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Stability
Fluid instability can occur in many different situations. The fluid stability section has therefore been divided into three sub-sections, each of which demonstrates fluid instabilities with different fundamental causes. Each of the fluid stability movies is found on this page in its respective subsection, with an explanation of the movie and a basic general explanation of the root phenomenon.
Rayleigh- Taylor Instability
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The classic Rayleigh- Taylor instability is demonstrated in the first example calculation. When a high density fluid is placed over a low density fluid an unstable condition exists, which causes the two fluids to exchange places. In this example an initial perturbation is placed in the center in order to initiate the instability. The size of the box is quite large so that the exchange of fluids takes place in a fairly turbulent manner. An asymmetric fluid exchange occurs when a step in fluid level is used as an initial condition, as shown in example 2. The large difference in the fluid behavior contrasted in the first two examples demonstrates the sensitivity to initial conditions for this type of instability. The effect of fluid viscosity is demonstrated in example 3. The same parameters of size, initial perturbation, and density difference were used, but with a much higher viscosity. The high viscosity serves to damp out the high degree of turbulence found in the previous examples allowing the large wavelength instabilities to dominate the flow behavior.
Benard Instability
When a layer of fluid is heated from below and cooled from above the resulting convective patterns are often called the Benard instability, The first example illustrates the transition to convection of an initially quiescent layer of fluid that has a vertically unstable temperature gradient. The fluid is low Prandtl number (Pr = 0.0125; high thermal conductivity). The second example shows the temperature contours from a slightly higher Prandtl number fluid (Pr = 2.25; low thermal conductivity)
Kelvin-Helmholtz Instability
The Kelvin-Helmholtz instability is another classic flow instability. This instability is characterized by waves that appear between two superimposed fluids of differing densities and velocities. A familiar example are the ripples that form when wind flows over a pool of water.The first example demonstrates the Kelvin instability wherein two stably stratified fluids are flowing from left to right with the uppermost low density fluid travelling 3.5 times faster than the lower heavy fluid. Another example of the Kelvin-Helmholtz instability are waves that grow on jets of high or low density fluid, such as the hot bouyant jet of gas shown in the second example.