Improving numerical and experimental methods for the source level of propulsors

Underwater radiated noise (URN) from shipping is receiving increasing attention because of its negative impact on marine ecosystems, as shipping is known to be the dominant contributor to low-frequency continuous sound in the oceans. In work package 4 of the SATURN project several technical mitigation measures are being developed such as a propeller retrofit by a pumpjet and by a trochoidal propeller and using air bubbles to mitigate machinery noise and propeller cavitation noise. The development and evaluation of these mitigation measures requires computational and experimental tools, such as model-scale tests, that need to be further developed to give an accurate prediction of the performance at full-scale. The present document describes the development of these tools as performed in Task 4.2 of the SATURN project. A high-fidelity computational method for the analysis of propeller cavitation noise has been extended with various approaches proposed in literature to account for non-condensable gas. However, these approaches were found to be insufficient to describe the effect on cavitation dynamics, with the root of the problem lying in the incompressibility of the flow assumed in the flow solver. It is therefore advised to focus future studies on pursuing compressible or weakly- compressible flow modelling approaches and attempting to make them computationally-feasible for practical applications. A low-fidelity method based on a boundary element method has been used to predict the radiated noise of cavitating propellers, focusing on the noise by the cavitating tip vortex. The full-scale broadband spectrum of a ferry sailing at 18 knots was used to validate the prediction showing very good agreement. The low computational time allows the investigation of the effect of changes in geometry on radiated noise. Future extension of the method may include the contribution of sheet cavitation on the radiated noise and the noise at harmonics of blade passage frequency. For the prediction of the radiated noise by non-cavitating propeller flow several approaches using high-fidelity computational methods have been evaluated. Comparison with model-scale tests performed within a previous EU project showed that a RANS based method underpredicts the noise at harmonics of the blade passage frequency. For this URN prediction use is made of the acoustic analogy approach for which the FWH equations was selected. The impermeable approach was found to give more reliable results than the porous surface. Non-cavitating propeller noise was also analysed using LES computations coupled with the FWH acoustic analogy. The propeller was operating in uniform inflow but in the presence of a rudder. It was found that in the near field of the propeller the (quadrupole) sound generated by the propeller tip vortices is the dominant noise source but in the far field it is the blade loading that dominates the radiated noise. The presence of the rudder increases the loading sound. An experimental technique for the characterisation of an acoustic source has been analysed using near-field measurements performed in the distorted flow. The technique can separate the hydrodynamic pressure from the acoustic pressure using a single sensor rather than the two- sensor method developed previously while results are nearly identical. In addition, the use of flow- field (PIV) measurements performed at relative low sampling rate with acoustic measurements performed at high sampling rate also poses some challenges to identify the sound sources when making use of conditional analysis. Several strategies have been discussed to overcome this challenge such as down-sampling of the acoustic signal, PIV-field extrapolation, and real time recording of conditional events. Most promising seem the combination of PIV and CFD results to extrapolate the PIV data and the use of a conditional event provided the reference signal has good signal-to-noise ratio for the acoustic pressure. The effect of reverberation on the measured noise levels in a cavitation test facility has been investigated by mounting a sound source in the propeller plane of a ship model at zero speed and when moving. The reverberation radius was found to be in good agreement with available measurements for a stationary isolated noise source, hence in absence of ship model and ship speed. Even though the ship model does not affect the reverberation radius, it did change the directionality of the sound field. Measured sound levels upstream of the sound source differed from the equivalent free-field values when corrected for Lloyd-mirror, whereas below and downstream the sound source the difference was small.