drops bounce, with a focus on () the unconventional microscale and nanoscale physics required to predict transitions to/from merging and () the development of computational models. This naturally leads to the exploration of an array of other topics, such as the Leidenfrost effect and dynamic wetting, in which gas films also play a prominent role.

Computers & Fluids top

► A GRP-based tangential effects preserving, high resolution and efficient ghost fluid method for the simulation of two-dimensional multi-medium compressible flows
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s): Zhixin Huo, Zupeng Jia

► A new compact scheme-based Lax–Wendroff method for high fidelity simulations
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s): V.K. Suman, P. Sundaram, Soumyo Sengupta, Tapan K. Sengupta

► Numerical investigation about effects of flow sediment-storage variation on capacity modelling of morphological evolutions
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s): Peng Hu, Mobassir Azam, Wei Li, Linwei Dai, Hongyang Zhao

► Validation of a Eulerian–Lagrangian numerical algorithm for simulating ultra-coarse particles transported in horizontal and vertical hydraulic pipes
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s): Ruijie Zhao, Yuanhang Zhang, Xuzhen Zhang, Xikun Wang

► Investigation of radial basis function dynamic mesh method with rotation correction based on adaptive background mesh
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s): Han Tang, Guannan Zheng, Yuchen Zhang

► A data-driven multiscale model for reactive wetting simulations
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s): Jaideep Ray, Jeffrey S. Horner, Ian Winter, David J. Kemmenoe, Edward R. Arata, Michael Chandross, Scott A. Roberts, Anne M. Grillet

► Editorial Board
    

Publication date: 30 May 2024

Source: Computers & Fluids, Volume 276

Author(s):

► A finite element framework for fluid–structure interaction of turbulent cavitating flows with flexible structures
    

Publication date: Available online 18 May 2024

Source: Computers & Fluids

Author(s): Nihar B. Darbhamulla, Rajeev K. Jaiman

► An assessment of CFD-scale fluid–structure interaction simulations through comprehensive experimental data in cross-flow
    

Publication date: Available online 11 May 2024

Source: Computers & Fluids

Author(s): Daniele Vivaldi

► Adaptive Immersed Mesh Method (AIMM) for Fluid–Structure Interaction
    

Publication date: Available online 4 May 2024

Source: Computers & Fluids

Author(s): R. Nemer, A. Larcher, E. Hachem

International Journal of Computational Fluid Dynamics top

► A Large Time Step Wave-Adding Simple Scheme for Unsteady Compressible Flows
    9 May, 2024
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► A Direct Implicit Discretization Approach for Arbitrary Non-conformal Interface by Using a Virtual Cell
  26 Apr, 2024
Volume 37, Issue 7, August 2023, Page 656-674
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► Droplet Injection for Multiphase Rainfall Simulations on Dynamically Refined Mesh for Effective Interface Capturing
  17 Apr, 2024
Volume 37, Issue 7, August 2023, Page 607-636
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► A Level Set-Based Solver for Two-Phase Incompressible Flows: Extension to Magnetic Fluids
    5 Apr, 2024
Volume 37, Issue 7, August 2023, Page 565-606
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► Experimental and Numerical Investigation of Flow Characteristics Around Complex Bridge Piers in Different Geometries
    2 Apr, 2024
Volume 37, Issue 7, August 2023, Page 637-655
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► On the convergence of the Rhie–Chow stabilized Box method for the Stokes problem
  14 May, 2024
On the convergence of the Rhie–Chow stabilized Box method for the Stokes problem

Box method is a piecewise linear Petrov–Galerkin formulation on the Voronoi dual mesh of a Delaunay triangulation. Rhie–Chow interpolation is a technique widely adopted in finite volumes to stabilize the Stokes problem. This stabilization satisfies continuity, consistency, coercivity and inf-sup stability in a variational framework and the well posedness and the convergence of the Box method can be proved both theoretically and numerically.


