Wall-parallel stereo particle-image velocimetry measurements in the roughness sublayer of turbulent flow overlying highly irregular roughness

Abstract

Stereo particle-image velocimetry measurements were conducted in a streamwise–spanwise (x − z) plane deep within the roughness sublayer (y = 0.047δ; δ is the boundary-layer thickness) of a zero-pressure-gradient turbulent boundary layer overlying highly irregular surface roughness replicated from a turbine blade damaged by foreign-material deposition. The ensemble-averaged streamwise velocity defect revealed the tendency of the roughness to promote channeling of the flow in the form of low-momentum pathways (LMPs) and high-momentum pathways. Enhanced turbulent and vortical activity was observed both between and along the spanwise boundaries of these streamwise-elongated pathways. In particular, streamwise pathways of wall-normal vortex cores of opposing rotational sense were observed along the spanwise boundaries of the identified LMP in the rough-wall flow. Conditional averaging revealed that these counter-rotating vortical motions are associated with streamwise flow against the mean-flow direction and could perhaps be the origination mechanism of the LMPs. Two-point correlation coefficients of velocity and swirling strength reflected large-scale streamwise coherence of these quantities along and outboard of the identified LMP in the rough-wall flow, supporting the notion that the motions responsible for the LMP have large-scale, streamwise coherence. Finally, the influence of different topographical scales of the roughness on the flow in the roughness sublayer was explored using low-order models of the original, full surface as originally proposed by R. Mejia-Alvarez and K. T. Christensen [Phys. Fluids 22(1), 015106 (2010)]. While a model containing only the largest topographical scales qualitatively reproduced the features of the full-surface flow, additional intermediate topographical scales were required to quantitatively reproduce the statistical and structural nature of the full-surface flow in the roughness sublayer.