Spatio-temporal boundary dissipation measurement in Taylor-Couette flow using Diffusing-Wave Spectroscopy

TL;DR

This paper introduces a novel method using Diffusing-Wave Spectroscopy combined with high-speed imaging to measure and visualize the spatio-temporal boundary dissipation rate in Taylor-Couette flow, validated against theoretical and global measurements.

Contribution

The study demonstrates a new technique for direct, time-resolved imaging of boundary dissipation rates in complex flows using DWS and speckle pattern analysis.

Findings

Quantitative imaging of boundary dissipation in Taylor-Couette flow.

Validation of the method against theoretical and global measurements.

Observation of spatio-temporal dynamics beyond flow instabilities.

Abstract

Diffusing-Wave Spectroscopy (DWS) allows for the direct measurement of the squared strain-rate tensor. When combined with commonly available high-speed cameras, we show that DWS gives direct access to the spatio-temporal variations of the viscous dissipation rate of a Newtonian fluid flow. The method is demonstrated using a Taylor-Couette (TC) cell filled with a lipid emulsion or a \ch{TiO2} suspension. We image the boundary dissipation rate in a quantitative and time-resolved fashion by shining coherent light at the experimental cell and measuring the local correlation time of the speckle pattern. The results are validated by comparison with the theoretical prediction for an ideal TC flow and with global measurements using a photomultiplier tube and a photon correlator. We illustrate the method by characterizing the spatial organization of the boundary dissipation rate past the Taylor-Couette instability threshold, and its spatio-temporal dynamics in the wavy vortex flow that arises beyond a secondary instability threshold. This study paves the way for direct imaging of the dissipation rate in a large variety of flows, including turbulent ones.