Numerical investigation of the flow over an axisymmetric hill using LES, DES, and RANS

The flow around an axisymmetric hill, mounted in a channel with a fully developed approach flow, is investigated. The flow contains complex structures such as a turbulent boundary layer with several unsteady separations and reattachments. It is highly three-dimensional due to both streamwise and spanwise pressure gradients on the leeside of the hill. The shallowness of the separation region makes the flow a very demanding test case for any computational fluid dynamics model. Three different strategies are used in this study: Reynolds-averaged Navier–Stokes (RANS), large eddy simulation (LES), and detached eddy simulation (DES). The computed flow, in terms of velocity and pressure profiles, compared with measurement data and the results show that LES and DES are indeed capable of handling this complicated flow in a correct way whereas RANS clearly fails to predict several important flow features. Furthermore, the influence of the size of the computational domain, the grid resolution and the inflow boundary conditions is also studied. It is found that the pressure field is sensitive to the location of the inlet and the DES model is very sensitive to the inlet boundary condition on the eddy viscosity. To significantly improve the predictions, it is believed that the near-wall resolution must be increased substantially, in particular in the spanwise direction, or a better wall handling has to be incorporated.

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