How did Ludwig Prandtl suggest the condition known as the Prandtl Condition?
Prandtl condition: The Prandtl condition was suggested by the German physicist Ludwig Prandtl to identify possible boundary layer separation points of incompressible flows. Ludwig Prandtl:  In the 1920s he developed the mathematical basis for the fundamental principles of subsonic aerodynamics in particular; and in general up to and including transonic velocities.
In the 1920s he developed the mathematical basis for the fundamental principles of subsonic aerodynamics in particular; and in general up to and including transonic velocities.