Conventional wind tunnels are great, but some aerodynamic testing requires facilities of a different nature. The video above is from the WindEEE dome, a hexagonal chamber with sixty fans on one wall, eight directional fans on the other five walls, and six fans in the upper chamber. Each is individually computer controlled, allowing the researchers to create straight flows as well as complex vortical ones. The video shows their tornado flow, which stands 5 m tall and swirls at 30 m/s. They can also move the tornado around the chamber at 2 m/s. This capability enables a kind of scale-model analysis of tornadoes and their impact that’s not possible in most facilities. You can read more about the dome at New Scientist or the WindEEE website. (Video credit: New Scientist/WindEEE; submitted by entropy-perturbation)
Search results for: “art”
Dancing Droplets
What makes drops of food coloring able to dance, chase, sort themselves, or align with one another? This unexpected behavior is a consequence of food coloring consisting of two mixed liquids: water and propylene glycol. Both have their own surface tension properties and evaporation rates, which ultimately drives the behavior you see in the animations above. Both long-range and short-range interactions are observed. The former are due to vapor from each droplet adsorbing onto the glass around the droplet, thereby changing the local surface tension and causing nearby drops to feel an attractive force. The short-range effects are also surface-tension-driven. Droplets with lower surface tension will naturally try to flow toward areas of higher surface tension, which causes them to “chase” dissimilar adjacent drops. You can learn more about the research in the videos linked below (especially the last two), or you can read about the work in this article or the original research paper. (Image credit: N. Cira et al., source videos 1, 2, 3, 4; GIFs via freshphotons; submitted by entropy-perturbation)
“Jack and the Giant”
This fantastic music video by Kim Pimmel is a beautiful merger of art and fluid dynamics. Using household goods (and some slightly more exotic ferrofluid), the video shows how mesmerizing diffusion, buoyancy, Marangoni flow, and other fluid effects can be up close. It may also be the first time I’ve ever seen fluid dynamics–specifically bubbles–used as characters! Also be sure to check out some of his previous videos, many of which also feature cool fluid dynamics. (Video credit and submission: K. Pimmel)
Reader Question: When Mercury Meets Lava
Reader lucondri asks:
What happens when mercury touches lava?
That’s an interesting thought experiment, but hopefully no one tries it any time soon given mercury’s toxicity. So, what might happen? Mercury has a boiling point just under 630 Kelvin, and, although the temperature of molten lava varies, it’s between 970 and 1470 Kelvin when it first erupts. So mercury would definitely vaporize (i.e. boil) on contact with lava. (Again, this is very bad for anyone nearby.) If you’re curious what boiling liquid mercury looks like, wonder no further.
Molten lava is much, much hotter than the boiling point of mercury, though, so there’s a possibility that the mercury won’t boil away instantly. This is because of the Leidenfrost effect, where a thin layer of vapor forms between a liquid and an extremely hot surface. The vapor has such low friction that the liquid can essentially skate across a surface, and it doesn’t boil away instantly because the vapor insulates it from the extreme heat. After some digging, I found a paper that placed the Leidenfrost temperature of mercury between about 850 and 950 Kelvin, meaning that fresh lava is probably hot enough to generate mercury Leidenfrost drops.
So pouring a lot of mercury on lava will probably result in some boiling, but there’s also a good chance that it will form a bunch of skittering mercury droplets that will stick around awhile before they evaporate into toxic mercury gas. That said, it’s a lot easier and safer to watch awesome Leidenfrost drop videos with other liquids. (Collage credit: N.Sharp; images sources: Z. T. Jackson, and A.Biance)
Magnetic Putty
Sometimes fluids are slow-moving enough that it takes timelapse techniques to reveal the flow. Fog is one example, and, as seen above, magnetic silly putty is another. The putty is an unusual fluid in a couple of ways. First, having been impregnated with ferromagnetic nanoparticles, it is sensitive to magnetic fields, making it a sort of ferrofluid. And secondly, being silly putty, it’s a non-Newtonian fluid, meaning that it has a nonlinear response to deformation – a fact that will be familiar to anyone who has tried to knead putty versus striking it. With a strong enough magnet, the putty makes for an impressively tenacious creeping flow. (Video credit: I. Parks; via io9; submitted by Chad W.)
When Lava Meets Ice
What happens when lava meets ice or water? Artists and geologists are working together to explore these interactions by melting crushed basalt and pouring it onto different substrates. Ice is their classic example; instead of melting instantly through the ice, the lava is so hot that it creates a layer of steam between it and the ice. This steam helps the lava flow due to lower friction while also insulating the ice from the lava. It’s an example of the Leidenfrost effect. The end result is a very bubbly lava flow thanks to the steam trying to escape through the viscous lava. (Video credit: Science Channel; submitted by @jchawner)
Reader Question: Rippling Runoff
Reader junolivi asks:
When shallow water (like runoff from melting snow) flows across pavement, it creates small repeated wave-like ripples. What creates that texture and why isn’t it just a steady flow?
