The instantaneous structure of turbulent boundary layers over surfaces with spanwise heterogeneity

Abstract

Turbulent flows over surfaces with extensive roughness variation in the spanwise direction induce a secondary flow in the form of streamwise aligned counter-rotating vortices. In this study, we conduct cross-plane stereoscopic particle image velocimetry (SPIV) measurements over surfaces constructed from spanwise-alternating smooth and rough strips to emulate spanwise heterogeneity in a rough-walled surface. The half-spanwise-wavelength of the alternating rough and smooth strips S varies from S/δ = 0.32–3.63, relative to the spanwise-averaged boundary layer thickness, δ. Two limiting cases are observed when S/δ ≫ 1 and ≪ 1, where the size of secondary flow is limited by δ and S, respectively...