Hydroacoustic reduction mechanism for the bionic hydrofoil

A bionic hydrofoil with the features of trailing-edge serrations, leading-edge wavy protuberances, and wavy surface was proposed for the radiation noise reduction. Large eddy simulations were conducted for the near-field unsteady turbulent flows at Reynolds number (Re) 1.0×105 and zero angle of attack to obtain the radiation noise sources. A Ffowcs Williams and Hawkings acoustic analogy was applied to the far-field sound calculations for two sets of bionic hydrofoils with different amplitudes and wavelengths of the wavy edge. The results show that the bionic hydrofoil effectively reduces its drag, the maximum drag reduction reaches 18.15% and the smallest mean drag exists at the medium amplitude or wavelength. The bionic hydrofoil also effectively reduces the turbulent radiation noise; the maximum overall sound pressure level reduction reaches 20 dB in the considered cases. It is found that, as the wavy edge amplitude increases to 2 mm, the line spectra disappear in the sound pressure level (SPL) profile. The SPL reduces by a value more than 25 dB. The wavy surface of the bionic hydrofoil triggers streamwise vortices, which break the long spanwise vortex structures into the small-scale vortices and make the flow separation shift forward. As the amplitude of the wavy edge increases, the vorticity of the streamwise vortices increases. This makes the flow separation shift more forward and more violently breaks the downstream vortices. It turns out that the velocity fluctuation in the wake is attenuated, leading to the radiation noise reduction.