Towards roughness-based drag reduction in cross-flow dominated flows

Recent theoretical results are presented from our ongoing study investigating the distinct convective instability properties of the boundary-layer flow over rough rotating disks. In this study, radial anisotropic surface roughness (concentric grooves) is modelled using the partial-slip approach of Miklavčič & Wang (2004) and the surface-geometry approach of Yoon et. Al (2007). An energy analysis reveals that for both instability modes, the main contributors to the energy balance are the energy production by the Reynolds stresses and conventional viscous dissipation. For the Type I mode, energy dissipation increases and the Reynolds-stress energy production decreases with roughness under both models. This suggests a clear stabilising effect of the anisotropic roughness on the Type I mode. For the Type II mode, the Reynolds-stress energy production increases with roughness under both models. However, the energy dissipation of the Type II mode decreases with the roughness under the surface-geometry model and increases under the partial-slip model. This sensitivity to the precise form of the anisotropic roughness suggests that maximising dissipation by an appropriately designed roughness can theoretically lead to an overall beneficial stabilisation of both the Type I and Type II modes. This is a potential route to overall boundary-layer-transition delay and drag reduction in cross-flow dominated flows.