Turbulent flow over sinusoidal bumps and hills derived from large eddy simulations

We describe recent developments in constructing and utilizing a large-eddy simulation code for high Reynolds number boundary layer flow over small-scale topography. The code adopts a Boussinesq approximation and utilizes a conventional terrain following grid. The numerical procedure is similar to our flat-wall code, namely pseudospectral differencing in horizontal (transformed) coordinates, finite difference in the vertical and third-order Runge-Kutta time stepping. The code is an advance over the algorithm described by Sullivan etal (2008), since it acommodates 3-D topography and has an improved pressure solver. We use the LES to examine the 3-D time-evolving structures downstream of steep bumps and hills. The code successfully replicates the large-scale structures first reported by Gong etal (1996) for small values of roughness for flow over sinusoidal bumps. The flow changes markedly with higher roughness and the onset of flow separation. Preliminary computations are also carried out, using as many as 4096 processors, for turbulent flow past very steep hills with maximum slope ak = 0.6.