Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "stratification"

Though often spotted in water waves or clouds, the Kelvin-Helmholtz instability is easily demonstrated in the lab as well. Here a tank with two layers of liquid - fresh water on top and denser blue-dyed saltwater on the bottom - is used to generate the instability. When level, the two layers are stationary and stable due to their stratification. Upon tilting, the denser blue liquid sinks to the lower end of the tank while the freshwater shifts upward. When the relative velocity of these two fluids reaches a critical point, their interface becomes unstable, forming the distinctive wave crests that tumble over to mix the two layers. (Video credit: M. Stuart)

This video has a fun and simple demonstration of the importance of fluid density in buoyancy and stratification. Fresh water (red) and salt water (blue) are released together into a small tank. Being lighter and less dense, the red water settles on top of the blue water, though some internal waves muddy their interface. After the water settles, a gate is placed between them once more and one side is thoroughly mixed to create a third fluid density (purple), which, when released, settles between the red and blue layers. In addition to displaying buoyancy, this demo does a great job of showing the internal waves that can occur within a fluid, especially one of varying density like the ocean. (Video credit: UVic Climate Modeling Group)

One of the most commonly observed fluid instabilities is the Rayleigh-Taylor instability, which occurs between fluids of differing densities.  It’s most often seen when a denser fluid sits over a lower density fluid. In the video above, this is demonstrated experimentally: a lower density green fluid mixes in with the clear, higher density fluid.  This is the classical case in which each initial region of fluid is uniform in density prior to the removal of the barrier.  But what happens when each zone has its own variation in density? This is the second case.  Before the barrier is removed, each region of the tank has a varying—or stratified—fluid density.  In this case, the unmixed fluids are stably stratified, meaning that the fluid density increases with depth. At the barrier interface, the two separate fluids are still unstably stratified—with the denser fluid on top—so when the barrier is removed, the Rayleigh-Taylor instability still drives their mixing. Because of the stable stratification within the original unmixed fluids, the mixing region after the barrier’s removal is more limited. (Video credit: M. D. Wykes and S. B. Dalziel; via PhysicsCentral by APS)

In large-scale geophysical flows, rotation and density gradients often play major roles in the structures that form. Here the UCLA SPINLab demonstrates how large, essentially flat vortices—pancake vortices—form in rotating, stratified fluids. The stratification, in this case, is due to the density difference between salt water and fresh water; salt water is denser and therefore less buoyant, so it sinks toward the bottom of the tank. Note how the pancake vortex only forms when the fluid is both stratified and rotating.  If it lacks one of the two, the structures will be very different. (Video credit: O. Aubert et al./SPINLab UCLA)

This image, taken from a direct numerical simulation, shows turbulence in a stably stratified flow in which lighter fluid sits atop a denser fluid. In the image lighter colors represent denser fluid. Turbulence is created by the shear forces caused when the lighter fluid on top moves faster than the denser fluid on the bottom; however the stable stratification will tend to counteract or stabilize the turbulence. Note the vast variety and detail of the scales involved in turbulence; this is what makes it such a difficult process to simulate and model. (Image credit: G. Matheou and D. Chung, NASA/JPL-Caltech)

Home to storms capable of lasting for a hundred years or more, Jupiter's atmosphere is a highly turbulent place. Currently, no comprehensive theory exists to explain the symmetry of Jupiter’s bands of clouds and the persistence of vortices such as the Great Red Spot, however, the mixing and stratification visible on the planet remains a beautiful reminder of the power of fluid dynamics. (Photo credits:Cassini - 1, 2,  Voyager 1, New Horizons - 1, 2)