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Annual Review of Fluid Mechanics top

► Fluid Dynamics of Polar Vortices on Earth, Mars, and Titan
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 265-289, January 2023.
► Particle Rafts and Armored Droplets
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 459-480, January 2023.
► Sharp Interface Methods for Simulation and Analysis of Free Surface Flows with Singularities: Breakup and Coalescence
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 707-747, January 2023.
► A Perspective on the State of Aerospace Computational Fluid Dynamics Technology
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 431-457, January 2023.
► 3D Lagrangian Particle Tracking in Fluid Mechanics
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 511-540, January 2023.
► Evaporation of Sessile Droplets
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 481-509, January 2023.
► Gas-Liquid Foam Dynamics: From Structural Elements to Continuum Descriptions
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 323-350, January 2023.
► Transition to Turbulence in Pipe Flow
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 575-602, January 2023.
► Icebergs Melting
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 377-402, January 2023.
► Elasto-Inertial Turbulence
  19 Jan, 2023
Annual Review of Fluid Mechanics, Volume 55, Issue 1, Page 675-705, January 2023.

Computers & Fluids top

► Neural vortex method: From finite Lagrangian particles to infinite dimensional Eulerian dynamics
    

Publication date: Available online 1 February 2023

Source: Computers & Fluids

Author(s): Shiying Xiong, Xingzhe He, Yunjin Tong, Yitong Deng, Bo Zhu

► A sufficient condition for free-stream preserving WENO schemes on curvilinear grids of complex geometries
    

Publication date: Available online 1 February 2023

Source: Computers & Fluids

Author(s): Hongmin Su, Jinsheng Cai, Kun Qu, Shucheng Pan

► RANS thermal modelling of a natural convection boundary layer at low Prandtl number
    

Publication date: Available online 2 February 2023

Source: Computers & Fluids

Author(s): Agustín Villa Ortiz, Lilla Koloszar

► Editorial Board
    

Publication date: 15 March 2023

Source: Computers & Fluids, Volume 253

Author(s):

► Revisiting RANS prediction of transitional flow on T3A flat plate subject to various freestream turbulences
    

Publication date: Available online 1 February 2023

Source: Computers & Fluids

Author(s): Tingyun Yin, Giorgio Pavesi, Shouqi Yuan

► A consistent volume-of-fluid approach for direct numerical simulation of the aerodynamic breakup of a vaporizing drop
    

Publication date: Available online 1 February 2023

Source: Computers & Fluids

Author(s): Bradley Boyd, Yue Ling

► Kinetic consistent MHD algorithm for incompressible conductive fluids
    

Publication date: Available online 19 November 2022

Source: Computers & Fluids

Author(s): Boris Chetverushkin, Andrey Saveliev, Valeri Saveliev

► Multi-fidelity vortex simulations of rotor flows: Validation against detailed wake measurements
    

Publication date: Available online 20 January 2023

Source: Computers & Fluids

Author(s): Néstor Ramos-García, Aliza Abraham, Thomas Leweke, Jens Nørkær Sørensen

► Numerical simulation of bubbly flows by the improved lattice Boltzmann method for incompressible two-phase flows
    

Publication date: Available online 30 January 2023

Source: Computers & Fluids

Author(s): Satoshi Saito, Masato Yoshino, Kosuke Suzuki

► Residual estimation for grid modification in wall-modeled large eddy simulation using unstructured high-order methods
    

Publication date: Available online 30 January 2023

Source: Computers & Fluids

Author(s): Marcel Blind, Ali Berk Kahraman, Johan Larsson, Andrea Beck

International Journal of Computational Fluid Dynamics top

► Vorticity Confinement Technique and Blade Element Method for Accurate Propeller Modelling
  11 Jan, 2023
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► Numerical Investigation of a Simplified Wing–body Junction Flow
    6 Jan, 2023
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► Heat Transfer of Aggregate in a Drying Drum Based on the Multi-Scale Model and Fluid-Solid Coupling
  30 Dec, 2022
Volume 36, Issue 6, July 2022, Page 506-517
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► Data-Driven Proxy Modeling of Water Front Propagation in Porous Media
  27 Dec, 2022
Volume 36, Issue 6, July 2022, Page 465-487
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► Study of Falling Condensate Droplets on Parallelepiped Solid Surface Using Hybrid 3D MRT-LBM
  13 Dec, 2022
Volume 36, Issue 6, July 2022, Page 488-505
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► Implementation and Validation of the SST Delayed eXtra-LES Model for Complex Turbulent Flows
  12 Dec, 2022
Volume 36, Issue 6, July 2022, Page 441-464
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► A Conceptual Alternative Machine Learning-Based Method for Mesh Sensitivities Calculation in a Turbomachinery Blades Optimisation Framework
  28 Mar, 2022
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► An unconditionally stable third order scheme for mixed convection flow between parallel plates with oscillatory boundary conditions
    2 Feb, 2023
An unconditionally stable third order scheme for mixed convection flow between parallel plates with oscillatory boundary conditions

It was asserted that no multistep method with more than two steps is A-stable (Altunkaya et al. 2017). However, this article proposes a third-order multistep method that is always stable for time-dependent partial differential equations. A third-order numerical scheme has been proposed for solving parabolic equations. The scheme is linear and unconditionally stable. The comparison showed less error than the existing backward Euler scheme with fourth-order spatial discretization. The proposed scheme solves the nonlinearized partial differential equation due to the compact scheme implementation technique.


