CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home >

CFD Journal Feeds

Annual Review of Fluid Mechanics top

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

Computers & Fluids top

► Editorial Board
    

Publication date: 30 October 2020

Source: Computers & Fluids, Volume 211

Author(s):

► Moving least-squares aided finite element method (MLS-FEM): A powerful means to predict pressure discontinuities of multi-phase flow fields and reduce spurious currents
    

Publication date: 30 October 2020

Source: Computers & Fluids, Volume 211

Author(s): Mehdi Mostafaiyan, Sven Wießner, Gert Heinrich

► Flow over a hydrofoil in the wake of a propeller
    

Publication date: Available online 14 September 2020

Source: Computers & Fluids

Author(s): Antonio Posa, Riccardo Broglia, Elias Balaras

► A Segregated Spectral Element Method for the 2D Transient Incompressible Navier-Stokes Equations
    

Publication date: Available online 27 June 2020

Source: Computers & Fluids

Author(s): Wenqiang He, Guoliang Qin, Yazhou Wang, Zhenzhong Bao

► Moving meshes in complex configurations using the composite sliding grid method
    

Publication date: Available online 6 August 2019

Source: Computers & Fluids

Author(s): Mehdi Falsafioon, Sina Arabi, Ricardo Camarero, Francois Guibault

► Numerical simulation of nonlinear interactions in a naturally transitional flat plate boundary layer
    

Publication date: Available online 13 March 2020

Source: Computers & Fluids

Author(s): Amir Banari, Martin Gehrke, Christian F. Janßen, Thomas Rung

► Implementation of a stable high-order overset grid method for high-fidelity simulations
    

Publication date: 30 October 2020

Source: Computers & Fluids, Volume 211

Author(s): Mathieu Deuse, Richard D. Sandberg

► Data-driven surrogate modeling of multiphase flows using machine learning techniques
    

Publication date: 30 October 2020

Source: Computers & Fluids, Volume 211

Author(s): Himakar Ganti, Prashant Khare

► A high-order discontinuous Galerkin solver for the incompressible RANS equations coupled to the <math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg" class="math"><mrow><mi>k</mi><mo linebreak="goodbreak">−</mo><mi>ϵ</mi></mrow></math> turbulence model
    

Publication date: 15 November 2020

Source: Computers & Fluids, Volume 212

Author(s): Marco Tiberga, Aldo Hennink, Jan Leen Kloosterman, Danny Lathouwers

► Application of projection and immersed boundary methods to simulating heat and mass transport in membrane distillation
    

Publication date: 15 November 2020

Source: Computers & Fluids, Volume 212

Author(s): Jincheng Lou, Jacob Johnston, Nils Tilton

International Journal of Computational Fluid Dynamics top

► Massively Parallel Location and Exchange Tools for Unstructured Meshes
    8 Sep, 2020
.
► SPH Modelling of Dam-break Floods, with Damage Assessment to Electrical Substations
    2 Sep, 2020
.
► Study of a Falling Rigid Particle Passing Around Obstacles in a Fluid Channel
  25 Aug, 2020
.
► Combined Vorticity Confinement and TVD Approaches for Accurate Vortex Modelling
  13 Aug, 2020
.
► Correction
  13 Aug, 2020
Volume 34, Issue 6, July 2020, Page I-I
.
► Recent Features and Industrial Applications of the Hybrid SPH-FE Method
  11 Aug, 2020
.
► Integration of TRIZ and CFD to New Product Development Process
  21 Jul, 2020
Volume 34, Issue 6, July 2020, Page 418-437
.
► A Generic Performance Analysis Technique Applied to Different CFD Methods for HPC
  17 Jul, 2020
.
► An Open Boundary Condition for High-order Solutions of Magnetohydrodynamics on Unstructured Grids
  14 Jul, 2020
Volume 34, Issue 6, July 2020, Page 438-456
.
► A Priori Sub-grid Modelling Using Artificial Neural Networks
  13 Jul, 2020
Volume 34, Issue 6, July 2020, Page 397-417
.

International Journal for Numerical Methods in Fluids top

► An exact Riemann solver for one‐dimensional multi‐material elastic‐plastic ows with Mie‐Grüuneisen equation of state without vacuum
  17 Sep, 2020

Abstract

In this paper, we present exact Riemann solvers for the Riemann problem and the half Riemann problem, respectively, for one‐dimensional multi‐material elastic‐plastic flows with the Mie‐Grüneisen equation of state(EOS), hypo‐elastic constitutive model and the von Mises' yielding condition. We firstly analyze the Jacobian matrices in the elastic and plastic states, and then build the relations of different variables across different type of waves. Based on these formulations, an exact Riemann solver is constructed with totally thirty‐six possible cases of wave structures. A large number of tests prove the rightness of the new exact Riemann solver. Moreover, an exact Riemann solver is also deduced for the half Riemann problem and its validity is tested by two examples.

► A robust overset assembly method for multiple overlapping bodies
  15 Sep, 2020
A robust overset assembly method for multiple overlapping bodies

1. We present a new overset assembly method allowing for multiple overlapping bodies by searching the donor cell before identifying the interpolation boundary. 2. It shows good ability to eliminate “isolated island” and “orphan point” problems by selecting interpolation cells only in areas with donor cells. 3. It was properly validated by several cases and the efficiency of the overset assembly was significantly higher than other similar overset assembly programs.


