It is difficult to give a rigorous definition of a separated flow, but it is relatively easy to learn how to distinguishing it from an attached flow. Just look at the 15 videos. Notice the main features of separated flows:
1. Fluid particles leave the vicinity of the surface at the separation point.
2. In the eddy the velocity is small.
3. In the eddy the fluid moves backwards.
Video 1. Separated flow past an aerofoil. Notice the flow reversal in the eddy, small value of the velocity in the eddy, and how the fluid particles leave the vicinity of the surface at the separation point.
Video 2. Attached flow past thin Joukowski aerofoil. Compare with Video 3.
Video 3. Separated flow past thick Joukowski aerofoil. Compare with Video 2.
Video 4. Attached flow in a confusor. Notice that the flow accelerates. Compare with Video 5.
Video 5. Separated flow in a diffusor. Notice that in the absence of separation the flow would decelerate. Compare with Video 4.
Video 6. Separated flow past a forward-facing step. Notice the separation eddy upstream of the step.
Video 7. This flow has a section where the fluid flows backwards, but this flow is not separated.
Video 8. Separated flow past a pentagon.
Video 9. Flow past a randomly selected object almost always is separated. The object in the movie is not an exception.
Video 10. Karman vortex street in the flow past a flat plate. Compare with Video 11 and notice which has a wider wake (hence greater drag). Check the enrichment lesson, which also uses this video.
Video 11. Karman vortex street in the flow past a semi-circle. Compare with Video 10 and notice which has a wider wake (hence greater drag).
Video 13. Separated flow past a circular cylinder at a medium Reynolds number. After a transient it becomes steady.
Video 14. Development of a Karman vortex street in a flow past a circular cylinder and a moderate Reynolds number.
Video 15. Marginal separation near the leading edge of an aerofoil. Notice the small size of the separation bubble, the loss of stability in the separated shear layer due to Kelvin-Helmholtz instability, and the reattachment of the boundary layer.