Zhenhua Xia

Flow patterns in the sedimentation of an elliptical particle

We study the dynamics of a single two-dimensional elliptical particle sedimenting in a Newtonian fluid using numerical simulations. The main emphasis in this work is to study the effect of boundaries on the flow patterns observed during sedimentation. The simulations were performed using a multi-block lattice Boltzmann method as well as a finite-element technique and the results are shown to be consistent. We have conducted a detailed study on the effects of density ratio, aspect ratio and the channel blockage ratio on the flow patterns during sedimentation. As the channel blockage ratio is varied, our results show that there are five distinct modes of sedimentation: oscillating, tumbling along the wall, vertical sedimentation, horizontal sedimentation and an inclined mode where the particle sediments with a non-trivial orientation to the vertical. The inclined mode is shown to form a smooth bridge between the vertical and horizontal modes of sedimentation. For narrow channels, the mode of sedimentation is found to be sensitively dependent on the initial orientation of the particle. We present a phase diagram showing the transitions between the various modes of sedimentation with changing blockage ratio of the channel.

Constrained large-eddy simulation of separated flow in a channel with streamwise-periodic constrictions

Constrained large-eddy simulation (CLES) method has been recently developed by Chen and his colleagues for simulating attached and detached wall-bounded turbulent flows. In CLES, the whole domain is simulated using large-eddy simulation (LES) while a Reynolds stress constraint is enforced on the subgrid-scale (SGS) stress model for near wall regions. In this paper, CLES is used to simulate the separated flow in a channel with streamwise-periodic constrictions at Re = 10,595. The results of CLES are compared with those of Reynolds-averaged Navier-Stokes (RANS) method, LES, detached eddy simulation (DES) and previous LES results by Breuer et al. and Ziefle et al. Although a coarse grid is used, our results from the present LES, DES and CLES do not show large deviations from the reference results using much finer grid resolution. The comparison also shows that CLES performs the best among different turbulence models tested, demonstrating that the CLES provides an excellent alternative model for separated flows. Furthermore, the cross-comparisons among different CLES implementations have been carried out. Our simulation results are in favor of using the constraint from algebraic RANS model or solving the RANS model equations in the whole domain with a length scale modification according to the idea from DES.

Constrained large-eddy simulation of laminar-turbulent transition in channel flow

A constrained large-eddy simulation (CLES) of a laminar-turbulent transition in a temporally developing channel flow is performed. First, we confirm the capability of CLES to simulate this transition problem using the a priori Reynolds stress estimated from a direct numerical simulation. Based on the analysis of the Reynolds stress during the transition process, an intermittency factor is introduced in the Reynolds-averaged Navier–Stokes equation (RANS) model to account for the transition property. Two simple approaches are used to construct the intermittency factor. One is based on the shape factor, and the other is based on the coefficients of Smagorinsky models. The CLES results using the intermittency modified RANS model can accurately predict the onset of the transition and the basic transition process, in a manner similar to a large eddy simulation with dynamics Smagorinsky model (LES-DSM). Meanwhile, CLES preserves its advantage over LES-DSM in the turbulent state. The present work illustrates that CLES can be used to simulate transitional flows.

Modulation to compressible homogenous turbulence by heavy point particles. I. Effect of particles’ density

In this paper, two-way interactions between heavy point particles and forced compressible homogenous turbulence are simulated by using a localized artificial diffusivity scheme and a Eulerian–Lagrangian approach. The initial turbulent Mach number is around 1.0 and the Taylor Reynolds number is around 110. Seven different simulations of 106 particles with different particle densities (or initial Stokes numbers) are considered. The statistics of the compressible turbulence, such as the turbulence Mach number, kinetic energy, dilatation, and the kinetic energy spectra, from different simulations are compared with each other and with the one-way undisturbed case. Our results show that the turbulence is suppressed if the two-way coupling backward interactions are considered, and the suppression is more obvious if the density of particles is higher. The kinetic energy spectrum at larger initial Stokes number (higher density) exhibits a reduction at low wave numbers and an augmentation at high wave numbers, whic...

Large eddy simulation of spanwise rotating turbulent channel flow with dynamic variants of eddy viscosity model

A fully developed spanwise rotating turbulent channel flow has been numerically investigated utilizing large-eddy simulation. Our focus is to assess the performances of the dynamic variants of eddy viscosity models, including dynamic Vreman’s model (DVM), dynamic wall adapting local eddy viscosity (DWALE) model, dynamic σ (Dσ) model, and the dynamic volumetric strain-stretching (DVSS) model, in this canonical flow. The results with dynamic Smagorinsky model (DSM) and direct numerical simulations (DNS) are used as references. Our results show that the DVM has a wrong asymptotic behavior in the near wall region, while the other three models can correctly predict it. In the high rotation case, the DWALE can get reliable mean velocity profile, but the turbulence intensities in the wall-normal and spanwise directions show clear deviations from DNS data. DVSS exhibits poor predictions on both the mean velocity profile and turbulence intensities. In all three cases, Dσ performs the best.

