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I came across this image:
My question is: given a general Cp plot, how does one tell from the shape of the curve if flow transitions or separates? Specifically, what features on the plot do I look out for that can tell me if flow transitions or separates (e.g. inflection points, plateaus, etc)
I know that a strong adverse pressure gradient causes separation which shows as a steep downward slope on the graph. What I don't know is where exactly on the graph transition or separation happens.
Separation happens at the separation point, when the steep pressure gradient suddenly changes to a plateau, and transition happens at the transition point when the plateau changes back to a steep pressure gradient. Reattachment is a little harder to discern: Here the slope of the pressure gradient, which is quite steep right past the transition point, changes back to something close to its inviscid value.
It helps to plot the inviscid pressure distribution (dashed lines) together with the "real", viscous one (solid lines), as shown below for an Eppler 502 at 3° angle of attack:
The blue line shows the pressure coefficient on the upper side and the red one on the lower side. On both sides there are laminar separation bubbles, but the one on the bottom is much bigger, so I will focus on this one. In a way, the plot lines, or more precisely the difference between inviscid and viscous pressures, mirror the behaviour of the boundary layer: With the separation the boundary layer thickens as the outer flow lines move away from the wing contour. With the transition the flow lines bend towards the contour again and at the reattachment they continue to follow the contour.
Detail view of the plot above.