Abstract

This work proposes an unconditionally stable third-order multistep technique for time-dependent partial differential equations. Its unconditional stability is proved by employing von Neumann stability analysis, and constructed Matlab code is another solid proof of the existence of the such scheme. The scheme is constructed on three consecutive time levels, and a compact fourth-order scheme is considered for spatial discretization. The convergence conditions are found when applied to the system of parabolic equations. The scheme is tested on two examples of flow between parallel plates. The mathematical model of heat and mass transfer of flow between parallel plates under the effects of viscous dissipation, thermal radiations, and chemical reaction is given and solved by the proposed scheme. The impact of some parameters, including radiation and reaction rate parameters, on velocity, temperature, and concentration profiles is also illustrated by graphs. The proposed scheme is also compared with the existing scheme, providing faster convergence than an existing one. The fundamental benefit of the proposed scheme is that it can give a compact fourth-order solution to parabolic equations.

► Addendum to “SWASHES: A compilation of shallow water analytic solutions for hydraulic and environmental studies”
    1 Feb, 2023
International Journal for Numerical Methods in Fluids, EarlyView.
► Stability Effect of Multidimensional Velocity Components in Numerical Flux SLAU
  31 Jan, 2023

Abstract

In computational fluid dynamics of compressible fluid flow, the simple low-dissipation advection upstream (SLAU) scheme formulated with multidimensional velocity components (normal and parallel to a cell interface) is a widely employed all-speed scheme. As a variant of SLAU, the mSLAU scheme, which adopts only a velocity component normal to the cell interface instead of multidimensional velocity components, is used for rotorcraft calculations. However, although mSLAU has been claimed to be empirically stable, it has been pointed out that using only the cell-interface-normal velocity component instead of the multidimensional velocity components causes numerical instability. Therefore, to clarify the roles of the multidimensional velocity components for computational stability, we solved some benchmark problems associated with using SLAU or mSLAU. We discovered that the multidimensional velocity components contributed to stability against poor-quality grids by isotropically producing a larger amount of numerical dissipation, especially in low-subsonic and hypersonic flows. Although mSLAU could practically treat moderate Mach number flows (approximately 0.1 < M < 1.0) when coupled with the minmod limiter, using only the cell-interface-normal velocity component can deteriorate convergence of calculations and lead to susceptibility in the grid geometry.

This article is protected by copyright. All rights reserved.

► A New Numerical Approach to Gardner Kawahara Equation in Magneto‐Acoustic Waves in Plasma Physics
  31 Jan, 2023

Abstract

The basic idea of this article is to investigate the numerical solutions of Gardner Kawahara equation, a particular case of extended Korteweg-de Vries (KdV) equation, by means of finite element method. For this purpose, a collocation finite element method based on trigonometric quintic B-spline basis functions is presented. The standard finite difference method is used to discretize time derivative and Crank-Nicolson approach is used to obtain more accurate numerical results. Then, von Neumann stability analysis is performed for the numerical scheme obtained using collocation finite element method. Several numerical examples are presented and discussed to exhibit the feasibility and capability of the finite element method and trigonometric B-spline basis functions. More specifically, the error norms L 2 and L are reported for numerous time and space discretization values in tables. Graphical representations of the solutions describing motion of wave are presented.

► A High Order Stabilized Solver for the Volume Averaged Navier‐Stokes Equations
  30 Jan, 2023

Abstract

The Volume-Averaged Navier-Stokes equations are used to study fluid flow in the presence of fixed or moving solids such as packed or fluidized beds. We develop a high-order finite element solver using both forms A and B of these equations. We introduce tailored stabilization techniques to prevent oscillations in regions of sharp gradients, to relax the Ladyzhenskaya-Babuska-Brezzi inf-sup condition, and to enhance the local mass conservation and the robustness of the formulation. We calculate the void fraction using the Particle Centroid Method. Using different drag models, we calculate the drag force exerted by the solids on the fluid. We implement the method of manufactured solution to verify our solver. We demonstrate that the model preserves the order of convergence of the underlying finite element discretization. Finally, we simulate gas flow through a randomly packed bed and study the pressure drop and mass conservation properties to validate our model.

► Adaptive geometric multigrid for the mixed finite cell formulation of Stokes and Navier‐Stokes equations
  28 Jan, 2023

Abstract

The unfitted finite element methods have emerged as a popular alternative to classical finite element methods for the solution of partial differential equations and allow modeling arbitrary geometries without the need for a boundary-conforming mesh. On the other hand, the efficient solution of the resultant system is a challenging task because of the numerical ill-conditioning that typically entails from the formulation of such methods. We use an adaptive geometric multigrid solver for the solution of the mixed finite cell formulation of saddle-point problems and investigate its convergence in the context of the Stokes and Navier-Stokes equations. We present two smoothers for the treatment of cutcells in the finite cell method and analyze their effectiveness for the model problems using a numerical benchmark. Results indicate that the presented multigrid method is capable of solving the model problems independently of the problem size and is robust with respect to the depth of the grid hierarchy.

