Separation control and drag reduction using roughness elements

In the present work the effects of different types of roughness elements on flow separation over a curved boundary is investigsted by means of direct numerical simulations. The geometry and boundary conditions are such that the basic physics of the flow over bluff bodies are represented. Two types of roughness elements are considered, dimples and spherical beads. The Reynolds number, Reh, based on the freestream velocity and height, varied from 3;000 to 30;000. The results are in good qualitative agreement with results for flow over bluff bodies with surface roughness. In particular, the roughness elements are very effective in causing transition of the boundary layer at a much lower Reynolds numbers 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 of the Reynolds number within the range considered in this study. The reason for this different behavior lies in the way the boundary layer grows between the two different roughness elements. For the spherical beads the transition shifts upstream and moves toward the stagnation point on the front of the bump 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.