Hydroacoustic Absorption and Amplification by Turbulence

TL;DR

This study experimentally demonstrates that underwater turbulence can significantly absorb or amplify acoustic waves at high frequencies, revealing a new, poorly understood interaction mechanism.

Contribution

It provides the first experimental evidence of turbulence-induced acoustic amplification and absorption at frequencies far above turbulent fluctuation frequencies, challenging existing theories.

Findings

Acoustic signals can be attenuated or amplified by over 60% without spectral broadening.

Amplification depends on wave frequency, not amplitude.

Turbulence affects wave amplitude even without mean flow, unlike laminar flow.

Abstract

Acoustic waves propagating through fluid media are significantly influenced by turbulence. This paper experimentally investigates the influence of underwater turbulence on the propagation characteristics of acoustic waves, revealing that acoustic waves can be absorbed or amplified at frequencies far exceeding the turbulent fluctuation frequency. The maximum observed attenuation or amplification of received signals exceeds 60%, with no spectral broadening. The amplification factor depends on the wave frequency rather than its amplitude. The study covers two flow conditions: pipe flow and free jet, driven by either a pump or hydraulic head difference. The frequency range generated by the hydroacoustic transducers covers 60 kHz to 4.4 MHz, while the wave propagation directions both parallel and perpendicular to the mean flow are considered. For each case, the amplitudes of all frequency components simultaneously decreases or increases under turbulence, with no new spectral components appearing. Turbulent fluctuations without mean motion can still alter the wave amplitude, while laminar flow has no effect on acoustic signals. Comparison with conventional theories and experiments indicates that mechanisms such as bubbles, resonance, scattering, or viscous dissipation cannot explain the observed phenomena. This indicates that there exists an incompletely understood new mechanism in the interaction between turbulence and acoustic waves.