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TURBULENT BOUNDARY LAYER STRUCTURE OVER SPARSELY-DISTRIBUTED ROUGHNESS

Bharathram Ganapathisubramani
Engineering and the Environment University of Southampton Tizard, Highfield, Southampton, SO17 1BJ, UK

Michael P. Schultz
Department of Naval Architecture and Ocean Engineering United States Naval Academy Annapolis, Maryland 21402 USA

Аннотация

Experiments were performed on a surface consisting of sparsely-distributed rigid circular cylinder roughness elements to examine the effects of sparseness on turbulence structure. Measurements were made using both two-component LDV and wide-field planar PIV techniques. The mean velocity profile in defect form conforms to outer-layer similarity. The streamwise and wall-normal turbulence intensities appear to exhibit outer-layer similarity for y > 3ks. However, the Reynolds shear stress is consistently higher compared to smooth-wall as well as other rough-wall data. The higher shear stress appear to be linked with strong ejection events that characterise the outer region of the flow. Two-point spatial correlations of both streamwise and wall-normal velocity fluctuations indicate that the turbulent structures in the outer region are coherent over much larger streamwise distances compared to smooth and other rough-wall cases. These results point to the fact that sparseness plays an important role in the outer-layer structure of turbulent boundary layers over rough walls.