Turbulent boundary layer structure over sparsely-distributed roughness

Experiments were performed on a surface consisting of sparsely-distributed rigid circular cylinder roughness elements to examine the effects of solidity on turbulence structure. The solidity ($\lambda$) of the roughness (defined as frontal area of the roughness elements per unit wall-parallel area) is 0.08. This value was chosen to lie in the ``sparse'' regime ($\lambda \leq$ 0.1) while maintaining a large equivalent sandgrain roughness height ($k_s$). Measurements were made using both two-component LDV and wide-field planar PIV techniques at two Reynolds numbers, $Re_\tau$ = 1800 and 4000. The results indicate that the relative roughness height of these elements ($k/\delta$) is approximately 0.09. The value of ${k_s}^+$ is greater than 350 in both cases indicating that the flow is in the fully-rough regime. The mean velocity profile in defect form at both Reynolds numbers conforms to outer-layer similarity. The Reynolds stresses appears to exhibit outer-layer similarity for $y > 3k_s$. However, differences between the two Reynolds numbers are observed closer to the wall. Specifically, a broad peak in the streamwise Reynolds normal stress is observed around $y/\delta$ = 0.12 for $Re_\tau$ = 4000. Further analysis using both the PIV and LDV data will be performed and presented.