Knots have long fascinated humans, appearing in art for thousands of years and generating entire fields of study. Until recently, however, the idea of a knotted fluid was purely theoretical. To knot fluids, researchers used 3D printing to create twisted hydrofoil shapes. When towed through water, fluid travels around the shape and spins up at the trailing edge, creating a knotted vortex ring. The knotted vortices were captured with 3D imaging, allowing scientists to observe how they evolve. So far the knots they’ve created have all been unstable, deforming until two vortex lines approach one another. Upon contact, the vortices disconnect and reconnect with one another, unraveling the knot. Intriguingly, these vortex reconnections seem remarkably similar to the vortex reconnections observed between quantized vortices in superfluids. (Video credit: D. Kleckner et al.)
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Sochi 2014: Bobsledding
Today bobsledding is an sport rife with modern technology and design techniques. In recent years, companies better known for their expertise in automobiles and Formula 1 racing have become players with BMW designing American sleds, McLaren making the UK sleds, and Ferrari providing for the Italian team. Like many winter gravity sports, contenders can be separated by as little as hundredths of a second. This makes aerodynamics a serious concern, but the variability of the sled’s position and orientation over a run makes realistically simulating the aerodynamics, either in a wind tunnel or computationally, extremely difficult. Additionally, the sport’s governing body restricts a sled’s dimensions, weight, shape, and other details; for example, bobsleds are not allowed to use vortex generators that would help maintain attached flow and reduce drag. Instead, designers try to shave drag elsewhere, in the shaping of the sled’s nose or by tweaking the back end of the sled to reduce the drag-inducing wake. Even the shape of the driver’s helmet is aerodynamically significant. (Image credits: Exa Corp, Getty Images, BMW)
FYFD is celebrating #Sochi2014 by looking at fluid dynamics in winter sports. Check out our previous posts on how skiers glide, the US speedskating suit controversy, and why ice is slippery.
Happy Valentine’s Day!
What can you do with a 7 x 7 grid of miniature vortex cannons? Why, make floating vortex hearts, of course. Happy Valentine’s Day from FYFD! (Video credit: D. Schulze/bitsbeauty; via Colossal)
Vibrations from Vortices
Vortex shedding frequently happens in the wakes of non-streamlined bodies as a result of flow around the obstacle. Newton’s third law states that forces come in equal and opposite pairs, meaning that the vortex shedding behind an obstacle is accompanied by a force on the obstacle. For a fixed cylinder, this is not always apparent, but for a pendulum, like the ones demonstrated in this video, this vortex-induced vibration causes significant motion. This same effect can make traffic lights and industrial chimneys sway. You’ve likely experienced it yourself as well, if while swimming you’ve ever spread your fingers underwater and spun in place. Try it sometime with your arm out and you’ll feel the vortices make your arm vibrate up and down as you spin. (Video credit: Harvard Natural Sciences Lecture Demonstrations)
Snow Rollers
Snow rollers are nature’s snowballs, formed when high winds roll a chunk of snow along the surface, allowing it to accumulate more and more material. They occur relatively rarely because their appearance is the culmination of several specific meteorological factors. To form rollers, the ground needs to be icy, with a layer of loose, wet snow above the ice. And, of course, it needs to be windy enough to move the snow without being so windy that snow breaks up. In the photos above, the snow roller got too large for the wind to continue moving it, but the wind didn’t stop blowing. Instead, the snow roller became an obstacle to the flow and a horseshoe vortex formed at its base. The spinning of the vortex dug out the trench in front of and along the sides of the snow roller. This same effect is often seen on the windward side of trees in winter. (Photo credit and submission: S. Benton)
Fluids Round-up – 11 January 2014
It’s a big fluids round-up today, so let’s get right to it.
- Over at txchnologist, there’s a great article on controlling combustion instabilities in rocket engines with sound.
- Quanta Magazine asks if knot theory can help unravel turbulence. (submitted by iamaponyrocket)
- SciAm takes a look at how FIFA finally got their aerodynamics right so that their video game football (soccer) balls fly correctly.
- The Smithsonian considers an important question: can you fry foods in space?
- The Navy unveiled a fantastic new facility for simulating ocean waves (via J. Ouellette)
- At SciAm, there’s a nice explanation of the polar vortex and its effects on recent freezing weather. For additional background, check out this excerpt from a presentation by meteorology professor Jennifer Francis. (via Nicholas Travers)
- Cold weather also brings a host of new viral videos; NatGeo explains some of the science behind instant snow, ice fog, and frozen bubbles. See also: our own explanation of the instant snow phenomenon.
- io9 looks at the physics of knuckleballs.
- Over at Wired, Rhett Allain questions whether dwarves should stand in floating barrels. Also on the subject of The Hobbit, here’s an analysis of fire-breathing in dragons.
- At SciAm, Kyle Hill explains how inertia lets one pour a drink toward the sky.
- SciAm reports on a manufacturing process for superhydrophobic paper.
- I don’t know what banking has to do with a pool of non-Newtonian fluids, but this Malaysian ad sure makes it look fun. (via physicsphysics and jmlinhart)
- Wired has a great write-up on the mantis shrimp, which kills its prey with cavitation.
