DOI: 10.1615/TSFP7
TURBULENT BOUNDARY LAYER STRUCTURE OVER SPARSELY-DISTRIBUTED ROUGHNESS
摘要
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.