INTERACTION OF OBLIQUE INSTABILITY WAVES WITH WEAK STREAMWISE VORTICES

This paper is concerned with the effect of a weak spanwise-variable me an-flow distortion on the growth of oblique instability waves in a Bla sius boundary layer. The streamwise component of the distortion veloci ty initially grows linearly with increasing streamwise distance, reach es a maximum, and eventually decays through the action of viscosity. T his decay occurs slowly and allows the distortion to destabilize the B lasius flow over a relatively large streamwise region. It is shown tha t even relatively weak distortions can cause certain oblique Rayleigh instability waves to grow much faster than the usual two-dimensional T ollmien-Schlichting waves that would be the dominant instability modes in the absence of the distortion. The oblique instability waves can t hen become large enough to interact nonlinearly within a common critic al layer. It is shown that the common amplitude of the interacting obl ique waves is governed by the amplitude evolution equation derived in Goldstein & Choi (1989). The implications of these results for Klebano ff-type transition are discussed.