aerodynamics Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...

, the Crow instability, or V.C.I. vortex crow instability, is an

inviscid Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

line-vortex instability, named after its discoverer S. C. Crow. The effect of the Crow instability can often be observed in the skies behind large aircraft, when the

wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates Lift (force), lift. The name is a misnomer because the cores of the vortex, vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes ...

interact with

contrail Contrails (; short for "condensation trails") or vapour trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several kilometres/miles above the Earth's surface. ...

s from the engines, producing visible distortions in the shape of the contrail.

Instability development

The Crow instability is a vortex pair instability, and typically goes through several stages: *A pair of counter rotating vortices act upon each other to amplify small

sinusoidal A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic wave whose waveform (shape) is the trigonometric sine function. In mechanics, as a linear motion over time, this is '' simple harmonic motion''; as rotation, it correspond ...

distortions in their vortex shapes (normally created by some initial disturbance in the system). *The waves develop into either symmetric or anti-symmetric modes, depending on the nature of the initial disturbance. *These distortions grow, both through interaction from one vortex on another, and also 'Self Induction' of a vortex with itself. This leads to an exponential growth in the vortex wave amplitude. *The vortex amplitudes reach a critical value and reconnect, forming a chain of

vortex ring A vortex ring, also called a toroidal vortex, is a torus-shaped vortex in a fluid; that is, a region where the fluid mostly spins around an imaginary axis line that forms a closed loop. The dominant flow in a vortex ring is said to be toroidal, ...

Aviation vortices

The wings of airplanes in flight produce at least one pair of

trailing vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates Lift (force), lift. The name is a misnomer because the cores of the vortex, vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes ...

. These vortices are a major source of

wake turbulence Wake turbulence is a disturbance in the Atmosphere of Earth, atmosphere that forms behind an aircraft as it passes through the air. It includes several components, the most significant of which are wingtip vortices and jet-wash, the rapidly moving ...

as they persist for a significant period of time after the airplane has passed. If the decay of trailing vortices were due solely to viscous effects in the core of each vortex, decay would be so slow that they would persist for hundreds of miles behind the airplane. In fact, these vortices only persist for tens of miles. The additional cause of the collapse of these vortices is large-scale instabilities such as Crow instability.McLean, Doug (2013) ''Understanding Aerodynamics''. Page 369. Wiley. ISBN 978-1-119-96751-4

References

Bibliography

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External links

Scientific American: The Crow Instability

Meteorological phenomena {{meteorology-stub