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)
When a droplet falls through an air/water interface, a vortex ring can form and fall through the liquid. In this video, the researchers investigate the effects of a stratified fluid interface on this falling vortex ring. In this case, a less dense fluid sits atop a denser one. Depending on the density of the initial falling droplet and the distance it travels through the first fluid, the behavior and break-up of the vortex ring when it hits the denser fluid differs. Here four different behaviors are demonstrated, including bouncing and trapping of the vortex ring. (Video credit: R. Camassa et al.)
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)
Accidental releases of combustible gases in unconfined spaces can be difficult to recreate in a laboratory environment. Here researchers simulate the conditions using detonation inside a soap film bubble. Combustible gases are pumped inside the soap film and then a spark creates ignition. The resulting flame propagation is visualized using high-speed schlieren photography, making the density gradients in the flame visible. When the mixture of hydrogen fuel to air is balanced, the flame is spherically symmetric with a high flame speed. In contrast, weaker mixtures of fuel/air produce slow flame speeds and mushroom-like flames that leave behind unreacted fuel. This is due to buoyant effects; the time scale associated with buoyancy is smaller than that of the flame speed and chemical reactions when the fuel/air mixture is lean. (Video credit: L. Leblanc et al.)
Convective cells form as fluid is heated from below. As the fluid near the bottom warms, its density decreases and buoyancy causes it to rise while cooler fluid descends to replace it. This fluid motion due to temperature gradients is called Rayleigh-Benard convection and the cells in which the motion occurs are called Benard cells. This particular type of convection is essentially what happens when a pot is placed on a hot stove, so the shapes are familiar. Similar shapes also form on the sun’s photosphere, where they are called granules.
Tornadogenesis—the formation of tornadoes—remains a topic of active research as there is relatively little direct experimental data, owing to the difficulty of prediction as well as measurement. Initially, a variation of wind speed at different altitudes in the atmosphere causes shearing, which can lead to the formation of a horizontal column of rotating air—a vortex line similar to a roll cloud. Beneath a developing storm, the updraft of warm local air can pull this vortex line upwards, creating vertical rotation in the cloud, thereby birthing a supercell. Supercells do not always spawn tornadoes, and the exact causes that result in tornadic or nontornadic supercells are not fully understood. However, the formation of tornadoes within the supercell seems dependent on the downdraft of cool air within the storm as well as stretching of the vortex line, which increases its rate of rotation. For more information, check out this explanatory video and some of the talks by Paul Markowski. (Thanks to mindscrib, aggieastronaut and others for their submissions related to this topic! Photo credits: P. Markowski and D. Zaras)
This timelapse video shows the formation and steady-state behavior of a buoyancy-driven plume created by a chemical reaction. As the plume accelerates upward, it develops a head, which in some cases detaches from the plume in the form of a vortex ring. A new head then develops before also detaching and accelerating upwards. (Video credit: M. Rogers)
In the frozen reaches of our planet, the atmosphere and ocean can interact in bizarre ways. Under calm ocean conditions when the air at sea level is much colder than the water temperature brinicles—the underwater equivalent to an icicle—can form. The cold air above rapidly freezes ocean water at the surface, concentrating water’s salt content into a very cold brine which sinks rapidly. As this brine descends, it freezes the water around it into an ice sheath. As the brinicle grows and eventually reaches the sea floor, its cold temperatures can wreak havoc on the creatures living there.