Generation of Tollmien-Schlichting waves by convecting gusts interacting with sound

A new mechanism is proposed for the generation of Tollmien-Schlichting (T-S ) waves by free-stream turbulence. For definiteness and self-consistency, t he mechanism is described mathematically by using a triple-deck formalism. The free-stream turbulence is represented by convecting gusts consisting of the so-called vortical and entropy waves of small amplitude. We show that suitable convecting gusts can interact with sound waves in the free stream to produce a forcing that has the same time and length scales as those of t he T-S waves, thereby exciting such waves in the vicinity of the lower bran ch of the neutral stability curve. The T-S waves so produced have the order of magnitude of epsilon(2)R(5/16), where epsilon is the amplitude of the f ree-stream disturbance and R the global Reynolds number. The scale conversi on is achieved without resorting to any non-homogeneity on the wall, and he nce the mechanism operates in a flat boundary layer. Furthermore, the T-S w aves so generated do not undergo any immediate decay, as they may do in som e other receptivity processes. For homogeneous isotropic free-stream turbul ence, the spectrum of the T-S waves is obtained. The efficiency of the rece ptivity mechanism is assessed by parametric studies.