Abstract

The finite volume method (FVM) is widely adopted in many different applications because of its built-in conservation properties, its ability to deal with arbitrary mesh and its computational efficiency. In this work, we consider the Rhie–Chow stabilized Box method (RCBM) for the approximation of the Stokes problem. The Box method (BM) is a piecewise linear Petrov–Galerkin formulation on the Voronoi dual mesh of a Delaunay triangulation, whereas the Rhie–Chow (RC) stabilization is a well known stabilization technique for FVM. The first part of the article provides a variational formulation of the RC stabilization and discusses the validity of crucial properties relevant for the well-posedness and convergence of RCBM. Moreover, a numerical exploration of the convergence properties of the method on 2D and 3D test cases is presented. The last part of the article considers the theoretically justification of the well-posedness of RCBM and the experimentally observed convergence rates. This latter justification hinges upon suitable assumptions, whose validity is numerically explored.

► A third‐order entropy condition scheme for hyperbolic conservation laws
    9 May, 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.

► Hybrid large eddy simulation and Lagrangian simulation of a compressible turbulent planar jet with a chemical reaction
    9 May, 2024
Hybrid large eddy simulation and Lagrangian simulation of a compressible turbulent planar jet with a chemical reaction

Large eddy simulation (LES) coupled with Lagrangian particle simulation (LPS) is performed for high-speed turbulent planar jets with a chemical reaction. The molecular diffusion is modeled by a mixing volume model for notional particles, which is extended for compressible reacting flows. The LES/LPS accurately predicts the progress of mixing and reaction in both subsonic and supersonic jets.


Abstract

Large eddy simulation (LES) coupled with Lagrangian particle simulation (LPS) is applied to investigate high-speed turbulent reacting flows. Here, LES solves a velocity field while LPS solves scalar transport equations with notional particles. Although LPS does not require sub-grid scale models for chemical source terms, molecular diffusion has to be modeled by a so-called mixing model, for which a mixing volume model (MVM), that is originally proposed for an inert scalar in incompressible flow, is extended to reactive scalars in compressible flows. The extended model is based on a relaxation process toward the average of nearby notional particles and assumes a common mixing timescale for all species. LES/LPS with the MVM is applied to a temporally-evolving compressible turbulent planar jet with an isothermal reaction and is tested by comparing the results with direct numerical simulation (DNS). The results show that LES/LPS well predicts the statistics of mass fractions. As the jet Mach number increases, the reaction progress delays due to the delayed jet development. This Mach number dependence is also well reproduced in LES/LPS. The mean molecular diffusion term of the product calculated as a function of its mass fraction also agrees well between LES/LPS and DNS. An important parameter for the MVM is the distance among particles, for which the requirement for accurate prediction is presented for the present test case. LES/LPS with the MVM is expected to be a promising method for investigating compressible turbulent reactive flows at a moderate computational cost.

► Employment of an efficient particle tracking algorithm based on barycentric coordinates in hybrid finite‐volume/probability‐density‐function Monte Carlo methods
    9 May, 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.

► High‐resolution mapping type WENO‐Z schemes for solving compressible flow
    9 May, 2024
High-resolution mapping type WENO-Z schemes for solving compressible flow

This paper presents two new improved WENO schemes: WENO-MZ, which uses a mapping function to increase the ratio of less smooth sub-stencils to smooth ones; WENO-MD, which includes a modifier function. Both schemes demonstrate higher shock capture capabilities and better resolution than existing WENO schemes. In addition, they require less computational time than WENO-M and WENO-AIM. This has been confirmed by both theoretical and numerical experiments.


Summary

This paper presents a new WENO-Z scheme (WENO-MZ) that incorporates a mapping function to enhance the weights of the less smooth sub-stencils. The mapping function uses an innovative approach to modify the weight ratio of the less smooth sub-stencil to the smooth stencil. In addition, we present the WENO-MD scheme, which is a further development of the WENO-MZ scheme that incorporates a modifier function. The WENO-MD scheme shows improvements over the WENO-MZ scheme by achieving an improved optimal order at critical points in higher orders and by increasing the proportion of less smooth sub-stencils. Theoretical and numerical experiments have shown that the newly developed methods have improved shock capture capabilities and resolution compared to WENO-JS, WENO-Z, WENO-M, WENO-D, and WENO-AIM, and also lead to significant computational time savings compared to WENO-M and WENO-AIM.

