  The relevance of the vortex-gas model to the large scale dynamics of
temporally evolving turbulent free shear layers in an inviscid incompressible
fluid has recently been established by extensive numerical simulations
(Suryanarayanan et al, Phys. Rev. E 89, 013009, 2014). Here, the effects of the
velocity ratio across a spatially evolving 2D free shear layer are investigated
by vortex-gas simulations, using a computational model based on Basu et al
(1992, 1995), but with a crucial improvement that ensures conservation of
global circulation. These are carried out for a range of values of the velocity
ratio parameter $\lambda= (U_1-U_2)/(U_1+U_2)$, where $U_1$ and $U_2$ ($< U_1$)
are respective velocities across the layer. The simulations show that the
conditions imposed at the beginning of the free shear layer and at the exit to
the domain can affect the flow evolution in their respective neighborhoods, the
latter being particularly strong as $\lambda \rightarrow 1$. In between the two
neighborhoods is a regime of universal self-preserving growth rate given by a
universal function of $\lambda$ The computed growth rates are located within
the scatter of experimental data on plane mixing layers, and in close agreement
with recent high Reynolds number experiments and 3D LES studies, past the
mixing transition, and support the view that free shear layer growth can be
largely explained by the 2D vortex dynamics of the quasi-two-dimensional large
scale structures.
