Mixing by internal waves quantified using combined PIV/PLIF technique

TL;DR

This paper introduces a combined PIV/PLIF technique to quantitatively measure internal wave-induced mixing and turbulent diffusivity in stratified fluids, demonstrating the role of Triadic Resonance Instability in enhancing mixing.

Contribution

It presents the first quantitative implementation of combined PIV/PLIF in continuously stratified fluids and validates the method through TRI characterization, enabling precise mixing measurements.

Findings

Internal waves cause measurable turbulent diffusivity up to 15 mm^2/s.

Triadic Resonance Instability significantly enhances mixing in stratified flows.

The combined PIV/PLIF technique accurately captures velocity and density fields in stratified fluids.

Abstract

We present a novel characterization of mixing events associated with the propagation and overturning of internal waves, studied thanks to the simultaneous use of Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) techniques. This combination of techniques had been developed earlier to provide an access to simultaneous velocity and density fields in two-layer stratified flows with interfacial gravity waves. Here, for the first time, we show how it is possible to implement it quantitatively in the case of a continuously stratified fluid where internal waves propagate in the bulk. We explain in details how the calibration of the PLIF data is performed by an iterative procedure, and we describe the precise spatial and temporal synchronizations of the PIV and PLIF measurements. We then validate the whole procedure by characterizing the Triadic Resonance Instability (TRI) of an internal wave mode. Very interestingly, the combined technique is then applied to a {precise} measurement of the turbulent diffusivity K_t associated with mixing events induced by an internal wave mode. Values up to K_t=15 mm^2\cdot s^{-1} are reached when TRI is present (well above the noise of our measurement, typically 1 mm^2\cdot s^{-1}), unambiguously confirming that TRI is a potential pathway to turbulent mixing in stratified flows. This work therefore provides a step on the path to new measurements for internal waves.