► A centered limited finite volume approximation of the momentum convection operator for low‐order nonconforming face‐centered discretizations
    9 May, 2024
A centered limited finite volume approximation of the momentum convection operator for low-order nonconforming face-centered discretizations

We propose in this article a discretization of the momentum convection operator for the approximation of the Navier–Stokes equations by the low-order nonconforming Rannacher–Turek finite element. This operator is of finite volume type, and its almost second order expression is derived by an algebraic MUSCL-like technique; it satisfies a discrete and local kinetic energy conservation identity. The stability, consistency, and accuracy of the resulting scheme is assessed by numerical experiments for incompressible, barotropic and compressible flows, including the Euler equations.


Abstract

We propose in this article a discretization of the momentum convection operator for fluid flow simulations on quadrangular or generalized hexahedral meshes. The space discretization is performed by the low-order nonconforming Rannacher–Turek finite element: the scalar unknowns are associated with the cells of the mesh while the velocities unknowns are associated with the edges or faces. The momentum convection operator is of finite volume type, and its expression is derived, as in MUSCL schemes, by a two-step technique: (i)$$ (i) $$ computation of a tentative flux, here, with a centered approximation of the velocity, and (ii)$$ (ii) $$ limitation of this flux using monotonicity arguments. The limitation procedure is of algebraic type, in the sense that its does not invoke any slope reconstruction, and is independent from the geometry of the cells. The derived discrete convection operator applies both to constant or variable density flows and may thus be implemented in a scheme for incompressible or compressible flows. To achieve this goal, we derive a discrete analogue of the computation ui(∂t(ρui)+div(ρuiu)=12∂t(ρui2)+12div(ρui2u)$$ {u}_i\kern0.3em \Big({\partial}_t\left(\rho {u}_i\right)+\operatorname{div}\left(\rho {u}_i\boldsymbol{u}\right)=\frac{1}{2}{\partial}_t\left(\rho {u}_i^2\right)+\frac{1}{2}\operatorname{div}\left(\rho {u}_i^2\boldsymbol{u}\right) $$ (with u$$ \boldsymbol{u} $$ the velocity, ui$$ {u}_i $$ one of its component, ρ$$ \rho $$ the density, and assuming that the mass balance holds) and discuss two applications of this result: first, we obtain stability results for a semi-implicit in time scheme for incompressible and barotropic compressible flows; second, we build a consistent, semi-implicit in time scheme that is based on the discretization of the internal energy balance rather than the total energy. The performance of the proposed discrete convection operator is assessed by numerical tests on the incompressible Navier–Stokes equations, the barotropic and the full compressible Navier–Stokes equations and the compressible Euler equations.

► Predicting aerosol transmission in airplanes: Benefits of a joint approach using experiments and simulation
    9 May, 2024
Predicting aerosol transmission in airplanes: Benefits of a joint approach using experiments and simulation

We simulate aerosol transmission in an aircraft cabin and compare simulated particle counts with measurements. Simulations were done using MESHFREE, a meshfree Lagrangian method for computational fluid dynamics. Measurements were performed at the Fraunhofer Flight Test Facility.


Summary

We investigate the transmission of aerosol particles in an airplane cabin with a joint approach using experiments and simulation. Experiments were conducted in a realistic aircraft cabin with heated dummies acting as passengers. A Sheffield head with an aerosol generator was used to emulate an infected passenger and particle numbers were measured at different locations throughout the cabin to quantify the exposure of other passengers. The same setting was simulated with a computational fluid dynamics model consisting of a Lagrange continuous phase for capturing the air flow, coupled with a Lagrange suspended discrete phase to represent the aerosols. Virtual measurements were derived from the simulation and compared with the experiments. Our main results are: the experimental setup provides good measurements well suited for model validation, the simulation does correctly reproduce the fundamental mechanisms of aerosol dispersion and simulations can help to improve the understanding of aerosol transmission for example by visualizing particle distributions. Furthermore, with findings from the simulation it was possible to crucially improve the experimental setup, proving that feedback between the numerical and the hardware world is indeed beneficial.

