Xingsi Han

Comparison of URANS, PANS, LES and DNS of flows around simplified ground vehicles with passive flow manipulation

Flow control of ground vehicles has recently attracted large interest in both industry and academia. The potential in energy savings seems to be considerable and at the moment both passive and active control strategies are explored.

Very Large Eddy Simulation of Passive Drag Control for a D-Shaped Cylinder

The numerical study reported here deals with the passive flow control around a two-dimensional D-shaped bluff body at a Reynolds number of Re=3.6×104. A small circular control cylinder located in the near wake behind the main bluff body is employed as a local disturbance of the shear layer and the wake. 3D simulations are carried out using a newly developed very large eddy simulation (VLES) method, based on the standard k − e turbulence model. The aim of this study is to validate the performance of this method for the complex flow control problem. Numerical results are compared with available experimental data, including global flow parameters and velocity profiles. Good agreements are observed. Numerical results suggest that the bubble recirculation length is increased by about 36% by the local disturbance of the small cylinder, which compares well to the experimental observations in which the length is increased by about 38%. A drag reduction of about 18% is observed in the VLES simulation, which is quite close to the experimental value of 17.5%. It is found that the VLES method is able to predict the flow control problem quite well.

A New Very Large Eddy Simulation Model for Simulation of Turbulent Flow

Among various hybrid RANS/LES methodologies, Speziale’s Very Large Eddy Simulation (VLES) is one that was early proposed and is a unified simulation approach that can change seamlessly from RANS to DNS depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (1) RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation (DES) concept; (2) An LES subgrid scale model can be reached by the introduction of a third length scale, i.e. integral turbulence length scale. Thus the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k − e model and Wilcox’s k − ω model. Applications are conducted for the turbulent channel flow at Re τ = 395 and turbulent flow past a square cylinder at Re = 22000. Results are compared with previous studies. It is demonstrated that the new method is quite effective in resolving the large flow structures, and can give satisfactory predictions on a very coarse mesh.

Study of active flow control for a simplified vehicle model using PANS turbulence model

Flow control has shown a potential in reducing the drag in vehicle aerodynamics. Its effects are highly related to the flow structures, including small ones, and thus accurate predictions of the flow field are needed for numerical investigation. The numerical study reported in the present paper deals with active flow control for a quasi-2D simplified vehicle model by a synthetic jet. A newly developed PANS turbulence model is used, based on the ζ − f RANS turbulence model. The aim is to validate the performance of this model for the complex flow control problem. Results are compared with previous studies using LES and experimental data, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS model predicts a drag reduction of 9.8%, which is close to previous LES results. The velocity profiles predicted by the PANS model agree well with experimental data for both natural and controlled cases. It is found that the PANS model is able to predict the flow control problem well, actually in a way not inferior to the LES method, and can thus be used for further flow control studies in vehicle aerodynamics.