Summary

A robust overset assembly method allowing for multiple overlapping bodies is presented. This method extends the overset assembly method to deal with the interference between objects and prevents the failure of the search for donor cells due to the complex shape of the models. The donor cells are searched before identifying the boundary of the chimera interpolation. By selecting interpolation cells only in areas with donor cells, the final overset interpolation boundary cells can always find their donor cells, which prevents orphan points and achieves excellent robustness. First, the implementation of the proposed assembly method is described in detail. Subsequently, the usability and efficiency of the method are verified using several cases. Finally, the integration strategy of the computational fluid dynamics solver based on overset interpolation with the modification of the boundary type is described and verified using practical cases with overlapping bodies. The results demonstrate the applicability of the proposed overset grid assembly method.

► Basic verification of a numerical framework applied to a morphology adaptive multifield two‐fluid model considering bubble motions
  15 Sep, 2020
Basic verification of a numerical framework applied to a morphology adaptive multifield two‐fluid model considering bubble motions

A multifield two‐fluid model is proposed, allowing for adaptive switching between different flow morphologies. A compact momentum interpolation method for multiphase flows is applied and virtual mass consistently added to this approach. Large‐scale interfaces are described with an appropriate drag model formulation and the stiff system is approximately resolved with a partial‐elimination algorithm, which is extended to multiple phases via a sum formulation. Solver capabilities are demonstrated in cases, where disperse and multiple continuous phases occur in the same location.


Summary

A morphology adaptive modeling framework is derived that is able to handle computationally efficiently dispersed as well as resolved interfacial structures coexisting in the computational domain with the same set of equations. The Eulerian multifield two‐fluid model is combined with the compact momentum interpolation method for multiple phases, which has been proposed in the literature as an extension to the Rhie‐Chow pressure‐velocity coupling. Additionally to the interfacial drag force, the virtual mass force is consistently accounted for in the model. Utilizing a specialized interfacial drag formulation, large interfacial structures can be described with the presented method in a volume‐of‐fluid‐like manner, additionally to the disperse description. The strong phase coupling due to the drag closure model in interfacial regions is resolved with a partial elimination algorithm, which is adapted to work in an approximate manner for more than two phases via a sum formulation. The presented model is implemented in the C++ library OpenFOAM and solver performance is compared with results obtained with the homogeneous model approach in two cases of a single rising gas bubble for two‐ and three‐dimensional space, respectively. Additionally, for both three‐dimensional cases, the results are compared with experimental data. Finally, the presented method's capability of representing dispersed and resolved interfacial structures at the same time is demonstrated with two test cases: a two‐dimensional gas bubble, rising in a liquid, which is laden with micro gas bubbles, and a two‐dimensional stagnant stratification of water and oil, sharing a large‐scale interface, which is penetrated by micro gas bubbles.

► Large‐scale simulation of shallow water waves via computation only on small staggered patches
  13 Sep, 2020

Abstract

Amultiscale computational scheme is developed to use given small micro‐scale simulations of complicated physicalwave processes to empower macro‐scale system‐level predictions. By coupling small patches of simulations over unsimulated space, large savings in computational time are realisable. Here we generalise the patch scheme to the case of wave systems on staggered grids in 2D space. Classic macro‐scale interpolation provides a generic coupling between patches that achieves consistency between the emergent macro‐scale simulation and the underlying micro‐scale dynamics. Spectral analysis indicates that the resultant scheme empowers feasible computation of large macro‐scale simulations of wave systems even with complicated underlying physics. As a example of the scheme’s application, we use it to simulate some simple scenarios of a given turbulent shallow water model.

► A stable hybrid Roe scheme on triangular grids
  13 Sep, 2020

Summary

Numerical shock instability is a common problem for shock‐capturing methods that try to resolve contact and shear waves with minimal diffusion. Most flux‐difference splitting and the AUSM family of schemes produce the carbuncle phenomenon on both structured and unstructured grids. The original Roe scheme is well known to generate shock anomalies and can lead to non‐entropic weak solutions to the Euler equations. A simple and robust approach for healing these numerical instabilities is to apply the hybrid technique incorporated with an efficient weighting switch function to control the amount of dissipation in the vicinity of shock waves. This paper proposes a simple, robust, and accurate hybrid Roe scheme (Roe+ scheme) by hybridizing the Roe scheme and the modified AUSMV+ scheme. A new normalized pressure/density‐based weighting switch function is proposed and applied to the scheme to minimize the numerical dissipation and maintain the robustness of the hybridization. The linearized discrete analysis is performed to evaluate the proposed scheme according to the perturbation damping mechanism of an odd‐even decoupling problem. The resulting recursive equations indicate that the hybridized mechanism damps all perturbations effectively. Finally, several numerical examples demonstrated that the Roe+ scheme provides an accurate, robust, and carbuncle‐free solution on both structured and unstructured triangular grids.

► A multipoint flux approximation with diamond stencil finite volume scheme for the two‐dimensional simulation of fluid flows in naturally fractured reservoirs using a hybrid‐grid method
  11 Sep, 2020
A multipoint flux approximation with diamond stencil finite volume scheme for the two‐dimensional simulation of fluid flows in naturally fractured reservoirs using a hybrid‐grid method

This paper demonstrates the capability of the MPFA‐D, coupled with a hybrid‐grid scheme, to model two‐phase flows in a naturally fractured porous media, with channels or barriers in different spatial positions, using unstructured grids, under any permeability tensors.


