Potential flow obeys Laplace's equation with certain boundary conditions (i.e. no fluid penetrates the solid body in flow, and far away from the body, the flow is uniform with a given velocity and pressure).
So let's condider the potential flow around a cylinder. After the fluid moves to the "top" of the cylinder (above the "+" in the image) it then bends around the surface instead of continuing in a straight line.
Why does it bend and not continue in a straight line from the point before the bending?
Since the flow is potential (no viscosity) the only thing causing the fluid to bend around can be the pressure field, but im not sure how this works.
EDIT: Thanks to the first comment I do realize that at the "top" of the cylinder surface there is a pressure gradient normal to the flow (i.e. lower pressure nearer the surface) which would cause flow turning at that point. But how is this pressure gradient established in the first place? It seems like one is arguing that the flow bends because of the pressure gradient, but there exists a pressure gradient only because the flow bends... Am I missing something here?