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

CFD Journal Feeds

Annual Review of Fluid Mechanics top

► Space-Time Correlations and Dynamic Coupling in Turbulent Flows
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 51-70, January 2017.
► Combustion and Engine-Core Noise
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 277-310, January 2017.
► Uncertainty Quantification in Aeroelasticity
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 361-386, January 2017.
► Simulation Methods for Particulate Flows and Concentrated Suspensions
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 171-193, January 2017.
► Model Reduction for Flow Analysis and Control
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 387-417, January 2017.
► Vapor Bubbles
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 221-248, January 2017.
► Saph and Schoder and the Friction Law of Blasius
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 575-582, January 2017.
► Recent Developments in the Fluid Dynamics of Tropical Cyclones
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 541-574, January 2017.
► Inflow Turbulence Generation Methods
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 23-49, January 2017.
► Blood Flow in the Microcirculation
    5 Jan, 2017
Annual Review of Fluid Mechanics, Volume 49, Issue 1, Page 443-461, January 2017.

Computers & Fluids top

► Chebyshev collocation spectral lattice Boltzmann method in generalized curvilinear coordinates
  25 Feb, 2017
Publication date: 26 March 2017
Source:Computers & Fluids, Volume 146
Author(s): K. Hejranfar, M. Hajihassanpour
In this work, the Chebyshev collocation spectral lattice Boltzmann method is implemented in the generalized curvilinear coordinates to provide an accurate and efficient low-speed LB-based flow solver to be capable of handling curved geometries with non-uniform grids. The low-speed form of the D2Q9 and D3Q19 lattice Boltzmann equations is transformed into the generalized curvilinear coordinates and then the spatial derivatives in the resulting equations are discretized by using the Chebyshev collocation spectral method and the temporal term is discretized with the fourth-order Runge–Kutta scheme to provide an accurate and efficient low-speed flow solver. All boundary conditions are implemented based on the solution of the governing equations in the generalized curvilinear coordinates. The accuracy and robustness of the solution methodology presented are demonstrated by computing different benchmark and practical low-speed flow problems that are 2D Couette flow between concentric moving cylinders, 2D flow in a gradual expansion duct, 2D regularized trapezoidal cavity flow, and 3D flow in curved ducts of rectangular cross-sections. Results obtained for these test cases are in good agreement with the existing analytical and numerical results. The computational efficiency of the proposed solution methodology based on the Chebyshev collocation spectral lattice Boltzmann method implemented in the generalized curvilinear coordinates is also examined by comparison with the developed second-order finite-difference lattice Boltzmann method that indicates the proposed method provides more accurate and efficient solutions in terms of the CPU time and memory usage. The study shows the present solution methodology is robust and accurate for solving 2D and 3D low-speed flows over practical geometries. Indications are that the solution algorithm based on the CCSLBM in the generalized curvilinear coordinates does not need any filtering or numerical dissipation for stability considerations and thus high accuracy solutions obtained by applying the CCSLBM can be used as benchmark solutions for the evaluation of other LBM-based flow solvers.

► An integral validation technique of RANS turbulence models
  25 Feb, 2017
Publication date: Available online 16 February 2017
Source:Computers & Fluids
Author(s): Ian Pond, Alireza Ebadi, Yves Dubief, Christopher M. White
An integral technique to validate Reynolds-averaged Navier-Stokes (RANS) turbulence models is presented. The technique has the advantage of providing a direct connection between wall fluxes and mean flow dynamics, thus providing the necessary means to evaluate if a model correctly predicts the flow physics. In turn, the technique provides needed information critical to the improved development of turbulence models. To assess the value of the technique, it is used to evaluate the performance of two low-Reynolds-number turbulence models against DNS of reciprocating channel flow with heat transfer. The evaluation demonstrates that the integral technique is an improved validation technique compared to standard validation techniques.

► Numerical modelling of the rise of Taylor bubbles through a change in pipe diameter
  25 Feb, 2017
Publication date: 22 April 2017
Source:Computers & Fluids, Volume 148
Author(s): Stephen Ambrose, Ian S. Lowndes, David M. Hargreaves, Barry Azzopardi
The rise of Taylor bubbles through expansions in vertical pipes is modelled using Computational Fluid Dynamics. The predictions from the models are compared against existing experimental work and show good agreement, both quantitatively and qualitatively. Many workers, including the present work, find that, as the bubble passes through the expansion, it will either remain intact or split into one or more daughter bubbles. We find that the critical length of bubble, defined as the maximum length that will pass through intact, is proportional to the cosecant of the angle of the expansion. Further, we show that for an abrupt expansion, the critical bubble length became unaffected by the walls of the upper pipe as the diameter was increased.

► Flow control of the wake vortex street of a circular cylinder by using a traveling wave wall at low Reynolds number
  25 Feb, 2017
Publication date: 2 March 2017
Source:Computers & Fluids, Volume 145
Author(s): Feng Xu, Wen-Li Chen, Wei-Feng Bai, Yi-Qing Xiao, Jin-Ping Ou
In the present study, a traveling wave wall (TWW) was employed to manipulate the vortex shedding behind a fixed circular cylinder based on a 2-D CFD numerical simulation method at a low Reynolds number of 200. The study mainly focused on four types of TWW propagation directions combined with nine different wave velocities to eliminate vortex shedding, and the lift and drag coefficients and vortex shedding modes at different propagation directions or velocities were analyzed in detail to access the effectiveness of the TWW flow control method. The control mechanism of eliminating wake stemming from the flow around a TWW-controlled circular cylinder was revealed by the boundary vorticity flux (BVF) and relative flow fields. The results show that the type of downstream propagating TWW can effectively eliminate the alternating wake. When the ratio of the wave velocity to the oncoming velocity is 2.0, the fluctuations of the lift coefficients descended to the lowest point. The averages of the drag coefficients decreased with increasing wave velocity and descended to the lowest point when the ratio of the wave velocity to the oncoming velocity was 5.0. Owing to the capture of small-scale vortices, there was a relatively larger level of vorticity in the wave valley of the TWW cylinder. The relative flow fields showed that the vortex shedding from the cylinder was completely eliminated and that the goal of eliminating the oscillating wake and suppressing the potential vortex-induced vibration (VIV) of flow around a cylinder was achieved.

