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

TL;DR

This study develops a coupled flow-acoustic boundary element framework to analyze noise generation by oscillating hydrofoils mimicking fish fins, revealing how combined motions influence acoustic emissions and potential for quieter swimming.

Contribution

It introduces a validated numerical framework coupling flow and acoustic solvers to study noise from biologically-inspired foil motions, highlighting the effects of combined heaving and pitching on noise levels.

Findings

Dipolar acoustic directivity observed across all motions.

Peak noise increases with reduced frequency and Strouhal number.

Combined heaving and pitching produce less noise than single motions.

Abstract

A transient two-dimensional acoustic boundary element solver is coupled to a potential flow boundary element solver via Powell's acoustic analogy to determine the acoustic emission of isolated hydrofoils performing biologically-inspired motions. The flow-acoustic boundary element framework is validated against experimental and asymptotic solutions for the noise produced by canonical vortex-body interactions. The numerical framework then characterizes the noise production of an oscillating foil, which is a simple representation of a fish caudal fin. A rigid NACA 0012 hydrofoil is subjected to 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. A dipolar acoustic directivity is found for all motions, frequencies, and amplitudes considered, and the peak noise level increases with both the reduced frequency and the Strouhal number. A combined heaving and pitching motion produces less noise than either a purely pitching or purely heaving foil at a fixed reduced frequency and amplitude of motion. Correlations of the lift and power coefficients with the peak root-mean-square acoustic pressure levels are determined, which could be utilized to develop long-range, quiet swimmers.