A condition that occurs at sufficiently high Reynolds numbers in which the surface streamlines break away from the surface.
Separation is due to the presence of a solid boundary, at which the no-slip condition—that is, the velocity of the fluid particles in contact with the surface is the velocity of that surface—is satisfied and vorticity is generated. Separation of a steady boundary layer at a plane or rounded rigid wall occurs whenever the velocity of the fluid just outside of the boundary layer decreases in the mean flow direction sufficiently rapidly and by a sufficient amount. This can be accomplished by the imposition of an opposing pressure gradient in the direction of flow.
Batchelor, G. K. 1967. Fluid Dynamics. Cambridge University Press, . p. 325.