Summary

Two‐phase flows of oil and water in naturally fractured reservoirs can be described by a system of nonlinear partial differential equations that comprises of an elliptic pressure equation and hyperbolic saturation equation coupled through the total velocity field. Modeling this problem is a great challenge, due to the complexity of the depositional environments, including inclined layers and fractures with different sizes and shapes, and random spatial distribution. In this work, to solve the pressure equation, we adopted a cell‐centered finite‐volume method with a multipoint flux approximation that uses the “diamond stencil” (MPFA‐D) coupled with a hybrid‐grid method (HyG) to deal with the fractures. The classical first‐order upwind method was used to solve the saturation equation, in its explicit and implicit versions. The MPFA‐D is a very robust and flexible formulation that is capable of handling highly heterogeneous and anisotropic domains using general polygonal meshes. In the strategy developed in this work, the mesh that discretize the domain must fit the spatial position of the fractures, so that they are associated to the control surfaces—as (n − 1)D cells—therefore, the calculation of the fluxes in these control surfaces is dependent on the pressures on fractures and on the adjacent volumes. In HyG, the fractures are expanded to nD in the computational domain. The proposed formulation presented quite remarkable results when compared with similar formulations using classical full pressure support and triangle pressure support methods, or even the with MPFA‐D itself when the fractures are treated as nD geometric entities.

► The double‐tree method: An O(n) unsteady aerodynamic lifting surface method
  11 Sep, 2020
The double‐tree method: An O(n) unsteady aerodynamic lifting surface method

A new method for reducing the computational cost of the unsteady vortex lattice method is introduced. The method reduces the order of computations from O(n 2) to O(n) by utilizing two tree structures in tandem. A number of case studies are analyzed to verify its effectiveness and show how the accuracy/efficiency trade‐off can be controlled.


Summary

Two new methods for reducing the computational cost of the unsteady vortex lattice method are developed. These methods use agglomeration to construct time‐saving tree structures by approximating the effect of either a group of vortex rings or query points. A case study shows that combining the two new O(log n) tree methods together results in an O(n) method, called the double‐tree method. Other case studies show that the trade‐off between accuracy and speed can be easily and reliably controlled by the agglomeration cutoff distance. For a flat plate with 5 × 200 panels analyzed over 20 time steps, the double‐tree method is 7 times faster than the unsteady vortex lattice method with a <5% difference in the force distribution and total lift coefficient. The case studies suggest that the computational benefit will increase for the same level of accuracy if the size of the problem is increased, making the method beneficial for full‐aircraft analysis within optimization or dynamic load analysis, where the computational cost of the unsteady vortex lattice method can be large.

► A third‐order compact nonlinear scheme for compressible flow simulations
  11 Sep, 2020
A third‐order compact nonlinear scheme for compressible flow simulations

Simulation of Rayleigh‐Taylor instability problem: The proposed scheme, TCNS, together with an accurate/less dissipative Riemann solver, HLLC, obtains significantly high resolutions for small scales


Summary

Reynolds‐averaged Navier‐Stokes simulations based on second‐order numerical methods are widely used by commercial codes and work as dominating tools for most industrial applications. They, however, suffer from limitations in accurate and reliable predictions of skin‐friction drag and aerodynamic heating, as well as in simulations of complex flows such as large‐scale separation and transition. A remedy for this is the development of high‐order schemes, by which numerically induced dissipation and dispersion errors of low‐order schemes can be effectively reduced. Weighted compact nonlinear schemes (WCNSs) are a family of high‐resolution nonlinear shock‐capturing methods. A stencil‐selection procedure is introduced in the proposed work with an aim to improve the nonlinear weight of the third‐order WCNS. By using the approximate dispersion relation analysis, it is demonstrated that the new scheme has reduced dissipation and dispersion errors, compared with WCNSs using two typical nonlinear weights. Improvements are also achieved by the new scheme in numerical tests such as the double Mach reflection problem and the Rayleigh‐Taylor instability simulation, which are characterized by strong shock discontinuities and rich small scales, respectively. The new scheme is therefore highly favored in the simulation of flow problems involving strong discontinuities and multiscales phenomena.

► Simulation of compressibility of entrapped air in an incompressible free surface flow using a pressure‐based method for unified equations
  11 Sep, 2020
Simulation of compressibility of entrapped air in an incompressible free surface flow using a pressure‐based method for unified equations

In this article, a pressure‐based method is developed to solve the unified conservation laws for incompressible and compressible fluids. The proposed method is validated based on the comparison of the pressure fluctuations due to an oscillating water column in a closed tube and a free drop of a liquid block in a tank. The developed code is applied to simulate the compression and expansion of the entrapped air in a dam break flow.


Summary

A pressure‐based method is developed to solve the unified conservation laws for incompressible and compressible fluids. A polytropic law is used to model the compressibility of a gas and decouple the energy equation. The pressure field is calculated by solving a single‐pressure Poisson equation for the entire flow domain. The effects of the compressibility of the gas are reflected in the source term of the Poisson equation. The continuities of pressure and normal velocity across a material interface are achieved without any additional treatment along the interface. To validate the developed method, the oscillation of a water column in a closed tube due to the compression and expansion of air in the tube is simulated. The computed time history of the pressure at the end wall of the tube is in good agreement with other computational results. The free drop of a water column in a closed tank is simulated. The time history of the pressure at the center of the bottom of the tank shows good agreement with other reported results. The developed code is applied to simulate the air cushion effect of entrapped air in a dam break flow. The computed result is in good agreement with other experimental and computational results until the air is entrapped. As the entrapped air pocket undergoes rapid pulsation, the pressure field of water around the air pocket oscillates synchronously.