► Issue Information
    9 May, 2024
International Journal for Numerical Methods in Fluids, Volume 96, Issue 6, June 2024.
► Conservative solution transfer between anisotropic meshes for time‐accurate hybridized discontinuous Galerkin methods
    9 May, 2024
Conservative solution transfer between anisotropic meshes for time-accurate hybridized discontinuous Galerkin methods

Previously developed anisotropic mesh adaptation framework is coupled with a hybridized discontinuous Galerkin (HDG) solver for general time-dependent balance laws. Special emphasis is placed on the solution transfer between anisotropically adapted meshes such that the conservation of physically relevant quantities is preserved and the accuracy of high-order method is not compromised. This is achieved by so called Galerkin projection on each element of the mesh. These properties are verified by means of test cases having both smooth and discontinuous solutions.


Abstract

We present a hybridized discontinuous Galerkin (HDG) solver for general time-dependent balance laws. In particular, we focus on a coupling of the solution process for unsteady problems with our anisotropic mesh refinement framework. The goal is to properly resolve all relevant unsteady features with the smallest possible number of mesh elements, and hence to reduce the computational cost of numerical simulations while maintaining its accuracy. A crucial step is then to transfer the numerical solution between two meshes, as the anisotropic mesh adaptation is producing highly skewed, non-nested sequences of triangular grids. For this purpose, we adopt the Galerkin projection for the HDG solution transfer as it preserves the conservation of physically relevant quantities and does not compromise the accuracy of high-order method. We present numerical experiments verifying these properties of the anisotropically adaptive HDG method.

► A parallel grad‐div stabilized finite element algorithm for the Navier–Stokes equations with a nonlinear damping term
    9 May, 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.

Journal of Computational Physics top

► BEM-based fast frequency sweep for acoustic scattering by periodic slab
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Yuta Honshuku, Hiroshi Isakari

► Broadband topology optimization of three-dimensional structural-acoustic interaction with reduced order isogeometric FEM/BEM
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Leilei Chen, Haojie Lian, Hao-Wen Dong, Peng Yu, Shujie Jiang, Stéphane P.A. Bordas

► Approaches for fast Brownian dynamics simulation with constraints
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): William T. Funkenbusch, Kevin S. Silmore, James W. Swan

► Perfectly matched layers for the Boltzmann equation: Stability and sensitivity analysis
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Marco Sutti, Jan S. Hesthaven

► Efficient and scalable path-planning algorithms for curvature constrained motion in the Hamilton-Jacobi formulation
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Christian Parkinson, Isabelle Boyle

► A non-overlapping optimization-based domain decomposition approach to component-based model reduction of incompressible flows
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Tommaso Taddei, Xuejun Xu, Lei Zhang

► A general fourth-order mesoscopic multiple-relaxation-time lattice Boltzmann model and its macroscopic finite-difference scheme for two-dimensional diffusion equations
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Ying Chen, Zhenhua Chai, Baochang Shi

► A numerical approach for nonlinear transmission line analysis with bidirectional coupling to lumped-element and particle-in-cell models
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Jingwen Xu, Zili Chen, Yu Wang, Shimin Yu, Hongyu Wang, Wei Jiang, Ya Zhang

► A Fourier spectral immersed boundary method with exact translation invariance, improved boundary resolution, and a divergence-free velocity field
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Zhe Chen, Charles S. Peskin

► A boundary condition-enhanced direct-forcing immersed boundary method for simulations of three-dimensional phoretic particles in incompressible flows
    

Publication date: 15 July 2024

Source: Journal of Computational Physics, Volume 509

Author(s): Xiaojue Zhu, Yibo Chen, Kai Leong Chong, Detlef Lohse, Roberto Verzicco

