Structural subgrid-scale modeling for large-eddy simulation: A review

Accurately modeling nonlinear interactions in turbulence is one of the key challenges for large-eddy simulation（LES） of turbulence. In this article, we review recent studies on structural subgrid scale modeling, focusing on evaluating how well these models predict the effects of small scales. The article discusses a priori and a posteriori test results. Other nonlinear models are briefly discussed, and future prospects are noted.

Nonlinear turbulence models for predicting strong curvature effects

Prediction of the characteristics of turbulent flows with strong streamline curvature, such as flows in turbomachines, curved channel flows, flows around airfoils and buildings, is of great importance in engineering applications and poses a very practical challenge for turbulence modeling. In this paper, we analyze qualitatively the curvature effects on the structure of turbulence and conduct numerical simulations of a turbulent Uduct flow with a number of turbulence models in order to assess their overall performance. The models evaluated in this work are some typical linear eddy viscosity turbulence models, nonlinear eddy viscosity turbulence models （NLEVM） （quadratic and cubic）, a quadratic explicit algebraic stress model （EASM） and a Reynolds stress model （RSM） developed based on the second-moment closure. Our numerical results show that a cubic NLEVM that performs considerably well in other benchmark turbulent flows, such as the Craft, Launder and Suga model and the Huang and Ma model, is able to capture the major features of the highly curved turbulent U-duct flow, including the damping of turbulence near the convex wall, the enhancement of turbulence near the concave wall, and the subsequent turbulent flow separation. The predictions of the cubic models are quite close to that of the RSM, in relatively good agreement with the experimental data, which suggests that these models may be employed to simulate the turbulent curved flows in engineering applications.

Turbulent Transport Coefficients in Turbulent Flows of Liquid Sodium

The direct measurements of turbulent viscosity and effective magnetic diffusivity in turbulent flow of electro-conductive fluids under moderate magnetic Reynolds number,i.e.,1<Rm<Rm*,where Rm* denotes the dynamo threshold,are reported.The measurements are performed in a nonstationary turbulent flow of liquid sodium,generated in a closed toroidal channel.The peak level of the Reynolds number reached 3 000 000,which corresponds to magnetic Reynolds number about 30.

Numerical analysis of cavitation shedding flow around a three-dimensional hydrofoil using an improved filter-based model

The cavitation shedding flow around a 3-D Clark-Y hydrofoil is simulated by using an improved filter-based model （FBM） and a mass transfer cavitation model with the consideration of the maximum density ratio effect between the liquid and the vapor. The unsteady cloud cavity shedding features around the Clark-Y hydrofoil are accurately captured based on an improved FBM model and a suitable maximum density ratio. Numerical results show that the predicted cavitation patterns and evolutions compare well with the experimental visualizations, and the prediction errors of the time-averaged lift coefficient, drag coefficient and Strouhal number St for the cavitation number o- = 0.8, the angle of attack a= 8°at a Reynolds number Re = 7 x 10^5 are only 3.29%, 2.36% and 9.58%, respectively. It is observed that the cavitation shedding flow patterns are closely associated with the vortex structures identified by the Q- criterion method. The predicted cloud cavitation shedding flow shows clearly three typical stages： （1） Initiation of the attached sheet cavity, the growth toward the trailing edge. （2） The formation and development of the re-entrant jet flow. （3） Large scale cloud cavity sheds downstream. Numerical results also indicate that the non-uniform adverse pressure gradient is the main driving force of the re-entrant jet, which results in the U-shaped cavity and the 3-D bubbly structure during the cloud cavity shedding.

Calculation of tip vortex cavitation flows around three-dimensional hydrofoils and propellers using a nonlinear k-ε turbulence model

Simulations of tip vortex wetted flows and cavitating flows are carried out by using a RANS model. Two types of turbule- nce models, with and without the Boussinesq turbulent-viscosity hypothesis, are adopted in comparing with experimental results regarding the vorticity, the strain rate and the Reynolds shear stress distributions in the vortex region. The numerical results imply that the spatial phase shift between the mean strain rate and the Reynolds stresses can be accurately modeled by the nonlinear κ-ε turbulence model, the tip vortex cavitation region can only be predicted using the nonlinear κ-ε turbulence model. The mecha- nism of the over-dissipation due to the turbulence model is analyzed in terms of the turbulence production, which is one of the dominant source terms in the transport equations of energy.