► A proper orthogonal decomposition variational multiscale meshless interpolating element‐free Galerkin method for incompressible magnetohydrodynamics flow
  11 Sep, 2020
A proper orthogonal decomposition variational multiscale meshless interpolating element‐free Galerkin method for incompressible magnetohydrodynamics flow


Summary

In the recent decade, the meshless methods have been handled for solving most of PDEs due to easiness of the meshless methods. One of the popular meshless methods is the element‐free Galerkin (EFG) method that was first proposed for solving some problems in the solid mechanics. The test and trial functions of the EFG are based on the special basis. Recently, some modifications have been developed to improve the EFG method. One of these improvements is the variational multiscale EFG procedure. In the current article, the shape functions of interpolation moving least squares approximation have been applied to the variational multiscale EFG technique for solving the incompressible magnetohydrodynamics flow. In order to reduce the elapsed CPU time of simulation, we employ a reduced‐order model based on the proper orthogonal decomposition technique. The current combination can be referred to as the reduced‐order variational multiscale EFG technique. To illustrate the reduction in CPU time used as well as the efficiency of the proposed method, we applied it for the two‐dimensional cases.

Journal of Computational Physics top

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

Publication date: Available online 9 September 2020

Source: Journal of Computational Physics

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

► A weighted Shifted Boundary Method for fee surface flow problems
    

Publication date: Available online 10 September 2020

Source: Journal of Computational Physics

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

► A spectral deferred correction method for incompressible flow with variable viscosity
    

Publication date: Available online 10 September 2020

Source: Journal of Computational Physics

Author(s): Jörg Stiller

► A Neural Network based Shock Detection and Localization Approach for Discontinuous Galerkin Methods
    

Publication date: Available online 9 September 2020

Source: Journal of Computational Physics

Author(s): Andrea D. Beck, Jonas Zeifang, Anna Schwarz, David G. Flad

► Boundary treatment of implicit-explicit Runge-Kutta method for hyperbolic systems with source terms
    

Publication date: Available online 9 September 2020

Source: Journal of Computational Physics

Author(s): Weifeng Zhao, Juntao Huang

► A robust and efficient segregated algorithm for fluid flow: The EPPL method
    

Publication date: Available online 5 September 2020

Source: Journal of Computational Physics

Author(s): HuiJie Zhang, WeiBing Zhu, Hong Chen

► A comparative study of the delta-Eddington and Galerkin quadrature methods for highly forward scattering of photons in random media
    

Publication date: Available online 5 September 2020

Source: Journal of Computational Physics

Author(s): Hiroyuki Fujii, Go Chiba, Yukio Yamada, Yoko Hoshi, Kazumichi Kobayashi, Masao Watanabe

► A conservative discontinuous Galerkin discretization for the chemically reacting Navier-Stokes equations
    

Publication date: Available online 8 September 2020

Source: Journal of Computational Physics

Author(s): Ryan F. Johnson, Andrew D. Kercher

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

Publication date: Available online 11 September 2020

Source: Journal of Computational Physics

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

► QTT-isogeometric solver in two dimensions
    

Publication date: Available online 11 September 2020

Source: Journal of Computational Physics

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

Journal of Turbulence top

► On the importance of the drag coefficient modelling in the double averaged Navier-Stokes equations for prediction of the roughness effects
    9 Sep, 2020
.
► Actuator line method applied to grid turbulence generation for large-Eddy simulations
  13 Aug, 2020
.
► Controlling Rayleigh–Bénard convection via reinforcement learning
  29 Jul, 2020
.
► Large eddy simulation of the periodic cavity evolution and the turbulence characteristics around a Delft Twist-11 hydrofoil
  27 Jul, 2020
Volume 21, Issue 7, July 2020, Page 386-405
.
► Wall-shear stress fluctuations in a supersonic turbulent boundary layer over an expansion corner
  23 Jul, 2020
Volume 21, Issue 7, July 2020, Page 355-374
.
► Towards a self-consistent Boltzmann's kinetic model of fluid turbulence
  22 Jul, 2020
Volume 21, Issue 7, July 2020, Page 375-385
.
► A perspective on machine learning in turbulent flows
  24 Apr, 2020
.
► Monte Carlo science
  19 Mar, 2020
.
► Neural network models for the anisotropic Reynolds stress tensor in turbulent channel flow
  24 Dec, 2019
.

