Tag: fluid dynamics

Soap Bubble Coalescence
Droplets falling onto a bath of the same liquid will sometimes coalesce via a series of increasingly smaller droplets in a process known as the coalescence cascade. Soap bubbles, it turns out, can exhibit a similar partial coalescence. When a bubble nears a soap film and the air between them drains away, coalesce can begin. If the the soap film beneath the bubble ruptures, some air from the inside of the bubble can escape. Part of the bubble coalesces with the soap film and a smaller daughter bubble is left behind. The researchers observed this process happen up to three times before the bubble coalesced completely. Alternatively, if the soap film did not rupture, the air inside the bubble had no escape, and the bubble would coalesce into a hemispherical lens atop the soap film. (Video credit: G. Pucci et al.; via KeSimpulan)
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Don’t forget about our FYFD survey! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here. Please take a few minutes to participate and share!
July 27, 2015
Suppressing Instability
The Rayleigh Taylor instability is a common fluid phenomenon in which the interface between fluids of differing densities becomes unstable. It’s what’s responsible for all those awesome pictures of milk in ice coffee. For many years, fluid dynamicists theorized that the instability might be inhibited by rotation, which tends to suppress velocity changes along the axis of rotation. But actually creating an experiment demonstrating the effect was extremely difficult because any attempts to set a denser fluid over a lighter one before rotating it would kick off the instability. Recently, however, researchers succeeded in creating an experimental demonstration, seen in the video above. They did so by using magnetism. The initial set-up consists of two fluids of similar densities – a heavier, diamagnetic fluid on the bottom and a lighter, paramagnetic fluid floating on top. The tank was then spun up until both fluids were rotating like a rigid body. Then, the entire set-up was lowered into a vertically-oriented magnetic field. The paramagnetic fluid on top was attracted by the field while the diamagnetic fluid on the bottom was repelled. The end result is that the magnetic field created the effect of the upper fluid being heavier, thereby initiating the Rayleigh-Taylor instability. As you can see in the video, rotation does slow down–but not prevent–the instability. But it took some very clever and careful experimental design to show! (Video credit: K. Baldwin et al.)
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Don’t forget about our FYFD survey! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here. Please take a few minutes to participate and share!
July 24, 2015
Happy 5th, FYFD!
FYFD is 5 years old! Hard to believe it’s been five whole years. Thank you to everyone who has helped along the way, especially those of you who produce, submit, and share such beautiful fluid dynamics.
Thanks also to everyone who is participating in our reader survey. We’re getting a lot of great feedback. If you haven’t taken it yet, there’s still time!
And, finally, in honor of five years of FYFD, I present you with the five most popular FYFD posts of all time:
1. Swimming through surface tension – Originally posted 7 Feb 2013
2. Bioluminescence as a defense mechanism – Originally posted 4 Sep 2014
3. Liquid mushroom – Originally posted 19 Feb 2013
4. Dancing droplets – Originally posted 30 Mar 2015
5. Stepping on lava – Originally posted 19 Dec 2014
July 23, 2015
Alligators Water Dancing
Amorous alligators call to mates with a behavior known as water dancing. Their audible bellows are accompanied by infrasonic sound–vibrations below the 20 Hz limit of human hearing. These vibrations from their lungs excite Faraday waves in the water near the alligator’s back and make the surface explode in a dance of jets and atomized droplets. I’ve seen similar results in other instances of vibration, but this may be the only example of this I’ve seen in the wild. Researchers studying the phenomenon noted that the frequency of sound the alligators emit corresponds to a wavelength equal to the spacing of the raised scales, or scutes, on the alligators’ backs. They hypothesize that the shape of the scutes helps males create the display. (Image credit: N. Marven, source; research credit: P. Moriarty and R. Holt; h/t to io9)
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Don’t forget about our FYFD survey! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here. Please take a few minutes to participate and share!
