Active control of turbulent boundary layers

An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer ($Re_\theta \,{=}\, 1960$) using a linear active control scheme. System identification in the boundary layer provides optimal transfer functions that predict the downstream characteristics of the streamwise velocity fluctuations. Enhanced detection techniques isolate the large-scale turbulent motion and improve the downstream correlations, resulting in greater controllability. The control is applied using a spanwise array of resonant synthetic jet actuators that introduce pairs of streamwise vortices into the flow. Control results show that a maximum reduction of 30% in the streamwise velocity fluctuations is achieved. This reduction is greatest at the point of optimization but spans a few hundred viscous lengths downstream of the actuator, about 50 viscous lengths in the wall-normal direction and 150 viscous lengths in the spanwise direction. The wall pressure fluctuation and the mean wall shear stress (measured approximately using mean velocity profiles near the wall) were reduced by 15% and 7% respectively. The bursting frequency, based on VITA event detection was also reduced by up to 23%.