Zhaoshun Zhang

A new dynamic subgrid eddy viscosity model with application to turbulent channel flow

A new subgrid eddy viscosity model is proposed for large eddy simulation of turbulent flows. The new model is based on the exact energy transport equation between resolved and unresolved scale turbulence. The subgrid eddy viscosity of new model is proportional to the skewness of longitudinal velocity increment, which measures the ratio of cascade energy to the dissipation. The new model is verified in isotropic turbulence and tested in turbulent channel flow with satisfaction.

Large eddy simulation of rotating turbulent channel flow with a new dynamic global-coefficient nonlinear subgrid stress model

In this paper, a new dynamic global-coefficient nonlinear subgrid scale (SGS) model is proposed for large eddy simulation (LES) of rotating turbulent channel flow. The basic model is a nonlinear model with a tensorial polynomial relation between the SGS stress and the resolved strain rate tensor. A new dynamic procedure is proposed to determine the model coefficients of the nonlinear model. The new dynamic method is derived from the globally averaged transport equation of the Reynolds shear stress, on which the rotation has strong and direct effects. The new dynamic nonlinear SGS model is examined in rotating turbulent channel at Re=umh/ν=7000, Ro=2Ωh/um =0.3 and 0.6, where Reynolds number Re and Rotation number Ro are defined by bulk mean velocity um , half channel width h, kinematic viscosity ν and angular velocity of spanwise rotation Ω. The statistical results obtained from the new model agree well with those from direct numerical simulation (DNS). The new model also successfully predicts the major st...

Subgrid modeling of anisotropic rotating homogeneous turbulence

We investigate subgrid modeling of anisotropic rotating turbulence with a dynamic equation of structure functions of the filtered velocity field. The local volume-averaged structure function equation of rotating turbulence is introduced and an eddy viscosity subgrid model is obtained. The resulting subgrid model is similar to that of the study of Cui et al. [Phys. Fluids 16, 2835 (2004)]. It is directly related to the transfer term: the third-order structure function. This term can be computed dynamically during large eddy simulations (LES). Tests are successfully carried out in LES of decaying, rotating, homogeneous turbulence at high Reynolds numbers. Results are in excellent agreement when compared with those of Cambon et al. [J. Fluid Mech. 337, 303 (1997)].

High Accurate Finite Volume Method for Large Eddy Simulation of Complex Turbulent Flows

This paper proposes a finite volume method with compact fourth order accuracy scheme for large eddy simulation (LES). Two-dimensional lid-driven cavity flow and a flow over an oscillating plate are used as examples to verify both the accuracy and convenience of the proposed scheme. A turbulent channel flow and a turbulent flow over a backward facing step are numerically tested for its effectiveness by LES with dynamic Smagorinsky subgrid model. Immersed boundary method (IBM) is applied in this paper to deal with flows with complex configuration so that the boundary condition on the rigid wall can be satisfied well. A curved channel flow and a flow around an airfoil of NACA0012 are computed with immersed boundary method, and comparison with experimental data is also made, showing that the higher accurate finite volume method for LES is proved to be a promising numerical method.

Dependence of turbulent scalar flux on molecular Prandtl number

The dependence of turbulent scalar flux on molecular Prandtl number is studied by direct numerical simulation of statistically stationary isotropic turbulence with uniform mean gradient of temperature. Both Reynolds averaged scalar flux and subgrid scalar flux are investigated at molecular Prandtl numbers ranging from 0.1 to 3.0. In order to consider the Reynolds number effect, two cases of grid resolution are computed, i.e., 1283 and 2563, with the Taylor-scale Reynolds numbers equaling 30 and 50, respectively. The turbulent Prandtl number is used to characterize the turbulent scalar flux. It is found that both Reynolds averaged turbulent Prandtl number (simplified as turbulent Prandtl number hereafter) and subgrid Prandtl number change with molecular Prandtl number significantly. The turbulent Prandtl number has been found to be a linearly reciprocal function of molecular Prandtl number, whereas the subgrid Prandtl number takes a minimum around Pr=1. The appearance of minimum subgrid Prandtl number arou...

