FYFD

Category: Phenomena

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    Lift Over Wings

    One of the most vexing topics for fluid dynamicists and their audiences is the subject of how wings generate lift. As discussed in the video above, there are a number of common but flawed explanations for this. Perhaps the most common one argues that the shape of the wing requires air moving over the top to move farther in the same amount of time, therefore moving faster. The flaw here, as my advisor used to say, is that there is no Conservation of Who-You-Were-Sitting-Next-To-When-You-Started. Nothing requires that air moving over the top and bottom of a wing meet up again. In fact, the air moving over the top of the wing outpaces air moving underneath it.

    In the Sixty Symbols video, the conclusion presented is that any complete explanation requires use of three conservation principles: mass, momentum, and energy. In essence, though, this is like saying that airplanes fly because the Navier-Stokes equations say they do. It’s not a terribly satisfying answer to someone uninterested in the mathematics.

    Part of the reason that so many explanations exist – here’s one the video didn’t touch on using circulation – is that no one has presented a simple, intuitive, and complete explanation. This is not to say that we don’t understand lift on fixed wings – we do! It’s just tough to simplify without oversimplifying.

    Here’s the bottom line, though: the shape of the wing forces air moving around it to change direction and move downward. By Newton’s 3rd law (equal and opposite reactions), that means the air pushes the wing up, thereby creating lift. (Video credit: Sixty Symbols)

  • Sunset Vortices

    Sunset Vortices

    Often our atmosphere’s transparency masks the beautiful flows around us. This spectacular image shows a flight landing in Munich just after sunrise. Low-hanging clouds get sliced by the airplane’s passage and curl into its wake. The swirls are a result of the plane’s wingtip vortices, which wrap from the high-pressure underside of the wing toward the low-pressure upperside. The vortices stretch behind in the plane’s wake, creating turbulence that can be dangerous to following planes. In fact, these vortices are a major determining factor in the frequency of take-off and landing on a given runway. The larger a plane, the larger its wingtip vortices and the more time it takes for the turbulence of its passage to dissipate to a safe level for the next aircraft. (Image credit: T. Harsch; submitted by Larry S.)

  • Jupiter On Display

    Jupiter On Display

    The rich detail of Jupiter’s atmosphere is on full display in this enhanced-color image from the Juno spacecraft. (Full resolution version here – trust me, you want to click that link.) To the north, on the left side of the image, Jupiter’s Great Red Spot swirls. To the center and right, the cloud bands of Jupiter’s southern region are coming into view. The color enhancements really highlight eddies on the edge of these bands. These are examples of Kelvin-Helmholtz instabilities caused by shear between cloud bands moving at different speeds. Within the bands, smaller vortices spin. Some of these are anti-cyclones, high-pressure storm systems found all over the planet. Jupiter’s atmosphere still holds many mysteries for scientists, but I love how every gorgeous image Juno sends back shows fluid physics written larger than life across our solar system’s biggest planet. (Image credit: NASA/JPL-Caltech/SwRI/MSSS/G. Eichstädt /S. Doran; via Gizmodo)

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    Life at the Interface

    Water striders are masters of life at the interface of water and air. Their spindly legs are skinnier than the capillary length of water, meaning that, at their size, surface tension is strong enough to overcome gravitational effects. Thus, their feet leave dimples on the interface, but the water itself holds them up. To keep from getting accidentally drenched (and thus weighed down), the striders are covered in tiny hairs that trap a layer of air that makes them hydrophobic or water-repellent. To get around, these masters of the interface use their middle legs in a manner similar to oars. They push against the dimple around their legs, which generates vortices under the surface and helps propel them. Even more impressive, the water strider can jump off the surface, a feat that requires remarkable adaptation in order to maximize the jump without breaking surface tension. (Video credit: Deep Look)

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    Paintball Collisions

    In their latest video, the Slow Mo Guys collide paintballs in mid-air, creating some pretty great paint splashes. The high-speed video does a nice job of revealing some of the typical stages a splash goes through. Initially, the paint spreads in a liquid sheet. As it expands and (necessarily) thins, holes form and grow, driving the paint into string-like ligaments. These ligaments are also stretching and eventually break up into an spray of droplets, much like the jet dripping from your faucet does. If you’d like to see some more awesome high-speed liquid collisions, check out what happens when a solid projectile hits a falling drop and then look at when a laser pulse hits a droplet. (Image and video credit: The Slow Mo Guys; submitted by Omar M.)

