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.