EXPERIMENTS ON LOCALIZED DISTURBANCES IN A FLAT-PLATE BOUNDARY-LAYER - PART 2 - INTERACTION BETWEEN LOCALIZED DISTURBANCES AND TS-WAVES

Previous studies on boundary layer transition at moderate levels of fr ee stream turbulence (FST) have shown that the transition process can be promoted by the introduction of Tollmien-Schlichting (TS) waves. In the present work the interaction between localized boundary layer dis turbances and controlled TS-waves is studied experimentally. The local ized disturbances are generated either from a controlled free stream p erturbation, or by means of suction or injection through a slot in the hat plate surface. Both methods result in boundary layer disturbances dominated by elongated streamwise streaks of high and low velocity in the streamwise component. A strong interaction is observed preferably for high frequency TS-waves, which are damped when generated separate ly, and the interaction starts as a local amplification of a wide band of low-frequency oblique waves. The later stages of the transition pr ocess can be identified as a non-linear interaction between the obliqu e structures, leading to regeneration of new and stronger streamwise s treaks. (C) Elsevier, Paris.