Discrete-roughness Effects in a Three-dimensional Boundary Layer on an Airfoil by Means of DNS

The effects of a spanwise row of cylindrical roughness elements in a laminar, compressible, three-dimensional boundary layer on a wing profile are investigated by direct numerical simulations (DNS). Large, so-called effective elements, are capable of directly tripping turbulent flow. This can be either promoted by a strong convective or a global instability behind or around the element. In the former case, elements appear either sub-effective or effective, depending on the unsteady background disturbance level. For one investigated height, a convective instability is found as long as the background disturbance level is low. If the disturbance level is deliberately increased to a higher level for a limited time frame, the flow field remains turbulent, even after the controlled input is switched off again.