Why do the bottoms of shower curtains drift in toward the water coming from the shower head?
We all know that moment. You’re minding your own business, scrubbing away, and all of a sudden, the shower curtain billows up and grabs you. Scientists have debated the cause of this behavior for years. Some argued that the curtain billowed due to hot air rising from the shower. Others claimed the fast-moving spray caused lift that pulled the curtain up. But fifteen years ago, one scientist tackled the problem computationally. He performed a numerical simulation of a shower head spraying into a bath and found that this spray of droplets creates a weak horizontal vortex in the shower.
This shower vortex has a low-pressure core at the middle, which is thought to provide the suction that causes the shower curtain to billow. The scientist, David Schmidt, was awarded the 2001 Ig Nobel Prize for his work. (Image credits: N. Paix, D. Schmidt; research credit: D. Schmidt)
In flight, airplane wings produce dramatic wingtip vortices. These vortices reduce the amount of lift a 3D wing produces relative to a 2D one. How much they influence the lift depends on both the strength and proximity of the vortex. The stronger and closer it is, the more detrimental its effect. One way airplane designers reduce the effects of wingtip vortices is by adding an extra section, called a winglet, to the end of the wing. Among other effects, the winglet moves the wingtip vortex further away from the main wing, which reduces its influence and allows the airplane to regain some of the lift that would otherwise be lost. (Image credits: A. Wielandt et al., source)
Evolution often requires compromise between competing effects. Large-eared bats, for example, rely on the size of their ears to aid their echolocation, but such large ears can hurt them aerodynamically, thus limiting their flight. Results from a recent experiment, however, suggest that large ears are not a total loss aerodynamically speaking. Researchers used particle image velocimetry to study the wakes behind free-flying, large-eared bats and found significant downward flow behind the bats’ bodies. This indicates that the bats generate some lift with their ears, body, and/or tail. The position and tilt of the ears in flight is similar to forward swept wings, which the authors suggest could help contract the wake behind the ears and reduce its negative influence on flow over the wings. Although the evidence is not yet conclusive, the study does suggest that large ears may be more aerodynamically beneficial than they appear. (Image credit: L. Johansson et al./Lund University, source; via Jalopnik)
The next FYFD webcast will be this Saturday, May 21st at 1pm EDT. My guests will be Professor Jean Hertzberg of the University of Colorado at Boulder and Professor Kate Goodman of the University of Colorado at Denver. Dr. Hertzberg is the creator of the course Flow Visualization, an interdisciplinary course combining engineering, art, and fluid dynamics. It’s a class (and website) that’s been an inspiration for me and FYFD since the early days! Dr. Goodman, an expert in engineering education, earned her PhD studying the Flow Viz course and its impact. This will be wide-ranging discussion – with everything from experimental fluid dynamics and engineering education to art, photography, and hopefully even cardiac fluid dynamics!
As nocturnal hunters, owls are aerodynamically optimized for stealthy flying. This clip from BBC Earth demonstrates just how quiet a barn owl is in flight compared to a pigeon or a peregrine falcon. The owl’s large wingspan relative to its body size gives it enough lift that it does not have to flap often, allowing it to glide instead, but this is far from its only stealthy adaptation. Owl feathers feature a serrated leading edge that helps break flow over the wing into smaller, quieter vortices. Their fringe-like trailing edge breaks flow up even further and acts to damp noise from airflow. The downy feathers of the owl’s body also help muffle any noise from the bird’s movement, allowing the barn owl to fly almost silently. (Video credit: BBC Earth; via Gizmodo)
Bumblebees are small all-weather foragers, capable of flying despite tough conditions. Given the trouble that micro air vehicles have when flying in gusty winds, bumblebees can help engineers to understand how nature successfully deals with turbulence. Under smooth laminar conditions like those shown in the animation above, bumblebees stay aloft by beating their wings forward and backward in a figure-8-like motion. On both the forward downstroke and the backward upstroke, you’ll notice a blue bulge near the front of the bee’s wing. This is a leading-edge vortex, which provides much of the bee’s lift.
Researchers were curious how adding turbulence would affect their virtual bee’s flight. The still image above shows the bee in moderate freestream turbulence (shown in cyan). Surprisingly, this outside turbulence has very little effect on the flow generated by the bee, shown in pink. In fact, the researchers found that the bees could fly through turbulence without a significant increase in power. Too much turbulence does make it hard for the bee to control its flight, though. The bee’s shape makes it prone to rolling, and the researchers estimated, based on a bee’s 20 ms reaction time, that bumblebees can probably only correct that roll and maintain controlled flight at turbulence intensities less than 63% of the mean wind speed. (Image credits: T. Engels et al., source; via Physics Focus)
Plants and other non-motile organisms have developed some clever methods to disperse their seeds and spores for reproduction. Some plants use vortex rings for dispersal; others make their seeds aerodynamic. Low ground-dwellers like mushrooms must contend with a lack of wind to lift their spores and carry them away. Instead, they use evaporative cooling to generate their own air currents.
Mushroom caps contain a lot of water and, as that water evaporates, it cools air near the mushroom, just as sweat evaporating off your skin cools you. That cooler, denser air tends to spread, carrying the spores outward. At the same time, the freshly evaporated water vapor is less dense than the surrounding air, so it rises. This combination of rising and spreading is capable of carrying spores tens of centimeters into the air, where the wind is stronger and able to carry spores further. (Image credit: New Atlantis, source; research credit: E. Dressaire et al.)
Hummingbirds are incredible flyers, especially when it comes to hovering. To hover stationary and stable enough to feed, the hummingbird’s flapping pattern not only has to generate enough lift, or vertical force, to counteract their weight, but the bird must balance any forward or backward forces generated during flapping.
As you can see in the animations above, when hovering the hummingbird’s wings move forward and back rather than up and down. When slowed down even further, the figure-8 motion of the wings becomes apparent. This careful motion is key to the hover; it allows the bird to generate about 70% of its lift on the downstroke when the wings move forward and creates the remainder of the lift needed on the upstroke. For much more high-speed footage of hummingbirds, check out the full BBC Earth Unplugged video, but be warned: you may experience a cuteness overdose! (Image credit: BBC Earth Unplugged, source)
Major volcanic eruptions can be accompanied by pyroclastic flows, a mixture of rock and hot gases capable of burying entire cities, as happened in Pompeii when Mt. Vesuvius erupted in 79 C.E. For even larger eruptions, such as the one at Peach Spring Caldera some 18.8 million years ago, the pyroclastic flow can be powerful enough to move half-meter-sized blocks of rock more than 150 km from the epicenter. Through observations of these deposits, experiments like the one above, and modeling, researchers were able to deduce that the Peach Spring pyroclastic flow must have been quite dense and flowed at speeds between 5 – 20 m/s for 2.5 – 10 hours! Dense, relatively slow-moving pyroclastic flows can pick up large rocks (simulated in the experiment with large metal beads) both through shear and because their speed generates low pressure that lifts the rocks so that they get swept along by the current. (Image credit: O. Roche et al., source)
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 gravity
I 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 thatfast. 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 fasterthan 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.
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.)
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