► A constrained proper orthogonal decomposition model for upscaling permeability
  26 Jan, 2023
A constrained proper orthogonal decomposition model for upscaling permeability

New Findings Applying the models to practical datasets, statistics from the error analysis shows classical POD algorithm seems to be more preferred for LRA. However, since non-negativity of permeability datasets is a priority, the constrained POD (non-negative POD) algorithm described in this article is more appropriate. Results shows that the NPOD model has the capability of using minimal number of modes to reconstruct the permeability image while still retaining the geological details from the original data as opposed to the POD model.


Abstract

Reservoir modeling and simulation are vital components of modern reservoir management processes. Despite the advances in computing power and the advent of smart technologies, the implementation of model-based operational/control strategies has been limited by the inherent complexity of reservoir models. Thus, reduce order models that not only reduce the cost of the implementation but also provide geological consistent prediction are essential. This article introduces reduced-order models based on the proper orthogonal decomposition (POD) coupled with linear interpolation for upscaling. First, using POD-based models, low rank approximate (LRA) are obtained by projecting the high dimensional permeability dataset to a low dimensional subspace spanned by its trajectories to decorrelate the dataset. Next, the LRA is integrated into the interpolation algorithm to generate upscaled values. This technique is highly scalable since low-rank approximations are achieved by the variation in the number of modes used for reconstruction. To test the validity and reliability of the model, we show its application to the practical dataset from SPE10 benchmark case2. From statistics of the error analysis, the classical POD algorithm seems to be more preferred for LRA; however, since non-negativity of the permeability data set is a priority, the constrained POD (non-negative POD) algorithm described in this article is more appropriate. Finally, we compared the POD-based models to a traditional industry-standard upscaling technique (e.g., arithmetic mean) to highlight our model benefits/performance. Results show that the POD-based models, particularly the non-negative POD model, produce considerably less error than the arithmetic mean model in the upscaling process.

► Dry and wet boundary treatment and improvement of a TVD‐MacCormack scheme in shallow water flow
  26 Jan, 2023
Dry and wet boundary treatment and improvement of a TVD-MacCormack scheme in shallow water flow

A simple and efficient dry-wet boundary treatment method is proposed. We also improve difference scheme to solve the problem of distortion of left and right traveling waves propagating in the dry bed.


Abstract

To solve shallow water equation, this paper proposes a simple and easy-to-operate dry-wet boundary treatment method based on the total variation diminishing (TVD)- MacCormack scheme. The method requires to judge dry and wet nodes before the calculation of prediction step and correction step, respectively. Then, the dry nodes are fictitiously celled and the topographic variables are reset. Moreover, previous researches show that when the same differential scheme was used, the left and right traveling waves showed over predicted computational fluxes during the downstream dry bed flow evolution, which led to distorted values and non-real physical phenomena. To solve the problem, the difference scheme for prediction step and correction step is modified, and a new finite difference scheme improvement method is proposed. Finally, the numerical solutions are compared with the analytical solution results by five classical cases to verify the rationality of the proposed method in this paper.

► Energy‐consistent formulation of the pressure‐free two‐fluid model
  24 Jan, 2023
Energy-consistent formulation of the pressure-free two-fluid model

The pressure-free two-fluid model is a model for stratified incompressible flow in ducts, in which the pressure is eliminated through intricate use of the constraints. This article proposes a modification to the model based on the requirement of energy conservation, which makes it consistent with the original pressure-including two-fluid model. An energy-conserving discretization is applied to the improved model, and is extended with an energy-conserving discretization of the source terms due to gravity acting in the streamwise direction.


Abstract

The pressure-free two-fluid model (PFTFM) is a recent reformulation of the one-dimensional two-fluid model (TFM) for stratified incompressible flow in ducts (including pipes and channels), in which the pressure is eliminated through intricate use of the volume constraint. The disadvantage of the PFTFM was that the volumetric flow rate had to be specified as an input, even though it is an unknown quantity in case of periodic boundary conditions. In this work, we derive an expression for the volumetric flow rate that is based on the demand for energy (and momentum) conservation. This leads to PFTFM solutions that match those of the TFM, justifying the validity and necessity of the derived choice of volumetric flow rate. Furthermore, we extend an energy-conserving spatial discretization of the TFM, in the form of a finite volume scheme, to the PFTFM. We propose a discretization of the volumetric flow rate that yields discrete momentum and energy conservation. The discretization is extended with an energy-conserving discretization of the source terms related to gravity acting in the streamwise direction. Our numerical experiments confirm that the discrete energy is conserved for different problem settings, including sloshing in an inclined closed tank, and a traveling wave in a periodic domain. The PFTFM solutions and the volumetric flow rates match the TFM solutions, with reduced computation time, and with exact momentum and energy conservation.

► A unified time discontinuous Galerkin space‐time finite element scheme for non‐Newtonian power law models
  24 Jan, 2023
A unified time discontinuous Galerkin space-time finite element scheme for non-Newtonian power law models

A unified Discontinuous Galerkin Time, Space-Time Finite Element scheme is developed and is applied for discretizing time dependent viscous shear-thinning p-power law fluid flow models. A stability bound is given. The efficiency of the method is investigated by solving associated benchmark problems.


Abstract

In this work, a stabilized time Discontinuous Galerkin, Space-Time Finite Element (tDG-ST-FE) scheme is presented for discretizing time-dependent viscous shear-thinning fluid flow models, which exhibit a usual power-law stress strain relation. The development of the proposed numerical scheme based mainly on a unified weak space-time formulation, where simple streamline-upwind terms have been added in the numerical scheme, for stabilizing the discretization of the associated temporal and convective terms. The original time interval is partitioned into time subintervals, resulting in a subdivision of the space-time cylinder into space-time subdomains. Discontinuous Galerkin techniques are applied for the time discretization between the space-time subdomain interfaces. A stability bound is given for the derived ST-FE scheme. In the last part numerical examples on benchmark problems are presented for testing the efficiency of the proposed method.

Journal of Computational Physics top

► An efficient hybrid multi-resolution WCNS scheme for solving compressible flows
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Zhenming Wang, Jun Zhu, Chunwu Wang, Ning Zhao

► A projection-based Characteristic Mapping method for tracer transport on the sphere
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Seth Taylor, Jean-Christophe Nave

► Linearly implicit and high-order energy-preserving relaxation schemes for highly oscillatory Hamiltonian systems
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Dongfang Li, Xiaoxi Li, Zhimin Zhang

► Efficient parallel strategy for molecular plasmonics – A numerical tool for integrating Maxwell-Schrödinger equations in three dimensions
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Maxim Sukharev

► A FFT accelerated fourth order finite difference method for solving three-dimensional elliptic interface problems
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Yiming Ren, Shan Zhao

► A metalearning approach for Physics-Informed Neural Networks (PINNs): Application to parameterized PDEs
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Michael Penwarden, Shandian Zhe, Akil Narayan, Robert M. Kirby

► Deep learning-enhanced ensemble-based data assimilation for high-dimensional nonlinear dynamical systems
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Ashesh Chattopadhyay, Ebrahim Nabizadeh, Eviatar Bach, Pedram Hassanzadeh

► Uncertainty quantification in scientific machine learning: Methods, metrics, and comparisons
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Apostolos F. Psaros, Xuhui Meng, Zongren Zou, Ling Guo, George Em Karniadakis

► Conditional moment methods for polydisperse cavitating flows
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Spencer H. Bryngelson, Rodney O. Fox, Tim Colonius

► Energy landscape analysis for two-phase multi-component NVT flash systems by using ETD type high-index saddle dynamics
    

Publication date: 15 March 2023

Source: Journal of Computational Physics, Volume 477

Author(s): Yuze Zhang, Xuguang Yang, Lei Zhang, Yiteng Li, Tao Zhang, Shuyu Sun

Journal of Turbulence top

► An improved method for coherent structure identification based on mutual K-nearest neighbors
  20 Dec, 2022
Volume 23, Issue 11-12, November - December 2022, Page 655-673
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► DNS predictions of NOx production in developing turbulent mixing layers with non-premixed hydrogen–air combustion
  13 Dec, 2022
Volume 23, Issue 11-12, November - December 2022, Page 636-654
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► High-enthalpy effects on turbulent coherent structures over a curved compression corner
  11 Dec, 2022
Volume 23, Issue 11-12, November - December 2022, Page 615-635
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► A systematic study of a droplet breakup process in decaying homogeneous isotropic turbulence using a mesoscopic simulation approach
  21 Nov, 2022
Volume 23, Issue 11-12, November - December 2022, Page 567-614
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► Influence of turbulent structure on the heat transfer of Rayleigh–Bénard convection with triangular roughness element
  16 Nov, 2022
Volume 23, Issue 11-12, November - December 2022, Page 549-566
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► Reflections on roughness modelling in turbulent flow
  23 Oct, 2022
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► Effects of form-induced velocity in rough-wall turbulent channel flows
  11 Oct, 2022
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Physics of Fluids top

► Buoyant fluid injections at high viscosity contrasts in an inclined closed-end pipe
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
This paper studies the buoyant miscible injection of a high-viscosity fluid in a pipe filled with a low-viscosity fluid. The injection is carried out via an eccentric inner pipe inside an inclined closed-end outer pipe. A heavy fluid is injected into a light fluid at a constant density difference. Although the density difference is small, the buoyancy force, quantified via the Archimedes number (Ar), remains large. Our research relies on non-intrusive experimental methods, via a mix of high-speed camera imaging, ultrasound Doppler velocimetry, planar laser induced fluorescence, and particle image velocimetry techniques, accompanied by complementary numerical simulations. The effects of the viscosity ratio (M), the Reynolds number (Re) and the inclination angle (β) are analyzed on the injection/placement flow dynamics. Accordingly, a detailed description of the flow is presented, in terms of the concentration and velocity fields, the average front velocity of the heavy fluid ([math]), the mixing index, and the flow regimes. The findings reveal that [math] is mainly governed by an inertial-buoyant balance, allowing us to develop a correlation for [math] vs Ar, M, Re and β. The results also show that a heavy fluid front separation occurs when M is small, β is large (i.e., near-vertical inclinations), and Re is large. This observation permits us to classify the flows into separation and non-separation regimes, in a dimensionless group plane based on a combination of the aforementioned dimensionless numbers.
► Structural variations of endothelial cell monolayer under startup shear conditions
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
We study the response of an endothelial cell monolayer lining the bottom surface of a cartesian Couette geometry in variations of critical shearing parameters that affect the fluid environment, such as the gap distance between the upper moving and the bottom stationary plates and the velocity of the moving plate. Specifically, we propose an in silico rheometric emulation based on startup shear experiments in a representative two-dimensional domain of the monolayer that accounts for the interaction of the blood plasma and the deformable multilayer poroelastic endothelial cells. We present quantitative predictions for the shear and normal stresses on each cell compartment (membrane, cytoplasm, and nucleus) and their structural changes. We show that the variation of the Wall Shear Stress (WSS) along the cell membrane is considered significant and strongly dependent on the shape of the cell, while membrane thinning is more prominent at the locus of high WSS in the range of physiological velocities. However, under extreme velocities, wall thinning prevails at the locus of flow stagnation.
► Deep learning model for two-fluid flows
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
Various industries rely on numerical tools to simulate multiphase flows due to the wide occurrence of this phenomenon in nature, manufacturing processes, or the human body. However, the significant computation burden required for such simulations directs the research interest toward incorporating data-based approaches in the solution loop. Although these approaches returned significant results in various domains, incorporating them in the computational fluid dynamics (CFD) field is wrangled by their casting aside of the already known governing constitutional laws along with the natural incompatibility of various models with unstructured irregular discretization spaces. This work suggests a coupling framework, between a traditional finite element CFD solver and a deep learning model, for tackling multiphase fluid flows without migrating the benefits of physics-enriched traditional solvers. The tailored model architecture, along with the coupling framework, allows tackling the required problem with a dynamically adapted unstructured irregular triangular mesh, thus dodging the limitation of traditional convolution neural networks. Moreover, the various ingredients that allowed the model to simulate the complex and computation-demanding Navier–Stokes flow equation, such as relying on a sequential validation dataset while exposing the model training to a noise inherited from the quality of its inferring, along with the proper choice of model inputs, are highlighted and elaborated throughout this paper. To the authors' knowledge, this work is the first of its type to introduce a data-based graph-based approach for solving multiphase flow problems with a level-set interface capturing method.
► Long-term degradation of high molar mass poly(ethylene oxide) in a turbulent pilot-scale pipe flow
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
The long-term drag reduction capability of poly(ethylene oxide) with a nominal molar weight of [math] g/mol dissolved in water was investigated in a pilot-scale pipe flow device (inner diameter of test section 26 mm) at a Reynolds number of 105. A total loss of the initially high (75%) drag reduction capability was observed over a flow distance of several ∼10 km while the molar weight of the polymer was still [math] g/mol. Mechanical degradation in the turbulent flow as well as ageing of the polymer dissolved in water caused this loss in drag reduction capability. A simple ansatz of two independent, statistical polymer chain scission mechanisms was used to describe the polymer degradation empirically using a modified Brostow model. This empirical description was applied successfully and suggested that the polymer exhibited at least 15 cleavage points for mechanical degradation.
► Virtual laboratory experiments on the interaction of a vortex with small-scale topography
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
This study presents numerical analogs of laboratory experiments designed to explore the interaction of broad geophysical flows with irregular small-scale bathymetry. The previously reported “sandpaper” theory offered a succinct description of the cumulative effect of small-scale topographic features on large-scale flow patterns. However, initial investigations have been conducted using numerical models with simplified quasi-geostrophic equations that may inadequately represent the dynamics realized in the world’s oceans. This investigation advances previous efforts by using a fully nonlinear Navier–Stokes model configured for rotating tank experiments to (i) validate the theory and (ii) offer guidance for future physical experiments that will confirm theoretical ideas.
► Lagrangian mixing of pulsatile flows in constricted tubes
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
Several Lagrangian methods were used to analyze the mixing processes in an experimental model of a constricted artery under a pulsatile flow. Upstream Reynolds number Re was changed between 1187 and 1999, while the pulsatile period T was fixed at 0.96 s. Velocity fields were acquired using Digital Particle Image Velocimetry for a region of interest (ROI) located downstream of the constriction. The flow is composed of a central jet and a recirculation region near the wall where the vortex forms and sheds. To study the mixing processes, finite-time Lyapunov exponents (FTLE) fields and concentration maps were computed. Two Lagrangian coherent structures (LCS) responsible for mixing fluid were found from FTLE ridges. A first LCS delimits the trailing edge of the vortex, separating the flow that enters the ROI between successive periods. A second LCS delimits the leading edge of the vortex. This LCS concentrates the highest particle agglomeration, as verified by the concentration maps. Moreover, from particle residence time maps, the probability of a fluid particle leaving the ROI before one cycle was measured. As Re increases, the probability of leaving the ROI increases from 0.6 to 0.95. Final position maps [math] were introduced to evaluate the flow mixing between different subregions of the ROI. These maps allowed us to compute an exchange index between subregions, [math], which shows the main region responsible for the mixing increase with Re. Finally, by integrating the results of the different Lagrangian methods, a comprehensive description of the mixing and transport of the flow was provided.
► Convective instabilities in a laminar shock-wave/boundary-layer interaction
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
Linear stability analyses are performed to study the dynamics of linear convective instability mechanisms in a laminar shock-wave/boundary-layer interaction at Mach 1.7. In order to account for all two-dimensional gradients elliptically, we introduce perturbations into an initial-value problem that are found as solutions to an eigenvalue problem formulated in a moving frame of reference. We demonstrate that this methodology provides results that are independent of the numerical setup, frame speed, and type of eigensolutions used as initial conditions. The obtained time-integrated wave packets are then Fourier-transformed to recover individual-frequency amplification curves. This allows us to determine the dominant spanwise wavenumber and frequency yielding the largest amplification of perturbations in the shock-induced recirculation bubble. By decomposing the temporal wave-packet growth rate into the physical energy-production processes, we provide an in-depth characterization of the convective instability mechanisms in the shock-wave/boundary-layer interaction. For the particular case studied, the largest growth rate is achieved in the near-vicinity of the bubble apex due to the wall-normal (productive) and streamwise (destructive) Reynolds-stress energy-production terms. We also observe that the Reynolds heat-flux effects are similar but contribute to a smaller extent.
► Generation of a net flow due to fixed oblique beam structures in the nucleate boiling region
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
Effective utilization of unused heat below 200 °C is essential for a sustainable society. In this study, we propose a thermally driven water pump using fixed oblique beam structures with bubbles in the nucleate boiling region (approximately, 100–130 °C). Here, the oblique beam structure breaks the symmetry of the bubble force, and thus, they provide a net flow. Specifically, by using six fixed oblique beams along a circular fluidic channel, we observed a net flow of an average flow velocity of ∼40 mm/s and an average volume flow rate of ∼10 000 mm3/s (∼0.01 l/s) at the superheat of ∼22 K. Our findings should contribute to the effective use of unused heat such as factory waste heat and environmental energy.
► Statistics of geometrical invariants of magnetic field gradient tensors in the turbulent magnetosheath based on magnetospheric multiscale mission
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
Geometrical invariants of magnetic field gradient tensors are used to classify the topological structures of a magnetic field. This study presents a statistical analysis on the geometrical invariants of magnetic field gradient based on high-quality data measured by magnetospheric multiscale mission in turbulent magnetosheath. The method for the classification of velocity field topologies cannot be applied to magnetic field with strong intensity directly because the magnetic field cannot be transformed to zero by selecting a co-moving reference frame in which the velocity is zero. During a strong magnetic field, flux ropes and tubes are the most possible magnetic structures. Statistics in the plane formed by geometrical invariants show that about 23% are force-free structures comprised of 20.5% flux tubes and 79.5% flux ropes. The remaining actively evolved structures are comprised of 30% flux tubes and 70% flux ropes. Moreover, the conditional average of current density and Lorentz force decomposition in geometrical invariants plane are investigated. The results show that flux ropes carried more current density than flux tubes for the same geometrical invariants, and flux ropes tend to associate with magnetic pressure force and flux tubes tend to associate with magnetic tension.
► Data-based autonomously discovering method for nonlinear aerodynamic force of quasi-flat plate
    2 Feb, 2023
Physics of Fluids, Volume 35, Issue 2, February 2023.
Expression of nonlinear aerodynamic phenomena and calculation of nonlinear aeroelastic response require accurate and concise aeroelastic force function. In this paper, a group sparse regression method is used to reveal the nonlinear mapping aerodynamics relationship between motion and force from data. The aeroelastic force function discovered by this method balances modeling accuracy and simplicity. A quasi-flat plate in coupled vertical–torsional harmonic motion is employed as an experimental object in this work. Aerodynamic motion-force dataset is collected by forced motion test in wind tunnel, including 484 cases. The sparse regression analytic result shows that [math] ([math] is torsional displacement) can represent the nonlinearity in aerodynamic for all cases, even wind speed, amplitude, amplitude ratio, frequency ratio, and angle of attack are in different combinations.

