LDA MEASUREMENTS IN A TURBULENT BOUNDARY-LAYER OVER A D-TYPE ROUGH WALL

Two-component LDA measurements in smooth wall and d-type rough wall bo undary layers indicate that there are important differences in the tur bulence intensities over a significant distance above the two surfaces . All the Reynolds stresses-especially the shear stress-are larger ove r the d-type rough wall, reflecting the relatively strong momentum exc hange which occurs over the roughness cavities. Flow visualisations in dicate that this exchange is associated with strong ejections of cavit y fluid into the outer flow.