Large filter-feeders like the manta ray face the interesting challenge of obtaining enough small particulates like plankton to sustain an animal the size of a car. They do this through what is known as ram filter-feeding, essentially swimming open-mouthed through food-laden waters, filtering out the food, and releasing the water through their gills. Their internal filtration doesn’t simply catch particles like a colander does, though – it would be too easy for the ray’s filters to clog. Instead, the animals use several alternative methods to catch and redirect particles toward their esophagus. One, known as crossflow filtration, causes water to turn sharply through the filters. Heavier particles cannot accelerate that quickly, so they are carried onward. Another method, vortex filtration, works like a tiny centrifuge, spinning the water and ejecting the heavier particles back toward the esophagus. (Video credit: Science Friday; research credit: E. Paig-Tran, thesis)
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Singing Sand Dunes
Reports of singing sand dunes date at least as far back as 800 C.E. Strange as it sounds, about forty sites around the world have been associated with this phenomenon, in which avalanches of sand grains on the outer surface of the dune cause a deep, booming hum for up to several minutes. As you can hear in the video above, the sound of the dune is somewhat like a propeller-driven airplane. A leading explanation for this behavior is that it results not from the size or shape of the sand grains but from the structure of the underlying dune.
Measurements show that the booming sand dunes contain a hard packed layer of sand 1-2 meters below the surface. When sand at the surface is disturbed by the wind or sliding researchers, it creates vibrations. Those disturbances have trouble crossing into the air or into the harder layers below. Instead they resonate in the upper surface of the sand, which acts as a waveguide, reflecting and enhancing the sound, just as the body of a violin resonates to enhance the vibration of its strings. For more, check out this video from Caltech or the research paper linked below. (Video credit: N. Vriend; research credit: M. Hunt and N. Vriend, pdf)
Review: “Life in Moving Fluids”
If you liked the prairie dog post earlier this week and you’re interested in more examples of biological fluid dynamics, you may enjoy Steven Vogel’s “Life in Moving Fluids”. I’m often asked for suggestions of readable textbooks for those who want an introduction to fluid dynamics, and this book is a great option. It addresses a wide variety of basic fluids concepts without getting as bogged down mathematically as many of the engineering texts do. It is written as an introduction to fluid dynamics for working biologists, though, so it contains plenty of technical detail – including relevant equations, discussions of basic flow measurement techniques, and overviews of the early academic literature.
It is also chock full of interesting biological applications of fluid dynamics with examples ranging from the growth patterns of barnacles to the shape-shifting drag capabilities of trees. Vogel keeps a light-hearted tone and dry humor throughout and doesn’t shy away from puns.
I read a first edition of the book (copyright 1981). The second edition, from the mid ‘90s, has updated coverage of the research literature, but I dare say the the topic has exploded within the last 20 years, so your mileage may vary with regard to the literature review. However, age in no way impacts the quality of Vogel’s coverage of the basics of fluid dynamics, and I feel confident in recommending this as an introductory text for those who’d like to pursue fluids in more depth. (Images: S. Vogel/Princeton U. Press; h/t to Chris R.)
Dyeing the River
Every year Chicago dyes part of its river green to celebrate St. Patrick’s Day. This timelapse video gives a great view of the 2016 dyeing. If you watch closely, you’ll see that what’s being put in the river isn’t originally green. It’s actually an orange powder being distributed through flour sifters by the men on the boat. The exact formula is secret, but the dye is considered environmentally safe. To mix up the dye, a chase boat follows the dye boat, using its motor and wake structure to help add some turbulence to the river. It takes several passes to get the water uniformly green, but it requires a remarkably small amount of dye to do so, only about a paint can’s worth. So enjoy a little fluid dynamics today with your festivities! (Or, if you prefer to celebrate a different sort of fluid dynamics today, allow me to offer you the physics of Guinness.) (Video credit: Chris B Photo)
Prairie Dog Physics
One challenge facing burrowing mammals is ensuring sufficient oxygen within their den. Prairie dogs achieve this with a clever use of Bernoulli’s principle. They build multiple entrances to their tunnels. One of them, labeled as Entrance A above, is built with a raised mound of dirt, while the other, Entrance B, is not. The raised mound creates an obstacle for the wind to move around, which increases the wind velocity at Entrance A compared to the normal wind speed at Entrance B. From Bernoulli’s principle, we know that a higher velocity means a lower pressure, so there is a decreasing pressure gradient through the tunnel from Entrance B to Entrance A. That favorable pressure gradient pulls fresh air through the prairie dog tunnels, allowing the colony to breathe easy. (Image credits: N. Sharp; original prairie dog photos by jinterwas and J. Kubina; final images shared under Creative Commons; research credit: S. Vogel et al.; h/t to Chris R.)
Fluids Round-up
Last week was supposed to have a fluids round-up, but we were having too much fun walking on water instead. So here it is now!
