Acoustic signature of a propeller operating upstream of a hydrofoil

The Ffowcs-Williams and Hawkings acoustic analogy is utilized to analyze the signature of a system consisting of a propeller and a downstream hydrofoil, mimicking a rudder at 0° incidence. This study is carried out exploiting the database generated by Large-Eddy Simulations on a cylindrical mesh consisting of almost 2 × 109 grid points. Three rotational speeds of the propeller are considered. The analysis reveals that the major sources of sound are located at the leading edge of the hydrofoil, due to the impingement by the propeller wake. With the exception of small radial coordinates around the propeller wake, between two and four diameters from the propeller axis, where the non-linear sources of sound have the lead, most noise comes from the linear, loading sources on the surface of the hydrofoil, due to fluctuations of the hydrodynamic pressure. As a result, the azimuthal directivity of the sound pressure levels develops a dipole-like distribution, elongated in the direction of the span of the hydrofoil. The attenuation of the acoustic pressure along the radial direction is initially cubic, then quadratic, and eventually, within less than ten diameters away from the system, linear.