Experimental analysis of transition delay by means of roughness elements

An experimental activity aimed at the development of a passive method for viscous drag reduction in an aerodynamic body is presented. The reduction is achieved by delaying the laminar-turbulent transition of the boundary layer. The research is motivated by previous linear stability analysis where it was demonstrated that stable streaks in a boundary layer are capable to stabilize the growth of Tollmien-Schlichting waves. In this paper the results of a series of experiments on a flat plate are presented. Firstly, it is 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. Secondly, it is demonstrated that the generated streaks are really effective in reducing the growth of the Tollmien-Schlichting waves as predicted by the theory. Finally, it is shown that using this passive control technique it is possible to sensibly delay transition to turbulence