EXPERIMENTS IN A BOUNDARY-LAYER SUBJECTED TO FREE-STREAM TURBULENCE .2. THE ROLE OF TS WAVES IN THE TRANSITION PROCESS

The natural occurrence of Tollmien-Schlichting (TS) waves has so far o nly been observed in boundary layers subjected to moderate levels of f ree stream turbulence (Tu < 1%), owing to the difficulty in detecting small-amplitude waves in highly perturbed boundary layers. By introduc ing controlled oscillations with a vibrating ribbon, it is possible to study small-amplitude waves using phase-selective filtering technique s. In the present work, the effect of TS-waves on the transition is st udied at Tu = 1.5%. It is demonstrated that TS-waves can exist and dev elop in a similar way as in an undisturbed boundary layer. It is also found that TS-waves with quite small amplitudes are involved in nonlin ear interactions which lead to a regeneration of TS-waves in the whole unstable frequency band. This results in a significant increase in th e number of turbulent spots, which promote the onset of turbulence.