Investigation into the tip-gap flow and its Influence on ducted propeller tip-gap noise using acoustic analogies

Abstract

Ducted propellers are commonly used on naval vesselssuch as submarines as well as increasingly moreomnipresent autonomous underwater vehicles (AUVs).Understanding the noise signature of these vessels is ofcritical importance both from detection and concealmentperspective, but the detailed physics of how the tip of thepropeller blade interacts with the boundary layer on theduct are not sufficiently well understood. Present studyintroduces a series of simulations in which tip vorticesformed over finite-span lifting surfaces are investigatedwith the aim of better understanding interactions oftip-gap flow and its interaction with incident boundarylayers on radiated noise. Particular attention is devotedto characterising the nature of the incipient complex flowfeatures and their effect on the force coefficients, surfacepressure fluctuations. These reveal that variations offorce coefficients with tip-gap height may be used asan early indicator of tip vortex being suppressed as thegap is reduced. Use of the λ2criterion identifies theformation of strong, coherent vortical structures in the tipgap region. It is suggested that the interaction of thesestructures with the tip edges is likely to be in importantsource of noise, with a similar generation mechanism totrailing edge noise. Preliminary analysis of the acousticsignatures computed using Ffowcs Williams-Hawkingsacoustic analogy indicates high levels of directivity withmajority of the noise being generated by the foil ratherthan the duct. Presence of substantial amounts ofvorticity in the flow also suggests that accounting forthe non-linear quadrupole noise sources within the fluidmight be necessary to fully describe the noise pattern developed

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

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