► Robust and efficient adjoint solver for complex flow conditions
  25 Feb, 2017
Publication date: 22 April 2017
Source:Computers & Fluids, Volume 148
Author(s): Shenren Xu, Sebastian Timme
A key step in gradient-based aerodynamic shape optimisation using the Reynolds-averaged Navier–Stokes equations is to compute the adjoint solution. Adjoint equations inherit the linear stability and the stiffness of the nonlinear flow equations. Therefore for industrial cases with complex geometries at off-design flow conditions, solving the resulting stiff adjoint equation can be challenging. In this paper, Krylov subspace solvers enhanced by subspace recycling and preconditioned with incomplete lower-upper factorisation are used to solve the stiff adjoint equations arising from typical design and off-design conditions. Compared to the baseline matrix-forming adjoint solver based on the generalized minimal residual method, the proposed algorithm achieved memory reduction of up to a factor of two and convergence speedup of up to a factor of three, on industry-relevant cases. These test cases include the DLR-F6 and DLR-F11 configurations, a wing-body configuration in pre-shock buffet and a large civil aircraft with mesh sizes ranging from 3 to 30 million. The proposed method seems to be particularly effective for the more difficult flow conditions.

► Numerical simulations of compressible multicomponent and multiphase flow using a high-order targeted ENO (TENO) finite-volume method
  25 Feb, 2017
Publication date: 26 March 2017
Source:Computers & Fluids, Volume 146
Author(s): Ory Haimovich, Steven H. Frankel
High-order numerical simulations of compressible multicomponent and multiphase flows are challenging due to the need to resolve both complex flow features and sharp gradients associated with material interfaces or shocks with minimal spurious oscillations. Recently, in the context of the WENO family of schemes, increasing the ENO property and incorporating improved convergence properties near local extrema points, has resulted in the targeted ENO or TENO scheme. In this study, a robust high-order finite-volume method based on the TENO scheme is implemented and tested for simulating multicomponent and multiphase compressible flows. A fifth-order spatial reconstruction is combined with a high-resolution modified HLLC Riemann solver, adjusted for the six-equation formulation of the diffuse interface model, and a third-order TVD Runge–Kutta explicit time-stepping scheme. Multidimensional extension is handled utilizing Gauss–Legendre quadrature points to evaluate both the flux and gas void fraction inter-cell terms. Several challenging 1D and 2D test cases are performed and compared to previously published experimental data and numerical simulations where available. A parametric study of the user-defined threshold parameter in the TENO algorithm is also studied and the TENO scheme is found to be more robust and less dissipative than both the WENO-Z and WENO-JS schemes.

► Analysis of non-conservative interpolation techniques in overset grid finite-volume methods
  25 Feb, 2017
Publication date: 22 April 2017
Source:Computers & Fluids, Volume 148
Author(s): S. Völkner, J. Brunswig, T. Rung
This paper reports on the challenges of using an overset grid approach when simulating incompressible flows with a cell-centred finite-volume solver. The focal point is on the grid coupling approach. Usually, overset grid methods for unstructured three-dimensional grids apply a local interpolation of field values onto the partner grid as a coupling procedure. Such local interpolation is obviously insufficient to guarantee that the global sum of the mass fluxes across the overlapping interfaces vanishes. Hence, the inherent mass conservation of finite volume methods is violated by the non-conservative property of the overset grid coupling. Since incompressible finite-volume solvers directly use the mass defect when solving for the pressure, severe pressure fluctuations may be provoked. A sensitivity study of the residual mass defect on overlapping grids and the related pressure fluctuations is performed for a variety of coupling strategies. Emphasis is given to five different aspects, i.e. relative grid motion, the order of accuracy of the employed baseline interpolation, the formulation of global mass conservation enforcing practices, the resolution quality of the overlapping grids and multiphase-flow issues. Examples included refer to a simple 2D cylinder flow, a 2D/3D lid-driven cavity flow, a 2D body force disturbed channel flow and 2D oscillating hydrofoil flows in fully wetted single-phase and immiscible two-phase conditions.Results reveal that transient flows and simulations with relative grid motion are subject to significant mass and pressure fluctuations. High-order interpolation and grid refinement prove beneficial, but can still be afflicted with severe disturbances, particularly when the resolution properties of the overlapping grids disagree. Two-phase flows with a high density ratio show a distinct compensating behaviour, while the introduction of flux or wetted volume correction practices leads to notable improvements for single-phase flows in both homogeneous and heterogeneous resolution conditions.

► Compressibility effects on the first global instability mode of the vortex formed in a regularized lid-driven cavity flow
  25 Feb, 2017
Publication date: 2 March 2017
Source:Computers & Fluids, Volume 145
Author(s): Yuya Ohmichi, Kojiro Suzuki
The effect of compressibility on the stability of a two-dimensional (2D) lid-driven cavity flow was investigated using a global linear stability analysis in this study. In a previous study, Bergamo et al. (2015) [12] revealed that compressibility has a stabilizing effect on dominant instability modes of this flow. However, the detailed mechanism of this stabilization effect has not been elucidated. The present study focused on the mechanism of this stabilizing effect on the first instability mode which causes a periodic oscillation to a primary vortex formed in the cavity. Our results show that the compressibility has a stabilizing effect on this flow due to the baroclinic torque and vorticity-dilatation term which appear in the vorticity transport equation. The distribution of these terms show that the vorticity-dilatation effect suppresses the changes in the perturbed vorticity (i.e., stabilizes the flow), and the baroclinic torque deforms the perturbed vorticity distribution. Furthermore, the effect of baroclinic torque was quantitatively estimated by solving the forced compressible stability problem in which the corresponding linearized equations are artificially forced to cancel the baroclinic torque. The results clearly show that the baroclinic torque decreases the growth rate of the first instability mode. However, the quantitatively estimated variation in the critical Reynolds number caused by the baroclinic torque indicates this term is not the most dominant mechanism of the stabilization effect and dilatation effects would be more significant.

► An adaptive numerical method for free surface flows passing rigidly mounted obstacles
  25 Feb, 2017
Publication date: 22 April 2017
Source:Computers & Fluids, Volume 148
Author(s): Kirill D. Nikitin, Maxim A. Olshanskii, Kirill M. Terekhov, Yuri V. Vassilevski, Ruslan M. Yanbarisov
The paper develops a method for the numerical simulation of a free-surface flow of incompressible viscous fluid around a streamlined body. The body is a rigid stationary construction partially submerged in the fluid. The application we are interested in the paper is a flow around a surface mounted offshore oil platform. The numerical method builds on a hybrid finite volume / finite difference discretization using adaptive octree cubic meshes. The mesh is dynamically refined towards the free surface and the construction. Special care is taken to devise a discretization for the case of curvilinear boundaries and interfaces immersed in the octree Cartesian background computational mesh. To demonstrate the accuracy of the method, we show the results for two benchmark problems: the sloshing 3D container and the channel laminar flow passing the 3D cylinder of circular cross-section. Further, we simulate numerically a flow with surface waves around an offshore oil platform for the realistic set of geophysical data.

