Response of a turbulent boundary layer to steady, square-wave-type transverse wall-forcing

Abstract

This study investigates the spatial evolution of a zero pressure gradient turbulent boundary layer (TBL) imposed by a square-wave (SqW) of steady spanwise wall-forcing, which varies along the streamwise direction (𝑥). The SqW wall-forcing is imposed experimentally via a series of streamwise periodic belts running in opposite spanwise directions, following the methodology of Knoop et al. [Exp. Fluids 65, 65 (2024)], with the streamwise extent increased to beyond ∼11 times the boundary layer thickness (𝛿𝑜) in the present study. This unique setup is leveraged to investigate the influence of viscous-scaled wavelength of SqW wall-forcing on the turbulent drag reduction efficacy for 𝜆+𝑥=471 (..