Wave forerunners of streamwise structures in a swept wing boundary layer

In a model experiment wave packets (forerunners) have been detected for the first time in the flow regions preceding the fronts of streamwise structures in a swept wing boundary layer. The characteristics of the wave packets and the generating streamwise structures and the dynamics of their downstream development are investigated. It is shown that the forerunners can transform into turbulent spots thus leading to laminar-turbulent transition. Certain components of the forerunners are compared with a periodic instability wave.