Infection of subcritical roughness wakes by unstable neighbors

The detailed mechanisms by which a wake of a subcritical discrete roughness element in a zero-pressure gradient laminar boundary layer is destabilized by background disturbances are analyzed. This knowledge is relevant for extending the understanding and control of boundary transition caused by an isolated discrete roughness element to a splatter involving discrete roughness elements of various sizes. Direct numerical simulations are performed for a range of configurations involving multiple discrete roughness elements and the results are analyzed using stability analysis and vorticity dynamics points of view. It is found that transition behind large discrete roughness elements can destabilize the lifted-up vortical structure downstream of a subcritical roughness wake by causing the top of the vortex lines to clump or by causing the entire structure to sway from side to side. The mechanism depends upon the relative locations of the roughness elements. Broader impacts of these findings on distributed roughness transition, wedge spreading on rough surfaces, and mitigation strategies are discussed.