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)
Category: Phenomena
Breaking Jets Into Drops
A falling stream of water will break into droplets due to the Plateau-Rayleigh instability. Small disturbances can create a wavy perturbation in the falling jet. Under the right conditions, the pressure caused by surface tension will be larger in the narrower regions and smaller in the wider ones. This imbalance will drive flow toward the wider regions and away from the narrower ones, thereby increasing the waviness in the jet. Eventually, the wavy jet breaks into droplets, which enclose the same volume of water with less surface area than the perturbed jet did. The instability is named for Joseph Plateau and Lord Rayleigh, who studied it in the late 19th century and showed that a falling jet of a non-viscous fluid would break into droplets if the wavelength of its disturbance was larger than the jet’s circumference. (Image credit: N. Morberg)
Air Pressure in Flight
We live at the bottom of a sea of air, surrounded by a constant pressure equal to 101 kPa (14.7 psi) over our entire bodies. For the most part, we don’t notice the pressure air exerts on us. But if you’ve flown on a commercial airplane, you may have noticed some of the effects of changing that air pressure. Flexible sealed containers, like bags of chips or bottles of water, change their shape dramatically over the course of a flight because the air pressure inside them can be greater than the air cabin pressure at altitude. In the video above, Nick Moore measured his in-flight cabin pressure as 84 kPa (12psi), which is equivalent to about 1500 m (5000 ft) above sea level. Why do airlines keep the cabin pressure lower in flight? The biggest reason is because the airplane, like the in-flight snack, is a pressure vessel. At cruising altitudes the outside air pressure is about 24 kPa (3.5 psi). To keep the interior of the cabin habitable, the fuselage of the airplane has to hold a higher pressure. The larger the difference between the interior and exterior pressures, the greater the stress the airplane must withstand. Keeping the air pressure in flight a little lower makes the engineering a little easier and does the occupants no harm. (Video credit: N. Moore)
Sharkskin Instability
Homemade spaghetti noodles exhibit a roughened surface that’s the result of viscoelastic behavior known as the sharkskin instability. It’s usually observed in the industrial extrusion of polymer plastics. In the case of spaghetti, the long, complex polymer molecules necessary for the instability come from the proteins in eggs. The characteristically rough surface of the extruded material is caused by the transition from flow through the die to air. Inside the die, friction from the walls exerts a strong shear force on the outer part of the fluid while the inner portion flows freely. When the material exits the die, the sudden lack of friction on the outer portion of the fluid causes it to accelerate to the same velocity as the middle of the flow. This acceleration stretches the polymers until they snap free of the die; after the strained polymers relax, the material keeps a rough, saw-tooth pattern. In industry, the sharkskin instability can be prevented by regulating temperature or flow speed. In the case of spaghetti, though, Modernist Cuisine suggests the roughness is desirable because it helps trap the pasta sauce. Bon appetit! (Image credit: Modernist Cuisine)
Making Lava
In this video, NPR’s Adam Cole takes a trip to upstate New York to find out how to make lava – and not the kind with vinegar and baking soda! We’ve featured footage from this duo before. Since most lava flows don’t occur in predictable or controlled circumstances, it can be tough for scientists to study their fluid properties and flow behaviors. Set-ups like this one allow more precise experimentation, as well as opportunities to test other wild ideas. For more, check out the full video and the Syracuse University Lava Project. (Video credit: NPR Skunk Bear/A. Cole; via skunkbear)
Spinning Paint
Fluid dynamical behaviors are often the result of competing forces. Here paint flung from a spinning rod illustrates the effects of adhesion, surface tension, and centrifugal force. In general, surface tension tries to hold a fluid together, and adhesion allows it to stay attached to a surface. Centrifugal force, on the other hand, tends to push the fluid outward. As the spinning rod accelerates, centrifugal force wins over adhesion and the paint spirals outward. For awhile, surface tension manages to hold the paint together, stretching it into spiraling ligaments of fluid. But when centrifugal force overpowers surface tension as well, the ligaments of paint snap into smaller droplets, still flying outward. Check out the full video for more great slow motion shots, and be sure to look at photographer Fabian Oefner’s “Black Hole“ series, which inspired the video. (Image credit: BBC Earth Unplugged, source video)
Underwater Explosions
As dangerous as explosions are in air, they are even more destructive in water. Because air is a compressible fluid, some part of an explosion’s energy is directed into air compression. Water, on the other hand, is incompressible, which makes it an excellent conductor of shock waves. In the video above we see some simple underwater explosions using water bottles filled with dry ice or liquid nitrogen. The explosions pulsate after detonation due to the interplay between the expanding gases and the surrounding water. When the gases expand too quickly, the water pressure is able to compress the gases back down. When the water pushes too far, the gases re-expand and the cycle repeats until the explosion’s energy is expended. This pulsating change in pressure is part of what makes underwater explosions so dangerous, especially to humans. Note in the video how the balloons ripple and distort due to the changing pressure. Those same changes in pressure can cause major internal damage to people. (Video credit: The Backyard Scientist; submitted by logicalamaze)
Water in Oil
Pouring water on an oil fire is a quick way to cause almost explosive results. Since water is denser than oil, it quickly sinks to the bottom of a container, heating up as it does. When the water reaches its boiling point, it evaporates and expands as steam. That phase change involves a huge change in volume, a fact made especially clear in the video below. The steam expands and rises, throwing droplets of oil upward and outward. These smaller atomized droplets are easier to combust, which, in the case of the video above, causes a veritable cloud of flames if a fire has already started.
(Video credits: The Slow Mo Guys and N. Moore)
Coriolis
There’s an infamous supposition about drains swirling one way in the Northern Hemisphere and the other way in the Southern Hemisphere. Destin from Smarter Every Day and Derek from Veritasium have put the claim to the test with experiments on either side of the globe. First, go here and watch their synchronized videos side-by-side. (To synchronize, start the left video and pause it at the sync point. Then start the second video and unpause the first video when the second video hits the sync point.) I’ll wait here.
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That was awesome, right?! The demonstration doesn’t work with toilets because they’re driven by the placement of jets around the circumference. And your bathtub doesn’t usually work either because any residual vorticity in the tub gets magnified by conservation of angular momentum as it drains. It’s like a spinning ice skater pulling their arms in; the rotation speeds up. So, to get around that problem, Destin and Derek let their pools sit for a day to damp out any motion before draining. At that point, the Coriolis effect is strong enough to cause the pools to rotate in opposite directions when drained. You may wonder why the effect is so slight for the pools when it’s pretty stark with hurricanes and cyclones. The answer is a matter of scale. The pools are perhaps 2 meters wide, which means that the difference in latitude across the the pool is very slight and therefore, the differential speed imparted by the Earth’s rotation is also very small. Because hurricanes and cyclones are much larger, they experience stronger influence from the Coriolis effect. (Image credits: Smarter Every Day/Veritasium; via It’s Okay To Be Smart)
Convection from a Heat Source
Convection is a major driver in many flows in nature. In this film, the UCLA Spinlab demonstrates buoyant convection caused by a local heat source. They deposit dye on a submerged, continuously heated plate, then observe as the dye slowly rises with the heated (lower density) fluid. The surface forms a cap for the rising dye, which then spreads horizontally. Qualitatively similar flows can be seen in nature over volcanic eruptions or in thunderstorms when clouds reach the troposphere or a capping inversion. Be sure to check out the rest of the Spinlab’s videos. (Video credit: UCLA Spinlab; submitted by Jon B.)
