The acoustic signature of a propeller–hydrofoil system in the far field

Data from Large-Eddy Simulations on a grid consisting of 3.8 billion points are utilized to reconstruct the acoustic far field of a propeller–hydrofoil system, using the Ffowcs-Williams and Hawkings acoustic analogy. Results demonstrate that the lowest frequencies, around the shaft frequency, are dominated by the sound coming from the surface of the downstream hydrofoil, especially in its normal direction. In contrast, at the blade frequency and higher frequencies, the acoustic signature of the upstream propeller gains the lead, especially in the upstream and downstream directions. Minima of the sound pressure levels correlate in all cases with the spanwise direction of the hydrofoil. Despite the dramatic change of the flow conditions for growing incidence angles of the hydrofoil, characterized by a substantial rise of the turbulent fluctuations on its suction side, no similar increase in the acoustic pressure in the far field occurs at large incidence angles, even at the onset of separation phenomena, which are able to affect the highest frequencies only.