Abstract
We analyse transition mechanism and visualize vortical structure in the transition regions of the Blasius boundary layer directly simulated on a parallel-super computer. The simulation started with a initial flow given as the basic flow plus small disturbances. When the disturbance amplitude is smaller, the transition is induced by the well-known Tollmien-Schlichting wave instability. In this case. A vortices induced by the nonlinear mechanism appear and produce peak-valley structures on the Tollmien-Schlichting wave, which have been observed in the experiments. On the other case starting with a larger disturbance amplitude than the threshold value, transient growth of streamwise vortices becomes to be dominant. Then, the amplified streamwise vortices collapse to three-dimensional vortices by the nonlinear mechanism.
