Wind tunnel

Boundary layer

Physics

Reynolds number

Water tunnel

Mechanics

Laminar flow

Propeller

Noise (video)

Image (mathematics)

Vortex

Engineering

Rotor (electric)

Vortex shedding

Artificial intelligence

Turbulence

Marine engineering

Acoustics

Particle image velocimetry

Computer science

Quantum mechanics

Blade pitch

Advance ratio

Flow separation

AIAA Journal

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 from 0 to 0.6, corresponding to a tip Reynolds number ranging from to . The flow behavior on the blade surface and around the propeller is investigated with oil-flow visualizations and particle image velocimetry. At 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 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.

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