Tip gap noise mechanisms in ducted marine propellers due to unsteady flow features

Abstract

Ducted propellers exist in both aerodynamic and hydrodynamic contexts, and the former has received significantly more attention than the latter. This thesis is aimed at identifying the key noise mechanisms in marine ducted propellers. The properties of the fluid media have a significant impact on the relative importance of noise producing flow features and the differences in likely noise sources for aerodynamic and hydrodynamic propellers are discussed. This thesis uses time-resolved simulations to investigate the unsteady flow features in the tip gap region of a ducted propeller. By inferring the important noise mechanisms it is possible to identify specific criteria which can be used to produce noise mitigating designs. A variety of simplified geometries have been investigated beginning with an open tipped foil to provide a base line case. The complexity of the case is progressively increased to investigate the impact of the tip gap height, the inclusion of a no slip condition on the boundary surface above the tip, and the impact of foil thickness and camber. The numerical simulations have been carried out using an open source computational fluid dynamics software with both time-averaged, and time-resolved simulations conducted. Post processing techniques such as vortex identification criteria and an acoustic analogy have been used to infer the primary noise sources in the tip gap region. Non-dimensional quantities have been used to investigate the flow features and have shown good agreement with previous numerical and experimental work. The studies found that the most likely cause of noise within the tip gap region is due to the unsteady flow structures being shed through the tip gap and scattered over the sharp edge of the tip. The size and strength of these structures is highly dependent on the tip gap size, and explains why reduced tip gaps are associated with reduced rotor tip noise. The vortex structures are shown to be an accurate indicator of the hydrodynamic surface pressure variations leading to a more intuitive metric to use for improved designs. A simple design modification is made to the blade with the aim of removing the vortex structures within the tip gap region in order to reduce the strength of the unsteady pressure fluctuations. This improved design reduced the surface pressure spectra by greater than 30dB at certain, key frequencies.

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ORCID for Stephen Turnock: orcid.org/0000-0001-6288-0400

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