Rotating a fluid often produces different dynamical behavior than for a non-rotating fluid. Here this concept is demonstrated by dropping creamer into a tank of water. Both experiments produce a turbulent plume, but the way the plume spreads and diffuses is much different in the case of the rotating tank, thanks to the Coriolis effect. (Video credit: SPINLab UCLA)
Tag: plumes
Plumes Driven by Chemistry
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)
Underwater Plumes
During 2010’s Deepwater Horizon oil spill there were reports of underwater plumes of oil escaping collection. This video demonstrates how such a plume can form. There are two clips shown here; in both the tank is filled with salt water of varying salinity, with denser saltwater at the bottom. The first jet is a green alcohol/water mixture and the second is a red gauge oil. Both jets have the same density and flow rate, but they vary in their Reynolds number. The first turbulent jet gets trapped at the interface between the denser and lighter saltwater while the less turbulent red jet passes the interface with no difficulty. The researchers suggest that strong turbulence can create an emulsion, a mixture of two normally immiscible fluids–imagine shaking a container of oil and vinegar really well–which can lead to underwater trapping.
Coughing Contagions
Schlieren imaging has applications even in public health. This video demonstrates the spread of contagion via coughing with and without a mask on. Although air from the cougher’s lungs escapes the sides of the mask, it mostly rises on a thermal plume rather than projecting 1 to 2 meters forward in a turbulent jet as in the maskless case. Flu season is just starting. Don’t forget to get your flu shot!
Tank Shock Waves
High-speed video of a tank firing at 18000 fps shows shock waves made visible due to light distortion. When the air density changes (due to temperature or compression), it’s index of refraction changes, causing the background to appear distorted. Most of the video shows the subsonic development of the turbulent exhaust plume. Note the speed at which the exhaust moves relative to the airborne shrapnel. (submitted by Stephan)
Volcanic Ash Plume
Video footage of Iceland’s Grimsvotn volcano erupting shows a massive turbulent plume of ash. The largest scales of the plume are of the order of hundreds, if not thousands of meters, and the eddies of the plume appear to move very slowly, especially far from the base. According to Kolmogorov, however, at the smallest scales of the flow (< 1 mm), the turbulent motions are isotropic. No one has been able to achieve Reynolds numbers high enough to fully prove or disprove Kolmogorov’s hypothesis, but natural events like volcanic eruptions produce some of the largest Reynolds numbers on earth. (See also: interview with videographer; via Gizmodo, jshoer)
Godspeed, Discovery!
The space shuttle, despite three decades of service, remains a triumph of engineering. Although it is nominally a space vehicle, fluid dynamics are vital throughout its operation. From the combustion in the engine to the overexpansion of the exhaust gases; from the turbulent plume of the shuttle’s wake to the life support and waste management systems on orbit, fluid mechanics cannot be escaped. Countless simulations and experiments have helped determine the forces, temperatures, and flight profiles for the vehicle during ascent and re-entry. Experiments have flown as payloads and hundreds of astronauts have “performed experiments in fluid mechanics” in microgravity. Since STS-114, flow transition experiments have even been mounted on the orbiter wing. The effort and love put into making these machines fly is staggering, but all things end. Godspeed to Discovery and her crew on this, her final mission!
Smokestack Plumes
On a cold and windy day, the plume from a smokestack sometimes sinks downstream of the stack instead of immediately rising (Figure 1). This isn’t an effect of temperature–after all, the exhaust should be warm compared to the ambient, which would make it rise. It’s actually caused by vorticity.
Figure 2: Simple geometry (side view) In Figure 2, we see a simplified geometry. The wind is blowing from right to left, and its velocity varies with height due to the atmospheric boundary layer. Mathematically, vorticity is the curl of the velocity vector, and because we have a velocity gradient, there is positive (counterclockwise) vorticity generated.
Figure 3: Vortex lines (top view) According to Helmholtz, we can imagine this vorticity as a bunch of infinite vortex lines convecting toward the smokestack, shown in Figure 3. Those vortex lines pile up against the windward side of the smokestack–Helmholtz says that vortex lines can’t end in a fluid–and get stretched out in the wake of the stack. If we could stand upstream of the smokestack and look at the caught vortex line, we would see a downward velocity immediately behind the smokestack and an upward velocity to either side of the stack. It’s this downward velocity that pulls the smokestack’s plume downward.
Figure 4: Vortex wrapped around stack Now Helmholtz’s theories actually apply to inviscid flows and the real world has viscosity in it–slight though its effects might be–and that’s why this effect will fade. The vortex lines can’t sit against the smokestack forever; viscosity dissipates them.
Three Flows in One
These plumes of smoke demonstrate the three types of fluid flow: laminar, transitional, and turbulent. At the bottom of the photo, the plumes are smooth and orderly, as is typical for laminar flow. At the top, the smoke’s movement is chaotic and intermittent, full of turbulent eddies. Between these two stages, the flow is in transition; there is still some semblance of order to it, but disturbances in the plume are getting amplified and breaking down into turbulence.
Photo credit: J. Russo
