Particle image velocimetry studies of an incipient spot in the Blasius boundary layer

Spatial evolution of a small amplitude localized disturbance introduced into the laminar boundary layer of a flat plate has been studied experimentally using the particle image velocimetry (PIV) technique. PIV data have been acquired in the spanwise and wall normal planes. Long and well defined high and low speed streaks are seen in the spanwise plane. The number of streaks are found to increase in the downstream direction. Breathing mode type oscillation of the boundary layer is observed. Associated with the streaks and akin to the bypass transition, 'backward' and 'forward' jet like structures of the fluctuating velocity components are observed.