Active control of large-scale structures in turbulent boundary layers using wall-normal jets

This experimental study uses a spanwise array of nine wall-normal jets to actively perturb large-scale structures in a high Reynolds number turbulent boundary layer. The footprints of passing large-scale structures are sensed by an upstream spanwise array of hot-film sensors, which provides the input to the real-time controller. Various real-time large-scale control strategies can be tested with this facility. Downstream of the jet array, a further sensing array probes the efficacy of the control strategy. A large field of view streamwise / wall-normal plane PIV experiment also provides additional opportunities to investigate the interaction between the jet actuators and the large-scale structures. Results demonstrate that the degree of drag reduction attained is strongly correlated to how effectively large-scale positive fluctuations have been targeted by the control scheme. In addition, there is a strong correlation between the reduction of large-scale energy and the observed drag reduction. Such control effects persist to 2$\delta$ (approximately 28000 wall units) downstream of the control input. Further analysis of the near-wall small scale turbulence reveals that the near-wall cycle has been modified due to the altered footprint from the controlled large-scale events.