Investigation of rotor noise source mechanisms with forward speed simulation

Abstract

The noise of a model propeller was measured with and without forward speed simulation in the open test section of a low-speed wind tunnel at velocities up to 60 m/s and blade tip Mach numbers up to 0.6. Compared to the static tests, even low wind velocities reduced the propeller noise by up to 20 dB. Strong sound radiation occurred due to interaction of the propeller blades with the wake of the supporting beam when the propeller was mounted as a pusher propeller. At wind velocities above 20 m/s, the high-frequency part of the propeller noise depends strongly on the angle of attack of the blades. At moderate angles of attack, laminar vortex shedding noise dominates the high-frequency sound radiation. This noise component could be eliminated by tripping the boundary layer on the suction surface of the blades. The frequencies of maximum noise radiation were predicted fairly well by existing theories. The laminar vortex shedding noise disappears at higher angles of attack of the propeller blades. If the angle of attack is further increased, strong braodband noise occurs which is generated probably by the turbulent boundary layer and local flow separations.