Enhancement of the temporal growth rate of inviscid three-dimensional
instability
waves in free shear layers by deformation of the basic flow is studied.
The deformation
of a two-dimensional mixing layer is assumed to yield a base flow that
remains
unidirectional, but has a steady spanwise speed variation in addition to
the two-
dimensional shear. The computed growth rates for hyperbolic tangent base
flow,
perturbed this way, show enhanced instability in the sense that the neutral
waves
of the unperturbed flow exhibit positive growth rates. For each imposed
spanwise
periodicity, an oblique mode is selected that shows maximum growth rate.
The results
are consistent with related theoretical studies and with qualitative observations
in
experiments.