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Incompressible flow assumption is an essential step that simplifies numerical simulations for objects inside flowing water. However, incompressibility assumption creates a conflicting situation for sound propagation as the propagation speed is given by 0 = √ ⁄ where denotes the pressure and denotes the density. When = 0 is assumed to relieve simulations, speed of sound theoretically becomes infinite and therefore, induced pressures should be corrected with the acoustic analogy. This approach is called the "hybrid method" that combines hydrodynamic solver with hydroacoustic solver. Hydroacoustic solver adds compressibility effects to the incompressible hydrodynamic solver and uses hydrodynamic pressure to calculate acoustic pressure. Time step size is an important parameter to calculate acoustic pressure field in the fluid domain and an approach to determine the minimum value (for at least capturing the first blade passage frequency) is presented in this study. Another purpose of this paper is to investigate the effect of incompressibility on hydroacoustics and to analyze the necessity of utilizing the famous Ffowcs Williams-Hawkings (FWH) equation for predicting marine propeller noise; both for cavitating and non-cavitating cases. Our results are in line with other researchers; hydrodynamic pressure is sufficient to assess the hydroacoustic performance of marine propellers in the near-field due to having very low acoustic Mach numbers. Near-field results from the hydrodynamic solver are then extrapolated to the far-field by adopting ITTC distance normalization equation. However; this equation, which is actually the inverse distance law, is only valid for point noise sources in stationary flow. It is found out that eliminating FWH equation by coupling the incompressible hydrodynamic solver with ITTC distance normalization equation fails to produce satisfactory results. For cavitating cases, numerical results in this study show that implementation of FWH is required even in the near-field.

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This study focuses on the prediction of the hydrodynamic and hydroacoustic performance of a cavitating marine propeller in open water condition using Reynolds-averaged Navier-Stokes (RANS) and Detached Eddy Simulation (DES) solvers. The effectiveness of the methods is investigated for the recently introduced benchmark propeller that belongs to the research vessel 'The Princess Royal'. The main emphasis of the study is to examine the capabilities of the RANS and DES solvers for predicting the hydrodynamic performance of a propeller in the presence of sheet and tip vortex cavitation (TVC). In the numerical simulations of the cavitating propeller flow, the Schnerr-Sauer cavitation model based on a reduced Rayleigh-Plesset equation was used to model the sheet and tip vortex cavitation. An alternative Vorticity-based Adaptive Mesh Refinement (V-AMR) technique was employed for the accurate realisation of the TVC in the propeller's slipstream. In the hydroacoustic calculations, a porous Ffowcs Williams Hawkings equation (P-FWH) was employed together with the DES solver. The numerical hydrodynamic and hydroacoustic results are compared with those of experimental data for the benchmark propeller available from the University of Genova Cavitation Tunnel. The results show that both the RANS and DES solvers are successful for modelling of the sheet cavitation on the propeller blades. However, the prediction of the TVC extension using the RANS solver is found to be insufficient in comparison to the TVC prediction when using the DES method. This is due to the inherent modelling limitations of the RANS solver. In addition to hydrodynamic performance predictions, the overall noise spectrums were found in an agreement with the experimental data with discrepancies between the low and high-frequency region.