Discussion

The results of our studies prove the concept we proposed: a boundary layer can be stabilized at Tollmien-Schlichting waves by artificially forcing streaks of well-controlled shape and, above all, amplitude. This stabilization of TS waves can delay the "classical" transition to turbulence in the boundary layer. A control protocol based on this type of approach is attractive for essentially two reasons:

• (a) the control is passive, i.e. it consumes no energy (the lift-up effect amplifies the actuators' energy);

• (b) the actuators manipulate the flow from upstream, so there's no need to place them throughout the unstable region.

Before this type of approach can be considered for industrial applications, a number of questions need to be answered: can streaks of higher amplitude...