On a new form of marginal separation

Incompressible high Reynolds number flows past a slender airfoil have the distinguishing property that there exists a critical value of the angle of attack where the wall shear stress on the suction side vanishes in a single point but immediately recovers. This phenomenon is now commonly refered to as marginal separation. In the present study it is shown that a laminar boundary layer on a flat plate subjected to a linearly retarded external flow may experience a similar behaviour if the molecular complexity of the medium is sufficiently large. The role of the critical angle of attack is played by the critical value of the freestream density ρ c ∞ on a given isentrope. As in the case of a slender airfoil it is possible to construct an asymptotic theory which allows to calculate small separation bubbles which form if ρ ∞ becomes smaller than ρ c ∞ .