The role of surface texturing on the physics of boundary layer separation over a bump

In the present work the effects of different types of roughness elements on flow separation over a circular bump is investigated by means of direct numerical simulations. Two types of roughness elements are considered, dimples and spherical beads. The boundary condition setup and the Reynolds number range were selected to replicate in a qualitative sense the initial development of the boundary layer (growth, transition, separation) on full cylinders or spheres, which is primarily responsible for the trends in the behavior of their drag coefficient. Both roughness elements are very effective in causing transition of the boundary layer at a much lower Reynolds number when compared to a smooth surface. For the spherical beads the drag coefficient exhibits a minimum and quickly rises as the Reynolds number increases. For the dimples the minimum drag coefficient remains constant and independent on the Reynolds number within the range considered in this study. This behaviour agrees with experimental observations in the literature for similar types of roughness elements. The differences are due to the way the boundary layer grows over dimples or spherical beads. For the latter, transition shifts upstream and moves toward the stagnation point on the front of the body as the Reynolds number increases. An earlier transition means the boundary layer starts growing thicker earlier and has less momentum to overcome the adverse pressure gradient. As a result the separation point moves upstream too giving rise to increased drag. In contrast the transition and separation points are weakly dependent on the Reynolds number for the case of the dimples.