The results of an experimental investigation of the instability of a laminar boundary layer leading to transition are presented. Waves were introduced into the boundary layer of a flat plate using the vibrating-ribbon technique, and their development and subsequent breakdown were studied using the hot-wire anemometer as the principal measuring device. Certain significant features of the wave behavior have been revealed. The motions leading to transition are strongly three dimensional, and the nature of this three-dimensionality has been investigated. The effect of the wave on the mean flow and its behavior in relation to the Tollmien-Schlichting theory were also studied. It is demonstrated that associated with the wave growth there is present an energy-concentrating mechanism involving the transfer of wave energy from one spanwise position to another resulting in street like concentrations of wave energy. It is also shown that the initial breakdown of laminar flow is of small extent and may be loosely described as point like. Breakdown of the laminar flow occurs in the outer region of the layer, and intermittent separation was not involved in the transition process described herein.
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