Visualization of near-wall structures of an isolated cylindrical roughness element in a laminar boundary layer without pressure gradient

Abstract

The near-wall structures of a cylindrical roughness element were investigated experimentally. The element of aspect ratio η=3 and height k= 10 mm was placed inside the laminar flat-plate boundary layer of the laminar water channel, University of Stuttgart. The freestream velocity u∞ was altered to investigate roughness Reynolds numbers in the range Rekk=ukkν−1=380 to Rekk=950. Time-resolved surface visualizations with the Temperature-Sensitive Paint (TSP) method were performed to visualize the flow structures near the wall. Different formations of small scale structures in the unsteady wake at transitional conditions were detected. The results from the surface are compared to results from visualizations at the top edge of the cylindrical roughness element. Changes in the system of laminar horseshoe vortices and the recirculation zone are well captured.