* Two or more vortices that are approximately parallel and circulating in the same direction will quickly merge to form a single vortex. The [[Circulation (fluid dynamics)|circulation]] of the merged vortex will equal the sum of the [[Circulation (fluid dynamics)|circulations]] of the constituent vortices. For example, a sheet of small vortices flows from the trailing edge of the wing or propeller of an airplane when the wing is developing [[Lift (force)|lift]] or the propeller is developing [[thrust]]. In less than one wing [[Chord (aircraft)|chord]] downstream of the trailing edge of the wing these small vortices merge to form a single vortex. If viewed from the tail of the airplane, looking forward in the direction of flight, there is one [[Wingtip vortices|wingtip vortex]] trailing from the left-hand wing and circulating clockwise, and another wingtip vortex trailing from the right-hand wing and circulating anti-clockwise. The result is a region of downwash behind the wing, between the pair of [[wingtip vortices]]. These two [[wingtip vortices]] do not merge because they are circulating in opposite directions. | * Two or more vortices that are approximately parallel and circulating in the same direction will quickly merge to form a single vortex. The [[Circulation (fluid dynamics)|circulation]] of the merged vortex will equal the sum of the [[Circulation (fluid dynamics)|circulations]] of the constituent vortices. For example, a sheet of small vortices flows from the trailing edge of the wing or propeller of an airplane when the wing is developing [[Lift (force)|lift]] or the propeller is developing [[thrust]]. In less than one wing [[Chord (aircraft)|chord]] downstream of the trailing edge of the wing these small vortices merge to form a single vortex. If viewed from the tail of the airplane, looking forward in the direction of flight, there is one [[Wingtip vortices|wingtip vortex]] trailing from the left-hand wing and circulating clockwise, and another wingtip vortex trailing from the right-hand wing and circulating anti-clockwise. The result is a region of downwash behind the wing, between the pair of [[wingtip vortices]]. These two [[wingtip vortices]] do not merge because they are circulating in opposite directions. |