Though they may appear random at first glance, turbulent flows do possess structure. The video above shows a numerical simulation of a mixing layer, a flow in which two adjacent regions of fluid move with different velocities. The upper third of the frame shows a top view, and the bottom frame shows a side view, in which the upper fluid layer moves faster than the lower one. The difference in velocities creates shear which quickly drives the mixing layer into turbulence. But watch the chaos carefully, and your eye will pick out vortices rolling clockwise in the largest scales of the mixing layer. These features are known as coherent structures, and they are key to current efforts to understand and model turbulent flows. (Video credit: A. McMullan)
Finally, today’s lead image comes from our friends at Think Elephants, who study elephant intelligence over in Thailand and occasionally capture the animals’ mastery of fluid dynamics. Be sure to check them out and follow them on Twitter and Facebook.
When supersonic flow is achieved through a wind tunnel or rocket nozzle, the flow is said to have “started”. For this to happen, a shock wave must pass through, leaving supersonic flow in its wake. The series of images above show a shock wave passing through an ideal rocket nozzle contour. Flow is from the top to bottom. As the shock wave passes through the nozzle expansion, its interaction with the walls causes flow separation at the wall. This flow separation artificially narrows the rocket nozzle (see images on right), which hampers the acceleration of the air to its designed Mach number. It also causes turbulence and pressure fluctuations that can impact performance. (Image credit: B. Olson et al.)
Turbulence is an excellent mixer. Here two fluorescent dyes are injected into a turbulent water jet. Flow is from the bottom of the image toward the top. The dyes are quickly mixed into the background fluid by momentum convection, their concentration decreasing with increased distance from the source. Large-scale structures like the eddies visible in this image drive this convection of momentum in turbulent flows. In contrast, consider laminar flows, where momentum and molecular diffusion dominate how fluids move. In such laminar flows, it’s even possible to unmix two fluids, a feat that cannot be accomplished in the jet above. (Photo credit: M. Kree et al.; via @AIP_Publishing)
Air dancers—those long fabric tubes with fans blowing into the bottom—are a popular way for shops to draw attention. They bend and flutter, shake and kink, all due to the interaction of airflow in and around them with the fabric. When the interior flow is smooth and laminar, the tube will stand upright, with very little motion. As the air inside transitions, some fluttering of the tube can be observed. Ultimately, it is when the air flow becomes turbulent that the cloth really dances. Variations in the flow are strong enough at this point that the tube will occasionally buckle. Behind this constriction, the flow pressure increases until its force is enough to overcome the weight of the tube and lift it once more. (Video credit: A. Varsano)
Last night marked the 2013 Ig Nobel Prize Award Ceremony, in which researchers are honored for work that “makes people LAUGH and then THINK”. Historically, the field of fluid dynamics has been well-represented at the Ig Nobels with some 13 winners across the fields of Physics, Chemistry, Mathematics, and—yes—Fluid Dynamics since the awards were introduced in 1991. This is in stark contrast to the awards’ more famous cousins, the Nobel Prizes.
This is not to suggest that no fluid dynamicist has done work worthy of a Nobel Prize. Ludwig Prandtl, for example, revolutionized fluid dynamics with the concept of the boundary layer (pdf) in 1904 but never received the Nobel Prize for it, perhaps because the committee shied from giving the award for an achievement in classical physics. General consensus among fluid dynamicists is that anyone who can prove a solution for turbulence using the Navier-Stokes equation will likely receive a Nobel Prize in addition to a Millennium Prize. In the meantime, we carry on investigating fluids not for the chance at glory, but for the joy and beauty of the subject. (Image credits: Improbable Research and Wikipedia)
Alberto Seveso's gorgeous high-speed photos of ink diffusing in water have a dramatic sense of texture to them. Though still delicate, the whorls of fluid seem almost solid enough to touch. Watch the edges, though, and you can see thin wisps of color and hints of instabilities. Like cream poured into coffee, these ink sculptures are short-lived. Some of his works are available as prints or wallpapers (zip file). (Photo credit: Alberto Seveso)
Holidays involving fireworks deserve high-speed videos of hydrogen explosions. Although Periodic Table of Videos focuses on the chemistry involved in setting hydrogen on fire, there are some lovely fluid dynamics on display, too. There’s turbulence, combustion (obviously), and, if you watch closely, you can even see the initial vorticity caused by the rubber’s burst twisting the growing flames. (Video credit: Periodic Table of Videos)