Hi! I'm a bit confused about the source of drag in a 2D flow (actually I'm talking about air). I heard that if the viscosity of a fluid is 0 then there is no drag at all (no pressure and no friction drag). I think I understand why viscosity affects friction drag, but why does it have an effect on pressure drag? For example if we look at an airfoil, at the leading edge near the stagnation point there is a little overpressure because the air slows down near that point and this causes the pressure drag. I don't understand why this overpressure depends on the viscosity of the fluid. Thank you in advance!