This is a great question that’s probably crossed the mind of anyone who’s seen water running down the gutter of a street after a storm. The short answer is that this gravity-driven flow is becoming unstable.
Fluid dynamicists often like to characterize flows into two main types: laminar and turbulent. Most flows in nature are turbulent, like the wild swirls you see behind cars driving in the rain. But there are laminar flows in nature as well. Often flows that begin as laminar will become turbulent. This happens because those laminar flows are unstable to disturbances.
The classic example of stability is a ball on a hill. If the ball is at the top of the hill and you disturb it, it will roll down the hill because its original position was unstable. If, on the other hand, the ball is in a depression, then you can prod the ball and it will eventually settle back down into its original place because that position was stable. Another way of looking at it is that, in the unstable case, the disturbance–how far the ball is from its original position–grows uncontrollably. In the stable case, on the other hand, the disturbance can be initially large but eventually decays away to nothing.
There are many ways to disturb a laminar flow–surface roughness, vibrations, curvature, noise, etc., etc. These disturbances enter the flow and they can either grow (and become unstable) or decay (because the flow is stable to the disturbance). Just as one can look at the stability of a pendulum, one can mathematically examine the stability of a fluid flow. When one does this for water flowing down an incline, one finds that the flow is quite unstable, even in the ideal case of a pure, inviscid fluid flowing down a smooth wall.
The reason that one sees distinctive waves with a particular wavelength (assuming that they aren’t caused by local obstructions) is directly related to this idea of instability. Essentially, the waves are the disturbance, having grown large enough to see. One could imagine that any wavelength disturbance is possible in a flow, but mathematically, what one finds, is that different wavelengths have different growth rates associated with them. The wavelength we observe is the most unstable wavelength in the flow. This is the wavelength that grows so much quicker than the others that it just overwhelms them and trips the flow to turbulence. This is very common. For example, you can see distinctive waves showing up before the flow goes turbulent in both this mixing layer simulation and this boundary layer flow.
(Image credits: anataman, mo_cosmo; also special thanks to Garth G. who originally asked a similar question via email)
Asteroid Impact
I often receive questions about how fluids react to extremely hard and fast impacts. Some people wonder if there’s a regime where a fluid like water will react like a solid. In reality, nature works the opposite way. Striking a solid hard enough and fast enough makes it behave like a fluid. The video above shows a simulated impact of a 500-km asteroid in the Pacific Ocean. (Be sure to watch with captions on.) The impact rips 10 km off the crust of the Earth and sends a hypersonic shock wave of destruction around the entire Earth. There’s a strong resemblance in the asteroid impact to droplet impacts and splashes. Much of this has to do with the energy of impact. The asteroid’s kinetic (and, indeed, potential) energy prior to impact is enormous, and conservation of energy means that energy has to go somewhere. It’s that energy that vaporizes the oceans and fluidizes part of the Earth’s surface. That kinetic energy rips the orderly structure of solids apart and turns it effectively into a granular fluid. (Video credit: Discovery Channel; via J. Hertzberg)
How Eyelashes Work
New research shows that eyelashes divert airflow around the eye, serving as a passive filter that reduces dust collection and controls evaporation. Mammal hairs in places like the nose act as ram filters that trap the particles that hit them and which require regular cleaning via sneezing. Eyelashes, on the other hand, prevent dust collection by altering airflow at the surface of the eye. At the optimal length of roughly 1/3rd the width of an eye, eyelashes create a stagnation zone near the eye surface that forces air to travel above rather than through the eyelashes. This results in lower shear stress and lower flow speeds at the eye surface, both of which help reduce evaporation and shield the eye from dust. Lashes can get too long, though; the researchers found that longer lashes tended to channel higher flow speeds toward the eye surface, leading to faster evaporation rates. Thus, donning longer fake eyelashes may dry out your eyes. (Image credit: G. Diaz Fornaro; research credit: G. Amador et al.; via skunkbear)
Making a Bottle Resonate
If you’ve ever blown across the top of a bottle to make it play a note, then you’ve created a Helmholtz resonator. Air flow across the top of the bottle causes air in and around the bottle neck to vibrate up and down. Like a mass on a spring, the air oscillates with a particular frequency that depends on the system’s characteristics. We hear this vibration as a a deep hum, but in the high-speed video above, you’re actually seeing the vibration as smoke pulsing in and out of the bottle. Helmholtz resonance shows up more than just in blowing across beer bottles; it’s also a factor in many resonating instruments, like the guitar. To learn more about the physics and mathematics of the effect, check out this page from the University of New South Wales. (Video credit: N. Moore)