  • Pressing Non-Newtonian Fluids

    Pressing Non-Newtonian Fluids

    For many fluids, the relationship between force and deformation is not simple. The catch-all name for these materials is non-Newtonian fluids. In a recent episode, the Hydraulic Press Channel did some experiments extruding a couple non-Newtonian fluids: oobleck and a temperature-sensitive putty. What they demonstrated is that a fluid’s response to the forces it experiences can change depending on the rate at which force is applied.

    Take their putty example from the latter half of the video. When the hydraulic press pushes the putty slowly, it extrudes in a smooth, semi-solid string. When they increase the pressure driving the hydraulic press, it pushes the putty more quickly, causing it to spray out of the die in a shredded mess. What they actually did here is surpass a threshold for what’s known in manufacturing as the sharkskin instability. This behavior occurs due to long-chain polymer molecules in the fluid. Inside the die, flow near the walls is slowed down by friction but moves freely in the middle of the pipe. When the walls are suddenly gone, flow at the outside accelerates to match the inside of the stream, which stretches the polymers until they can snap free of the die. The result is the rough, saw-tooth-like pattern seen here. (Video and image credit: Hydraulic Press Channel, source)

  • Surge Flows

    Surge Flows

    Sandy beaches can be a great place to play with neat flows. In a recent video, Frank Howarth describes playing with beach rivers on the Oregon coast and observing a surge flow there. Under the right conditions, a current flowing over sand will build up sand ripples large enough that they form miniature dams in the flow. This traps additional water, which eventually collapses the sand ripples, releasing a surge of water. The surge tends to smooth out the sand and cause the ripple-making process to start over. It’s a fairly unusual phenomenon, but it’s one known to happen seasonally in a few specific places, like at Medano Creek in Colorado’s Great Sand Dunes National Park. There the snowmelt-fed creek surges during the late spring and early summer, releasing a fresh wave every 20 seconds or so. (Image credit: F. Howarth, source; h/t to Sebastian E.)

  • Solar Eclipses and Coronal Mass Ejections

    Solar Eclipses and Coronal Mass Ejections

    Observations of many solar phenomena have only become accessible to humans relatively recently with the advent of satellites. Prior to that, it simply wasn’t feasible to observe dynamics in the sun’s atmosphere, like solar prominences or coronal mass ejections – the sun was simply too bright to see them – except during the occasional total solar eclipse!

    In the 1970s, scientists identified massive bursts of solar plasma as coronal mass ejections. These solar storms are responsible for so-called space weather and, when directed toward Earth, can pose a hazard to technologies on the ground and astronauts in orbit. Scientists initially thought this was the first time such storms had been observed, but they later recognized that photographs and sketches of an 1860 total eclipse revealed that humanity had seen a coronal mass ejection more than 100 years before! Check out the NASA video below for the full story. You can also learn about some of the science that will be going on in today’s eclipse. And, for those in the U.S. today, have a fun and safe time viewing the ecliipse!  (Image credit: S. Habbal, M. Druckmüller and P. Aniol, source; video credit: NASA Goddard)

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    Chinese Spouting Bowl Physics

    In their newest video, the Slow Mo Guys recreated one of my favorite effects: vibration-driven droplet ejection. For this, they use a Chinese spouting bowl, which has handles that the player rubs after partially filling the bowl with water. By rubbing, a user excites a vibrational mode in the bowl. Watch the GIFs above and you can actually see the bowl deforming steadily back and forth. This is the fundamental mode, and it’s the same kind of vibration you’d get from, say, ringing a bell. 

    Without a high-speed camera, the bowl’s vibration is pretty hard to see, but it’s readily apparent from the water’s behavior in the bowl. In the video, Gav and Dan comment that the ripples (actually Faraday waves) on the water always start from the same four spots. That’s a direct result of the bowl’s movement; we see the waves starting from the points where the bowl is moving the most, the antinodes. In theory, at least, you could see different generation points if you manage to excite one of the bowl’s higher harmonics. The best part, of course, is that, once the vibration has reached a high enough amplitude, the droplets spontaneously start jumping from the water surface! (Video and image credits: The Slow Mo Guys; submitted by effyeah-artandfilm)

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    Flowing Ice

    Glaciers are kind of bizarre. Despite being very solid, they still flow, sometimes on the order of a meter a day. This flow is driven by gravity and the incredible weight of the dense ice. Near the base of the glacier, the pressure is great enough to cause some localized melting. (Very high pressures actually decrease the melting point of water.) Glaciers also move through plastic deformation – this is the internal slippage Joe refers to in the video when he compares glaciers to a deck of cards. Despite their vast differences from typical fluid flows, glacial flows are often still described by the same equations of motion used in the rest of fluid dynamics! (Video credit: It’s Okay to Be Smart; via PBS Digital Studios)