Understanding the drag reduction properties of the flow over k and d -type rough surfaces

Understanding the flow over rough surfaces is an important problem in fluids engineering. Here, we present studies on turbulent flow over various roughness types. The rough surfaces considered in this study were based on the k and d -type behaviour used to classify the different roughness types. Previous results demonstrated that drag reduction can only be achieved using d -type roughness arranged in the streamwise direction. This arrangement is also known as flow over riblets. Obtaining an exact solution for such a problem is very challenging, therefore the problem was solved computationally using the finite element method. The flow simulation cases considered for this study were; laminar, turbulent and fully turbulent. The domain is represented by a channel flow with one sided profield roughness. Simulations were carried out to study the k and d -type behaviour over transverse and streamwise roughness. We were able to identify and benchmark the k and d -type behaviour for the flow over a transverse surface. Some similarities between the transverse and the streamwise roughness were observed in terms of k and d-type behaviour, mainly the production of stable vortices between the roughness elements, but further research is required to fully identify and benchmark the k and d-type behaviour in the flow over streamwise roughness.