ACTIVE AERODYNAMIC CONTROL OF WAKE-AIRFOIL INTERACTION NOISE - EXPERIMENT

A proof of concept experiment has been performed that shows the potential for active aerodynamic control of rotor wake/stator interaction noise in a simplified manner. A single airfoil model representing the stator was fitted with a movable trailing-edge flap controlled by a servomotor. This flap allowed control of the unsteady lift of the airfoil and acted as the active aerodynamic element in the system. Upstream of the airfoil, a disturbance generator created a periodic wake, similar to an actual rotor blade wake, which impinged on the airfoil. A control system moved the motor-driven nap in the correct angular displacement rate and phase to reduce the unsteady load on the airfoil during the wake interaction. The motion could be arbitrary, permitting the airfoil-flap combination to respond to generic inflow disturbances. Noise reduction results showed that this concept works very well in the bandwidth at which the servomotor drive actuated the flap (currently up to 100 Hz). The peak-to-peak acoustic interaction pulse was reduced by a factor of 2, whereas the corresponding noise was reduced by a significant 10 dB at some frequencies. A companion paper by E. J. Kerschen, entitled ''Active Aerodynamic Control of Wake-Airfoil Interaction Noise-Theory,'' demonstrates the ability to predict the required flap motion to achieve these dramatic noise reductions.