No abstract

Noise reduction was achieved on a model helicopter rotor using pulsed dielectric barrier discharge (DBD) plasma actuators mounted at the blades' leading edges. The rotor was mounted on a bi-axial balance in an anechoic chamber, and the following three-point noise-reduction strategy was adopted: first, the blades' collective (geometric) angle was increased into the post-stall regime to achieve maximum baseline thrust; second, actuation was applied, increasing thrust and decreasing torque; third, the rotational speed was decreased to recover the original baseline thrust. This strategy resulted in an increase in the maximum Figure of Merit and a decrease in broadband and tonal noise components. Broadband and loading noise reduction, up to 3 dB, was due to boundary layer attachment, resulting in smaller and weaker turbulent eddies in the wake and the tip-vortex and, therefore, more minor turbulent surface pressure fluctuations. Noise reduction directivity associated with the blade passing frequency of up to 3 dB was consistent with Lowson's theory. Surprisingly, at most of the observer angles, both the plasma pulsation and ionization frequencies were not visible in the spectra. This directional acoustic signature was assumed to be due to the geometric mounting of the actuator and its effect on the boundary layer.

Noise reduction was studied on a model helicopter rotor using pulsed dielectric barrier discharge (DBD) plasma actuators mounted at the blades’ leading edges. The evaluation was performed in a fully anechoic chamber, where thrust and torque were measured concurrently with the acoustic signature using an array of 15 microphones. Acoustic measurements were made for both baseline and actuated cases at a fixed collective pitch angle under conditions of constant thrust. Actuation produced broadband and tonal noise reductions of up to 8 and 3 dB, respectively, while simultaneously increasing the rotor figure of merit by 25%. Broadband noise reductions were due to control of the otherwise separated boundary layer, resulting in smaller and weaker turbulent eddies in the wake and the tip-vortex and, therefore, smaller turbulent surface pressure fluctuations. Noise reduction directivity associated with the blade passing frequency of up to 3 dB was consistent with rotational noise theory. At most of the observer angles, both the plasma pulsation and ionization frequencies were not visible in the spectra above the motor self-noise signature. Peaks in the second harmonic were observed at specific observer angles and were assumed to be due to the geometric mounting of the actuator and amplification of the pulsations in the laminar separation bubble.