Injection as an active flow control method for large-scale, periodic, 2D, roughness.

The effects of active flow control on drag reduction, flow recirculation, and turbulence statistics in the wake of 2D large-scale roughness elements were experimentally investigated using Particle Image Velocimetry in a water channel. Specifically, using blowing as the method of active flow control, changes to the velocity field and turbulence statistics were measured. The model surface consisted of fourteen square acrylic bars (2D roughness elements aligned perpendicular to the flow direction) and flow was injected evenly from multiple, equally sized and spaced ports located on either the bar or the wall at two different velocities. Overall, five sets of contour plots were created from averaged vector fields displaying streamwise velocity, wall-normal velocity, Reynolds Shear Stress (RSS), and Turbulent Kinetic Energy (TKE) for cases of no injection, 0.75 m/s injected from the surface, 0.75 m/s injected from the bar, 1.65 m/s injected from the surface, and 1.65 m/s injected from the bar. The high velocity injection from the bar resulted in a large reduction in both turbulence magnitude and recirculation zone size compared to the other test cases as well as the baseline, while the low velocity bar injection case produced a lower yet discernable effect. The wall injection caused minor reduction in turbulence magnitude and recirculation zone size at low velocities; however, at high velocities the injection jet split the recirculation zone into two regions and pushed high turbulence regions away from the wall. Overall, a high velocity injection in the streamwise direction produced the greatest reduction in both recirculation zone size and turbulence magnitude, and therefore likely minimized drag over the surface.