Wall-Parallel PIV Measurements in Turbulent Boundary Layers with Highly Directional Surface Roughness

摘要

The potential of converging-diverging-aligned (hereafterreferred to as CDA) riblets on reorganising the large-scalecoherent structures that populate the logarithmic regionof turbulent boundary layers is investigated at moderateReynolds number using Particle Image Velocimetry(PIV). This work is motivated by the analysis of the premultipliedenergy spectra of streamwise velocity fluctuationspresented by Nugroho et al. [10], which shows anincrease (or decrease) in the large-scale turbulence energycontribution over the converging (or diverging) region ofthe CDA riblets. The ability of CDA riblets to generatelarge-scale counter-rotating roll-modes [11] suggests that apreferential arrangement of the naturally occurring largescalestructures may have been introduced. In this studywe examine this possible spanwise redistribution of thecoherent structures using instantaneous PIV fields measuredin wall-parallel (streamwise-spanwise) planes withinthe logarithmic region. The analysis will also briefly lookat the spanwise periodicity of the region of intense vorticity.