An Experimental Study of Boundary-Layer Transition

Transition on a smooth flat plate in a zero pressure gradient was studied in a wind tunnel using conventional surface-tube and hot-wire techniques. In the range of free-stream turbulence used (0.1% to 0.5%) it is shown that the laminar oscillations predicted by the Tollmien-Schlichting theory of laminar-layer stability play an important role in transition. The process of transition in this case is shown to be similar to the development of a turbulent wake from a vortex street, the amplified frequency feeding the rest of the spectrum. The effect of free-stream turbulence on the process was studied in terms of the energy concentrated at the amplified frequency. As anticipated, free-stream turbulence hastens the feeding of energy from the amplified frequency to the rest of the spectrum. It is indicated that, at a much higher level of free-stream turbulence than used in these t ests, laminar oscillations will play no part in transition and, possibly, the phenomenon is controlled by the laminar-separation theory proposed by G. I. Taylor.