► Geometrical parameter analysis on stabilizing the flow regime over a circular cylinder using two small rotating controllers
  25 Feb, 2017
Publication date: 2 March 2017
Source:Computers & Fluids, Volume 145
Author(s): M. Goodarzi, E. Khalili Dehkordi
Vortex shedding behind a cylindrical structure decreases its lifetime. Different active and passive methods have been proposed for suppressing the shed vortices. Two small rotating cylinders installing near to the main cylindrical structure can be actively used for this purpose. In the present research work, the impacts of the geometrical parameters on the effectiveness of the two rotating controllers, which were symmetrically installed neighbor to the main circular cylinder, have been numerically studied at a particular laminar flow regime. A finite volume approach has been used to simulate the unsteady flow around cylinders. Numerical computations illustrated that, both the main cylinder and its adjacent rotating controllers might be subjected to oscillatory forces. Also, numerical results showed that if rotating controllers were installed at an appropriate position, oscillatory exerted forces on the system of cylinders might be completely suppressed and flow regime became stable. Besides, the exerted drag forces on the main cylinder and also rotating controllers decreased when rotating controllers were installed at this particular position. Meanwhile, extensive analyses on the details of the flow field have been presented to discuss the mechanism of re-stabilizing the flow regime.

International Journal of Computational Fluid Dynamics top

► A dynamic framework for functional parameterisations of the eddy viscosity coefficient in two-dimensional turbulence
  16 Feb, 2017
.
► The influence of magnetic field on the physical explosion of a heavy gas cloud
    8 Feb, 2017
Volume 31, Issue 1, January 2017, Page 21-35
.
► A hybrid approach for nonlinear computational aeroacoustics predictions
  10 Jan, 2017
Volume 31, Issue 1, January 2017, Page 1-20
.
► Comparative studies of numerical methods for evaluating aerodynamic characteristics of two-dimensional airfoil at low Reynolds numbers
    5 Jan, 2017
Volume 31, Issue 1, January 2017, Page 57-67
.
► GPU accelerated simulations of three-dimensional flow of power-law fluids in a driven cube
    4 Jan, 2017
Volume 31, Issue 1, January 2017, Page 36-56
.
► Erratum
  20 Dec, 2016
Volume 31, Issue 1, January 2017, Page x-x
.
► Erratum
  18 Aug, 2014
.

International Journal for Numerical Methods in Fluids top

► An evolve-then-filter regularized reduced order model for convection-dominated flows
  24 Feb, 2017

Summary

In this paper, we propose a new evolve-then-filter reduced order model (EF-ROM). This is a regularized ROM (Reg-ROM), which aims to add numerical stabilization to proper orthogonal decomposition (POD) ROMs for convection-dominated flows. We also consider the Leray ROM (L-ROM). These two Reg-ROMs use explicit ROM spatial filtering to smooth (regularize) various terms in the ROMs. Two spatial filters are used: a POD projection onto a POD subspace (Proj) and a POD differential filter (DF). The four Reg-ROM/filter combinations are tested in the numerical simulation of the three-dimensional flow past a circular cylinder at a Reynolds number Re=1000. Overall, the most accurate Reg-ROM/filter combination is EF-ROM-DF. Furthermore, the spatial filter has a higher impact on the Reg-ROM than the regularization used. Indeed, the DF generally yields better results than Proj for both the EF-ROM and L-ROM. Finally, the CPU times of the four Reg-ROM/filter combinations are orders of magnitude lower than the CPU time of the DNS. Copyright © 2017 John Wiley & Sons, Ltd.

Thumbnail image of graphical abstract

In this paper, we propose a new evolve-then-filter reduced order model (EF-ROM), which is a regularized ROM (Reg-ROM) that increases the numerical stability of ROMs for convection-dominated flows. This new Reg-ROM uses two explicit ROM spatial filters (a differential filter and a projection) to smooth (regularize) various terms in the ROMs. The new EF-ROM produces accurate and efficient numerical approximations of a three-dimensional flow past a circular cylinder at a Reynolds number Re = 1000.

► Improvement of Moving Particle Semi-Implicit Method for Simulation of Progressive Water Waves
  21 Feb, 2017

SUMMARY

Precise simulation of the propagation of surface water waves, especially when involving breaking wave, takes a significant place in Computational Fluid Dynamics. Due to the strong nonlinear properties, the treatment of large surface deformation of free surface flow has always been a challenging work in the development of numerical models. In this paper, the Moving Particle Semi-implicit (MPS) method, an entirely Lagrangian method, is modified to simulate wave motion in a 2-D numerical wave flume preferably. In terms of consecutive pressure distribution, a new and simple free surface detection criterion is proposed to enhance the free surface recognition in the MPS method. In addition, a revised gradient model is deduced to diminish the effect of non-uniform particle distribution, and then to reduce the numerical wave attenuation occurring in the original MPS model. The applicability and stability of the improved MPS method are firstly demonstrated by the calculation of hydrostatic problem. It is revealed that these modifications are effective to suppress the pressure oscillation, weaken the local particle clustering and boost the stability of numerical algorithm. It is then applied to investigate the propagation of progressive waves on a flat bed and the wave breaking on a mild slope. Comparisons with the analytical solutions and experimental results indicate that the improved MPS model can give better results about the profiles and heights of surface waves in contrast with the previous MPS models.

► A finite volume scheme preserving extremum principle for convection–diffusion equations on polygonal meshes
  21 Feb, 2017

Summary

We propose a nonlinear finite volume scheme for convection–diffusion equation on polygonal meshes and prove that the discrete solution of the scheme satisfies the discrete extremum principle. The approximation of diffusive flux is based on an adaptive approach of choosing stencil in the construction of discrete normal flux, and the approximation of convection flux is based on the second-order upwind method with proper slope limiter. Our scheme is locally conservative and has only cell-centered unknowns. Numerical results show that our scheme can preserve discrete extremum principle and has almost second-order accuracy. Copyright © 2017 John Wiley & Sons, Ltd.

Thumbnail image of graphical abstract

We propose a nonlinear finite volume scheme for convection–diffusion equation on polygonal meshes and prove that the discrete solution of the scheme satisfies the discrete extremum principle. Our scheme is locally conservative and has only cell-centered unknowns. Numerical results show that our scheme can preserve discrete extremum principle and has almost second-order accuracy.

► A new volume-preserving and continuous interface reconstruction method for 2D multimaterial flow
  20 Feb, 2017

Summary

A new 2D interface reconstruction method which ensures continuity of the interface and preserves volume fractions is presented here. It is made of two steps, first the minimization of a cost functional based on volume fractions least square errors by using dynamic programming, a fast and efficient scheme well known in image processing, and then a local correction phase. In each cell, the interface is made of two line segments joining two edges. This new interface reconstruction method, called DPIR (Dynamic Programming Interface Reconstruction) has been coupled with various advection schemes, among them the Lagrange + remap scheme. With a reasonable computational cost, it has been observed in various test cases that DPIR is more accurate and less diffusive compared to other existing classical reconstruction methods. This article is protected by copyright. All rights reserved.

