Acoustic spectra of a gas-filled rotating spheroid

TL;DR

This study demonstrates that acoustic spectra can be used to measure internal flow velocities in opaque, rotating fluids by identifying and analyzing mode frequencies, including rotational splitting, through experiments and theoretical modeling.

Contribution

It introduces a comprehensive method combining experiments, finite-element calculations, and symmetry analysis to retrieve rotational effects from acoustic spectra in a rotating spheroid.

Findings

Successful measurement of Coriolis effects via acoustic spectra

First experimental determination of rotational splitting coefficients for multiple modes

Validation of acoustic modal analysis as a non-intrusive velocimetry technique

Abstract

The acoustic spectrum of a gas-filled resonating cavity can be used to indirectly probe its internal velocity field. This unconventional velocimetry method is particularly interesting for opaque fluid or rapidly rotating flows, which cannot be imaged with standard methods. This requires to (i) identify a large enough number of acoustic modes, (ii) accurately measure their frequencies, and (iii) compare with theoretical synthetic spectra. Relying on a dedicated experiment, an air-filled rotating spheroid of moderate ellipticity, our study addresses these three challenges. To do so, we use a comprehensive theoretical framework, together with finite-element calculations, and consider symmetry arguments. We show that the effects of the Coriolis force can be successfully retrieved through our acoustic measurements, providing the first experimental measurements of the rotational splitting (or Ledoux) coefficients for a large collection of modes. Our results pave the way for the modal acoustic velocimetry to be a robust, versatile, and non-intrusive method for mapping large-scale flows. Pages: 1-14