July 22, 2015
Raindrops in Puddles
Watching rain drops hit a puddle or lake is remarkably fascinating. Each drop creates a little cavity in the water surface when it impacts. Large, energetic drops will create a crown-shaped splash, like the ones in the upper animation. When the cavity below the surface collapses, the water rebounds into a pillar known as a Worthington jet. Look carefully and you’ll see some of those jets are energetic enough to produce a little satellite droplet that falls back and coalesces. Altogether it’s a beautifully complex process to watch happen over and over again. (Image credit: K. Weiner, source)
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Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
July 20, 2015
Magnus Effect
Putting a little bit of spin on an object can have a big aerodynamic effect, thanks to the Magnus effect. As demonstrated in the video above, backspin on a basketball dropped from a big height will send it flying out and away. The reason spinning objects generate these counterintuitive motions is because the air flow over them creates differential pressures. On the side of the ball spinning with the flow, air is accelerated, dropping the local pressure; whereas on the opposite side, the ball spinning against the direction of flow makes the flow separate and no longer flow smoothly along that side. This causes a high pressure on that side. Like the difference in pressure on either side of an airfoil, the pressure difference across the ball creates a force that pushes the ball toward the low pressure side. Check out some of the other places Magnus effect shows up! (Video credit: Veritasium; submitted by Andrew C.)
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Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
July 17, 2015
The Kelvin-Helmholtz Instability
The Kelvin-Helmholtz instability is a pattern frequently found in nature. It has a distinctive shape, like a series of breaking ocean waves that curl over on themselves to create a string of vortices. The instability shows up when there is a velocity difference between two fluid layers. The unequal shear between the two layers magnifies any disturbance to their interface, which manifests in the fractal, overturning whorls seen in the numerical simulation above. You can find the Kelvin-Helmholtz instability in the lab, in the sky, in the ocean, on Jupiter and Mars–even on the sun! For more information on the methods used to create the simulation above, check out the full paper. (Video and research credit: K. Schaal et al.)
July 15, 2015
Recreating Hurricanes
Hurricane-related winds and storm surge cause massive damage every year. Understanding and being able to predict the impact of these storms on coastal structures can help save lives and properties. Until recently the most ferocious of hurricanes–category 5 storms that feature winds above 250 kph (150 mph)–could not be recreated in a laboratory scale. Now the University of Miami’s SUSTAIN (SUrge-STructure-Atmosphere INteraction) facility can produce category-5 equivalent winds, waves, and surge in a controlled environment. The massive test section measures 18 m x 6 m x 2 m and can be filled with over 140,000 liters of saltwater. The acrylic walls of the facility let researchers use optical flow diagnostics like particle image velocimetry (PIV) to measure flow anywhere in the test section. Some of their planned studies include experiments on how oil spills behave in storms and how strong aquaculture nets must be to maintain their catch through a storm. It will also be used to study interactions between buildings and storm surge. For more, check out their website or this video from the Weather Channel. (Image credits: Gort Photography, AFP/K. Sheridan, AP Photo/W. Lee; SUSTAIN Laboratory)
July 13, 2015
Cars Helping Cyclists
This year’s Tour de France opened with an individual time trial stage in which riders competed solo against the clock. But, according to numerical simulations, some riders may get an unfair aerodynamic advantage in the race if they have a following car. The top image shows the pressure fields around a rider with a car following 5 meters behind versus 10 meters behind. The size of the car means that it displaces air well in advance of its arrival. By following a rider closely, that car’s high pressure region can help fill in a cyclist’s wake, thereby reducing the drag the rider experiences. For a short time trial like the 13.8 km race that kicked off this year’s tour, a rider whose car follows at 5 meter could save 6 seconds over one whose car followed at the regulation 10 meter distance. (As it happens, the stage was decided by a 5 second margin.) Since not all riders get a team follow car, it’s especially important to ensure that those who do aren’t receiving an additional advantage. For more about cycling aerodynamics, check out our previous cycling posts and Tour de France series. (Image credit: TU Eindhoven, EPA/J. Jumelet; via phys.org; submitted by @NathanMechEng)
July 10, 2015
How the Grand Canyon Formed
The Grand Canyon is a monument to the power of water, air, and time. In this video from It’s Okay To Be Smart, Joe Hanson describes the formation of the Grand Canyon – from the ancient oceans that created its many layers to the tectonic upthrusts that eventually created the Colorado River that continues to cut through the Canyon’s rocks today. Fluid dynamics play a major role in the geology of the Grand Canyon, whether it’s in the mantle convection that helps drive plate tectonics or the sedimentation that builds and erodes rock layers. (Video credit: It’s Okay To Be Smart)
July 8, 2015