Acoustic noise measurement in counter-rotating propellers

Abstract

The acoustic noise and flow condition of tandem propellers in a horizontal axis-type tidal stream power unit, which is composed of counter-rotating propellers and double-rotational armature-type generator without traditional stators, were experimentally investigated. The front and rear propellers rotate in opposite directions and drive the inner and outer armatures, respectively; the rotational torque of the front propeller coincides with the torque of the rear propeller. The front blade profile exerts a great effect on noise generation from the tandem propellers, but the effect of the rear blade profile is comparatively small. The sound pressure level is largely concentrated at low frequencies, and most predominant frequencies can be predicted by Hanson’s method. These predominant frequencies are mainly caused by the tip vortex shedding from the front and rear blades in the counter-rotating propellers. The vortex shedding from the front blade does not affect the rear blade. As a result, the acoustic noise is weakened as long as the diameter of the rear propeller is smaller than the diameter of the front propeller.