Physics of Fluids top

► Two-phase flow boiling in a microfluidic channel at high mass flux
  17 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
We report the experimental investigations of two-phase flow boiling heat transfer characteristics of a refrigerant in a microfluidic channel at a high mass flux (more than 1000 kg/m2 s). We investigate the heat transfer coefficients at a heat flux range of 7.63 kW/m2–49.46 kW/m2, mass flux range of 600 kg/m2 s–1400 kg/m2 s (high mass flux), and saturation temperature range of 23 °C–31 °C. We propose the new two-phase flow boiling heat transfer correlation of a refrigerant, which is used as the working fluid for the present experiments, at the microfluidic scale. We experimentally establish the functional relationship of two-phase flow boiling heat transfer correlation of the refrigerant during flow boiling in a rectangular microchannel with the Reynolds number, the boiling number, and the Weber number. We believe that the inferences of this study may provide a design basis for the micro-heat exchanger, typically used for thermal management in electronic devices, micro-electro-mechanical systems, and electric vehicle battery cooling system.
► Anomalous features in internal cylinder flow instabilities subject to uncertain rotational effects
  17 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
We study the flow dynamics inside a high-speed rotating cylinder after introducing strong symmetry-breaking disturbance factors at cylinder wall motion. We propose and formulate a mathematically robust stochastic model for the rotational motion of the cylinder wall alongside the stochastic representation of incompressible Navier–Stokes equations. We employ a comprehensive stochastic computational fluid dynamics framework combining the spectral/hp element method and the probabilistic collocation method to obtain high-fidelity realizations of our mathematical model in order to quantify the propagation of parametric uncertainty for dynamics-representative quantities of interests. We observe that the modeled symmetry-breaking disturbances cause a flow instability arising from the wall. Utilizing global sensitivity analysis approaches, we identify the dominant source of uncertainty in our proposed model. We next perform a qualitative and quantitative statistical analysis on the fluctuating fields characterizing the fingerprints and measures of intense and rapidly evolving non-Gaussian behavior through space and time. We claim that such non-Gaussian statistics essentially emerge and evolve due to an intensified presence of coherent vortical motions initially triggered by the flow instability due to the symmetry-breaking rotation of the cylinder. We show that this mechanism causes memory effects in the flow dynamics in a way that noticeable anomaly in the time-scaling of enstrophy record is observed in the long run apart from the onset of instability. Our findings suggest an effective strategy to exploit controlled flow instabilities in order to enhance the turbulent mixing in engineering applications.
► Correlation between linear and nonlinear material functions under large amplitude oscillatory shear
  17 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
Fourier transform rheology is the most frequently used method to interpret the nonlinear rheological behavior of complex fluids under large amplitude oscillatory shear (LAOS). However, the unclear relationship between the higher harmonics and the fundamental harmonic obscures the physical meaning of the nonlinear functions. Here, we hypothesize that all the nonlinear oscillatory shear functions and normal stress functions can be expressed as linear combinations of linear viscoelastic functions or their derivatives at different frequencies under both strain-controlled LAOS (LAOStrain) and stress-controlled LAOS (LAOStress). We check this hypothesis using the time-strain separable Wagner model, Giesekus model, and modified Leonov model. We find such correlations between the nonlinear material functions and the linear material functions are intrinsic for viscoelastic liquids under LAOStrain, and for viscoelastic solids under LAOStress. Finally, these correlations are justified by a viscoelastic standard polydimethylsiloxane, an ethylene–octene multiblock copolymer melt, and a typical simple yield stress material (0.25 wt. % Carbopol).
► On determining the power-law fluid friction factor in a partially porous channel using the lattice Boltzmann method
  17 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
In the present work, the power-law fluid flow in a channel partially filled with a porous medium is numerically investigated using the lattice Boltzmann method (LBM). The porous domain, placed in the lower half of the channel, is represented according to a heterogeneous approach by a matrix of solid square disconnected blocks. The apparent viscosity of the power-law fluid is computed by locally varying the LBM relaxation factor. The results show the influence of geometry (porosity, number of obstacles, and hydraulic diameter), inertia (Reynolds number), and fluid properties (power-law index) over the partially porous-to-impermeable channel friction factor ratio. In general, the higher the porosity and the lower the number of obstacles, Reynolds number, and power-law index, the lower the friction factor. Finally, a correlation for the friction factor ratio as a function of the free region hydraulic diameter, permeability, and power-law index is presented for a specific channel configuration.
► Validation of finite element analysis strategy to investigate acoustic levitation in a two-axis acoustic levitator
  16 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
A two-axis acoustic levitator can be used to generate a standing pressure wave capable of levitating solid and liquid particles at appropriate input conditions. This work proposes a simulation framework to investigate the two-axis levitation particle stability using a commercial, computational fluid dynamics software based on the harmonic solution to the acoustic wave equation. The simulation produced predictions of the standing wave that include a strong “+” shaped pattern of nodes and anti-nodes that are aligned with the levitator axes. To verify the simulation, a levitator was built and used to generate the standing wave. The field was probed with a microphone and a motorized-scanning system. After scaling the simulated pressure to the measured pressure, the magnitudes of the sound pressure level at corresponding high-pressure locations were different by no more than 5%. This is the first time a measurement of a two-axis levitator standing pressure wave has been presented and shown to verify simulations. As an additional verification, the authors consulted high speed camera measurements of a reference-levitator transducer, which was found to have a maximum peak-to-peak displacement of 50 ± 5 μm. The reference-levitator is known to levitate water at 160 dB. The system for this work was simulated to match the operation of the reference-levitator so that it produced sound pressure levels of 160 dB. This pressure was achieved when the transducer maximum peak-to-peak displacement was 50.8 µm. The agreement between the two levitators’ displacements provides good justification that the modeling approach presented here produces reliable results.
► Comprehensive study of monatomic fluid flow through elliptical carbon nanotubes
  16 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