► Gradient-based nodal limiters for artificial diffusion operators in finite element schemes for transport equations
  20 Feb, 2017

Summary

This paper presents new linearity-preserving nodal limiters for enforcing discrete maximum principles in continuous (linear or bilinear) finite element approximations to transport problems with steep fronts. In the process of algebraic flux correction, the oscillatory antidiffusive part of a high-order base discretization is decomposed into a set of internodal fluxes and constrained to be local extremum dim inishing. The proposed nodal limiter functions are designed to be continuous and satisfy the principle of linearity preservation that implies the preservation of second-order accuracy in smooth regions. The use of limited nodal gradients makes it possible to circumvent angle conditions and guarantee that the discrete maximum principle holds on arbitrary meshes. A numerical study is performed for linear convection and anisotropic diffusion problems on uniform and distorted meshes in two space dimensions. Copyright © 2017 John Wiley & Sons, Ltd.

Thumbnail image of graphical abstract

Edge-based limiting techniques are proposed for enforcing local maximum principles in continuous finite element schemes for transport equations. Different generalizations of a one-dimensional jump and average limiter are considered and improved step by step. The use of limited nodal gradients makes it possible to circumvent angle conditions that apply to other local extremum diminishing and linearity-preserving limiters.

► Viscous regularization of the full set of nonequilibrium-diffusion grey radiation-hydrodynamic equations
  17 Feb, 2017

Summary

In [1], a viscous regularization technique, based on the local entropy residual, was proposed to stabilize the nonequilibrium-diffusion Grey Radiation-Hydrodynamic equations using an artificial viscosity technique. This viscous regularization is modulated by the local entropy production and is consistent with the entropy minimum principle. However, the work in [1] was only based on the hyperbolic parts of the Grey Radiation-Hydrodynamic equations and thus omitted the relaxation and diffusion terms present in the material energy and radiation energy equations. Here, we extend the theoretical grounds for the method and derive an entropy minimum principle for the full set of nonequilibrium-diffusion Grey Radiation Hydrodynamic equations. This further strengthens the applicability of the entropy viscosity method as a stabilization technique for radiation-hydrodynamic shock simulations. Radiative shock calculations using constant and temperature-dependent opacities are compared against semi-analytical reference solutions and we present a procedure to perform spatial convergence studies of such simulations. This article is protected by copyright. All rights reserved.

► Development of circular function-based gas-kinetic scheme (CGKS) on moving grids for unsteady flows through oscillating cascades
  15 Feb, 2017

SUMMARY

In this paper, the circular function-based gas-kinetic scheme (CGKS), which was originally developed for simulation of flows on stationary grids, is extended to solve moving boundary problems on moving grids. Particularly, the unsteady flows through oscillating cascades are our major interests. The main idea of the CGKS is to discretize the macroscopic equations by the finite volume method while the fluxes at the cell interface are evaluated by locally reconstructing the solution of the continuous Boltzmann BGK equation. The present solver is based on the fact that the modified Boltzmann equation which is expressed in a moving frame of reference can recover the corresponding macroscopic equations with Chapman-Enskog (C-E) expansion analysis. Different from the original Maxwellian function-based GKS, in order to improve the computational efficiency, a simple circular function is used to describe the equilibrium state of distribution function. Considering that the concerned cascade oscillating problems belong to cases that the motion of surface boundary is known a prior, the dynamic mesh method is suitable and is adopted in the present work. To achieve the mesh deformation with high quality and efficiency, a hybrid dynamic mesh method named RBFs-TFI is presented and applied for cascade geometries. For validation, several numerical test cases involving a wide range are investigated. Numerical results show that the developed CGKS on moving grids is well applied for cascade oscillating flows. And for some cases where nonlinear effects are strong, the solution accuracy could be effectively improved by using the present method.

► Issue Information
  14 Feb, 2017

No abstract is available for this article.

► Dynamically Correct Formulations of the Linearised Navier-Stokes Equations
  13 Feb, 2017

Summary

Motivated by the need to efficiently obtain low-order models of fluid flows around complex geometries for the purpose of feedback control system design, this paper considers the effect on system dynamics of basing plant models on different formulations of the linearised Navier-Stokes equations. We consider the dynamics of a single computational node formed by spatial discretisation of the governing equations in both primitive variables (momentum equation & continuity equation) and pressure Poisson equation (PPE) formulations. This reveals fundamental numerical differences at the nodal level, whose effects on the system dynamics at the full system level are exemplified by considering the corresponding formulations of a two-dimensional (2D) channel flow, subjected to a variety of different boundary conditions. This article is protected by copyright. All rights reserved.

► A Finite Volume scheme for solving Anisotropic Diffusion on ALE-AMR grids
  11 Feb, 2017

Summary

In the context of High Energy Density Physics (HEDP) and more precisely in the field of laser plasma interaction, Lagrangian schemes are commonly used. The lack of robustness due to strong grid deformations requires the regularization of the mesh through the use of Arbitrary Lagrangian Eulerian (ALE) methods. Theses methods usually add some diffusion and a loss of precision is observed. We propose to use Adaptive Mesh Refinement (AMR) techniques to reduce this loss of accuracy. This work focus on the resolution of the anisotropic diffusion operator on ALE-AMR grids. redIn this paper, we describe a second-order accurate cell-centered finite volume method for solving anisotropic diffusion on AMR type grids. The scheme describe here is based on local flux approximation which can be derived through the use of a finite difference approximation, leading to the CCLADNS scheme. In this paper, we discuss the 2D and 3D extension of the CCLADNS scheme to AMR meshes. This article is protected by copyright. All rights reserved.

Journal of Computational Physics top

► A weakly compressible SPH method based on a low-dissipation Riemann solver
  25 Feb, 2017
Publication date: 15 April 2017
Source:Journal of Computational Physics, Volume 335
Author(s): C. Zhang, X.Y. Hu, N.A. Adams
We present a low-dissipation weakly-compressible SPH method for modeling free-surface flows exhibiting violent events such as impact and breaking. The key idea is to modify a Riemann solver which determines the interaction between particles by a simple limiter to decrease the intrinsic numerical dissipation. The modified Riemann solver is also extended for imposing wall boundary conditions. Numerical tests show that the method resolves free-surface flows accurately and produces smooth, accurate pressure fields. The method is compatible with the hydrostatic solution and exhibits considerably less numerical damping of the mechanical energy than previous methods.