Journal of Turbulence top

► Evaluation of flow and flame characteristics of turbulent bluff-body CH4-H2 flame using LES-FPV approach
  11 May, 2024
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► Large eddy simulations of sheet-to-cloud cavitation transitions with special emphasis on the simultaneous existence of the re-entrant jet and shock waves
    8 May, 2024
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► Theory and simulations of confined periodic turbulence
    3 May, 2024
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► Spectral energy transfer analysis of a forced homogeneous isotropic turbulence using triple decomposition of velocity gradient tensor
  22 Mar, 2024
Volume 25, Issue 1-3, January - March 2024, Page 125-143
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► Multi-spatial scale and multi-frequency resolution Proper Orthogonal Decomposition for patterns of wall-streamwise pressure gradient fluctuation of impinging jets
  21 Mar, 2024
Volume 25, Issue 1-3, January - March 2024, Page 83-104
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► Effects of axial distance on flow behaviours of continuous jet impinging on a flat surface using a finite confined nozzle
  19 Mar, 2024
Volume 25, Issue 1-3, January - March 2024, Page 105-124
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► Laminar to turbulent transitions of pipe flows by resonances of natural frequencies of components of test sections
  23 Feb, 2024
Volume 25, Issue 1-3, January - March 2024, Page 62-82
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► Variational calculus in hybrid turbulence transport models with passive scalar
  25 Jan, 2024
Volume 25, Issue 1-3, January - March 2024, Page 16-61
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► Exciting turbulence in an elongated domain
    9 Jan, 2024
Volume 25, Issue 1-3, January - March 2024, Page 1-15
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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

► Efficient harmonic resolvent analysis via time stepping
  18 May, 2024

Abstract

We present an extension of the RSVD- \(\Delta t\)  algorithm initially developed for resolvent analysis of statistically stationary flows to handle harmonic resolvent analysis of time-periodic flows. The harmonic resolvent operator, as proposed by Padovan et al. (J Fluid Mech 900, 2020), characterizes the linearized dynamics of time-periodic flows in the frequency domain, and its singular value decomposition reveals forcing and response modes with optimal energetic gain. However, computing harmonic resolvent modes poses challenges due to (i) the coupling of all \(N_{\omega }\) retained frequencies into a single harmonic resolvent operator and (ii) the singularity or near-singularity of the operator, making harmonic resolvent analysis considerably more computationally expensive than a standard resolvent analysis. To overcome these challenges, the RSVD- \(\Delta t\)  algorithm leverages time stepping of the underlying time-periodic linearized Navier–Stokes operator, which is \(N_{\omega }\) times smaller than the harmonic resolvent operator, to compute the action of the harmonic resolvent operator. We develop strategies to minimize the algorithm’s CPU and memory consumption, and our results demonstrate that these costs scale linearly with the problem dimension. We validate the RSVD- \(\Delta t\)  algorithm by computing modes for a periodically varying Ginzburg–Landau equation and demonstrate its performance using the flow over an airfoil.

Graphical abstract

► Numerical simulations of dam-break flows of viscoplastic fluids via shallow water equations
  13 May, 2024

Abstract

This paper presents simulations of dam-break flows of Herschel–Bulkley viscoplastic fluids over complex topographies using the shallow water equations (SWE). In particular, this study aims to assess the effects of rheological parameters: power-law index (n), consistency index (K), and yield stress ( \(\tau _{c}\) ), on flow height and velocity over different topographies. Three practical examples of dam-break flow cases are considered: a dam-break on an inclined flat surface, a dam-break over a non-flat topography, and a dam-break over a wet bed (downstream containing an initial fluid level). The effects of bed slope and depth ratios (the ratio between upstream and downstream fluid levels) on flow behaviour are also analyzed. The numerical results are compared with experimental data from the literature and are found to be in good agreement. Results show that for both dry and wet bed conditions, the fluid front position, peak height, and mean velocity decrease when any of the three rheological parameters are increased. However, based on a parametric sensitivity analysis, the power-law index appears to be the dominant factor in dictating fluid behaviour. Moreover, by increasing the bed slope and/or depth ratio, the wave-frontal position moves further downstream. Furthermore, the presence of an obstacle is observed to cause the formation of an upsurge that moves in the upstream direction, which increases by increasing any of the three rheological parameters. This study is useful for an in-depth understanding of the effects of rheology on catastrophic gravity-driven flows of non-Newtonian fluids (like lava or mud flows) for risk assessment and mitigation.

