Boundary-Layer Transition in the Presence of Streamwise Vortices

Results of an experimental investigation of instability leading to transition in the subsonic boundary-layer flow along a flat plate are presented. A series of wings was placed outside the boundary layer to produce streamwise vortices, which in turn made the boundary layer three-dimensional—i.e., periodic in thickness in the spanwise direction. Hot-wire measurements were made to trace the downstream development of the disturbance or wave created by the vibrating ribbon. As the wave travels downstream, it is deformed into a three-dimensional configuration by the three-dimensionality of the boundary-layer flow, but it is eventually damped out so long as it remains small in intensity. I t is only after the wave intensity exceeds a certain amount (which depends on the degree of boundary-layer threedimensionality) that the nonlinear effect manifests itself by the rapid amplification of wave intensity, the rapid increase in wave three-dimensionality, and the distortion in mean velocity profile. The appearance of nonlinear development inevitably leads to the breakdown of laminar flow, and hence the onset of turbulence. There is present a mechanism by which the energy is transferred from one spanwise position to another so that the breakdown of laminar flow occurs as a consequence of three-dimensional development of the wave as a whole.