► A stochastic asymptotic-preserving scheme for a kinetic-fluid model for disperse two-phase flows with uncertainty
  25 Feb, 2017
Publication date: 15 April 2017
Source:Journal of Computational Physics, Volume 335
Author(s): Shi Jin, Ruiwen Shu
In this paper we consider a kinetic-fluid model for disperse two-phase flows with uncertainty. We propose a stochastic asymptotic-preserving (s-AP) scheme in the generalized polynomial chaos stochastic Galerkin (gPC-sG) framework, which allows the efficient computation of the problem in both kinetic and hydrodynamic regimes. The s-AP property is proved by deriving the equilibrium of the gPC version of the Fokker–Planck operator. The coefficient matrices that arise in a Helmholtz equation and a Poisson equation, essential ingredients of the algorithms, are proved to be positive definite under reasonable and mild assumptions. The computation of the gPC version of a translation operator that arises in the inversion of the Fokker–Planck operator is accelerated by a spectrally accurate splitting method. Numerical examples illustrate the s-AP property and the efficiency of the gPC-sG method in various asymptotic regimes.

► Non-deteriorating time domain numerical algorithms for Maxwell's electrodynamics
  25 Feb, 2017
Publication date: 1 May 2017
Source:Journal of Computational Physics, Volume 336
Author(s): S. Petropavlovsky, S. Tsynkov
The Huygens' principle and lacunae can help construct efficient far-field closures for the numerical simulation of unsteady waves propagating over unbounded regions. Those closures can be either standalone or combined with other techniques for the treatment of artificial outer boundaries. A standalone lacunae-based closure can be thought of as a special artificial boundary condition (ABC) that is provably free from any error associated with the domain truncation. If combined with a different type of ABC or a perfectly matched layer (PML), a lacunae-based approach can help remove any long-time deterioration (e.g., instability) that arises at the outer boundary regardless of why it occurs in the first place.A specific difficulty associated with Maxwell's equations of electromagnetism is that in general their solutions do not have classical lacunae and rather have quasi-lacunae. Unlike in the classical case, the field inside the quasi-lacunae is not zero; instead, there is an electrostatic solution driven by the electric charges that accumulate over time. In our previous work [23], we have shown that quasi-lacunae can also be used for building the far-field closures. However, for achieving a provably non-deteriorating performance over arbitrarily long time intervals, the accumulated charges need to be known ahead of time.The main contribution of the current paper is that we remove this limitation and modify the algorithm in such a way that one can rather avoid the accumulation of charge all together. Accordingly, the field inside the quasi-lacunae becomes equal to zero, which facilitates obtaining the temporally uniform error estimates as in the case of classical lacunae. The performance of the modified algorithm is corroborated by a series of numerical simulations. The range of problems that the new method can address includes important combined formulations, for which the interior subproblem may be non-Huygens', and only the exterior subproblem, i.e., the far field, is Huygens'.

► A finite volume method for two-sided fractional diffusion equations on non-uniform meshes
  25 Feb, 2017
Publication date: 15 April 2017
Source:Journal of Computational Physics, Volume 335
Author(s): Alex Simmons, Qianqian Yang, Timothy Moroney
We derive a finite volume method for two-sided fractional diffusion equations with Riemann–Liouville derivatives in one spatial dimension. The method applies to non-uniform meshes, with arbitrary nodal spacing. The discretisation utilises the integral definition of the fractional derivatives, and we show that it leads to a diagonally dominant matrix representation, and a provably stable numerical scheme. Being a finite volume method, the numerical scheme is fully conservative, and the ability to locally refine the mesh can produce solutions with more accuracy for the same number of nodes compared to a uniform mesh, as we demonstrate numerically.

► Multi-fidelity Gaussian process regression for prediction of random fields
  25 Feb, 2017
Publication date: 1 May 2017
Source:Journal of Computational Physics, Volume 336
Author(s): L. Parussini, D. Venturi, P. Perdikaris, G.E. Karniadakis
We propose a new multi-fidelity Gaussian process regression (GPR) approach for prediction of random fields based on observations of surrogate models or hierarchies of surrogate models. Our method builds upon recent work on recursive Bayesian techniques, in particular recursive co-kriging, and extends it to vector-valued fields and various types of covariances, including separable and non-separable ones. The framework we propose is general and can be used to perform uncertainty propagation and quantification in model-based simulations, multi-fidelity data fusion, and surrogate-based optimization. We demonstrate the effectiveness of the proposed recursive GPR techniques through various examples. Specifically, we study the stochastic Burgers equation and the stochastic Oberbeck–Boussinesq equations describing natural convection within a square enclosure. In both cases we find that the standard deviation of the Gaussian predictors as well as the absolute errors relative to benchmark stochastic solutions are very small, suggesting that the proposed multi-fidelity GPR approaches can yield highly accurate results.

► Kernel reconstruction methods for Doppler broadening — Temperature interpolation by linear combination of reference cross sections at optimally chosen temperatures
  25 Feb, 2017
Publication date: 15 April 2017
Source:Journal of Computational Physics, Volume 335
Author(s): Pablo Ducru, Colin Josey, Karia Dibert, Vladimir Sobes, Benoit Forget, Kord Smith
This article establishes a new family of methods to perform temperature interpolation of nuclear interactions cross sections, reaction rates, or cross sections times the energy. One of these quantities at temperature T is approximated as a linear combination of quantities at reference temperatures (Tj). The problem is formalized in a cross section independent fashion by considering the kernels of the different operators that convert cross section related quantities from a temperature T0 to a higher temperature T — namely the Doppler broadening operation. Doppler broadening interpolation of nuclear cross sections is thus here performed by reconstructing the kernel of the operation at a given temperature T by means of linear combination of kernels at reference temperatures (Tj). The choice of the L2 metric yields optimal linear interpolation coefficients in the form of the solutions of a linear algebraic system inversion. The optimization of the choice of reference temperatures (Tj) is then undertaken so as to best reconstruct, in the L sense, the kernels over a given temperature range [Tmin,Tmax]. The performance of these kernel reconstruction methods is then assessed in light of previous temperature interpolation methods by testing them upon isotope 238U. Temperature-optimized free Doppler kernel reconstruction significantly outperforms all previous interpolation-based methods, achieving 0.1% relative error on temperature interpolation of 238U total cross section over the temperature range [300 K,3000 K] with only 9 reference temperatures.

