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The density in these experiments is visualized using microencapsulated thermochromic liquid crystals. These are roughly 10 micron diameter plastic spheres filled with a liquid crystal which reflects different wavelengths of light depending on its temperature (and to some degree the viewing angle). Here clear is the coldest (less than 25 degrees celcius as at the bottom) and blue is the warmest (equal to or greater than 30 degrees celcius---throughout most of the fluid). The entire tank was first warmed to 30 degrees before a thin river of cold water (22.5 degrees) was pumped in. Depending on the speed everything from very stable flows (as the first image shows) to very unstable flows are observed.
Side view of a stable `river' flowing down an inclined plane. The direction of flow is from left to right in all images. Here you can actually see that the flow starts out thicker and is accelerating toward some terminal velocity. As it does so mass conservation requires that the layer become thinner. At slightly higher velocities a Kelvin-Helmholtz instability becomes visible and starts to mix the `river water' with the overlying fluid. The higher the velocity the more unstable and more energetic the flow becomes and, hence, the more mixing that occurs. The amount of mixing can become quite significant!
geophysics