The way in which a flow is constrained by a solid surface is one important way to classify flows. - **Internal Flow:** Flow completely enclosed by a [[Solid|solid]]. e.g. flow though a pipe or nozzle. - **External Flow:** Flow over a body immersed in a [[Fluid|fluid]]. e.g. flow over an airplane wing (airfoil). - **Open-Channel Flow:** Flow in a channel which is exposed to a free surface (zero [[Shear Stress|shear]]). e.g. flow of a river.