PIV study of the turbulent structure of a boundary layer developing over a rough surface

PIV measurements at relatively low Reynolds number in a turbulent boundary layer over a three-dimensional roughened surface, consisting of py-ramidal rows, are presented. Measurements were taken in a streamwise wall normal plane intercepting the apex of a row of pyramids and the diagonal of the square base. The results shown in the present paper point out the non-homogeneity of the flow in the roughness sub-layer. The different flow behaviours along the ascendant and the descendent part of the pyramids and in the region between two consecutive pyramids were investigated. Low values of the streamwise component of the mean velocity and high values of the Reynolds shear stresses are present in the down-stream part of the pyramids, near their base. In order to understand the origin of the high tur-bulence activity observed downstream the pyramids, near their base, a conditional analysis based on the quadrant method was performed. Particularly, the flow events producing the high values of the Reynolds shear stress were analyzed. Two-point streamwise velocity autocorrelation functions show a loss of longitudinal coherence due to the action of the surface roughness.