Journal of Conference Abstracts

The Life Cycle of a Stratified Shear Layer

Colm-cille Caulfield (c.p.caulfield@bris.ac.uk)¹ & Richard Peltier (peltier@atmosp.physics.utoronto.ca)²

¹School of Mathematics, University of Bristol, University Walk Bristol, BS8 1TW, U.K.

²Dept of Physics, University of Toronto, 60 St George St Toronto, ON M5S 1A7, Canada

We investigate the detailed nature of the "mixing transition" through which intense turbulence may develop in stratified free shear layers. We explicitly quantify the time-evolving irreversible mixing which occurs within the flow, which requires us to consider in detail the evolution of streamwise vortical streaks, which develop once the primary Kelvin-Helmholtz billow saturates.

Using the numerical data from a sequence of three-dimensional simulations with varying stratification, we accurately track the nonlinear amplification of these intermediate coherent structures, verifying that they are well-predicted by secondary stability analysis, and are due to a convective destabilization of the periphery of a Kelvin-Helmholtz billow. At all times we calculate the minimal potential energy of the system accessible by (notional) adiabatic rearrangement of fluid parcels, and so quantify continuously the irreversible "mixing".

The nonlinear amplification of the streamwise vortices is driven principally by the mean shear. Vortex stretching leads eventually to a violent subcritical vortex-vortex collision which drives the dominant mixing process in the flow life cycle. An appropriate definition of the "mixing efficiency" implies that the irreversible small-scale mixing of the density which is triggered by shear layer transition leads inevitably to a density "staircase", with regions of well-mixed fluid separated by narrow regions of relatively strong density gradient.