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Bistable Phenomenon of the Aerodynamic Forces on a Square Prism with the Aspect Ratio of 5

The flow around a finite-length square prism with aspect ratio of 5 is numerical investigated using LES at Red = 3900. The prism is mounted on a flat wall, with one end free. Based on the simulation results, it is found that the near wake is highly three dimensional under the effects of free-end downwash flow. The shear layers from prism side walls and free end form an arch-type structure. There are two typical flow modes presence in the near wake: first, the spanwise vortices are staggered arranged similar to that in 2D cylinder wake; second, the spanwise vortices are quasi-symmetrically arranged. These two modes occur alternately and intermittently. When the first mode occurs, the pressure on the prism side surface fluctuates periodically, corresponding to large values of drag and fluctuating lift coefficients; when the second modes occurs, there is no obvious pressure fluctuation on prism side surfaces, and the correspond drag and fluctuation life coefficients are significantly smaller than those for the first mode.