Influence of streamwise structures on the development of traveling waves.

For the first time, investigations have been performed on the influence of stationary (zero-frequency) streamwise (streaky) structures on traveling instability waves in the framework of the theory of three-wave resonance. Computations have been conducted for a two-dimensional boundary-layer at a free-stream Mach number M=2 and for a three-dimensional swept-wing boundary-layer at M=4. Usually, streamwise stationary structures, which are periodic in the spanwise direction, lead to faster growth of unsteady waves than their linear amplification. This facilitates an earlier laminar-turbulent transition. However, the performed investigation reveals a principal possibility of reducing linear growth rates of most unstable nonstationary perturbations when stationary vortices of sufficiently large amplitude are introduced into the flow. This is realizable at a certain ("optimal") phase ratio of waves in the resonant triplet. [ABSTRACT FROM AUTHOR]