Experiments in Fluids | 2021
Effects of localized blowing on the turbulent boundary layer over 2D roughness
Abstract
Planar particle image velocimetry was utilized to investigate the effects of localized blowing (injection) on the turbulent boundary layer over two-dimensional k-type roughness. The experiments were performed at a Reynolds number of 100,000, based on the boundary layer thickness and the free-stream velocity. The roughness was composed of identical transverse square bars at a pitch to height ratio $$p/k = 11$$\n ; the bars occupied $$\\sim 13-14\\%$$\n of the boundary layer thickness. In addition to a baseline no-injection case, localized blowing through five small spanwise jets was considered at two rates and for two injection locations. The volumetric flow rate through the jets was below 0.12% of the overall flow rate, resulting in a small perturbation to the flow field. As such, the overall flow organization is maintained across the five considered cases (baseline and four injection cases). However, the localized blowing alters the time-averaged streamwise velocity, boundary layer characteristics, Reynolds shear stress, and in-plane turbulent kinetic energy. For example, the localized blowing considered here can induce an increase up to $$\\sim 6\\%$$\n in the time-averaged streamwise velocity and a reduction up to $$\\sim 20\\%$$\n in the maximum streamwise-averaged Reynolds shear stress and turbulent kinetic energy. The blowing-induced deviations from the baseline case extend far within the boundary layer; however, two-point correlations and proper orthogonal decomposition analyses provide evidence for a similar turbulence structure far above the roughness, despite the blowing-induced deviations in the aforementioned flow quantities.