► A multi-layer integral model for locally-heated thin film flow
  25 Feb, 2017
Publication date: 1 May 2017
Source:Journal of Computational Physics, Volume 336
Author(s): E.D. Kay, S. Hibberd, H. Power
Based on an approach used to model environmental flows such as rivers and estuaries, we develop a new multi-layered model for thin liquid film flow on a locally-heated inclined plane. The film is segmented into layers of equal thickness with the velocity and temperature of each governed by a momentum and energy equation integrated across each layer individually. Matching conditions applied between the layers ensure the continuity of down-plane velocity, temperature, stress and heat flux. Variation in surface tension of the liquid with temperature is considered so that local heating induces a surface shear stress which leads to variation in the film height profile (the Marangoni effect). Moderate inertia and heat convection effects are also included.In the absence of Marangoni effects, when the film height is uniform, we test the accuracy of the model by comparing it against a solution of the full heat equation using finite differences. The multi-layer model offers significant improvements over that of a single layer. Notably, with a sufficient number of layers, the solution does not exhibit local regions of negative temperature often predicted using a single-layer model.With Marangoni effects included the film height varies however we find heat convection can mitigate this variation by reducing the surface temperature gradient and hence the surface shear stress. Numerical results corresponding to the flow of water on a vertical plane show that very thin films are dominated by the Marangoni shear stress which can be sufficiently strong to overcome gravity leading to a recirculation in the velocity field. This effect reduces with increasing film thickness and the recirculation eventually disappears. In this case heating is confined entirely to the interior of the film leading to a uniform height profile.

Graphical abstract

image
► A defect corrected finite element approach for the accurate evaluation of magnetic fields on unstructured grids
  25 Feb, 2017
Publication date: 15 April 2017
Source:Journal of Computational Physics, Volume 335
Author(s): Ulrich Römer, Sebastian Schöps, Herbert De Gersem
In electromagnetic simulations of magnets and machines, one is often interested in a highly accurate and local evaluation of the magnetic field uniformity. Based on local post-processing of the solution, a defect correction scheme is proposed as an easy to realize alternative to higher order finite element or hybrid approaches. Radial basis functions (RBFs) are key for the generality of the method, which in particular can handle unstructured grids. Also, contrary to conventional finite element basis functions, higher derivatives of the solution can be evaluated, as required, e.g., for deflection magnets. Defect correction is applied to obtain a solution with improved accuracy and adjoint techniques are used to estimate the remaining error for a specific quantity of interest. Significantly improved (local) convergence orders are obtained. The scheme is also applied to the simulation of a Stern–Gerlach magnet currently in operation.

► A multigrid method for linear systems arising from time-dependent two-dimensional space-fractional diffusion equations
  25 Feb, 2017
Publication date: 1 May 2017
Source:Journal of Computational Physics, Volume 336
Author(s): Xue-lei Lin, Michael K. Ng, Hai-Wei Sun
In this paper, we study a V-cycle multigrid method for linear systems arising from time-dependent two-dimensional space-fractional diffusion equations. The coefficient matrices of the linear systems are structured such that their matrix-vector multiplications can be computed efficiently. The main advantage using the multigrid method is to handle the space-fractional diffusion equations on non-rectangular domains, and to solve the linear systems with non-constant coefficients more effectively. The main idea of the proposed multigrid method is to employ two banded splitting iteration schemes as pre-smoother and post-smoother. The pre-smoother and the post-smoother take banded splitting of the coefficient matrix under the x-dominant ordering and the y-dominant ordering, respectively. We prove the convergence of the proposed two banded splitting iteration schemes in the sense of infinity norm. Results of numerical experiments for time-dependent two-dimensional space-fractional diffusion equations on rectangular, L-shape and U-shape domains are reported to demonstrate that both computational time and iteration number required by the proposed method are significantly smaller than those of the other tested methods.

► Seismic modeling with radial basis function-generated finite differences (RBF-FD) – a simplified treatment of interfaces
  25 Feb, 2017
Publication date: 15 April 2017
Source:Journal of Computational Physics, Volume 335
Author(s): Bradley Martin, Bengt Fornberg
In a previous study of seismic modeling with radial basis function-generated finite differences (RBF-FD), we outlined a numerical method for solving 2-D wave equations in domains with material interfaces between different regions. The method was applicable on a mesh-free set of data nodes. It included all information about interfaces within the weights of the stencils (allowing the use of traditional time integrators), and was shown to solve problems of the 2-D elastic wave equation to 3rd-order accuracy. In the present paper, we discuss a refinement of that method that makes it simpler to implement. It can also improve accuracy for the case of smoothly-variable model parameter values near interfaces. We give several test cases that demonstrate the method solving 2-D elastic wave equation problems to 4th-order accuracy, even in the presence of smoothly-curved interfaces with jump discontinuities in the model parameters.

Journal of Turbulence top

► Finite element-based large eddy simulation using a combination of the variational multiscale method and the dynamic Smagorinsky model
  23 Feb, 2017
.
► Optimal frequency-response sensitivity of compressible flow over roughness elements
    7 Feb, 2017
Volume 18, Issue 4, April 2017, Page 338-351
.
► Coherent structure extraction in turbulent channel flow using boundary adapted wavelets
    6 Feb, 2017
Volume 18, Issue 4, April 2017, Page 352-372
.
► Self-similarity and flow characteristics of vertical-axis wind turbine wakes: an LES study
    1 Feb, 2017
Volume 18, Issue 4, April 2017, Page 373-389
.
► Two-equation hybrid RANS–LES models: a novel way to treat k and ω at inlets and at embedded interfaces
  31 Jan, 2017
Volume 18, Issue 4, April 2017, Page 291-315
.
► Modulation of a methane Bunsen flame by upstream perturbations
  31 Jan, 2017
Volume 18, Issue 4, April 2017, Page 316-337
.

Physics of Fluids top

► History effect on the Reynolds stress in turbulent swirling flow
  24 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► Simulation of liquid jet atomization coupled with forced perturbation
  24 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► A novel investigation of a micropolar fluid characterized by nonlinear constitutive diffusion model in boundary layer flow and heat transfer
  24 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► Onset of Rayleigh-Bénard convection for intermediate aspect ratio cylindrical containers
  24 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► Tortuosity correction of Kozeny’s hydraulic diameter of a porous medium
  23 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► On the mixing enhancement in annular flows
  23 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► A theoretical approximation of the shock standoff distance for supersonic flows around a circular cylinder
  22 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► Shear-thinning of molecular fluids in Couette flow
  22 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► Editorial: In defense of science—What would John do?
  22 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.
► Anomalous structural response of nematic colloidal platelets subjected to large amplitude stress oscillations
  22 Feb, 2017
Physics of Fluids, Volume 29, Issue 2, February 2017.

