Laminar boundary layer separation, fast and slow

It is well known classical boundary layer theory fails if pow separation oc curs. In the limit of large Reynolds number this failure can be avoided if the interaction between the viscous wall layer and the external inviscid re gion is accounted for. The form of the resulting interaction equations cruc ially depends on the route towards separation. If a firmly attached boundar y layer is separated by the action of a rapid pressure increase separation is governed by the triple deck: equations e.g. the classical boundary layer equations supplemented with appropriate matching and interaction condition s. However, if the approach towards separation is much slower the formation of a marginally separated flow region is described by a nonlinear integrod ifferential equation. Representative solutions of both sets of interaction equations indicate that boundary layer separation often is accompanied with a loss of uniqueness.