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Spatially evolving vortex-gas turbulent free shear layers: Part 2. Coherent structure dynamics in vorticity and concentration fields

This paper examines the mechanisms of coherent structure interactions in spatially evolving turbulent free shear layers at different values of the velocity ratio parameter λ=$(U_1-U_2)/(U_1+U_2)$, where $U_1$ and $U_2 (\leq U_1)$ are the free stream velocities on either side of the layer. The study employs the point-vortex (or vortex-gas) model presented in part I (arXiv:1509.00603) which predicts spreading rates that are in the close neighborhood of results from most high Reynolds number experiments and 3D simulations. The present (2D) simulations show that the well-known steep-growth merger events among neighboring structures of nearly equal size (Brown & Roshko 1974) account for more than 70% of the overall growth at λ< 0.63. However the relative contribution of such 'hard merger' events decreases gradually with increasing λ, and accounts for only 27% of the total growth at the single-stream limit (λ = 1). It is shown that the rest of the contribution to layer growth is largely due to the increasing differential in size between neighboring structures as λ increases from 0.6 to 1.0, and takes place through two different routes. The first occurs via what may be called 'soft' mergers, involving extraction of little patches or filaments of vorticity over time from appreciably smaller (usually upstream) structures. This is consistent with earlier computational work on asymmetric two-vortex mergers (Yasuda & Flierl, 1995). The second route involves assimilation of disorganized 'vortex dust', which contains remnants of disrupted earlier structures not yet transferred to surviving ones. A combination of these processes is shown to cause an apparently nearly continuous increase in structure size with downstream distance at λ = 1, of the kind reported by D'Ovidio & Coats (2013)and attributed to mixing transition ...