Crow Instability

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Behind airplanes in flight, water vapor from the engine exhaust will sometimes condense in the wingtip vortices, thereby forming visible contrails.  The two initially parallel vortex lines are unstable and any small perturbation to them–a slight crosswind, for example–will cause an instability known as the Crow instability. The contrails become wavy, with the amplitude of the wave growing exponentially in time due to interactions between the two vortices. Eventually, the vortex lines can touch and pinch off into vortex rings. The effect is also quite noticeable when smoke generators are used on a plane, and there are some great examples in this air show video between 3:41:00 and 3:44:00. (Video credit: M. Landy-Gyebnar; h/t to Urs)

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