Category: Phenomena

American Football Aerodynamics
Like many sports balls, the American football’s shape and construction make a big difference in its aerodynamics. Unlike the international football (soccer ball), which undergoes significant redesigns every few years thanks to the World Cup, the American football has been largely unchanged for decades. The images above come from a computational fluid dynamics (CFD) simulation of a spiraling football in flight. Although the surface is lightly dimpled, the largest impact on aerodynamics comes from the laces and the air valve (just visible in the upper right image). Both of these features protrude into the flow and add energy and turbulence to the boundary layer. By doing so, they help keep flow attached along the football longer, which helps it fly farther and more predictably. For more, check out the video of the CFD simulation. (Image credits: CD-adapco; via engineering.com)
February 2, 2015
Inside a Can of Compressed Air
Many gases are stored in liquid form at high pressures. This video takes a look at tetrafluoroethane, better known as the substance in compressed air cans used for dusting electronics. At atmospheric pressure, tetrafluoroethane boils at about -26 degrees Celsius, but in an air duster, at around 7 atmospheres of pressure, it is a liquid. As demonstrated in the video, releasing the pressure causes the liquid to boil off. Even exposed to atmospheric pressure, though, the liquid doesn’t boil off instantly – the act of boiling requires thermal energy and, without a sufficient source of heat, the liquid consumes its own heat until it drops to a temperature below the boiling point. As it warms up from the surrounding air, it will start boiling again. I don’t recommend trying to open up an air duster can at home, though. High-pressure containers can be dangerous to open up, and tetrafluoroethane is now being phased out in some parts of the world due to its high global warming potential. (Video credit: N. Moore)
January 30, 2015
Below a Surfer’s Wave
From below a plunging breaking wave–the classic surfer’s wave–looks like a giant vortex tube. Smaller rib vortices, the rings around the main vortex in the photo above, can form where there are variations along the breaking wave. As the wave rolls on, it stretches the vorticity variations along the wave’s span. When stretched, vortices spin up and intensify; this is a result of conservation of angular momentum. Check out more amazing photos of waves in Ray Collins’ portfolio. (Photo credit: R. Collins; via The Inertia)
January 28, 2015
Melt Fracture in Plastics
Liquid plastics are often extruded–or pressure-driven through a die–during manufacturing. Early on manufacturers discovered that they could only extrude plastic at low flow rates, otherwise the plastic’s surface begins undulating in what became known as melt fracture. These corrugations result from the viscoelasticity of the plastic. Viscoelastic fluids have a response to deformation that is part viscous–like any fluid–and part elastic. At low flow rates, viscous forces dominate in the plastic, but at higher speeds, elasticity increases and the polymers in the plastic get stretched along the direction of flow. In response to this stretching, the polymers exert normal stresses, much like a rubber band that’s being stretched. Because this force acts only along the flow direction, different parts of the fluid are experiencing different forces, and these internal stresses cause the plastic to change shape. (Image credit: D. Bonn et al.)
January 26, 2015
Lava Coiling
It’s tough to get much closer to flowing lava than this video of freshly forming coastline in Hawaii. Lava is complex fluid, with viscous properties that vary significantly with chemical composition, temperature and deformation. Here, despite being very viscous, the lava flows quickly–perhaps even turbulently. Several times it forms a heap and even shows signs of the rope-coiling instability familiar from viscous fluids like honey. All in all, it’s quite mesmerizing. (Video credit: K. Singson; submitted by Stuart B.)
January 21, 2015
Fire-Breathing
In this high-speed video, the Slow Mo Guys demonstrate fire-breathing. Rather than using a liquid fuel like kerosene, they utilize cornstarch, which is both easily flammable and non-volatile thanks to its powdered form. Blowing out the cornstarch creates a turbulent jet of cornstarch and air. Combine that with a combustion source, and the cornstarch quickly deflagrates, meaning that the flame propagates via heat transfer. When neighboring regions of cornstarch become hot enough, they ignite and the flame front expands. You can observe this in the flame growth shown in the video; just after ignition the cornstarch jet is much wider than the fire and it takes some time for the flames to catch up with the jet. Although a liquid-fueled fireball operates by the same principles, it can look rather different. For comparison, check out this high-speed video of a WD-40 fireball. And, hopefully it goes without saying, but don’t try this stuff at home. (Video credit: The Slow Mo Guys)
January 12, 2015
Hand Dryers and Atomization
Some newer electric hand dryers, like the Dyson Airblade, use jets of high-speed air to dry hands faster than traditional models. Much of their effectiveness comes from the rapid atomization–or break-up into tiny droplets–of water on one’s hands. This is demonstrated in the animation above, which comes from a high-speed video of a water drop falling through the jets of a homemade dryer. Breaking up the water quickly disperses the microdroplets but it also speeds up evaporation by greatly increasing the exposed surface area of the water. This is similar to how you can get instant snow from throwing boiling water if it’s cold enough outside. (Image credit: tesla500, source video; submitted by Nick)
January 9, 2015
Interrupting Sediments
The pier at Progreso extends 6.5 kilometers into the Gulf of Mexico, creating an artificial obstruction to ocean flow and sediment transport near the shore. The first 2 kilometers of the pier are built on arches that allow some flow through, but the newer sections do not. Prevailing winds act from the east-northeast, driving flow roughly right to left in the image. The sediment traces flow around the pier and reveals the complicated flow-shadow downstream of the newer parts of the pier. (Image credit: NASA Earth Observatory)
January 7, 2015
Skydiving in Wind Tunnels
Skydivers and freefall acrobats utilize vertical wind tunnels as ground training facilities. Low-speed acrobatics, like gymnastics, relies on inertial forces and angular momentum for flips and attitude changes. But at freefall speeds, aerodynamic forces are much larger, and an acrobat’s orientation relative to the flow has a big effect on his stability and maneuverability. Simple movements of an arm or leg can significantly alter one’s aerodynamics, allowing the acrobats to choreograph controlled and synchronized motion. (Video credit: Red Bull)
Author’s note – After much consideration, I’ve decided to move FYFD to a MWF posting schedule for the time being. Working full-time has its limitations, and I believe the less frequent posting schedule will allow me to dedicate more time to generating new content like FYFD videos. This was a tough decision, but I hope it will help FYFD grow in the long-term. – Nicole
January 5, 2015
Lava-Driven Waterspouts
Seven waterspouts align as lava from the Hawaiian volcano Kilauea pours into the ocean in this striking photo from photographer Bruce Omori. Like many waterspouts–and their landbound cousins dust devils–these vortices are driven by variations in temperature and moisture content. Near the ocean surface, air and water vapor heated by the lava create a warm, moist layer beneath cooler, dry air. As the warm air rises, other air is drawn in by the low pressure left behind. Any residual vorticity in the incoming air gets magnified by conservation of angular momentum, like a spinning ice skater pulling her arms in. This creates the vortices, which are made visible by entrained steam and/or moisture condensing from the rising air. (Photo credit: B. Omori, via HPOTD; submitted by jshoer)
January 2, 2015