Guixiang Cui

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.

An improved velocity increment model based on Kolmogorov equation of filtered velocity

The velocity increment (VI) model, which was introduced by Brun et al., is improved by employing the Kolmogorov equation of filtered velocity in this paper. This model has two different formulations: a dynamic formulation and a simplified constant form in high Reynolds number turbulence. A priori tests in isotropic turbulence and wall-bounded turbulence are performed. A posteriori tests of decaying turbulence and channel Poiseuille flow are made to testify the model performance, especially on the energy backscatter. The simple constant coefficient formulation has good performance, and avoids the ensemble average operation, which exists in other subgrid models. This constant improved VI model is particularly proposed in complicated large-eddy simulation projects.

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...

Strengthened opposition control for skin-friction reduction in wall-bounded turbulent flows

An opposition control scheme with strengthened control input is proposed and tested in turbulent channel flows at friction Reynolds number Reτ = 180 by direct numerical simulations. When the detection plane is located at less than 20 wall units, the drag reduction rate can be greatly enhanced by increasing the control amplitude parameter. The maximum drag reduction rate achieved in the present study is around 33%, which is much higher than the best value of 25% reported in literature. The strengthened control can be more efficient to attain a given drag reduction rate. Based on the total shear stress at the virtual wall established between the real wall and the detection plane by the control, a new friction velocity is proposed and the corresponding coordinate transform is made. Scaled by the proposed friction velocity, the wall-normal velocity fluctuation and the Reynolds shear stress of the controlled flows are collapsed well with those of the uncontrolled flow in the new coordinate. Based on the similarity, a relation between drag reduction rate and the effectiveness of the virtual wall is deduced, which disclosed that the elevation and residual Reynolds shear stress at the virtual wall are the key parameters to determine the drag reduction rate. The conclusion are also validated at Reτ = 395 and 590. The decrease of the drag reduction rate with the increase of the Reynolds number is attributed to the enhanced residual Reynolds shear stress at the virtual wall.

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)].

Direct numerical simulation of spatially developing turbulent boundary layers with opposition control

Opposition control of spatially developing turbulent boundary layers for skin friction drag reduction is studied by direct numerical simulations. The boundary layer extends 800?0 in the streamwise (x) direction, with ?0 denoting the momentum thickness at the flow inlet. The Reynolds number, based on the external flow velocity and the momentum thickness, ranges from 300 to 860. Opposition control applied in different streamwise ranges, i.e. and as well as the uncontrolled case, are simulated. Statistical results and instantaneous flow fields are presented, with special attention paid to the spatial evolution properties of the boundary layer flow with control and the underlying mechanism. It is observed that a long spatial transient region after the control start and a long recovery region after the control end are present in the streamwise direction. A maximum drag reduction rate of about 22% is obtained as the transient region is passed, and an overshoot in the local skin friction coefficient (Cf) occurs in the recovery region. A new identity is derived for dynamical decomposition of Cf. Reduction of Cf by opposition control and overshoot of Cf in the recovery region are explained by quantifying the contributions from the viscous shear stress term, the Reynolds shear stress term, the mean convection term and other terms.

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...

Large eddy simulation of flow and scalar transport in a vegetated channel

Predicting flow and mass transport in vegetated regions has a broad range of applications in ecology and engineering practice. This paper presents large eddy simulation (LES) of turbulent flow and scalar transport within a fully developed open-channel with submerged vegetation. To properly represent the scalar transport, an additional diffusivity was introduced within the canopy to account for the contribution of stem wakes, which were not resolved by the LES, to turbulent diffusion. The LES produced good agreement with the velocity and concentration fields measured in a flume experiment. The simulation revealed a secondary flow distributed symmetrically about the channel centerline, which differed significantly from the circulation in a bare channel. The secondary circulation accelerated the vertical spread of the plume both within and above the canopy layer. Quadrant analysis was used to identify the form and shape of canopy-scale turbulent structures within and above the vegetation canopy. Within the canopy, sweep events contributed more to momentum transfer than ejection events, whereas the opposite occurred above the canopy. The coherent structures were similar to those observed in terrestrial canopies, but smaller in scale due to the constraint of the water surface.

Direct numerical simulation of turbulent flow in a rotating square duct

A fully developed turbulent flow in a rotating straight square duct is simulated by direct numerical simulations at Reτ = 300 and 0 ≤ Roτ ≤ 40. The rotating axis is parallel to two opposite walls of the duct and normal to the main flow. Variations of the turbulence statistics with the rotation rate are presented, and a comparison with the rotating turbulent channel flow is discussed. Rich secondary flow patterns in the cross section are observed by varying the rotation rate. The appearance of a pair of additional vortices above the pressure wall is carefully examined, and the underlying mechanism is explained according to the budget analysis of the mean momentum equations.