Experimental Investigation of the Effect of Axial and Azimuthal Separation of Individual Blades of a Propeller on Noise Generation

Abstract:
This paper experimentally investigates the effect of positioning of individual blades of a two bladed propeller around the rotor hub on overall noise generated by it. An experimental setup was created to measure noise and performance in an anechoic chamber to carry out parametric study in which axial and azimuthal separations between the two blades were introduced through a custom built rotor hub and balancing weight. The propeller noise that is dominated by tonal components associated with blade passage frequency appears to be influenced by azimuthal separation between individual blades and the broadband components generated by turbulent blade-wake interactions is primarily affected by the axial separation between individual blades. From the present study, it is identified that the rotor configurations with 60° azimuthal and 6 mm (3.2% of rotor radius) axial separation resulted in up to 4.4 dB reduction in Overall Sound Pressure Level (OASPL) and 63.9% reduction in acoustic energy, while the 120° configuration with 6 mm (3.2% of radius) axial separation showed up to 3.8 dB reduction in OASPL and 58.7% reduction in acoustic energy through redistribution in acoustic energy across wider frequency range. Flow-field measurements using Particle Image Velocimetry (PIV) revealed that these improvements are associated with merging of tip vortices, thereby reducing blade-wake interactions, demonstrating that controlled blade spacing through hub modification is an effective passive strategy for reducing UAV propeller noise without affecting the performance significantly.

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