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

Mixing by internal waves quantified using combined PIV/PLIF technique