Large-eddy simulation of plane channel flow with Vreman's model

ABSTRACT In this paper, large-eddy simulations of Vremans model (VM) have been carried out to investigate its performances in a temporal transitional channel flow and in high Reynolds number turbulent channel flows. As a preliminary work, it is found that cubic root of the cell volume is the best choice of filter width for both VM and dynamic VM based on Germano identity (DVM), according to comparative studies and a-posteriori analyses at Reτ = 590. VM and DVM are then used to simulate the temporal laminar–turbulent transitional channel flow, and the results turn out that VM and DVM are capable to simulate this temporal transient flow. In simulating high Reynolds number turbulent channel flows with a relatively coarse grid resolution, DVM itself shares the same weakness as the dynamic Smagorinsky model, while it can successfully predict the mean velocity profile and skin friction coefficient when it is coupled with the constrained large eddy simulation methodology. The coupling highly promotes the capability of Vremans model, offering a new promising approach to simulate high Reynolds number wall-bounded turbulent flows.

Comparisons of different implementations of turbulence modelling in lattice Boltzmann method

In this paper, we present an alternative approach for the turbulence modelling in the single-relaxation-time lattice Boltzmann method (LBM) framework by treating the turbulence term as an extra forcing term, in addition to the traditional approach of modifying the relaxation time. We compare these two different approaches and their mixture in large-eddy simulation (LES) of three-dimensional decaying isotropic homogenous turbulence using the Smagorinsky model and the mixed similarity model. When the LES was conducted using the Smagorinsky model, where the Boussinesq eddy-viscosity approximation is adopted, the results showed that these three different implementations are equivalent. However, when the mixed similarity model is adopted, which is beyond the Boussinesq eddy-viscosity approximation, our results showed that an equivalent eddy-viscosity will lead to errors, while the forcing approach is more straightforward and accurate. This provides an alternative and more general framework of simulation of turbulence with models in LBM, especially when the Boussinesq eddy-viscosity approximation is invalid.

Constrained Large Eddy Simulation of Wall-Bounded Turbulent Flows

We present a novel simulation tool-constrained large eddy simulation (CLES), for numerical experiments on the wall-bounded turbulent flows. Different from the traditional large eddy simulation(LES) and the available hybrid RANS/LES approaches, the CLES method computes the whole flow domain by solving the LES equations with a Reynolds-stress-constrained (RSC) subgrid-scale (SGS) stress model in the near-wall region and a traditional SGS stress model in the rest.The CLES approach is validated by simulating the turbulent channel flow and flow around a circular cylinder. With the same grid resolutions, CLES can successfully simulate all these flow regimes as well as DES and other available methods. For the case of attached flows, CLES is able to eliminate the non-physical Log-Layer Mismatch problem in traditional hybrid RANS/LES methods successfully, and to predict mean velocity profile, turbulent stresses and skin friction coefficient more accurately compared with the DES. For the case of detached flows, the performance of CLES is comparable to DES.

A new identification method in sampled quadrant analysis for wall-bounded turbulence

In this paper, a new identification method in sampled quadrant analysis was introduced to single out the larger ejection-type (Q2) and sweep-type (Q4) motions which directly contribute to the total Reynolds shear stress in an average sense. Different from previous ones, the threshold Rc in the present method is not an adjustable parameter, but a determined value by data. The singled-out objects by using the present method form 3D “force structures” that directly contribute to the skin-friction coefficient.

Quad-Decomposition of Velocity Field in Spanwise-Rotating Turbulent Plane Couette Flows

Abstract In this paper, we propose a new quad-decomposition approach for the instantaneous flow field which has the secondary flows. Different from the previously reported quad-decomposition, where the velocity field is decomposed into a mean part, a streamwise part and a cross-flow part of the secondary flow, and the residual three-dimensional fluctuation part, our new quad-decomposition separates the flow field into a mean part, a streamwise streaks-related part, a cross-flow roll-cells-related part and a residual cross-flow fluctuation part. These two decomposition approaches are used to explore the underlying physics of the energy balance and transfer among different shares of the turbulent kinetic energy in spanwise-rotating turbulent plane Couette flows. The new quad-decomposition can provide clear pictures of the energy transfer from the streaks to the residual cross-flow fluctuations due to the system rotation and the correlation between pressure and streamwise velocity fluctuations’ gradient, in addition to the bridge role of the cross-flow roll-cells-related field between the mean field and the residual fluctuation field, which can also be demonstrated by the previous quad-decomposition.