Theoretical and Computational Fluid Dynamics top

► Numerical modeling of the impact pressure in a compressible liquid medium: application to the slap phase of the locomotion of a basilisk lizard
    6 Feb, 2017

Abstract

The forces acting on a solid body just at the time of impact on the surface of a medium with very low compressibility, such as water, can be quantified at acoustic time scales. This is necessary in wide range of applications varying from large-scale ship designs to the walking or running mechanisms of small creatures such as the basilisk lizard. In order to characterize such forces, a numerical model is developed in this study and is validated using analytical expressions of pressure as a function of the speed of sound-wave propagation in water. The computational results not only accurately match the analytical values but are also able to effectively capture the propagation of acoustic waves in water. The model is further applied to a case study wherein the impact impulse required by the basilisk lizard to assist in its walking on the water surface is evaluated. The numerical results are found to be in agreement with the closest available experimental data. The model and approach are thus proposed to evaluate impact forces for wide range of applications.

► Model of non-stationary, inhomogeneous turbulence
    1 Feb, 2017

Abstract

We compare results from a spectral model for non-stationary, inhomogeneous turbulence (Besnard et al. in Theor Comp Fluid Dyn 8:1–35, 1996) with direct numerical simulation (DNS) data of a shear-free mixing layer (SFML) (Tordella et al. in Phys Rev E 77:016309, 2008). The SFML is used as a test case in which the efficacy of the model closure for the physical-space transport of the fluid velocity field can be tested in a flow with inhomogeneity, without the additional complexity of mean-flow coupling. The model is able to capture certain features of the SFML quite well for intermediate to long times, including the evolution of the mixing-layer width and turbulent kinetic energy. At short-times, and for more sensitive statistics such as the generation of the velocity field anisotropy, the model is less accurate. We propose two possible causes for the discrepancies. The first is the local approximation to the pressure-transport and the second is the a priori spherical averaging used to reduce the dimensionality of the solution space of the model, from wavevector to wavenumber space. DNS data are then used to gauge the relative importance of both possible deficiencies in the model.

► A zonally symmetric model for the monsoon-Hadley circulation with stochastic convective forcing
    1 Feb, 2017

Abstract

Idealized models of reduced complexity are important tools to understand key processes underlying a complex system. In climate science in particular, they are important for helping the community improve our ability to predict the effect of climate change on the earth system. Climate models are large computer codes based on the discretization of the fluid dynamics equations on grids of horizontal resolution in the order of 100 km, whereas unresolved processes are handled by subgrid models. For instance, simple models are routinely used to help understand the interactions between small-scale processes due to atmospheric moist convection and large-scale circulation patterns. Here, a zonally symmetric model for the monsoon circulation is presented and solved numerically. The model is based on the Galerkin projection of the primitive equations of atmospheric synoptic dynamics onto the first modes of vertical structure to represent free tropospheric circulation and is coupled to a bulk atmospheric boundary layer (ABL) model. The model carries bulk equations for water vapor in both the free troposphere and the ABL, while the processes of convection and precipitation are represented through a stochastic model for clouds. The model equations are coupled through advective nonlinearities, and the resulting system is not conservative and not necessarily hyperbolic. This makes the design of a numerical method for the solution of this system particularly difficult. Here, we develop a numerical scheme based on the operator time-splitting strategy, which decomposes the system into three pieces: a conservative part and two purely advective parts, each of which is solved iteratively using an appropriate method. The conservative system is solved via a central scheme, which does not require hyperbolicity since it avoids the Riemann problem by design. One of the advective parts is a hyperbolic diagonal matrix, which is easily handled by classical methods for hyperbolic equations, while the other advective part is a nilpotent matrix, which is solved via the method of lines. Validation tests using a synthetic exact solution are presented, and formal second-order convergence under grid refinement is demonstrated. Moreover, the model is tested under realistic monsoon conditions, and the ability of the model to simulate key features of the monsoon circulation is illustrated in two distinct parameter regimes.

► Stability and modal analysis of shock/boundary layer interactions
    1 Feb, 2017

Abstract

The dynamics of oblique shock wave/turbulent boundary layer interactions is analyzed by mining a large-eddy simulation (LES) database for various strengths of the incoming shock. The flow dynamics is first analyzed by means of dynamic mode decomposition (DMD), which highlights the simultaneous occurrence of two types of flow modes, namely a low-frequency type associated with breathing motion of the separation bubble, accompanied by flapping motion of the reflected shock, and a high-frequency type associated with the propagation of instability waves past the interaction zone. Global linear stability analysis performed on the mean LES flow fields yields a single unstable zero-frequency mode, plus a variety of marginally stable low-frequency modes whose stability margin decreases with the strength of the interaction. The least stable linear modes are grouped into two classes, one of which bears striking resemblance to the breathing mode recovered from DMD and another class associated with revolving motion within the separation bubble. The results of the modal and linear stability analysis support the notion that low-frequency dynamics is intrinsic to the interaction zone, but some continuous forcing from the upstream boundary layer may be required to keep the system near a limit cycle. This can be modeled as a weakly damped oscillator with forcing, as in the early empirical model by Plotkin (AIAA J 13:1036–1040, 1975).

► A numerical investigation into the effects of Reynolds number on the flow mechanism induced by a tubercled leading edge
    1 Feb, 2017

Abstract

Leading-edge modifications based on designs inspired by the protrusions on the pectoral flippers of the humpback whale (tubercles) have been the subject of research for the past decade primarily due to their flow control potential in ameliorating stall characteristics. Previous studies have demonstrated that, in the transitional flow regime, full-span wings with tubercled leading edges outperform unmodified wings at high attack angles. The flow mechanism associated with such enhanced loading traits is, however, still being investigated. Also, the performance of full-span tubercled wings in the turbulent regime is largely unexplored. The present study aims to investigate Reynolds number effects on the flow mechanism induced by a full-span tubercled wing with the NACA-0021 cross-sectional profile in the transitional and near-turbulent regimes using computational fluid dynamics. The analysis of the flow field suggests that, with the exception of a few different flow features, the same underlying flow mechanism, involving the presence of transverse and streamwise vorticity, is at play in both cases. With regard to lift-generation characteristics, the numerical simulation results indicate that in contrast to the transitional flow regime, where the unmodified NACA-0021 undergoes a sudden loss of lift, in the turbulent regime, the baseline foil experiences gradual stall and produces more lift than the tubercled foil. This observation highlights the importance of considerations regarding the Reynolds number effects and the stall characteristics of the baseline foil, in the industrial applications of tubercled lifting bodies.

