On predicting the sound from a cavitating marine propeller in a tunnel

Model tests in a cavitation tunnel are commonly used in order to predict the radiated noise from full-scale marine propellers. Correction factors and scaling procedures are commonly used in estimating the effects of background noise and the propeller model scale. Less attention is paid to the influence of the tunnel related factors. Above a certain frequency, i.e. first non-plane wave mode cut-on frequency, the sound pressure varies over the tunnel cross-section and non-plane waves are introduced. Below this frequency, only plane waves propagate in the tunnel and no attenuation due to geometrical spreading occurs in the far field of the propeller. Thus, it is difficult to apply source level scaling procedures based on, e.g., spherical spreading of the sound field. Also, rigid tunnel walls near the propeller load it acoustically and may alter the fundamental sound emission compared to emission of the same source in free field conditions. In this paper, sound emission and immission of acoustic sources in a free space and in a waveguide are discussed and sound from a cavitating model-size marine propeller in a tunnel is investigated with numerical methods.