Graphical abstract

► The fluidic pinball with symmetric forcing displays steady, periodic, quasi-periodic, and chaotic dynamics
    1 May, 2024

Abstract

We numerically investigate the fluidic pinball under symmetric forcing and find seven flow regimes under different rotation speeds. The fluidic pinball consists of three rotatable cylinders placed at the vertices of an equilateral triangle pointing upstream in a uniform oncoming flow. The starting point is the unforced asymmetric periodic vortex shedding at Reynolds number Re = 100 based on the cylinder diameter. The flow is symmetrically actuated by rotating the two rear cylinders at constant speed |b| up to three times the oncoming velocity in both directions. Counterclockwise (b > 0) and clockwise (b < 0) rotation of the bottom cylinder correspond to boat tailing and base bleeding, respectively. A total of seven distinct flow regimes are observed, including a steady flow, three symmetric/asymmetric periodic types of shedding, two symmetric/asymmetric quasi-periodic behaviors, and a chaotic dynamics. The vortex shedding features multiple coupled oscillator modes, including in-phase, anti-phase, and out-of-phase synchronization and non-synchronization. These shedding regimes are analyzed employing the temporal evolution of the aerodynamic forces and a dynamical mode decomposition of the wake flow. The kaleidoscope of unforced and forced dynamics promotes the fluidic pinball as a challenging modeling and control benchmark.

Graphic abstract

► A one-dimensional mathematical model for shear-induced droplet formation in co-flowing fluids
  22 Apr, 2024

Abstract

Shear-induced droplet formation is important in many industrial applications, primarily focusing on droplet sizes and pinch-off frequency. We propose a one-dimensional mathematical model that describes the effect of shear forces on the droplet interface evolution. The aim of this paper is to simulate paraffin wax droplets in a co-flowing fluid using the proposed model to estimate the droplet volume rate for different flow velocities. Thus, the study focuses only on the dripping regime. This one-dimensional model has a single parameter that arises from the force balance on the interface. This parameter is related to the shear layer thickness and hence influenced by the change in quantities like velocity, viscosity, and surface tension. The correlation describing the dependence of the parameter on these quantities using non-dimensional numbers is presented. The model is then cross-validated with the previous computational and experimental data. We use PETSc, an open-source solver toolkit, to implement our model using a mixed finite element discretization. We present the simulation results for liquid paraffin wax under fast-moving airflow with a range of velocities.

Graphical abstract

► Contribution of wedge and bulk viscous forces in droplets moving on inclined surfaces
  17 Apr, 2024

Abstract

Employing direct numerical simulations, we investigate water and water-glycerol (85 wt%) droplets ( \(\sim \) 25 µL) moving on smooth surfaces, with contact angles of around 90 \(^{\circ }\) , at varying inclinations. Our focus is on elucidating the relative contribution of local viscous forces in the wedge and bulk regions in droplets to the total viscous force. We observe that, for fast-moving droplets, both regions contribute comparably, while the contribution of the wedge region dominates in slow-moving cases. Comparisons with existing estimates reveal the inadequacy of previous predictions in capturing the contributions of wedge and bulk viscous forces in fast-moving droplets. Furthermore, we demonstrate that droplets with identical velocities can exhibit disparate viscous forces due to variations in internal fluid dynamics.

