In fluid mechanics, external flow is a flow that boundary layers develop freely, without constraints imposed by adjacent surfaces.
[1][2] It can be defined as the flow of a fluid around a body that is completely submerged in it.
Examples include fluid motion over a flat plate (inclined or parallel to the free stream velocity) and flow over curved surfaces such as a sphere, cylinder, airfoil, or turbine blade, water flowing around submarines, and air flowing around a truck;[3] a 2000 paper analyzing the latter used computational fluid dynamics to model the three-dimensional flow structure and pressure distribution on the external surface of the truck.
[3] In a 2008 paper, external flow was said to be "arguably is the most common and best studied case in soft matter systems.
[4] The term can also be used simply to describe flow in any body of fluid external to the system under consideration.