Data on turbulent mixing and other turbulent-flow phenomena suggest that a
(mixing) transition, originally documented to occur in shear layers, also o
ccurs in jets, as well as in other flows and may be regarded as a universal
phenomenon of turbulence. The resulting fully-developed turbulent flow req
uires an outer-scale Reynolds number of Re = U delta/nu greater than or sim
ilar to 1-2 x 10(4), or a Taylor Reynolds number of Re-T = mu' lambda(T) /
nu greater than or similar to 100-140, to be sustained. A proposal based on
the relative magnitude of dimensional spatial scales is offered to explain
this behaviour.