High amplitude streaks in boundary layers: a new passive mechanism for transition delay

This work is part of an experimental activity aimed at developing a passive control method for viscous drag reduction of aerodynamic bodies. The drag reduction is obtained by delaying the laminar-turbulent transition of the boundary layer in which strong stable streamwise streaks are generated. These streaks are indeed successful in stabilizing the growth of Tollmien-Schlichting waves as well as localized disturbances. A first series of experiments have shown that by means of suitable circular roughness elements it was possible to generate stable steady streaks of amplitudes up to 12% of the free stream velocity. Here we explore the possibility to generate streaks of much larger amplitude by using a row of Miniature Vortex Generators (MVGs) similar to those used to prevent separation. Experiments show that amplitudes larger than 30% can be obtained without having any secondary instability acting on the streaks. The increase of drag due to the streaky base flow is also quantified. Finally, it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one.