Helioseismology in a bottle: modal acoustic velocimetry

TL;DR

This paper introduces a novel laboratory technique called modal acoustic velocimetry, inspired by helioseismology, to measure the internal flow of rotating fluids in spherical cavities, especially useful for opaque liquids like metals.

Contribution

It demonstrates the first application of helioseismology-inspired acoustic mode analysis to determine internal flow profiles in a laboratory setting.

Findings

Successfully measured azimuthal velocity profiles using acoustic mode splittings.

Validated the method against hot film anemometer measurements.

Showed potential for studying opaque rotating fluids in laboratory experiments.

Abstract

Measurement of the differential rotation of the Sun's interior is one of the great achievements of helioseismology, providing important constraints for stellar physics. The technique relies on observing and analyzing rotationally-induced splittings of p-modes in the star. Here we demonstrate the first use of the technique in a laboratory setting. We apply it in a spherical cavity with a spinning central core (spherical-Couette flow) to determine the mean azimuthal velocity of the air filling the cavity. We excite a number of acoustic resonances (analogous to p-modes in the Sun) using a speaker and record the response with an array of small microphones on the outer sphere. Many observed acoustic modes show rotationally-induced splittings, which allow us to perform an inversion to determine the air's azimuthal velocity as a function of both radius and latitude. We validate the method by comparing the velocity field obtained through inversion against the velocity profile measured with a calibrated hot film anemometer. This 'modal acoustic velocimetry' technique has great potential for laboratory setups involving rotating fluids in axisymmetric cavities. It will be useful especially in liquid metals where direct optical methods are unsuitable and ultrasonic techniques very challenging at best.