High amplitude steady streaks in flat plate boundary layers

This experimental activity is part of a large research program aimed at developing a passive control method for viscous drag reduction of aerodynamic bodies. The drag reduction, accomplished by delaying the laminar-turbulent transition of the boundary layer, is obtained by generating stable laminar streamwise streaks inside the boundary layer, which are known to stabilize the growth of Tollmien-Schlichting waves as well as localized disturbances. A first series of experiments have shown that by means of suitable roughness elements it is possible to generate stable steady streaks of amplitudes up to 12% of the free stream velocity. In this paper we explore the possibility to generate streaks of much larger amplitude by using a row of Miniature Vortex Generators (MVGs). Experiments have been performed showing that amplitudes up to 30% can be obtained without having any secondary instability acting on the streaks. The induced drag due to the streaky base flow is quantified and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one.