To achieve a realistic model of a carbon nanotube (CNT) membrane, a good understanding of the effects associated with CNT deformations is a key issue. In this study, using molecular dynamics simulation, argon flow through elliptical CNTs is studied. Two armchair CNTs (6, 6) and (10, 10) were considered. The results demonstrated non-uniform dependency of the flow rate to eccentricity of the tube, leading to an unexpectedly increased flow rate in some cases. The effects of tube size, temperature, and pressure gradient are investigated, and longitudinal variations of the interatomic potential and average axial velocity in different segments of the cross section are presented to justify the abnormal behavior of the flow rate with eccentricity. The results showed a significant deviation from the macroscale expectations and approved elliptical deformation as a non-negligible change in the overall flow rate, which should be considered in predictive models of CNT membranes.
► Dynamics of large-scale circulation and energy transfer mechanism in turbulent Rayleigh–Bénard convection in a cubic cell
  16 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
We present the characteristics and dynamics of large-scale circulation (LSC) in turbulent Rayleigh–Bénard convection (RBC) inside a cubic cell. The simulations are carried out for a Rayleigh number range of 2 × 106 ≤ Ra ≤ 109 and using air (at Prandtl number Pr = 0.7) as the working fluid. Using the Fourier mode analysis, the strength, orientation, and associated dynamics of LSC are characterized. Following previous two-dimensional studies in RBC, we propose a mechanism of flow reversals based on the dynamics of corner vortices, which is less attempted in three-dimensional counterparts. We observe that the plane containing LSC is generally aligned along one of the diagonals of the box accompanied by a four-roll structure in the other. In addition to the primary roll, two secondary corner-roll structures are also observed in the LSC plane, which grow in size and destabilize the LSC, resulting in partial (ΔΦ1 ≈ π/2) and complete (ΔΦ1 ≈ π) reversals. In addition to previously reported rotation-led reorientations, we also observe cessation events that are rare in cubic cells. We observe that as the Rayleigh number is increased from Ra = 2 × 106 to 107, the number of reorientations reduces by one third. With an increase in Ra, the strength of LSC (SLSC) increases and the corner rolls reduce in size, which leads to the reduction in the occurrence of reorientations. At higher Rayleigh numbers (Ra > 108), the strength saturates around SLSC ≈ 0.75. To connect the dynamics between different coherent structures, we evaluate the turbulent kinetic energy (TKE) budget. Notably, our novel approach to study the variation of TKE along the azimuthal direction helps in identifying the dynamical coupling between the LSC and non-LSC planes. The analysis suggests that TKE is generally produced in localized regions in both the planes, while its dissipation mainly happens in the vicinity of the plane that contains LSC. The transport mechanism redistributes the energy between these planes and thus sustains the LSC and other coherent structures.
► A comparative turbulent flow study of unconfined orthogonal and oblique slot impinging jet using large-eddy simulation
  16 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
The incompressible turbulent flow field of the slot impinging jet has been studied numerically at a Reynolds number of 7900 and d = 6w using large-eddy simulation with the wall adapting local eddy-viscosity subgrid-scale model for the angles of impingement 70° and 90°. The validity of the computation is confirmed by reasonable comparisons of the wall shear stress, pressure variation over the impingement plate, jet-centerline velocity, and second-order turbulent properties with past experimental and numerical results. The turbulent stress, turbulent length scales, and turbulent structure sizes are observed to be increased in the oblique impingement. The appearance of the oblate spheroid-shaped, three-dimensional isotropic, and prolate spheroid-shaped turbulence has been marked in the wall-jet region using the anisotropy invariant map. The power spectra of the fluctuating field maintain the −5/3 slope in the inertial subrange, which as expected becomes steeper in the dissipation range, as stated by Kolmogorov. Both positively skewed and negatively skewed fluctuations are seen in the flow field, and their probability density functions suggest that the fluctuation range increases in the case of oblique impingement. The involvement of various shearing and swirling structures has been investigated employing the proper orthogonal decomposition, the Q- function, and the λ2- function, where the isosurface of vorticity components is used to represent the direction of rotations.
► Dynamics of bacterial deposition in evaporating drops
  15 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
Evaporation of drops almost always deposits their suspended particles at the drop edge. The dynamics of this process and the resulting patterns depend upon various parameters related to the liquid, substrate, and particles. An interesting scenario is interactions among the particles leading to inhomogeneous depositions characterized by distinct edge-growth dynamics. Here, we study a more complex system with bacteria inside the evaporating drop. Bacteria interact like sticky particles forming inhomogeneous clusters, however, with edge-growth dynamics as that of non-interacting particles. We hypothesis that this contradicting behavior is due to the increased randomness introduced by bacteria–substrate interactions. Our findings have importance in understanding the patterns and their formation in growth systems of soft matter.
► Coupled radiative and convective heat transfer in enclosures: Effect of inner heater–enclosure wall emissivity contrast
  15 Sep, 2020
Physics of Fluids, Volume 32, Issue 9, September 2020.
The problem of thermal radiation in the presence of nonuniform emissivity arising through different types surfaces involved in thermal-control systems is addressed. In particular, its effect on natural convection driven by an inner hot plate kept inside a square enclosure is studied. The enclosure considered is either horizontally or vertically cooled, and two different primary orientations of the inner hot plate are considered. The corresponding governing partial differential equations were solved by the finite volume method on a uniform and regular grid system. While doing so, the net radiation method was used to determine the radiative surface fluxes. The effect of two opposing emissivity contrasts between the inner hot plate and enclosure walls is studied for the Rayleigh numbers Ra ≤ 107. The flow and heat transfer mechanisms at the resulting steady state are discussed via isotherms, streamlines, and average Nusselt number [math]. The findings arrived out of this comprehensive study shows that prominent heat transfer enhancement occurs when the emissivity of the inner hot plate is higher. Significant changes introduced by the emissivity contrast in the velocity and temperature fields can be seen for higher Rayleigh numbers. Moreover, better heat removal through the combined radiation and convection mechanism is observed invariably for the vertical hot plate in the presence of emissivity contrast. It is found that the heat transfer can be augmented up to around 35% through a good knowledge of the emissivity contrast.

