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

• Published in: Physics of fluids (1994) 2013-11, Vol.25 (11), p.015106
• Main Authors: Mejia-Alvarez, R, Christensen, K T
• Format: Article
• Language: English
• Subjects: Boundaries; Coherence; Computational fluid dynamics; Fluid dynamics; Fluid flow; Pathways; Roughness; Turbulence; Turbulent flow; Velocimetry; Velocity measurement
• ISSN: 1070-6631; 0031-9171; 1089-7666
• DOI: 10.1063/1.4832377

Stereo particle-image velocimetry measurements were conducted in a streamwise-spanwise (x -- z) plane deep within the roughness sublayer (y = 0.047 delta ; delta 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. Fluids22(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.