Simulation of rough wall turbulent channel flow using a parametric forcing approach

The effects of rough walls on turbulent channel flow are simulated by adding a simple forcing term to the Navier-Stokes equations. An extensive parameter study is used to explore the effects of this term on turbulent channel flow. By varying the roughness height parameter and the roughness factor contained in the forcing term it is possible to simulate roughnesses of various strengths ranging from transitionally rough to very rough cases. A good qualitative agreement is found with the results of recent experimental and numerical studies of rough-wall turbulent channel flow for the mean flow statistics. The streamwise velocity shows a very good preservation of the outer layer similarity for both its mean and its fluctuations. The flow structure shows a weakening of the streamwise streaks above the roughness sublayer. Within the roughness sublayer the turbulent structure changes to a more isotropic state, while close to the wall a mixing layer-like turbulent state emerges in extremely rough cases. The roughness force term is a possible candidate for a wall model in large eddy and direct numerical simulations where the small-scale structure of the flow over a rough surface can be neglected