Modelling the acoustic radiation of propeller tip vortex cavitation in potential flow simulations

Propeller cavitation and its effect on underwater radiated noise (URN) are topics of increasing interest within the marine hydrodynamics community. Numerical prediction of the URN characteristics of a given propeller design is vital in achieving the design of low-noise propellers. In this context, the current paper investigates different acoustic models for the numerical evaluation of propeller tip vortex cavitation-induced- noise, using potential flow simulations. Two acoustic modelling approaches are discussed, considering an elongated cylindrical representation of helical tip vortex cavities. Model 2D is a two-dimensional geometrical representation of the tip vortex cavity, wherein equivalent acoustic sources are defined along the cavity surface. Model 1D is an axisymmetric one-dimensional model with equivalent point sources that are distributed along the cavity axis. The applicability and limitations of the two acoustic models are assessed through a verification study of an analytical case for harmonic oscillations of a circular cylinder. The influence and sensitivity of important parameters are discussed, along with their relevance and application when extended to typical tip vortex cavity structures emanating from ship propellers. Finally, the practicality of the acoustic models for propeller tip vortex cavities is investigated through a validation study for the 'Princess Royal' propeller. The hydrodynamic flow solution is obtained from a potential flow solver, which includes flow models for sheet cavitation and tip vortex cavitation. Tip vortex cavity acoustics is obtained using the time-varying tip vortex cavity geometry from the potential flow solution. Acoustic contributions due to the blade rotation and sheet cavity fluctuations are separately computed. The simulation results are then compared with experimental measurements.