Flow over a swept wing behaves very differently than a straight fixed wing or an airfoil. Instead of flowing straight along the chord of the wing in a two-dimensional fashion, air is also directed along the wing, parallel to the leading edge. The above oil flow visualization on a swept wing airplane model shows this curvature of streamlines. As a result of this three-dimensional flow behavior, boundary layers on swept wings are subject to the crossflow instability, which manifests as co-rotating vortices aligned to within a few degrees of the streamlines. Triggering this boundary layer instability can lead to turbulence and higher drag for the aircraft.
Tag: crossflow vortices
Transition to Turbulence
Smoke introduced into the boundary layer of a cone rotating in a stream highlights the transition from laminar to turbulent flow. On the left side of the picture, the boundary layer is uniform and steady, i.e. laminar, until environmental disturbances cause the formation of spiral vortices. These vortices remain stable until further growing disturbances cause them to develop a lacy structure, which soon breaks down into fully turbulent flow. Understanding the underlying physics of these disturbances and their growth is part of the field of stability and transition in fluid mechanics. (Photo credit: R. Kobayashi, Y. Kohama, and M. Kurosawa; taken from Van Dyke’s An Album of Fluid Motion)
