Experimental and numerical investigation of the sound field produced by a shrouded UAV propeller

This paper investigates the effect of an acoustically rigid shroud on the tonal noise produced by an unmanned aerial vehicle (UAV) propeller. Enclosing the propeller with a shroud alters the steady loading on the propeller blades, thereby changing the steady loading tones radiated from the sources on the propeller surface. In this paper it is shown that the time-average rotating pressure field on the inner surface of the shroud also makes a significant contribution to the tonal noise. The aerodynamics of a shrouded and an isolated propeller was investigated using computational fluid dynamics (CFD) simulations. The tones produced by the steady loading and thickness sources on the propeller, and the time-average rotating pressure field on the surface of the shroud were calculated using an acoustic analogy approach and data from the CFD simulations. These calculations show that the tonal noise radiated from the loading sources on the inner surface of the shroud make a significant contribution to the tonal noise levels and that the contribution from the loading sources on the other surfaces of the shroud are negligible. A probe microphone was used to measure the time-average rotating pressure field on the inner surface of the shroud and these measurements showed good agreement with the CFD simulation. These measurements were then used to calculate the contribution of the loading sources on the inner surface of the shroud to the total noise measured experimentally. High resolution acoustic measurements in the near-field of a shrouded and an isolated propeller were also taken. The near-field measurements were then projected to the acoustic far-field and these levels are compared with numerical predictions. The results show that the effect of the shroud is to decrease the tonal noise for observers below the rotational plane of the propeller.