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In [[fluid mechanics]], '''external flow''' is a flow that [[boundary layer]]s develop freely, without constraints imposed by adjacent surfaces.<ref name="AIP"/><ref name="dynamics"/> Accordingly, there will always exist a region of the flow outside the boundary layer in which velocity, temperature, and/or [[concentration gradient]]s are negligible. 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]], airwater flowing around an airplanesubmarines, and waterair flowing around submarinesa truck;<ref name="truck"/> a 2000 paper analyzing the latter used [[computational flow dynamics]] to model the three-dimensional flow structure and pressure distribution on the external surface of the truck.<ref name="truck"/> In a 2008 paper, external flow was said to be "arguably is the most common and best studied case in [[soft matter systems]].<ref name="stochastic"/>
The term can also be used simply to describe flow in any body of fluid external to the system under consideration.<ref name="external"/><ref name="asme"/>
