Detection of acoustic vortex generation from rotors using a virtual rotating receiver method

Abstract

Acoustic vortices with orbital angular momentum (OAM) exhibit significant potential in high-speed communication, particle manipulation, and analogue physics. However, existing methods of acoustic vortex generation, whether passive or active, are often cumbersome and rely on supplementary electronic devices to provide sound sources. In this study, we propose that acoustic vortex generation is an intrinsic characteristic of rotating rotors and develop a theory for the generation of acoustic vortices from rotor tonal sources. The theory indicates that the operating frequency and topological charge of the acoustic vortices can be tuned by the rotational frequency and number of blades of rotors. To validate the proposed theory, we employ the virtual rotating receiver (VRR) method to measure the rotational Doppler-shifted spectra of sound fields generated by single and coaxial twin rotors. Analyses of both the Doppler frequency shift and the phase differences between adjacent VRRs reveal an extreme Doppler shift at the location of the OAM handedness reversal, conclusively demonstrating that the generated sound fields are acoustic vortices. Our work has potential applications in multiplexing OAM beams for acoustic communication, contactless detection of rotor speed and number of blades during rotor operation, as well as noise reduction and energy harvesting through the manipulation of acoustic OAM.