Hello all, I'm trying to nail down why the viscosity of air helps it bend around a curve. I have a specific, simplified example in mind to cut away other phenomena. Imagine the region immediately behind the camber of the top of a typical airfoil. That is, the place where the slope of the airfoil first becomes negative. Why do the air streams bend down to follow the surface. I know that it is due to a pressure differential between the streams in the boundary layer, but what causes this pressure differential? It cannot simply be the wake effect (as the foil moves forward, the region immediately behind the curve is deprived of air since it was where the win "used to be.") Clearly, such an effect occurs regardless of viscosity. The instantaneous absence of air in the wake cannot be the sole reason as evidenced by the turbulence and separation of boundary layer found in the wake of a sphere. So, I'm trying to understand what it is about the viscosity of air that causes a pressure differential that (in term) allows this bending to occur. A separate, but related question, I can see how the pressure differential allows for "bending" of air streams around a surface, but the literature I have read suggests this pressure differential is required even after a bend to keep air streams parallel to a surface, or else the layers separate. Why is that necessary? If two air streams are parallel to one another, why is any pressure gradient needed to keep them from separating? I suspect it has something to do with one stream having a different velocity than the other but would appreciate a clear description.