Recovery of vortex packet organization in perturbed turbulent boundary layers

Turbulent boundary layers with Reτ=2500 were perturbed by an array of cylinders projecting outward from the wall, and the flow organization downstream was investigated at multiple measurement heights in the logarithmic region. Two array heights were considered: H=0.2δ, extending through the log region and H=δ, extending to the top of the boundary layer. Results from instantaneous PIV in wall-parallel planes and a vortex packet identification algorithm clearly showed a bottom-up mechanism for packet recovery downstream of the H=δ array, even though streamwise velocity statistics remained strongly perturbed. In contrast, some indications of top-down recovery were observed for the flow perturbed by the shorter H=0.2δ (H+=500) array. In this case, however, packet structures closer to the wall at z+=125 remained altered beyond the end of the measurement domain 7δ downstream of the cylinders even though streamwise velocity statistics relaxed nearly to the unperturbed values.