Closed-loop control of vortex-airfoil interaction noise

Closed-loop controlled interactions between an airfoil and impinging vortices were experimentally investigated. This work aims to minimize the fluctuating flow pressure (p) at the leading edge of the airfoil, which is a major source of the blade-vortex interaction noises commonly seen in rotorcrafts. Piezo-ceramic actuators were used to create a local surface perturbation near the leading edge of the airfoil in order to alter the airfoil-vortex interaction. Two closed-loop control schemes were investigated, which deployed p and the streamwise fluctuating flow velocity (u) as the feedback signal, respectively. While the control effect on p was measured using a fast response pressure transducer, the oncoming vortical flow was monitored using a particle image velocimetry and a hot wire. It was found that the control scheme based on the feedback signal u led to a pronounced impairment in the strength of oncoming vortices and meanwhile a maximum reduction in p by 39%, outperforming the control scheme based on the feedback signal p. Physics behind the observations is discussed.