Reynolds-stress enhancement associated with a short fetch of roughness in wall turbulence

Particle-image velocimetry experiments are performed to study the response of smooth-wall turbulent channel flow to a short fetch of roughness (10 outer length scales long). The roughness studied herein is replicated from a surface scan of a damaged turbine blade and contains both large- and small-scale surface defects attributable to pitting, deposition, and spallation. Quadrant analysis is used to investigate the characteristics of Reynolds-stress-producing events within the internal layer that develops over the roughness. The mean Reynolds stress is dramatically increased in the presence of the roughness as compared to the smooth-wall baseline owing to an increased number of extremely intense ejections and sweeps. In contrast, inward and outward interactions, as well as relatively weak ejection and sweep events, are found to be insensitive to the surface conditions. While stress and space fractions are found to be insensitive to the surface topology when all Reynolds-stress-producing events are included, stress and space fractions for only the most intense ejection and sweep events vary significantly with the local surface topology.