Abstract
The present study addresses an experimental and analytical investigation of the scattering of the tonal noise of low-speed propeller by a half-cylinder and a soft free surface. The interest is marine propeller noise of surface ships at the first harmonics of the blade passing frequency. Indeed, installed marine propellers correspond to very compact configurations. Both sizes blade tip radius and the distance to the ship hull are small compared to the wavelength. Thus, in view of the extremely low Helmholtz numbers, some amplification due to an asymptotic scattering regime is expected. Measurements made in air in similar conditions, in anechoic chamber confirmed that noise from small-scale isolated propeller is dramatically intensified near a half cylinder with reflecting plane. The problem is formulated analytically by using the formalism of source-modes near a scattering half cylinder combined with the effect of the free surface in two dimensions. In this configuration, the diffraction is found to induce an amplification but with a different wave pattern to what the diffraction is with a cylinder only. The results show that the installation effect is crucial for the analysis of marine propeller noise at the very low frequencies.
Published Version
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