PIV measurements of turbulent boundary layer over a rod-roughened wall

The effects of surface roughness on a turbulent boundary layer (TBL) were investigated by performing particle image velocimetry (PIV) measurements. The Reynolds number based on the momentum thickness was about Reθ = 1000–1500. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the ratio of the boundary layer thickness to the roughness height was k/δ = 0.025. Along the streamwise direction, self-preserving forms of the streamwise mean velocity and the turbulent Reynolds stress were obtained. Spatially developing characteristics of the rough wall TBL were examined. Introduction of the roughness elements augmented turbulent stresses in the region of y < 4–5ks, where ks is an effective sand roughness height. We found that the effective sand roughness height (ks) is a more appropriate length scale for representing the extent of roughness effect rather than the roughness height (k). By using PIV measurements, the effects of surface roughness on the turbulence structure in the roughness sublayer were examined and compared with previous direct numerical simulation (DNS) results. Iso-contours of the mean velocity and Reynolds stress in the roughness sublayer obtained by PIV were in very good agreement with DNS findings. Vortex structures induced by the rod-roughness and strong sweep events were also examined.