Rayleigh-Taylor instability

    The Rayleigh-Taylor instability, or RT instability (after Lord Rayleigh and G. I. Taylor), occurs any time a dense, heavy fluid is being accelerated by light fluid. This is the case with a cloud and shock system, or when a fluid of a certain density floats above a fluid of lesser density, such as dense oil floating above water.

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Two completely plane-parallel layers of immiscible fluid are in equilibrium, but the slightest perturbation leads to release of potential energy, as the heavier material moves down under the (effective) gravitational field, and the lighter material is displaced upwards. As the instability develops, downward-moving irregularities ('dimples') are quickly magnified into sets of inter-penetrating RT fingers. The upward-moving, lighter material behaves like 'Spherical Cap Bubbles'.

This process is evident not only in many terrestrial examples, from salt domes to weather inversions, but also in astrophysics and electrohydrodynamics. RT fingers are especially obvious in the Crab Nebula, in which the expanding pulsar wind nebula powered by the Crab pulsar is sweeping up ejected material from the supernova explosion 1000 years ago.

Note that the RT instability is not to be confused with the Rayleigh instability (or Plateau-Rayleigh instability) of a liquid jet. This latter instability, sometimes called the hosepipe (or firehose) instability, occurs due to surface tension, which acts to break a cylindrical jet into a stream of droplets having the same volume but lower surface area.

See also

• Richtmyer-Meshkov instability
• Kelvin-Helmholtz instability
• Mushroom cloud
• Plateau-Rayleigh instability
• Salt fingering

References

Original research papers:

Rayleigh, Lord (John William Strutt), "Investigation of the character of the equilibrium of an incompressible heavy fluid of variable density," Proceedings of the London Mathematical Society, Vol. 14, pages 170 - 177 (1883). (Original paper is available at: https://www.irphe.univ-mrs.fr/~clanet/otherpaperfile/articles/Rayleigh/rayleigh1883.pdf .)

Taylor, Sir Geoffrey Ingram, "The instability of liquid surfaces when accelerated in a direction perpendicular to their planes," Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 201, No. 1065, pages 192 - 196 (22 March 1950).