Organised large structure in the post-transition mixing layer. Part 3. Dynamics of the spatial growth

The first two parts of this study showed by experiment and by large-eddy simulation that, in mixing layers formed of processions of spanwise-oriented vortex structures, there is a fundamental change in the dynamics of the large-structure growth at what has been called the ‘mixing’ transition. This third part examines the relationship of the post-transition ‘organised’ flow to the mixing layer of classical turbulence theory. Further large-eddy simulations are presented which, like some of the experiments reported in Part 1, have captured the mixing layer in both its classical and organised turbulent states, thereby allowing them to be characterised separately. The simulation results are then used to calibrate model-free integral analyses of the dynamics of the mixing layer's spatial growth. It is shown that the organised and classical flows are alternative self-preserving turbulent flow states involving fundamentally different exchanges of mass, momentum and energy, one or other of which emerges naturally in particular realisations depending on the initial conditions.