Laminar Separation Bubble Noise on a Propeller Operating at Low Reynolds Numbers

This paper explains the presence and relevance of noise caused by a laminar separation bubble (LSB) on a propeller operating at a low Reynolds number. Microphone measurements of a propeller with both clean and forced boundary-layer transition blades are carried out in an anechoic wind tunnel by varying the propeller advance ratio J from 0 to 0.6, corresponding to a tip Reynolds number ranging from 4.3⋅104 to 105. The flow behavior on the blade surface and around the propeller is investigated with oil-flow visualizations and particle image velocimetry. At J=0.4 and 0.6, vortex shedding from the LSB causes high-frequency noise that appears as a hump in the far-field noise spectra. Forcing the location of the boundary-layer transition suppresses the LSB and, consequently, the hump, reducing the noise emission of about 5 and 10 dB at J=0.4 and 0.6, respectively. The fact that the hump is caused by LSB vortex shedding noise is further assessed by using a semi-empirical noise model; it shows that the hump is constituted by tones of different amplitudes and frequencies, emitted at different spanwise sections along the blade.