Graphical abstract

► Modal-based generalised quasilinear approximations for turbulent plane Couette flow
  15 Apr, 2024

Abstract

We study generalised quasilinear (GQL) approximations applied to turbulent plane Couette flow. The GQL framework is explored in conjunction with a Galerkin reduced-order model (ROM) recently developed by Cavalieri and Nogueira (Phys Rev Fluids 7:102601, 2022), which considers controllability modes of the linearised Navier–Stokes system as basis functions, representing coherent structures in the flow. The velocity field is decomposed into two groups: one composed by high-controllability modes and the other by low-controllability modes. The former group is solved with the full nonlinear equations, whereas the equations for the latter are linearised. We also consider a new GQL framework wherein the linearised equations for the low-controllability modes are driven by nonlinear interactions of modes in the first group, which are characterised by large-scale coherent structures. It is shown that GQL-ROMs successfully recover the statistics of the full model with relatively high controllability thresholds and sparser nonlinear operators. Driven GQL-ROMs were found to converge more rapidly than standard GQL approximations, providing accurate description of the statistics with a larger number of linearised modes. This indicates that the forcing of linearised flow structures by large-scale coherent structures is an important feature of turbulence dynamics that should be considered in GQL models. The results presented here reveal that further model reductions are attainable with GQL-ROMs, which can be valuable to extend these models to larger Reynolds numbers.

Graphical abstract

► Resolvent model for aeroacoustics of trailing edge noise
    8 Apr, 2024

Abstract

This study presents a physics-based, low-order model for the trailing edge (TE) noise generated by an airfoil at low angle of attack. The approach employs incompressible resolvent analysis of the mean flow to extract relevant spanwise-coherent structures in the transitional boundary layer and near wake. These structures are integrated into Curle’s solution to Lighthill’s acoustic analogy to obtain the scattered acoustic field. The model has the advantage of predicting surface pressure fluctuations from first principles, avoiding reliance on empirical models, but with a free amplitude set by simulation data. The model is evaluated for the transitional flow ( \(\text {Re} = 5e4\) ) around a NACA0012 airfoil at 3 deg angle of attack, which features TE noise with multiple tones. The mean flow is obtained from a compressible large eddy simulation, and spectral proper orthogonal decomposition (SPOD) is employed to extract the main hydrodynamic and acoustic features of the flow. Comparisons between resolvent and SPOD demonstrate that the physics-based model accurately captures the leading coherent structures at the main tones’ frequencies, resulting in a good agreement of the reconstructed acoustic power with that of the SPOD (within 4 dB). Discrepancies are observed at high frequencies, likely linked to nonlinearities that are not considered in the resolvent analysis. The model’s directivity aligns well with the data at low Helmholtz numbers, but it fails at high frequencies where the back-scattered pressure plays a significant role in directivity. This modeling approach opens the way for efficient optimization of airfoil shapes in combination with low-fidelity mean flow solvers to reduce TE noise.

Graphical abstract

► Analysis of transient and intermittent flows using a multidimensional empirical mode decomposition
    1 Apr, 2024

Abstract

Modal decomposition techniques are important tools for the analysis of unsteady flows and, in order to provide meaningful insights with respect to coherent structures and their characteristic frequencies, the modes must possess a robust spatial support. In this context, although widely used, methods based on singular value decomposition (SVD) may produce modes that are difficult to interpret when applied to problems dominated by intermittent and transient events. Fortunately, specific modal decomposition techniques have been recently developed to analyze such problems, but a proper comparison between them is still lacking from the literature. Therefore, this work compares two recent methods: the fast adaptive multivariate empirical mode decomposition (FA-MVEMD) and the multiresolution dynamic mode decomposition (mrDMD). These techniques are employed here for the study of flow databases involving transient and intermittent dynamics. Specifically, the investigated problems include an SD7003 airfoil subjected to deep dynamic stall conditions, and a steady NACA0012 airfoil operating at a transitional Reynolds number. In the former case, the methods are employed to investigate the onset and evolution of the dynamic stall vortex (DSV), while in the latter case, intermittent vortex pairing is analyzed. We show that the combination of a multidimensional EMD with the Hilbert transform provides modes with superior spatial support when compared to the mrDMD, also allowing the characterization of instantaneous frequencies of coherent structures. Moreover, the EMD also condenses a larger amount of information within a single intrinsic mode function (IMF).

Graphical abstract

► 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


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