  In this paper, the solutions of Navier-Stokes equations with Dirichlet
boundary conditions governing 2-D incompressible fluid flows are considered. A
condition for boundary layer separation, which is determined by initial values
and external forces, is obtained. More importantly, the condition can predict
directly when and where boundary layer separation will occur. The main
technical tool is geometric theory of incompressible flows developed by T. Ma
and S.Wang.
