Influence of bottom boundary conditions on flow past a three-dimensional hill

Three-dimensional (3D) obstacles on the ocean bottom are common sites for turbulence and mixing. Simulations are conducted for a conical hill, a canonical example of 3D obstacles. Motivated by the use of slip boundary condition (BC) in the literature on geophysical wakes, we examine the sensitivity of the flow to BCs (slip instead of no-slip) on the flat wall and on the obstacle surface. When the bottom is modeled entirely with the free-slip BC, vorticity generation is purely due to the curvature of the obstacle. The no-slip BC allows the formation of a bottom boundary layer which, after separation due to the adverse pressure gradient of curved surfaces, sheds vorticity into the wake. Significant changes occur in the structure of lee vortices and wake turbulence when the BC is changed. For instance, the boundary layer on the flat bottom suppresses the unsteady oscillatory behavior of the separated boundary layer behind the hill so as to delay the formation of coherent lee vortices to locations that are further downstream. The characteristics of the downslope jet formed as part of the hydraulic response in stratified flow and the internal wave field are also altered by the choice of BC.