When reading about flow separation I generally see a diagram like this:
This shows that the boundary layer slows as it passes over the wing, eventually creating an adverse pressure gradient. The effect being more pronounced close to the wing it will cause a reversal of flow and separation of the boundary layer.
I can easily see how this leads to flow separation, but I’m unsure as to what causes the boundary layer to slow down like this. I’ve read that the effect is independent of surface friction.
I know the air at the surface is stationary wrt the wing and its velocity increases the further away it gets. But if I look at the arrows in the diagram and select, say, the second one from the surface, it slows down as it goes along the wing, eventually reversing. At first I thought that the still air at the surface was somehow affecting it, but I don’t know how (shear stress?). But even at that, stationary air can not make it flow backwards.
So, what causes the flow closer to the surface to slow?