– NASA has asked Congress for funding for new X-plane programs to explore solutions for greener airliners and quieter sonic booms to enable next-generation air travel. Popular Science, Gizmodo, and Ars Technica take a closer look at the proposed projects. I won’t lie – as an aerospace engineer I am hugely in favor of this. The first ‘A’ in NASA has been neglected for quite a while and projects like these are needed if we want to advance the state-of-the-art in aeronautics.
– The New York Times’ ScienceTake video series took a look back at their most popular videos, and 3 of the top 5 videos are fluid dynamics-related. Because we are just that awesome. (via Rebecca M)
– I made a guest appearance on last week’s Improbable Research podcast, where we talked about bizarre experiments trying to unravel swimming.
– Physics Girl shows us 5 weird ways to blow out a candle. There’s some neat and potentially non-intuitive fluid dynamics involved!
– SciShow offers an explanation of why we sneeze. Spoiler alert: it’s more than just to get rid of irritants.
– Fluid dynamics made the short list for NPR’s Golden Mole awards with the discovery of dancing droplets. Here’s Skunkbear’s take on it.
– Ernst Mach, of Mach number fame, was also a bit of an artist and philosopher. (via @JenLucPiquant)
– It’s not quite fluid dynamics, but this Slow Mo Guys video of spinning burning steel wool might be their most beautiful video yet. Check it out!
(Image credit: NASA)
Molten Salt in Water
In his latest video, The Backyard Scientist explores what happens when molten salt (sodium chloride) gets poured into water. As you can see, the results are quite dramatic! He demonstrates pretty convincingly that the effect is physical – not chemical. The extreme difference in temperature between the liquid water (< 100 degrees Celsius) and the molten salt (> 800 degrees Celsius) causes the water to instantly vaporize due to the Leidenfrost effect. This vapor layer protects the liquid water from the molten salt – until it doesn’t. When some driving force causes a drop of water to touch the salt without that protective vapor layer, the extreme temperature difference superheats the water, causing it to expand violently, which drives more water into salt and feeds the explosion.
But why don’t the other molten salts he tests explode? Sodium carbonate, the third salt he tests, has a melting point of 851 degrees Celsius, 50 degrees hotter than sodium chloride. Yet for that test, the Leidenfrost effect prevents any contact between the two liquids. The key in this case, I hypothesize, is not simply the temperature difference between the water and salt, but the difference in fluid properties between sodium chloride and sodium carbonate. The breakdown of the vapor layer and subsequent contact between the water and the molten salt depends in part on instabilities in the fluids. A cavity where instabilities can grow more easily is one where the Leidenfrost effect is less likely to protect and separate the two fluids. And, in fact, it turns out that the surface tension of molten sodium chloride is significantly lower than that of molten sodium carbonate! A lower surface tension value means that the molten sodium chloride breaks into droplets more easily and its vapor cavity will respond more strongly to fluid instabilities, making it more likely to come in contact with liquid water and, thus, cause explosions. (Image/video credit: The Backyard Scientist; submitted by Simon H)
Webcast Teaser Reel
Saturday I topped off a week of water-walking physics by holding a webcast with Professor Tadd Truscott and PhD student Randy Hurd of The Splash Lab. We had an absolutely blast talking about skipping balls, aesthetics and art, sailing, STEM outreach, and much more. The video above is a short teaser for the webcast – you can watch the full hour here. There are demos, a lab tour, and even a chance to learn about how I do FYFD. If you’d like to see or take part in future webcasts, you can do so by becoming an FYFD patron! (Video credit: FYFD)
Humans Running on Water
How fast does a speedster like The Flash or Dash Parr from The Incredibles have to go to run on water? As we saw from other water-walkers like the basilisk lizard and the western and Clark’s grebes, any large creature wanting to run on water needs to generate the necessary vertical impulse by hitting the water hard, pushing off against the cavity that creates, and pulling their foot up before the cavity collapses around it.
Using basilisk lizards as our guide, we can build a simplified hydrodynamic model (following Glasheen and McMahon and Minetti et al.) to describe this process and predict a speedster’s necessary speed. If we assume our runner removes their foot before the cavity collapses, we have a relatively simple relation to satisfy, namely: the vertical impulse from the slap combined with the vertical impulse from the push, or stroke, must equal or exceed the impulse from the runner’s weight:
(Impulse from slap) + (Impulse from stroke) >= Impulse from runner’s weight
The impulse from the runner’s weight is relatively straightforward. It depends on the runner’s mass, gravity, and the time it takes the runner to complete a step. The other two terms are a bit more complicated and require some approximations. One is that we’ll treat the runner’s foot like a circular disk – this makes it easier to figure out the drag while the runner pushes against the water. Ultimately, the model requires five variables (four, if we assume that we’re on Earth):
– the runner’s mass
– the area of the runner’s foot
– the depth the runner’s foot reaches underwater
– the time it takes the runner to take one step
– the acceleration due to gravityI will spare you the math, but I’ve created an online calculator (now with English or metric versions) with the model, so you can follow along with my math or play around with your own numbers.
