The effects of roughness on the boundary layer development of a circular cylinder

This paper focuses on the effects of surface roughness in the flow past a circular cylinder at different Reynolds numbers. Large eddy simulations of the flow, from subcritical to transcritical Reynolds numbers and at relatively high equivalent sand grain roughness of ks / D = 0:02 are performed. In order to determine the effects of the surface roughness on the boundary layer transition and as a consequence on the wake topology, results are compared to literature available data for the rough and smooth cylinders. Results show that surface roughness triggers the transition to turbulence in the boundary layer at all Reynolds numbers, thus leading to an early separation caused by the increased drag and momentum deficit. In fact, even at subcritical Reynolds numbers boundary layer instabilities are triggered in the roughness sublayer which eventually lead to the transition to turbulence and the separation before the cylinder apex. For the transcritical Reynolds number (i.e. Re = 4:2x105), transition to turbulence is observed in the attached boundary layer. Largest changes in the flow topology are observed at Re = 4:2x105, as the wake is wider than that of the smooth cylinder at these Reynolds numbers, with larger Reynolds stresses along the boundary layer and the near wake.