Enhancement of three-dimensional instability of free shear layers

Enhancement of the temporal growth rate of inviscid three-dimensional insta bility waves in free shear layers by deformation of the basic flow is studi ed. The deformation of a two-dimensional mixing layer is assumed to yield a base flow that remains unidirectional, but has a steady spanwise speed var iation in addition to the two-dimensional shear. The computed growth rates for hyperbolic tangent base flow, perturbed this way, show enhanced instabi lity in the sense that the neutral waves of the unperturbed flow exhibit po sitive growth rates. For each imposed spanwise periodicity, an oblique mode is selected that shows maximum growth rate. The results are consistent wit h related theoretical studies and with qualitative observations in experime nts.