PIV measurements of turbulent flow overlying large, cubic- and hexagonally-packed hemisphere arrays

Turbulent flow overlying periodic arrays (cubic- and hexagonally-packed) of large hemispheres was examined experimentally. Flow in the vicinity of the individual roughness elements was successfully examined by conducting particle-image velocimetry measurements in a refractive-index matched flume where the refractive index of the working fluid matched that of the acrylic hemispheres. The spatial structure of flow in the roughness sublayer is spatially heterogeneous and significantly influenced by the topographic pattern of the hemispheres. However, structural coherence was observed outside the roughness sublayer, independent of the measurement location and packing arrangement. Consistent with previous studies, it appears that mean shear near the inflection point of the velocity profile plays an important role in generating high-intensity Reynolds shear stress events and the contribution of roughness-scale energy within the roughness sublayer was apparent in spatial spectra of velocity. In the outer layer, despite the small relative-submergence condition, first- and second- order velocity statistics in all rough-wall conditions showed outer-layer similarity.