Rayleigh-Taylor instability

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Image:HD-Rayleigh-Taylor.gif 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.

Two completely plane-parallel layers of immiscible fluid are stable, 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 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 astronomy. RT fingers are especially obvious in the Crab Nebula, in which hot gas from the explosion is ramming into the surrounding interstellar medium, and they give rise to the familiar clumpy appearance of material in these and several other astronomical objects.

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, sometime 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.