Non-linear noise from a ship propeller in open sea condition
In the present paper, we study the hydrodynamic noise generated by a ship propeller in open sea conditions. We use Large Eddy Simulation for the hydrodynamic field whereas the acoustic field is reconstructed by applying the advective form of the Ffowcs-Williams and Hawkings equation. A dynamic Lagrangian model is adopted for the closure of the subgrid-scale stresses and a wall-layer model allows to skip the resolution of the viscous sub-layer. The acoustic equation is formulated in the integral form and solved through direct integration of the volume terms. The propeller herein considered is a benchmark case, whose fluid dynamic data are available in the literature. A grid of about 3 ×106 cells is able to reproduce accurately both integral quantities like thrust and torque over the propeller, and turbulence propagating downstream in the wake.
Different noise generation mechanisms are investigated separately. The linear terms give rise to a narrow-band noise spectrum, with a mean peak at the blade frequency and other peaks at frequencies multiple of the rotational one. The non-linear quadrupole term reveals a broad band noise spectrum; the shaft vortex provides the largest contribution to the non-linear part of the noise propagated in the far field.