A ghost-cell immersed boundary method for large eddy simulation of flows in complex geometries

An efficient ghost-cell immersed boundary (IB) method is proposed for large eddy simulations of three-dimensional incompressible flow in complex geometries. In the framework of finite volume method, the Navier–Stokes equations are integrated using an explicit time advancement scheme on a collocated mesh. Since the IB method is known to generate an unphysical velocity field inside the IB that violates the mass conservation of the cells near the IB, a new IB treatment is devised to eliminate the unphysical velocity generated near the IB and to improve the pressure distribution on the body surface. To validate the proposed method, both laminar and turbulent flow cases are presented. In particular, large eddy simulations were performed to simulate the turbulent flows over a circular cylinder and a sphere at subcritical Reynolds numbers. The computed results show good agreements with the published numerical and experimental data.

Large-Eddy Simulation of Flow Over a Vegetation-Like Canopy Modelled as Arrays of Bluff-Body Elements

Turbulent flow over a vegetation canopy under neutral atmospheric conditions is investigated using large-eddy simulation. Each model tree, which consists of a sphere-shaped tree crown and a cylindrical trunk, is fully resolved. The resulting turbulence statistics and the drag force on the vegetation agree well with measurements from the corresponding wind-tunnel experiment described by Böhm et al. (Boundary-Layer Meteorol, 146:393–419, 2013). Statistically, this kind of model canopy exhibits both vegetation and bluff-body-flow characteristics. The time-averaged flow skims over the top of the underlying canopy, forming a low-momentum recirculation zone on the lee-side of the bluff elements, which causes significant dispersive stress within the canopy layer. Two other numerical representations of vegetation canopies, referred to as the drag-element and drag-crown approaches, have also been developed to assess the performance of simulations. Turbulence statistics suggest that the canopy shear layer interferes with wakes behind stems and crowns. The drag-crown approach yields better agreement between numerical results and experimental measurements than does the traditional drag-element approach, thus providing a promising numerical model for simulating canopy turbulence.

Study on the analogy between velocity and temperature fluctuations in the turbulent rotating channel flows

Turbulent rotating channel flow is of great interest in engineering practice, such as the cooling of the blades in turbo machinery. This paper investigates the turbulence characteristics of temperature fluctuations in rotating turbulent channel flow by direct numerical simulation. It is known that the analogy exists between temperature and velocity fluctuations in non-rotating channel flows. In this paper the analogous property is investigated between temperature and streamwise velocity fluctuations in rotating turbulent channel flows and it has been found that the analogy exists at pressures side in near wall region but the analogy is violated at the suction wall. The influence of the large scale flow structure in rotating channel, known as the Taylor-Gortler vortex, on the characteristics of temperature fluctuations are revealed.

Effects of plane strain on inhomogeneous turbulence

The influence of the mean plane strain on the turbulence transportation is investigated by large eddy simulation (LES) in the shearless turbulence mixing layer. It is found that the mean strains enhance the turbulent fluctuations in the mixing region. Compression in the inhomogeneous direction can greatly increase the transport of turbulent kinetic energy by triple correlation terms, while stretching in the inhomogeneous direction decreases the turbulence transportation. The gradient diffusion models for turbulent transportation are evaluated and it is found that the intermittency consideration can improve the prediction ability of the gradient-type models for the triple correlation terms.

Flow patterns and dissipation of turbulent kinetic energy in near-wall turbulence

It is disclosed the flow patterns of turbulent flows in near wall region by means of topological analysis. It has found that there exists a close relationship between the topological characteristics and the spikes which are dominated by the strong normal velocity fluctuations near the wall. It is also shown that the dissipation of turbulent kinetic energy is closely related to the spike events.