Laminar boundary-layer separation near the leading edge of a thin airfoil is one of the principal factors that limits the lift force acting on the airfoil. The onset of separation is accompanied by the formation of a short separation bubble inside the boundary layer and does not affect the outer flow in the limit of high Reynolds number.
This commonly as marginal separation denoted phenomenon is studied for transonic external flow conditions and high Reynolds number as well as ideal gas flow and thermally insulated walls of the airfoil under consideration. The interaction between the boundary-layer and the external flow, which is a vital aspect of marginal separation, manifests itself in a triple deck structure of the flow. This strucure is studied using the method of matched asymptotic expansions. As in the case of marginal separation in incompressible flow it is found that the wall shear stress distribution in the portion of the boundary-layer, which is affected by the interaction, is connected to the induced pressure gradient via an integral equation. A numerical scheme for solving this equation is presented and wall shear stress distributions depending on the angle of attack of the airfoil under consideration are calculated. This scheme also offers the possibility to study the effect of small obstacles, which are situated in the portion of the outer flow affected by the interaction, on the wall shear stress distribution. When calculating this effect it is found that small obstacles can be effectively used to control the onset of marginal flow separation.