► Influence of hydrodynamic slip on convective transport in flow past a circular cylinder
    1 Feb, 2017

Abstract

The presence of a finite tangential velocity on a hydrodynamically slipping surface is known to reduce vorticity production in bluff body flows substantially while at the same time enhancing its convection downstream and into the wake. Here, we investigate the effect of hydrodynamic slippage on the convective heat transfer (scalar transport) from a heated isothermal circular cylinder placed in a uniform cross-flow of an incompressible fluid through analytical and simulation techniques. At low Reynolds ( \({\textit{Re}}\ll 1\) ) and high Péclet ( \({\textit{Pe}}\gg 1\) ) numbers, our theoretical analysis based on Oseen and thermal boundary layer equations allows for an explicit determination of the dependence of the thermal transport on the non-dimensional slip length \(l_s\) . In this case, the surface-averaged Nusselt number, Nu transitions gradually between the asymptotic limits of \(Nu \sim {\textit{Pe}}^{1/3}\) and \(Nu \sim {\textit{Pe}}^{1/2}\) for no-slip ( \(l_s \rightarrow 0\) ) and shear-free ( \(l_s \rightarrow \infty \) ) boundaries, respectively. Boundary layer analysis also shows that the scaling \(Nu \sim {\textit{Pe}}^{1/2}\) holds for a shear-free cylinder surface in the asymptotic limit of \({\textit{Re}}\gg 1\) so that the corresponding heat transfer rate becomes independent of the fluid viscosity. At finite \({\textit{Re}}\) , results from our two-dimensional simulations confirm the scaling \(Nu \sim {\textit{Pe}}^{1/2}\) for a shear-free boundary over the range \(0.1 \le {\textit{Re}}\le 10^3\) and \(0.1\le {\textit{Pr}}\le 10\) . A gradual transition from the lower asymptotic limit corresponding to a no-slip surface, to the upper limit for a shear-free boundary, with \(l_s\) , is observed in both the maximum slip velocity and the Nu. The local time-averaged Nusselt number \(Nu_{\theta }\) for a shear-free surface exceeds the one for a no-slip surface all along the cylinder boundary except over the downstream portion where unsteady separation and flow reversal lead to an appreciable rise in the local heat transfer rates, especially at high \({\textit{Re}}\) and Pr. At a Reynolds number of \(10^3\) , the formation of secondary recirculating eddy pairs results in appearance of additional local maxima in \(Nu_{\theta }\) at locations that are in close proximity to the mean secondary stagnation points. As a consequence, Nu exhibits a non-monotonic variation with \(l_s\) increasing initially from its lowermost value for a no-slip surface and then decreasing before rising gradually toward the upper asymptotic limit for a shear-free cylinder. A non-monotonic dependence of the spanwise-averaged Nu on \(l_s\) is observed in three dimensions as well with the three-dimensional wake instabilities that appear at sufficiently low \(l_s\) , strongly influencing the convective thermal transport from the cylinder. The analogy between heat transfer and single-component mass transfer implies that our results can directly be applied to determine the dependency of convective mass transfer of a single solute on hydrodynamic slip length in similar configurations through straightforward replacement of Nu and \({\textit{Pr}}\) with Sherwood and Schmidt numbers, respectively.

► Excitation of unsteady Görtler vortices by localized surface nonuniformities
    1 Feb, 2017

Abstract

A combined theoretical and numerical analysis of an experiment devoted to the excitation of Görtler vortices by localized stationary or vibrating surface nonuniformities in a boundary layer over a concave surface is performed. A numerical model of generation of small-amplitude disturbances and their downstream propagation based on parabolic equations is developed. In the framework of this model, the optimal and the modal parts of excited disturbance are defined as solutions of initial-value problems with initial values being, respectively, the optimal disturbance and the leading local mode at the location of the source. It is shown that a representation of excited disturbance as a sum of the optimal part and a remainder makes it possible to describe its generation and downstream propagation, as well as to predict satisfactorily the corresponding receptivity coefficient. In contrast, the representation based on the modal part provides only coarse information about excitation and propagation of disturbance in the range of parameters under investigation. However, it is found that the receptivity coefficients estimated using the modal parts can be reinterpreted to preserve their practical significance. A corresponding procedure was developed. The theoretical and experimental receptivity coefficients are estimated and compared. It is found that the receptivity magnitudes grow significantly with the disturbance frequency. Variation of the span-wise scale of the nonuniformities affects weakly the receptivity characteristics at zero frequency. However, at high frequencies, the efficiency of excitation of Görtler vortices depends substantially on the span-wise scale.

► Reducing the pressure drag of a D-shaped bluff body using linear feedback control
  18 Jan, 2017

Abstract

The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.

► Cluster-based control of a separating flow over a smoothly contoured ramp
  17 Jan, 2017

Abstract

The ability to manipulate and control fluid flows is of great importance in many scientific and engineering applications. The proposed closed-loop control framework addresses a key issue of model-based control: The actuation effect often results from slow dynamics of strongly nonlinear interactions which the flow reveals at timescales much longer than the prediction horizon of any model. Hence, we employ a probabilistic approach based on a cluster-based discretization of the Liouville equation for the evolution of the probability distribution. The proposed methodology frames high-dimensional, nonlinear dynamics into low-dimensional, probabilistic, linear dynamics which considerably simplifies the optimal control problem while preserving nonlinear actuation mechanisms. The data-driven approach builds upon a state space discretization using a clustering algorithm which groups kinematically similar flow states into a low number of clusters. The temporal evolution of the probability distribution on this set of clusters is then described by a control-dependent Markov model. This Markov model can be used as predictor for the ergodic probability distribution for a particular control law. This probability distribution approximates the long-term behavior of the original system on which basis the optimal control law is determined. We examine how the approach can be used to improve the open-loop actuation in a separating flow dominated by Kelvin–Helmholtz shedding. For this purpose, the feature space, in which the model is learned, and the admissible control inputs are tailored to strongly oscillatory flows.

► Use of natural instabilities for generation of streamwise vortices in a laminar channel flow
  19 Dec, 2016

Abstract

An analysis of pressure-gradient-driven flows in channels with walls modified by transverse ribs has been carried out. The ribs have been introduced intentionally in order to generate streamwise vortices through centrifugally driven instabilities. The cost of their introduction, i.e. the additional pressure losses, have been determined. Linear stability theory has been used to determine conditions required for the formation of the vortices. It has been demonstrated that there exists a finite range of rib wave numbers capable of creating vortices. Within this range, there exists an optimal wave number which results in the minimum critical Reynolds number for the specified rib amplitude. The optimal wave numbers marginally depend on the rib positions and amplitudes. As the formation of the vortices can be interfered with by viscosity-driven instabilities, the critical conditions for the onset of such instabilities have also been determined. The rib geometries which result in the vortex formation with the smallest drag penalty and without interference from the viscosity-driven instabilities have been identified.


return

Layout Settings:

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