Experimental and Numerical Methods for Transition and Drag Predictions of Laminar Airfoils

The friction component is responsible for more than 40% of typical civil aircraft drag. As a consequence, the issue of laminar flow has been of prime importance in aeronautics for many years now. This article is focused on Tollmien-Schlichting-induced transition and drag predictions of two-dimensional laminar airfoils obtained with experimental and numerical methods. In 2012, a test campaign in the ONERA-S2MA wind tunnel, including infrared acquisitions, pressure sensors, and wake analyses, allowed substantial data to be obtained on such airfoils in transonic conditions. To complete this study, two-dimensional fluid dynamics computations have been performed, either with a Reynolds-averaged Navier-Stokes solver using transition criteria or with a boundary-layer code combined with direct stability analysis. Furthermore, experimental (wake survey) and numerical (far-field theory) techniques allowing airfoil drag breakdown have been employed. Wind-tunnel and computational fluid dynamics transition predictions have been compared. Good agreement has been observed but the transition criteria may show some limitations in particular situations, such as long separation bubble development. The gains in lift and drag due to laminar flow have been quantified (natural vs triggered transition). Concerning drag reduction, the importance of the viscous pressure component has been highlighted. Finally, the effects of parameters such as angle of attack, Mach number, and Reynolds number on transition location and drag have been investigated.