This paper explains the presence and relevance of noise caused by a laminar separation bubble (LSB) on a propeller operating at 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 · 10 4 to 10 5 . The flow behaviour 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 which 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.
|Title of host publication||28th AIAA/CEAS Aeroacoustics 2022 Conference|
|Number of pages||19|
|Publication status||Published - 2022|
|Event||28th AIAA/CEAS Aeroacoustics 2022 Conference - Southampton, United Kingdom|
Duration: 14 Jun 2022 → 17 Jun 2022
Conference number: 28
|Name||28th AIAA/CEAS Aeroacoustics Conference, 2022|
|Conference||28th AIAA/CEAS Aeroacoustics 2022 Conference|
|Period||14/06/22 → 17/06/22|
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