  A central but controversial issue in free turbulent shear flows has been the
universality (or otherwise) of their growth rates. We resolve this issue here
in the special case of a temporal 2D mixing layer in a point vortex gas by
extensive high-precision numerical simulations, utilizing for the first time a
powerful ensemble-averaging strategy. The simulations show that the momentum
thickness of such a mixing layer grows at the universal asymptotic rate of
0.0167 +/- 0.00017 times the velocity differential across the layer over a wide
range of initial conditions, often after very long transients.
