Experimental investigation of the noise emitted by two different propellers ingesting a planar boundary layer

Abstract

Abstract. Novel-aircraft concepts consider the possibility of placing the propulsor very close to the fuselage to ingest the incoming airframe boundary layer. In this configuration, the engine takes in flow at a reduced velocity, thus consuming less fuel in the combustion process. However, this induces a series of noise consequences that alter the noise perceived by an observer. The present work reports an experimental investigation to compare the far-field noise directivity emitted by two different propellers ingesting a boundary layer at two different states. The experiments have been performed in the anechoic wind tunnel at the University of Bristol. The experimental setup consists of a propeller placed in the proximity of a tangential flat plate, which represents a simplified model of a fuselage. Two tripping devices placed 1 m (6.5 rotor radii) upstream of the propeller have been used to generate distinct boundary layer thicknesses. Results from two distinct propellers with three and five blades have been compared, varying the advance ratio J from 0.56 to 0.98. Far-field noise has been acquired using a microphone array positioned in the plate plane. The data have been analysed in the frequency domain, providing an extensive characterization of the far-field directivity. Results show a general increase in noise when the propeller ingests a thicker boundary layer. Furthermore, a change in directivity pattern is observed varying the advance ratio, suggesting a variation of the underlying physics. Finally, considering different J, the overall noise emission appears to be dependent on the number of blades.