Ridge-induced secondary flows at high Reynolds number boundary layers

Significant research has recently been dedicated to the understanding of spanwise heterogenous roughness-driven secondary flows. These are shown to strongly modulate the baseflow, with significant alternations in the mean and turbulence properties. Despite the current vast body of work, most studies have been performed at low to moderate Reynolds numbers, restricting the investigation of secondary flows at high large-to-small scale separation. To address this, a turbulent boundary layer over a spanwise heterogeneous ridge-type topography is investigated. Using a floating-element drag balance and cross-plane stereoscopic particle image velocimetry, the skin friction and turbulence are examined. Three spanwise wave lengths at high Reynolds numbers are considered. The results showed that depending on the spanwise wavelength, the skin friction remains transitionally rough in a large portion of Reynolds numbers, but weakly asymptotes at high enough Reynolds numbers. Using the friction velocity, the inner and outer scaling of the mean velocity are examined to assess their impact on the roughness function and the wake region. Furthermore, the triple decomposition of the total shear stress allowed the quantification of the dispersive and turbulent stresses to the global momentum fluxes.