Acoustic far field of a propeller working in the wake of a hydrofoil

Abstract

The Ffowcs-Williams & Hawkings (FWH) acoustic analogy is adopted to reconstruct the acoustic far field of a system consisting of an upstream hydrofoil and a downstream propeller, considering the former at incidence angles of 0°, 10°, and 20°. Also comparisons against the same propeller working in isolated conditions are reported. Fluid dynamic data from earlier high-fidelity, Large-Eddy Simulations (LES) on a grid consisting of 1.7 × 109 points are utilized. The analysis demonstrates that, with some exceptions at the smallest frequencies, the acoustic far field is dominated by the loading sound coming from the propeller, achieving its highest values of acoustic pressure in the upstream and downstream directions. In contrast, the lowest values occur on the propeller plane, whose minima are aligned with the spanwise direction of the hydrofoil. A strong dependence on the incidence angle of the hydrofoil is found, although decreasing toward higher frequencies. Interestingly, while at the shaft and at the blade frequencies the acoustic pressure coming from the hydrofoil-propeller system is always higher than that from the open-water propeller working alone, as expected, at higher harmonics of the blade frequency this is not the case. This may be due to phenomena of destructive interactions across the acoustic sources on the surface of the propeller or the result of a shift of the acoustic signature toward even higher frequencies, beyond the range covered by the database available to the present study.