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The role of instability in the growth of a 2D, temporally evolving,`turbulent' free shear layer is analyzed using vortex-gas simulations thatcondense all dynamics into the kinematics of the Biot-Savart relation. Theinitial evolution of perturbations in a constant-vorticity layer is found to bein accurate agreement with the linear stability theory of Rayleigh. There isthen a stage of non-universal evolution of coherent structures that is closelyapproximated not by Rayleigh stability theory, but by the Karman-Rubach-Lamblinear instability of monopoles, until the neighboring coherent structuresmerge. After several mergers, the layer evolves eventually to a self-preservingreverse cascade, characterized by a universal spread rate found bySuryanarayanan et al. (Phys.Rev.E 89, 013009, 2014) and a universal value ofthe ratio of dominant spacing of structures ($\Lambda_f$) to the layerthickness ($\delta_\omega$). In this universal, self-preserving state, thelocal amplification of perturbation amplitudes is accurately predicted byRayleigh theory for the locally existing `base' flow. The model of Morris etal. (Proc.Roy.Soc. A 431, 219-243, 1990.), which computes the growth of thelayer by balancing the energy lost by the mean flow with the energy gain of theperturbation modes (computed from an application of Rayleigh theory), is shown,however, to provide a non-universal asymptotic state with initial conditiondependent spread-rate and spectra. The reason is that the predictions of theRayleigh instability, for a flow regime with coherent structures, are validonly at the special value of $\Lambda_f/\delta_\omega$ achieved in theuniversal self-preserving state.

The role of the `monopole' instability in the evolution of 2D turbulent free shear layers