Reynolds number effects on surface-induced secondary flows in turbulent boundary layers

This study explores the effect of friction Reynolds number (Re_ 3000 – 13\, 000) on secondary flows in three-dimensional turbulent boundary layers induced by spanwise surface heterogeneity. Using a combination of floating-element drag balance and high-resolution hot-wire anemometry, we examine how varying spanwise spacing (S/ where is the boundary layer thickness defined as the distance from the wall where streamwise mean velocity U = 0. 99U_) influences frictional drag, turbulence intensity, spectral energy distribution and the organisation of coherent structures. The results reveal that secondary flows modulate turbulence differently depending on S/, with strong near-wall effects at S/ 1 and outer-layer modulation at S/ 1. A robust spectral signature of secondary flows peaking at ₓ 3 and y 0. 5 emerges across all cases. This peak coexists with, or suppresses, very large-scale motions (VLSMs), depending on flow region and spacing. While VLSMs are suppressed in low-momentum pathways, they gradually recover in high-momentum pathways at higher S/ and Re_. These findings offer insights into the interplay between fluctuations caused by secondary motions and boundary layer structures at high Reynolds numbers.