Theoretical and Computational Fluid Dynamics top

► Floquet analysis on a viscous cylindrical fluid surface subject to a time-periodic radial acceleration
  14 Sep, 2020

Abstract

Parametrically excited standing waves are observed on a cylindrical fluid filament. This is the cylindrical analog of the Faraday instability in a flat surface or spherical droplet. Using Floquet theory, a linear stability analysis is carried out on a viscous cylindrical fluid surface, which is subjected to a time-periodic radial acceleration. Viscosity of the fluid has a significant impact on the critical forcing amplitude as well as the dispersion relation of the non-axisymmetric patterns. The effect of viscosity on the threshold of the pattern with azimuthal wavenumber \(m=1\) shows a different dependence from \(m>1\) . It is also observed that the effect of viscosity is greater on the threshold with higher m.

► Numerical simulations of buoyancy-driven flows using adaptive mesh refinement: structure and dynamics of a large-scale helium plume
    1 Sep, 2020

Abstract

The physical characteristics and evolution of a large-scale helium plume are examined through a series of numerical simulations with increasing physical resolution using adaptive mesh refinement (AMR). The five simulations each model a 1-m-diameter circular helium plume exiting into a \((4~\hbox {m})^3\) domain and differ solely with respect to the smallest scales resolved using the AMR, spanning resolutions from 15.6 mm down to 0.976 mm. As the physical resolution becomes finer, the helium–air shear layer and subsequent Kelvin–Helmholtz instability are better resolved, leading to a shift in the observed plume structure and dynamics. In particular, a critical resolution is found between 3.91 and 1.95 mm, below which the mean statistics and frequency content of the plume are altered by the development of a Rayleigh–Taylor (RT) instability near the centerline in close proximity to the plume base. Comparisons are made with prior experimental and computational results, revealing that the presence of the RT instability leads to reduced centerline axial velocities and higher puffing frequencies than when the instability is absent. An analysis of velocity and scalar gradient quantities, and the dynamics of the vorticity in particular, show that gravitational torque associated with the RT instability is responsible for substantial vorticity production in the flow. The grid-converged simulations performed here indicate that very high spatial resolutions are required to accurately capture the near-field structure and dynamics of large-scale plumes, particularly with respect to the development of fundamental flow instabilities.

► Mass transport in electrokinetic microflows with the wall reaction affecting the hydrodynamics
  29 Aug, 2020

Abstract

The mass transport in electrokinetically actuated microchannel flow is interesting when the wall reactions influence the wall potential, thereby affecting the hydrodynamics. This is the first work where the electro-osmotic flow is impacted by the chemical reactions. Since the wall potential is non-uniform, we have compared the results of the classical Poisson–Boltzmann equations with the generalized Poisson–Nernst–Planck model and investigated the applicability within the range of the operating conditions of the problem. The results provide fundamental understanding of the velocity profile within the channel and the wall concentration, which is significantly different from the classical species transport. The wall concentration is dependent on the electrokinetic parameters rather than the Reynolds and Peclet number solely. For constant volumetric flow rate, the resultant electro-osmotic velocity profile is not parabolic and exhibits higher convection close to the wall, leading to reduced solute polarization. The overall mass transport rate can be enhanced by more than two times with respect to non-electrical phenomena. The results will be useful in understanding the physics and provide operational know-how of electrokinetic-based applications related to capillary electrophoresis, electrochromatrogaphy and (bio-)chemical sensing.

► Investigation of pressure and the Lewis number effects in the context of algebraic flame surface density closure for LES of premixed turbulent combustion
  25 Aug, 2020

Abstract

Large scale industrial combustion devices, for example, internal combustion engines, gas turbine combustors, etc., operate under high-pressure conditions and utilize a variety of fuels. Unfortunately, the majority of the current numerical combustion modelling approaches are not fully validated for high-pressure and the non-unity Lewis number ( \(\hbox {Le} =\) thermal diffusivity/mass diffusivity) effects in premixed turbulent combustion. In any case, a numerical model needs to be checked for the effects of these parameters to guarantee generality of the model. In the present study, these two critical features of the models are numerically explored utilizing fundamental elements of several algebraic flame surface density reaction rate closure models accessible in the open literature. The Lewis number impact is likewise examined utilizing LES of recently published subgrid scale fractal flame surface density model, which indicated acceptable results for high and low-pressure methane fuelled applications. The computed numerical results are compared with an extensive experimental dataset for lean methane and propane fuels featuring various flow and turbulence conditions at operating pressures in the range of 1–10 bar. The quantitative results from most of the selected models do not show the experimentally observed trends at high-pressures and for non-unity Le number fuels. Modifications to the models are incorporated to reflect effects of these two important parameters utilizing a broad parametric investigation resulting in a satisfactory agreement with the experimental data.

► Oscillations of a periodically forced slightly eccentric spheroid in an unsteady viscous flow at low Reynolds numbers
    5 Aug, 2020

Abstract

The equations governing the dynamics of a periodically driven micro-spheroid in an unsteady viscous fluid at low Reynolds number are derived. Its oscillation properties in the presence/absence of memory forces are reported. The core part of the derivation is a perturbation analysis of motion of a sphere. The calculated solutions match with those available in the literature in the limiting case of a sphere. The dependence of the solutions on shape ( \(\alpha \) ), free oscillation frequency ( \(\omega _0\) ) and particle–fluid density ratio ( \(d_r\) ) is calculated. The maximum amplitude of the oscillations of an oblate spheroid is greater than that of a prolate spheroid, showing that the velocity disturbance for an oblate spheroid is higher in the presence/absence of the memory force. The increase in \(\alpha \) leads to the enhancement(reduction) of amplitude peaks in the case of the oblate (prolate) spheroid in the presence and more dominantly in the absence of the force. There is also a reduction in the amplitude of spheroid oscillations of many multiples due to the presence of the memory force. Stronger oscillation variations are observed on changing \(\omega _0\) or \(d_r\) compared to \(\alpha .\) The variations of the value of the phase are similar for both the spheroids on varying \(\omega _0\) and \(d_r\) , whereas they are reversed on varying \(\alpha .\) The linear scaling of amplitude on \(\alpha \) observed for the spheroids may give insight into the physics, especially regarding the quantum of velocity disturbances due to particle size. The slopes are high in the absence of the force, confirming that the presence of the force increases the resistance of spheroid motion, largely. The dependencies of oscillations on the parameters can be utilized for better separation of particles or for characterizing the suspension. The novelty of the problem and its analytical solutions might have value as tests in software for more complicated and realistic systems and hence strikes a good balance between complication and tractability.