Theoretical and Computational Fluid Dynamics top

► Enhancement of mixed convection heat transfer in a square cavity via a freely moving elastic ring
    2 Jan, 2023

Abstract

A freely moving elastic ring is used to enhance mixed convection heat transfer in a two-dimensional square cavity with three different Richardson (Ri) numbers of 0.1, 1.0, and 10. The multiple-relaxation time lattice Boltzmann method combined with the immersed boundary method is employed to simulate the mixed convection heat transfer and its interaction with the elastic ring in the cavity. Two different thermal conditions for the elastic ring, i.e., with and without thermal interaction, are considered. The results are given in terms of streamlines, isotherms, temperature distribution, and Nusselt (Nu) number. It was found that at the steady state, the ring accords to one of the streamlines in the cavity. In addition, for each investigated case, the Nu number decreases as the Ri number increases. Besides, the presence of the ring leads to a much higher heat transfer (Nu number) and a much earlier steady state as compared to the case with no ring. Finally, the values of the Nu number for both thermal conditions of the ring are about the same being slightly higher for the ring with thermal interaction.

Graphical abstract

► Control-volume study of flow field in a two-phase cyclonic separator in microgravity
  26 Dec, 2022

Abstract

The separation of two-phase flow is essential for many fluid systems in microgravity environments. The passive cyclonic separator is a prominent technology for this task. In the absence of gravity, the separators can operate in different parametric ranges than in normal gravity. The objective of the present investigation is to better understand the fluid physics involved in two-phase flow separation in microgravity by deriving the basic scaling laws for various important parameters. Combined approaches of control-volume analysis and numerical simulations are used to construct a system of equations that can accurately predict the gas core size under various conditions. The predictions are found to be in good agreement with the experimental data, both for pure liquid injection and two-phase flow injection cases. The control-volume equations are modified to include capillary effects and predict the critical condition for the collapse of the liquid layer in microgravity as the surface tension overcomes the centrifugal acceleration at the interface. It is shown that the results of the control-volume analysis can also be used to construct the operational map and to study the separation of a single bubble in microgravity.

Graphical abstract

► On the inviscid energetics of Mack’s first mode instability
  22 Dec, 2022

Abstract

High-speed boundary layer transition is dominated by the modal, exponential amplification of the oblique Mack’s first mode waves in two-dimensional boundary layers from Mach 1 up to freestream Mach numbers of 4.5 to 6.5 depending on the wall-to-adiabatic temperature ratio. At higher Mach numbers, the acoustic, planar Mack’s second mode waves become dominant. Although many theoretical, computational and experimental studies have focused on the supersonic boundary layer transition due to the oblique Mack’s first mode, several fundamental questions about the source of this instability and the reasons for its obliqueness remain unsolved. Here, we perform an inviscid energetics investigation and classify disturbances based on their energetics signature on a Blasius boundary layer for a range of Mach numbers. This approach builds insight into the fundamental mechanisms governing various types of instability. It is shown that first mode instability is distinct from Tollmien–Schlichting instability, being driven by a phase shifting between streamwise velocity and pressure perturbations in the vicinity of the generalized inflection point and insensitive to the viscous no-slip condition. Further, it is suggested that the obliqueness of the first mode is associated with an inviscid flow invariant.

Graphical abstract

► VOF with center of mass and Lagrangian particles (VCLP): a surface tracking and advection method for incompressible fluids
    1 Dec, 2022

Abstract

A novel surface tracking, and advection algorithm for incompressible fluid flows in two and three dimensions is presented. This method based on the volume-of-fluid (VOF) method, is named VOF-with-center-of-mass-and-Lagrangian-particles (VCLP), and it uses spatially and temporally localized Lagrangian particles (LPs) inside a finite volume framework. The fluid surface is recaptured and reconstructed piecewise using the mean slope and curvature. The fluid mass inside each cell is discretized spatially by LPs and distributed as blue noise. LPs are then advected cell by cell with a choice of two different advection schemes in time using interpolated velocity and approximated acceleration fields. VCLP continuously tracks the center of mass of the fluid parcels in the Lagrangian way and this helps to reduce the errors due to numerical acceleration that results from lack of information to reconstruct the interface accurately. VCLP’s performance is evaluated using standard benchmark tests in 2D and 3D such as translation, single vortex, deformation, and Zalesak’s tests from the literature. VCLP is applied to TSUNAMI2D, a 2D Navier–Stokes model to simulate shoaling and breaking of waves.

Graphical abstract

► Numerical tripping of high-speed turbulent boundary layers
    1 Dec, 2022

Abstract

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

Graphical abstract

► Optimal computational parameters for maximum accuracy and minimum cost of Arnoldi-based time-stepping methods for flow global stability analysis
    1 Dec, 2022

Abstract

Global instability analysis of flows is often performed via time-stepping methods, based on the Arnoldi algorithm. When setting up these methods, several computational parameters must be chosen, which affect intrinsic errors of the procedure, such as the truncation errors, the discretization error of the flow solver, the error associated with the nonlinear terms of the Navier–Stokes equations and the error associated with the limited size of the approximation of the Jacobian matrix. This paper develops theoretical equations for the estimation of optimal balance between accuracy and cost for each case. The 2D open cavity flow is used both for explaining the effect of the parameters on the accuracy and the cost of the solution, and for verifying the quality of the predictions. The equations demonstrate the impact of each parameter on the quality of the solution. For example, if higher-order methods are used for approaching a Fréchet derivative in the procedure, it is shown that the solution deteriorates more rapidly for larger grids or less accurate flow solvers. On the other hand, lower-order approximations are more sensitive to the initial disturbance magnitude. Nevertheless, for accurate flow solvers and moderate grid dimensions, first-order Fréchet derivative approximation with optimal computational parameters can provide 5 decimal place accurate eigenvalues. It is further shown that optimal parameters based on accuracy tend to also lead to the most cost-effective solution. The predictive equations, guidelines and conclusions are general, and, in principle, applicable to any flow, including 3D ones.

