Near-Wake Behavior of Discrete-Roughness Arrays in 2-d and 3-d Laminar Boundary Layers

Abstract

A spanwise row of medium-sized cylindrical roughness elements is introduced to two different base flows, a 2-d boundary layer and a respective 3-d boundary layer, as can be found on unswept and swept wings, respectively, both with strongly negative streamwise pressure gradient. For both cases, sub-effective and super-effective, i.e. directly flow tripping, elements are investigated. Similarities and differences in the flow fields around the roughness elements are highlighted. It turns out that, when comparing the results in a coordinate system aligned with the local flow direction at the boundary layer edge, the qualitative differences in the near wakes are rather subtle. Nevertheless, the elements in the 3-d boundary layer trigger steady crossflow vortices that will ultimately become secondarily unstable, and therefore lead at first to a continuously upstream-shifting transition location with increasing roughness height. Performance data for our direct numerical simulation code NS3D are given for the CRAY XE6 and the current test and development system CRAY XC40.