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 boundary layers indicate that there are important differences in the turbulence intensities over a significant distance above the two surfaces. All the Reynolds stresses-especially the shear stress-are larger over the d-type rough wall, reflecting the relatively strong momentum exchange which occurs over the roughness cavities. Flow visualisations indicate that this exchange is associated with strong ejections of cavity fluid into the outer flow.