Distributed Roughness Shielding in a Blasius Boundary Layer
The receptivity of distributed roughness to disturbances that undergo transient growth is not well understood. Some progress on this subject has been made by coordinated direct numerical simulations and experiments that investigate the laminar disturbances created by patches of distributed roughness with known surface topography. These results have shown that the presence of smaller amplitude distributed roughness can decrease the roughness wake created by larger roughness peaks. In the present work, this "shielding" effect is further examined. Combinations of distributed roughness patches and a discrete roughness element are studied in both experiments and direct numerical simulations. The distributed roughness patch and discrete roughness element are tested both separately and together. Surface shear stress images, velocity contours, and disturbance energies from the different cases are compared to each other to quantify the shielding effect. Both the DNS and the experiment show the presence of the shielding effect; adding distributed roughness around the discrete roughness reduced the amplitude of the roughness wake at the lower Reynolds number (Rek = 151) tested. At a higher Reynolds number (Rek = 220), the experimental results show that the added distributed roughness slightly delayed transition, while the DNS results show the same shielding effect that was seen at the lower Reynolds number condition.