► Solutions to aliasing in time-resolved flow data
    1 Dec, 2022

Abstract

Avoiding aliasing in time-resolved flow data obtained through high-fidelity simulations while keeping the computational and storage costs at acceptable levels is often a challenge. Well-established solutions such as increasing the sampling rate or low-pass filtering to reduce aliasing can be prohibitively expensive for large datasets. This paper provides a set of alternative strategies for identifying and mitigating aliasing that are applicable even to large datasets. We show how time-derivative data, which can be obtained directly from the governing equations, can be used to detect aliasing and to turn the ill-posed problem of removing aliasing from data into a well-posed problem, yielding a prediction of the true spectrum. Similarly, we show how spatial filtering can be used to remove aliasing for convective systems. We also propose strategies to prevent aliasing when generating a database, including a method tailored for computing nonlinear forcing terms that arise within the resolvent framework. These methods are demonstrated using a nonlinear Ginzburg–Landau model and large-eddy simulation data for a subsonic turbulent jet.

Graphical abstract

Wavenumber-frequency spectra of the forcing component for the streamwise momentum computed on the lipline near the jet nozzle.

► LBM study of natural convection heat transfer from a porous cylinder in an enclosure
    1 Dec, 2022

Abstract

Natural convection heat transfer from a porous cylinder put at various positions in a square, cooled enclosure, with air as the working fluid, is investigated in this work. The following setups are taken into account: The hot cylinder is placed in the middle of the enclosure, near the bottom, top, right sides, along diagonal as top-diagonal and bottom-diagonal. The cylinder and the enclosure walls are kept hot and cold, respectively. The lattice Boltzmann method is used to perform a numerical analysis for Rayleigh number \(10^{4}\le \) Ra \(\le 10^{6}\) and Darcy number \(10^{-6}\le \) Da \(\le 10^{-2}\) . The results are plotted as streamlines, isotherms, and local and mean Nusselt number values. The amount of heat transported from the heated porous cylinder is determined by varying Ra, Da, and the cylinder location. Even at a lower Rayleigh number ( \(10^{4}\) ), the average Nusselt number grows by nearly 70 % as the cylinder moves from the centre to the bottom and 105% as it moves to bottom-diagonal location when \({Da}=10^{-2}\) . At Ra \(=10^{6}\) and Da \(=10^{-2}\) , the heat transfer rate of the cylinder located near the corner of the enclosure at the bottom wall increases by approximately 33% when compared to the case of the cylinder in the centre. Convective effects are more noticeable when the cylinder is positioned towards the enclosure’s bottom wall. This research is applicable to electronic cooling applications in which a collection of electronic components is arranged in a circular pattern inside a cabinet.

Graphical abstract

► Dispersion of free-falling saliva droplets by two-dimensional vortical flows
    1 Dec, 2022

Abstract

The dispersion of respiratory saliva droplets by indoor wake structures may enhance the transmission of various infectious diseases, as the wake spreads virus-laden droplets across the room. Thus, this study analyzes the interaction between vortical wake structures and exhaled multi-component saliva droplets. A self-propelling analytically described dipolar vortex is chosen as a model wake flow, passing through a cloud of micron-sized evaporating saliva droplets. The droplets’ spatial location, velocity, diameter, and temperature are traced, coupled to their local flow field. For the first time, the wake structure decay is incorporated and analyzed, which is proved essential for accurately predicting the settling distances of the dispersed droplets. The model also considers the nonvolatile saliva components, adequately capturing the essence of droplet–aerosol transition and predicting the equilibrium diameter of the residual aerosols. Our analytic model reveals non-intuitive interactions between wake flows, droplet relaxation time, gravity, and transport phenomena. We reveal that given the right conditions, a virus-laden saliva droplet might translate to distances two orders of magnitude larger than the carrier-flow characteristic size. Moreover, accounting for the nonvolatile contents inside the droplet may lead to fundamentally different dispersion and settling behavior compared to non-evaporating particles or pure water droplets. Ergo, we suggest that the implementation of more complex evaporation models might be critical in high-fidelity simulations aspiring to assess the spread of airborne respiratory droplets.

Graphical Abstract

► Three-dimensional evolution of body and fluid motion near a wall
    1 Dec, 2022

Abstract

Evolution of three-dimensional body motion within surrounding three-dimensional fluid motion is addressed, each motion affecting the other significantly in a dynamic fluid–body interaction. This unsteady problem is set near a wall. The spatial three-dimensionality present is a new feature. For inviscid incompressible fluid, a basic nonlinear formulation is described, followed by a linearised form as a first exploration of parameter space and solution responses. The problem reduces to solving Poisson’s equation within the underbody planform, subject to mixed boundary conditions and to coupling with integral equations. Numerical and analytical properties show dependence mainly on the normal and pitch motions, as well as instability or bounded oscillations depending on the position of the centre of mass of the body, and a variety of three-dimensional shapes is examined.


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