The acoustic signature of a rudder in the wake of a propeller: Comparison between infinite and semi-infinite approximations

The acoustic analogy is adopted to reconstruct the sound pressure levels from a propeller–rudder system, using the database of instantaneous realizations of the solution from Large-Eddy Simulations, conducted on a cylindrical grid of 3.8 billion points. Results are compared across incidence angles and rudder geometries, approximated by hydrofoils of infinite and semi-infinite spanwise extents. The latter parameter does not affect significantly the acoustic pressure at no incidence, but its influence grows for increasing angles. Actually, trends are different at inner and outer radii. In the vicinity of the propeller wake, the acoustic signature is dominated by the non-linear sources of sound and is higher when the infinite approximation of the rudder geometry is considered, due to the massive separation affecting the suction side of the hydrofoil. In contrast, moving away along the radial direction the lead goes to the linear sources of sound on the surface of the hydrofoil, which are higher in the case of the semi-infinite approximation of the rudder. Surprisingly, separation phenomena, more significant on the suction side of the infinite hydrofoil, result in a lower loading component of the sound, at least in the range of low frequencies considered in the present study, maybe due to a shift of the acoustic signature of the system towards higher frequencies.