Experimental study of the initial growth of a localized turbulent patch in a stably stratified fluid

We present a laboratory experiment of the growth of a turbulent patch in a stably stratified fluid, due to a localized source of turbulence, generated by an oscillating grid. Synchronized and overlapping particle image velocimetry and planar laser induced fluorescence measurements have been conducted capturing the evolution of the patch through its initial growth until it reached a maximum size, followed by its collapse. The simultaneous measurements of density and velocity fields allow for a direct quantification of the degree of mixing within the patch, the propagation speed of the turbulent/non-turbulent interface and its thickness. The velocity measurements indicate significant non-equilibrium effects inside the patch which are not consistent with the classical used grid-action model. A local analysis of the turbulent/non-turbulent interface provides direct measurements of the entrainment velocity $w_{e}$ as compared to the local vertical velocity and turbulent intensity at the proximity of the interface. It is found that the entrainment rate $E$ is constrained in the range of $0{\div}0.1$ and that the local, gradient Richardson number at the interface is $\mathcal{O} (100)$. Finally, we show that the mean flow is responsible for the patch collapse.