RAYLEIGH-TAYLOR INSTABILITY

 The Rayleigh-Taylor instability induced by a magnetic field. Top - no rotation. Bottom - same setup rotating 24 times per minute.

The Rayleigh-Taylor instability induced by a magnetic field. Top - no rotation. Bottom - same setup rotating 24 times per minute.

The Rayleigh-Taylor instability describes the process by which two fluid layers of different density switch places, such as a heavy fluid resting on top of a light fluid under gravity. In a closed container, the exact procedure by which two fluids will switch positions (particularly if the fluid layers begins perfectly symmetric) is not immediately obvious, as they're in each other's way and filling all the space. By analogy, one can grasp the difficulty in switching positions by imagining a closed elevator perfectly packed with people, where a couple at the back must fight their way to reach the buttons at the front.  In this analogy, jamming occurs unless everyone moves cooperatively, like a flowing fluid. Real fluids will rearrange these layers via the growth of waves and plumes (see left), where the size and growth rate of these plumes is fundamental to many natural and industrial processes.

For example, simulations show that supernovae undergo the RTI, as the hot core tries to accelerate through the cooler more dense outer layers. The brightest, most energetic events in the universe must obey fluid dynamics, and the finger-like structures in the Crab nebula (a supernova leftover) show this. On Earth, deep salt deposits slowly grow through the overlaying more dense sediment beds above via the same mechanism, influencing plate tectonics. And in the laboratory, one attempt to generate energy from nuclear fusion (inertial confinement fusion, where lasers fired from all directions cause a fuel pellet to collapse) loses efficiency because of this same instability, and is believed to be the main reason this attempt at energy production has so far been unsuccessful. Clearly, experiments to understand and control this instability are required.

Experimentally, the RTI is difficult to observe, as any attempt to lay a heavy fluid on top of a light one will immediately become unstable before a complete layer has had time to develop. Additionally, standard techniques to rapidly switch the direction of acceleration to induce the instability in an otherwise stable system (such as placing the container in a rocket) induce unwanted flows and vibrations that interfere with the RTI.

 Collection of snapshots from the original, significantly higher quality, version of the video posted on FYFD. Left to right are zero, slow, faster and fastest spin rates respectively.

Collection of snapshots from the original, significantly higher quality, version of the video posted on FYFD. Left to right are zero, slow, faster and fastest spin rates respectively.

Our technique was thus: lay a light paramagnetic fluid on top of a heavy diamagetic fluid, such that it is stable under gravity, and lower it into a magnetic field, where at some point the effective weight of the top layer increases until surpasses that of the lower fluid, and the instability is triggered. We then used this technique to show that by rotating the chamber, we could induce a stabilizing Coriolis force, which inhibits large wavelength plumes, restricting the RTI to evolve via the growth of slower and smaller structures.

 

 

The popular Tumblr account under the family-friendly name F*%£ Yeah Fluid Dynamics thought one of our publication-attached videos on this rotation dampened instability was worthy of reposting.