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A transient two-dimensional acoustic boundary element solver is coupled to apotential flow boundary element solver via Powell's acoustic analogy todetermine the acoustic emission of isolated hydrofoils performingbiologically-inspired motions. The flow-acoustic boundary element framework isvalidated against experimental and asymptotic solutions for the noise producedby canonical vortex-body interactions. The numerical framework thencharacterizes the noise production of an oscillating foil, which is a simplerepresentation of a fish caudal fin. A rigid NACA 0012 hydrofoil is subjectedto combined heaving and pitching motions for Strouhal numbers ($0.03 < St < 1$)based on peak-to-peak amplitudes and chord-based reduced frequencies ($0.125 <f^* < 1$) that span the parameter space of many swimming fish species. Adipolar acoustic directivity is found for all motions, frequencies, andamplitudes considered, and the peak noise level increases with both the reducedfrequency and the Strouhal number. A combined heaving and pitching motionproduces less noise than either a purely pitching or purely heaving foil at afixed reduced frequency and amplitude of motion. Correlations of the lift andpower coefficients with the peak root-mean-square acoustic pressure levels aredetermined, which could be utilized to develop long-range, quiet swimmers.

On the noise generation and unsteady performance of combined heaving and pitching foils