On the Aeroacoustic Characterisation of Propellers During Vertical Descent

Propellers in vertical axial descent are exposed to complex aerodynamic interactions that most frequently result in unsteady loading and high noise levels, primarily resulting from ingestion of their own wake and disturbed inflow. This experimental study characterizes the aeroacoustic response of low Reynolds number propellers undergoing axial descent and establishes a direct correlation with loss of near-field flow coherence and alteration of far-field noise. Phase-locked stereoscopic Particle Image Velocimetry (PIV) and far-field acoustic recordings were obtained in four flight conditions: hover, vortex ring state, turbulent wake state, and windmill brake state. The data reveal an unambiguous transition of the far-field acoustic signature from tonal dominance during hover to broadband enhancement under windmill brake conditions. Proper Orthogonal Decomposition (POD) reflected systematic loss of vortex coherence, where the energy of the dominant mode fell from 42 percent during hover to 18 percent at 18 meters per second descent speed. Associated flow diagnostics reflected increased vorticity diffusion, reduced helicity, and increased axial turbulence, further substantiating the observed acoustic trends.