Cavitating wake dynamics and hydroacoustics performance of marine propeller with a nozzle

Using high-fidelity computational fluid dynamics modeling, the current work studies the cavitating turbulent flow of a ducted marine propeller and explores the physical mechanisms underpinning the underwater radiated noise. We employ the standard dynamic large-eddy simulation for the turbulent wake flow and the homogeneous Schnerr-Sauer model for the cavitation process, while the Ffowcs Williams-Hawkings acoustic analogy is used for hydroacoustic modeling. The modeling framework is validated against available experimental data, capturing a distinctive double-helical tip vortex cavitation and its qualitative patterns along the vortex trajectory. In comparison to the noncavitating scenario, the pressure fluctuation on the propeller surface is more ordered but energetic under cavitating conditions due to the periodic nature of the sheet cavity. This is reflected in the thrust spectrum in the form of stronger low-frequency tonal peaks and medium-frequency broadband components, while the high-frequency broadband components are relatively weaker. We show that cavitation enhances the monopole noise source due to fluid displacement by the cavity along with the dipole and quadrupole noise sources associated with the propeller surface and wake turbulence effects. Tonal noise with frequencies corresponding to harmonics of the blade passing frequency is also increased. Cavitating structures increase the hydroacoustic energy of the radiated noise at all orientations, particularly downstream, with an increase in the sound pressure levels by up to 20 dB. Finally, the addition of a duct nozzle inhibits cavitation originating from the propeller surface and its accompanying acoustic energy, although cavitating/vortical structures are now observed at new locations around the nozzle system. As a result, the overall radiated noise power is reduced in the ducted propeller configuration.