Click through to see how fast a human has to go to run on water.
So how fast would The Flash have to run? Barry Allen is grown man, roughly 75 kg in mass, with a foot area of about 314 cm^2. We can assume that he pushes his leg about 0.15 m into the water with each step. The best human sprinters run with a step time of 0.2 – 0.26 seconds, but Barry’s a metahuman, so we’ll give him the benefit of the doubt and say that he can take a step in 0.15 seconds. (Let’s be honest, he’s probably capable of faster than that!)
To keep from sinking, The Flash would have to strike his feet against the water at about 37 m/s. It’s a little tough to say exactly how that would translate into forward speed. Both basilisks and grebes strike the water at a higher speed than their forward velocity. Since their feet are parallel to the surface when they strike, the slap phase only gives them vertical impulse. Their forward velocity comes from the stroke phase where they can push off against the water. This suggests that a runner who generates a lot of their vertical impulse during the slap phase will be able to get more forward velocity out of the stroke phase because they can afford to push forward off the cavity instead of mostly up. That’s consistent with what we observe in the lizards and grebes; the grebe gets more of its impulse from the slap and its forward velocity is a larger percentage of its foot impact velocity compared to the basilisk.
Using the lizards and birds as our guide, we can estimate that The Flash, who gets about 45% of his necessary vertical impulse from slapping, will have a forward velocity of about 27 m/s or 98 kph. That’s a lot faster than any human has ever run – Usain Bolt has managed about 44.7 kph – but it’s not that fast. In CW’s The Flash TV show, his team estimates that he must run 650 miles per hour, or 1050 kph, to run on water. That is way faster than necessary!
How about Dash Parr, though? Dash is about 10 years old, so he’s a lot smaller than Barry. That means he has less mass to keep afloat (about 32 kg), but it also means that he has smaller feet (154 cm^2) and shorter legs (0.1 m foot depth). For the same stride rate, that means that Dash has to hit the water at 47 m/s, about 25% faster than Barry. It also means that Dash gets a tad more oomph from his slap (~46%) and runs across the water at 128 kph, about 30% faster than Barry has to go.
That’s totally doable for a superhero, but what about us regular humans? Sadly, our large mass and small feet won’t let us run on water like The Flash or Dash, but there are ways to bend the rules. One is to reduce gravity – this was the subject of an Ig Nobel prize-winning study by Minetti et al. The researchers put fins on volunteers, suspended them from a harness to reduce their effective weight, and got them to run in place in a pool. They found that fin-augmented humans could run on water in gravity about 20% of Earth’s.
Another technique is to increase a runner’s effective foot area without making them bother to lift the foot out of the water. In essence, a human can run on water across over-sized lily pads. In their study, Lothman and Ruina accomplished this with plywood pads laid out in a pool. The pads were buoyant enough to stay afloat at the water surface but would sink if a person stood still on them. But by running quickly from one to the next, their test subject was able to successfully run across water.
So, there you have it: speedsters can run on water without breaking autobahn speed limits and the rest of us can cheat. Be sure to check out the online calculator and play with the model yourself. And join me tomorrow for my special webcast with Professor Tadd Truscott and Randy Hurd, who inspired this foray into water-walking!
ETA: I updated the calculator so that there is now an English unit version as well as a metric unit version.
Research credits:
– Glasheen and McMahon, “A hydrodynamic model of locomotion in the basilisk lizard”, 1996.
– Minetti et al., “Humans running in place on water at simulated reduced gravity,” 2012.
– Lothman and Ruina, “Humans can run on water using big instantly-changable shoes,” 2012.
(Image credits: Disney, CW/DC Comics, Lothman and Ruina, source, and The Splash Lab)
Grebe Rushing Physics
As capable a water-runner as the common basilisk is, the western and Clark’s grebe is even more impressive. Not only do these birds weigh up to three times as much as an adult basilisk, but they start their water-walking from inside the water, which requires overcoming much more hydrodynamic force.
Like the lizards, grebes must slap the water with their feet to generate upward forces capable of supporting their weight above water. The birds take as many as 20 steps a second – an incredible and unmatched stride rate for a creature their size. Their feet impact the water at up at 4.5 m/s, which generates an impulse equivalent to 30-55% of the grebe’s weight. The rest of the necessary impulse comes from the stroke phase, where the bird pushes its foot down against the water.
When retracting its foot, the grebe extracts the foot with a sideways motion through the water – unlike the basilisk which pulls its foot out through the air cavity its stroke created. In order to reduce drag, the grebe’s foot collapses into a more streamlined shape as it gets pulled from the water, letting the bird set up for the next step. (Image/video credit: B. Struck, source; research credit: G. Clifton et al.)
This week FYFD is exploring the physics of walking on water, all leading up to a special webcast on March 5th with guests from The Splash Lab. The live webcast will be open to all FYFD patrons, so be sure to sign up if you want to tune in.