Experimental Investigation of Blade Wake Interaction Noise for a Hovering UAS Rotor

Small-scale unmanned aerial systems (UAS) operate with low tip Mach number rotors, making broadband noise a significant concern for public annoyance. The main broadband noise sources are high-frequency trailing edge noise and mid-frequency blade wake interaction (BWI) noise. The latter of which, is related to a rotor blade interacting with wake turbulence generated by a preceding blade. Due to the complex nature of the wake, it is important to identify turbulent structures which are relevant for BWI noise production. This information can for inform noise modelling and low-noise design efforts. For this reason hovering rotor experiments were conducted in the Penn State ARL Anechoic Room using an off-the-shelf 12 inch APC rotor operating at a tip Mach number of 0.25. The experiments combined high-speed (9 kHz) stereo-PIV measurements, force measurements, far-field acoustic measurements, and near-field wake pressure measurements. The results showed a mid-frequency range, which was coherent between the near-field wake pressure on the rotor slip-stream and the far-field acoustic pressure. Additionally, correlations between the near-field pressure and stereo-PIV measurements identified wake features which were relevant for BWI noise production.