► An ensemble Kalman filter approach to parameter estimation for patient-specific cardiovascular flow modeling
    1 Aug, 2020

Abstract

Many previous studies have shown that the fidelity of three-dimensional cardiovascular flow simulations depends strongly on inflow and outflow boundary conditions that accurately describe the characteristics of the larger vascular network. These boundary conditions are generally based on lower-dimensional models that represent the upstream or downstream flow behavior in some aggregated fashion. However, the parameters of these models are patient-specific, and no clear technique exists for determining them. In this work, an ensemble Kalman filter (EnKF) is implemented for the purpose of estimating parameters in cardiovascular models through the assimilation of specific patients’ clinical measurements. Two types of models are studied: a fully zero-dimensional model of the right heart and pulmonary circulation, and a coupled 0D–1D model of the lower leg. Model parameters are estimated using measurements from both healthy and hypertensive patients, and demonstrate that the EnKF is able to generate distinct parameter sets whose model predictions produce features unique to each measurement set. Attention is also given toward the quality of model predictions made in the absence of direct clinical counterparts, as well as techniques to improve filter robustness against shrinking ensemble covariance.

► Flowtaxis in the wakes of oscillating airfoils
    1 Aug, 2020

Abstract

Many aquatic organisms from copepods to harbor seals are able to detect and respond to flow disturbances. The physiological mechanisms underlying such behavior remain a challenge for current and future research. Here, we propose a simplified flow sensing scenario in which a mobile sensor reorients its heading in response to local flow stimuli, with the goal of tracing the wakes created by oscillating airfoils to their source. Specifically, we engineer a feedback control strategy where the sensory measurements are based on transverse vorticity gradients. Through numerical experiments, we assess the efficacy of the sensor in following topologically distinct wakes. We demonstrate that the strategy is robust to variations in the wake itself, and we arrive at empirical rules that the sensor’s initial position and orientation must satisfy in order to successfully locate the airfoil. We conclude by commenting on the relevance of the model and results to animal behavior and bio-inspired underwater robotics. We also discuss current and future opportunities for employing machine learning tools to devise and improve these sensory control strategies.

► Deep model predictive flow control with limited sensor data and online learning
    1 Aug, 2020

Abstract

The control of complex systems is of critical importance in many branches of science, engineering, and industry, many of which are governed by nonlinear partial differential equations. Controlling an unsteady fluid flow is particularly important, as flow control is a key enabler for technologies in energy (e.g., wind, tidal, and combustion), transportation (e.g., planes, trains, and automobiles), security (e.g., tracking airborne contamination), and health (e.g., artificial hearts and artificial respiration). However, the high-dimensional, nonlinear, and multi-scale dynamics make real-time feedback control infeasible. Fortunately, these high-dimensional systems exhibit dominant, low-dimensional patterns of activity that can be exploited for effective control in the sense that knowledge of the entire state of a system is not required. Advances in machine learning have the potential to revolutionize flow control given its ability to extract principled, low-rank feature spaces characterizing such complex systems. We present a novel deep learning model predictive control framework that exploits low-rank features of the flow in order to achieve considerable improvements to control performance. Instead of predicting the entire fluid state, we use a recurrent neural network (RNN) to accurately predict the control relevant quantities of the system, which are then embedded into an MPC framework to construct a feedback loop. In order to lower the data requirements and to improve the prediction accuracy and thus the control performance, incoming sensor data are used to update the RNN online. The results are validated using varying fluid flow examples of increasing complexity.

► Data-driven selection of actuators for optimal control of airfoil separation
    1 Aug, 2020

Abstract

We present a systematic approach for determining the optimal actuator location for separation control from input–output response data, gathered from numerical simulations or physical experiments. The Eigensystem realization algorithm is used to extract state-space descriptions from the response data associated with a candidate set of actuator locations. These system realizations are then used to determine the actuator location among the set that can drive the system output to an arbitrary value with minimal control effort. The solution of the corresponding minimum energy optimal control problem is evaluated by computing the generalized output controllability Gramian. We use the method to analyze high-fidelity numerical simulation data of the lift and separation angle responses to a pulse of localized body-force actuation from six distinct locations on the upper surface of a NACA 65(1)-412 airfoil. We find that the optimal location for controlling lift is different from the optimal location for controlling separation angle. In order to explain the physical mechanisms underlying these differences, we conduct controllability analyses of the flowfield by leveraging the dynamic mode decomposition with control algorithm. These modal analyses of flowfield response data reveal that excitation of coherent structures in the wake benefits lift control, whereas excitation of coherent structures in the shear layer benefits separation angle control.

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

Abstract

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


return

Layout Settings:

Entries per feed:
Display dates:
Width of titles:
Width of content: