Structure formation in marginally separated aerodynamic and related boundary layer flows

The present study deals with near critical marginally separated and triple deck flows. Asymptotic analysis then shows that unsteady three-dimensional disturbances of a steady two-dimensional critical state are governed by a nonlinear diffusion equation of Fisher's type in both cases. Solutions may exhibit finite-time blow-up but it is found that they can be extended to larger times, nevertheless. This in turn leads to the formation of characteristic flow structures localized in space and time which are universal in the sense that they are essentially independent of the initial form of the disturbances.