This short film for the 2016 Gallery of Fluid Motion features Montana State University students experiencing fluid dynamics in the classroom and in their daily lives. As in her previous film (which we deconstructed), Shanon Reckinger aims to illustrate some of our everyday interactions with fluids. This time identifying individual phenomena is left as an exercise for the viewer, but there are hints hidden in the classroom scenes. How many can you catch? I’ve labeled some of the ones I noticed in the tags. (Video credit: S. Reckinger et al.)
Tag: fluid dynamics
Crow Instability
Watching airplane contrails overhead, you may have noticed them transform into a daisy chain of distorted rings. This is an effect known as the Crow instability. The contrails themselves are the airplane’s wingtip vortices, made visible by water vapor condensed out of the engine exhaust. These two initially parallel vortex lines spin in opposite directions. A slight crosswind can disturb the initially straight lines, causing them to become wavy. This waviness increases over time until the vortex lines almost touch. Then the vortices pinch off and reconnect into a line of vortex rings that slowly dissipate. Be sure to check out the full-resolution version of this animation for maximum effect. (Image credit: J. Hertzberg, source)
Supercritical
Supercritical fluids are neither a gas nor a liquid. The video above shows a tube of pressurized xenon, initially below its boiling point of approximately ~16 deg C. As the temperature is raised, you see the meniscus that marks the liquid xenon disappear. At this point, the xenon has transitioned into the supercritical state. It takes up the entire tube – like a gas – but it is still capable of dissolving materials – like a liquid. At the same time, though, the xenon has no surface tension because there’s no liquid/vapor interface. Toward the end of the video, the temperature gets reduced and the xenon condenses back into a liquid state. Supercritical fluids can be used in a wide variety of industrial applications, including in decaffeination, dry cleaning, and refrigeration. (Video credit: wwwperiodictableru)
Swirling Pollen
This photo captures the chaotic mixing present in a simple puddle. Pine pollen strewn across the puddle’s surface acts as tracer particles, revealing some of the motion of the underlying water. As wind blows across the puddle, it moves the water through the formation of ripples and by shearing the surface. That deformation on the top of the puddle will cause further motion beneath the surface. With time and changing wind direction, the resulting pattern of flow can be very complex! (Photo credit: K. Jensen, original)
Pelican Surfing
Birds can be incredibly clever about using their surroundings to enhance their flight. Pelicans will even surf! As a line of waves rolls toward shore, it pushes a small updraft ahead of it – just like a line of mountains creates a windy updraft. Pelicans save energy by riding the updraft just like a surfer would ride the swell. Once the wave breaks, the air and water become turbulent and less useful, so the pelican cuts away to find his next ride. (Image and submission credit: N. Yarvin, source)
Fluorescein Ghosts
Fluorescein is a popular chemical for flow visualization, and, as this video from Shanks FX demonstrates, it’s not hard to extract from highlighters if you’d like to experiment with it yourself. Fluorescein can also be purchased in powder form, but it’s typically rendered into a dye before use. When dripped into water, it can leave behind ghostly glowing wakes. Happy Halloween! (Video credit: Shanks FX)
In other news, I am back from my vacation! Thanks again to Claire from Brilliant Botany for looking out for everything while I was gone. – Nicole
Non-Newtonian Splashes
What happens when a stream of liquid falls through a screen? As the above video shows, water creates a beautiful flower-like burst of fluid when it hits a screen. Adding a little polymer to the water makes it non-Newtonian and more viscous. When hitting the screen, this slows it down but doesn’t prevent the fluid from flowing.
Add enough polymer, though, and the fluid becomes what’s known as a yield-stress fluid. These fluids behave much like a solid–they don’t flow–until you apply a certain amount of stress. Then they’ll flow. If you’ve ever tried to get ketchup out of a glass bottle, then you’re familiar with how these yield-stress fluids act. When dropped onto a screen, the yield-stress fluid just forms a pile–unless the impact speed is high enough to create the necessary force to get the fluid to flow! (Video credit: B. Blackwell et al.)
Reflecting in a Stream
Total internal reflection traps three lasers in a stream of falling water. When light tries to pass from the water – a material with a high refractive index – to the air – which has a lower index of fraction – it can only do so if its angle of incidence is smaller than the critical angle. Here, the light impacts the water-air boundary at a large angle and rather than refracting across the interface – like the distorted view of a straw in a glass of water – the laser light is completely reflected. Instead of escaping, the laser light is trapped, becoming a ribbon of light that swirls inside the water stream until the light is diffused. (Image credits: L. Yarnell et al.; F. Batrack et al.)
The Pythagorean Cup
According to legend, Pythagoras invented a cup to prevent his students from drinking too greedily. If they overfilled the cup, it would immediately drain out all the fluid. The trick works thanks to a U-shaped tube in the center of the cup. As long as the liquid level is below the highest point in the U-tube, only the entrance side of the tube will be filled. As soon as the liquid level in the cup is higher, the weight of all that fluid forces liquid up and around the bend. This kicks off a siphoning effect that pulls all the fluid out. Coincidentally, this is the same way that toilet flushing works! Pulling the handle releases extra water into the bowl that raises the fluid level higher than the highest point in a U-bend. That establishes a siphon, which (provided nothing has clogged the pipe), empties the toilet bowl. (Video credit: Periodic Videos)
Shaking in the Wind
Sitting at a traffic stop on a windy day, you may have noticed the beam holding the traffic lights shaking steadily up and down. This phenomenon is called vortex-induced vibration. When the wind flows over the beam, it looks something like the flow animation shown above. Airflow follows the shape of the beam until near the backside, where the air separates from the surface and creates a vortex that sloughs off into the beam’s wake. These vortices form asymmetrically on the beam – first on one side, then the other. This creates unequal pressures on either side of the beam, and those pressure differences create a force that moves the beam. Because vortices are being steadily shed off the beam, it will keep moving back and forth as long as the wind is strong enough. (Image credits: traffic light – L. Sennick, source; cylinder – Aphex82/Wikimedia)
