5. Transition period

After the first two experimental campaigns in 2003 and 2004, it was time to ask the essential question: can we delay the transition to turbulence in the "classical" scenario by forcing streaks into the boundary layer? The fact that linear Tollmien-Schlichting waves are stabilized by streaks is a necessary condition for controllability, but not a sufficient one. The transition itself is triggered by the secondary instability of TS waves. In the absence of streaks, this secondary instability manifests itself when the rms value of the TS waves reaches an amplitude of the order of 1% U∞. There is no a priori guarantee that the presence of streaks does not significantly lower this secondary threshold by compensating for the effect of linear stabilization, and that therefore region (3) in figure