Why is the pressure on the inside of air flow curvature considered be low pressure and it is considered to be high pressure above? I read and seen videos that this is the case but I want to know why. Is it passed on the fact that usually a flow turn is caused by something and the pressure above is greater than that press below it?
Far away from the airfoil, pressure is atmospheric. What the pressure on airfoil surface is depends on both flow velocity there and the magnitude of curvature, among other factors. So if flow conditions and geometry are not identical on both sides of the airfoil, different pressures can develop on the two sides.