Closed-Loop Control of Unsteady Transient Growth Disturbances in a Blasius Boundary Layer using DBD Plasma Actuators

Plasma actuators have recently been shown to negate the effect of the transient growth instability occurring in a Blasius boundary layer for the purpose of delaying bypass transition. Specifically, during steady operation, the energy of a disturbance introduced via an array of static cylindrical roughness elements was reduced by up to 68\%, as shown by Hanson et al (Exp.\ Fluids, 2010). In the present work, the actuators used in the aforementioned study were integrated into a complete closed-loop control system capable of negating unsteady transient growth disturbances induced in a Blasius boundary layer established in a wind tunnel. Shear stress measurements from an array of hot-wires mounted just above the surface of the boundary-layer plate downstream of the actuators are used to provide feedback information about the state of the boundary layer. The effectiveness and robustness of the closed-loop controller are rigorously established based on both control-model simulations and experiments.