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.

Laminar Separation Bubble Noise on a Propeller Operating at Low Reynolds Numbers / Grande, Edoardo; Ragni, Daniele; Avallone, Francesco; Casalino, Damiano. - In: AIAA JOURNAL. - ISSN 0001-1452. - ELETTRONICO. - 60:9(2022), pp. 5324-5335. [10.2514/1.J061691]

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

Avallone, Francesco;Casalino, Damiano
2022

Abstract

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.
2022
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2976883