Localized Blowing for Near-Wake Flow and Vortical Structure Control in Turbulent Boundary Layers Over Periodic Two-Dimensional Roughness

Volumetric three-component flow measurements were made to investigate localized blowing (injection) as a control strategy for turbulent boundary layers over k-type two-dimensional (2D) roughness. The flow measurements were made using particle tracking velocimetry at a Reynolds number of 100,000, based on the freestream velocity and boundary layer thickness. The roughness occupied similar to 13% of the boundary layer thickness and consisted of transverse square bars positioned periodically at a pitch-to-height ratio of 11. Two cases were considered: a baseline case without blowing and a case with blowing through five spanwise jets issuing from the downstream face of the 11th bar. The results highlight the effectiveness of blowing in reducing the size of the recirculation zone and turbulence past the bar. Specifically, the spanwise-averaged flow field for the blowing case shows a 40% reduction in the reattachment length and similar to 25% reduction in the maximum Reynolds shear stress relative to the baseline case. Moreover, visualizations of the vortical structures past the bars for the baseline case display coherent spanwise vortices similar to those observed past isolated 2D bars and backward-facing steps; however, the spanwise vortices observed here exhibit more three-dimensionality likely due to the turbulence enhanced by upstream bars. Blowing disrupts these spanwise vortices and produces new vortical structures with a wall-normal sense of rotation, although significantly weaker than the spanwise vortices. As such, blowing results in a reduction in the spanwise-averaged spanwise vorticity characteristic of the flow over k-type 2D roughness. The disruption of the spanwise vortices and the reduction in the size of the recirculation zone are likely responsible for the reduction in the Reynolds shear stress and turbulent kinetic energy in the near wake.