- io9 tackles explaining one of the most vexing brain teasers in fluid dynamics, the Feynman sprinkler.
- 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.
(Photo credit: Think Elephants International/R. Shoer)
Is the Star Trek Voyager Opening Sequence Physically Realistic?
Today’s post is largely brought to you by the fact that I have been sick the past four days and my fiance and I have been bingeing on Star Trek Voyager. At some point, we began wondering about the sequence from 0:30-0:49 in which Voyager flies through a nebula and leaves a wake of von Karman vortices. Would a starship really leave that kind of wake in a nebula?
My first question was whether the nebula could be treated as a continuous fluid instead of a collection of particles. This is part of the continuum assumption that allows physicists to treat fluid properties like density, temperature, and velocity as well-defined quantities at all points. The continuum assumption is acceptable in flows where the Knudsen number is small. The Knudsen number is the ratio of the mean free path length to a characteristic flow length, in this case, Voyager’s size. The mean free path length is the average distance a particle travels before colliding with another particle. Nebulae are much less dense than our atmosphere, so the mean free path length is larger (~ 2 cm by my calculation) but still much smaller than Voyager’s length of 344 m. So it is reasonable to treat the nebula as a fluid.
As long as the nebula is acting like a fluid, it’s not unreasonable to see alternating vortices shed from Voyager. But are the vortices we see realistic relative to Voyager’s size and speed? Physicists use the dimensionless Strouhal number to describe oscillatory flows and vortex shedding. It’s a ratio of the vortex shedding frequency times the characteristic length to the flow’s velocity. We already know Voyager’s size, so we just need an estimate of its velocity and the number of vortices shed per second. I visually estimated these as 500 m/s and 2.5 vortices/second, respectively. That gives a Strouhal number of 0.28, very close to the value of 0.2 typically measured in the wake of a cylinder, the classical case for a von Karman vortex street.
So far Voyager’s wake is looking quite reasonable indeed. But what about its speed relative to the nebula’s speed of sound? If Voyager is moving faster than the local speed of sound, we might still see vortex shedding in the wake, but there would also be a bow shock off the ship’s leading edge. To answer this question, we need to know Voyager’s Mach number, its speed relative to the local speed of sound. After some digging through papers on nebulae, I found an equation to estimate speed of sound in a nebula (Eq 9 of Jin and Sui 2010) using the specific gas constant and temperature. Because nebulae are primarily composed of hydrogen, I approximated the nebula’s gas constant with hydrogen’s value and chose a representative temperature of 500 K (also based on Jin and Sui 2010). This gave a local speed of sound of 940 m/s, and set Voyager’s Mach number at 0.53, inside the subsonic range and well away from any shock wave formation.
Of course, these are all rough estimates and back-of-the-envelope fluid dynamics calculations, but my end conclusion is that Voyager’s vortex shedding wake through the nebula is realistic after all! (Video credit: Paramount; topic also requested by heuste11)
Volcanic Vortices from Etna
Italy’s Mount Etna is erupting again, producing a series of beautiful vortex rings. Like a dolphin’s bubble ring or a vortex cannon, the volcano’s rings are formed when gases are rapidly expelled through a narrow opening. Such formations are extremely common but are generally not visible to the eye. In this case, steam has gotten entrained into the rings to make them visible. Vortex rings can maintain their structure over substantial distances. The photographer of these rings noted that they lasted as many as ten minutes before dissipating. (Photo credit: T. Pfeiffer; via NatGeo)
Flow Behind a Cylinder
Flow over blunt bodies produces a series of alternating vortices that are shed behind an object. The image above shows the turbulent wake of a cylinder, with flow from right to left. Red and blue dyes are used to visualize the flow. This flow structure is known as a von Karman vortex street, named for aerodynamicist Theodore von Karman. The meander of the wake is caused by the shed vortices, each of which has a rotational sense opposite its predecessor. The rapid mixing of the two dyes is a result of the flow’s turbulence. In low Reynolds number laminar cases of this flow the structure of individual vortices is more visible. Similar flow structures are seen behind islands and in the wakes of flapping objects. (Photo credit: K. Manhart et al.)
Fluids Round-up – 13 October 2013
There were so many good fluids links this week that I decided for an off-week fluids round-up. Here we go!
- Jefferson Lab has a cool demo on how to make a cloud chamber using dry ice, isopropyl alcohol, and a radioactive source. There is all kinds of fun physics to explain in this one!
- io9 has a great article and videos on the efficiency of jellyfish propulsion (spoiler alert: there are vortex rings).
- Half-blimp, half-jet transport option could change shipping landscape. In a similar vein, Jalopnik takes a look back at the golden age of the dirigible.
- For the armchair daredevils, check out this 360-degree view of BASE jumping with a wingsuit off a Swiss mountain. (via Janeen M)
- Also from io9, an article on my favorite fluids demo: reversible laminar flow. If you’d like to try this at home, here’s a DIY version.
- National Geographic talks about the differences between hurricanes, cyclones, and typhoons.
- Finally, our lead video comes from #5facts and Sesame Street’s Grover who bring us 5 DIY science experiments, 4 of which are fluids demos. Sit back and enjoy!
(Video credit: #5facts/Sesame Street)