FYFD

Tag: physics

  • Turbojet Engines

    [original media no longer available]

    GE has a great new video with a straightforward explanation of the turbojet and the turbofan engines. The simplest description of the engines–suck, squeeze, bang, blow–sounds like a euphemism but it’s fairly accurate. The engines draw in air, compress it by making it flow through a series of small rotating blades, add fuel and combust the mixture, pull out energy through a turbine, and then blow the high-speed exhaust out the back to generate thrust. The thrust is key because it’s the force that overcomes drag on the plane and also generates the speed needed to create lift. There are two ways to significantly increase thrust: a) increase the mass flow rate of air through the engine, and/or b) increase the exhaust velocity. The turbojet engine draws in smaller amounts of air but generates very high exhaust velocities. The turbofan is today’s preferred commercial aircraft engine because it can generate thrust more efficiently at the desired aircraft velocity. The turbofan essentially has a turbojet engine in its center and is surrounded by a large air-bypass. Most of the air passing through the engine flows through the bypass and the fan. This increases its velocity only slightly, but it means that the engine accelerates much larger amounts of air without requiring much larger amounts of fuel. As an added bonus, the lower exhaust velocities of the turbofan engine make it much quieter in operation. (Video credit: General Electric)

  • Balloon Explosion

    Balloon Explosion

    These photos are shadowgraphs of a hydrogen flame exploding inside a balloon. The shadowgraph optical technique highlights density and temperature variations through their effect on a fluid’s refractive index. Here we see that the hydrogen flame has a strong cellular structure and is more turbulent than a methane flame. The cellular structure is a sign of an instability in the curved flame front. The instability and accompanying cellular appearance are a result of the complicated transport and reaction of fuel and oxidizer inside the flame. (Photo credits: P. Julien et al.)

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    Jupiter Timelapse

    This timelapse video shows Jupiter as seen by Voyager 1. In it, each second corresponds to approximately 1 Jupiter day, or 10 Earth hours. Be sure to fullscreen it so that you can appreciate the details. The timelapse highlights the differences in velocity (and even flow direction!) between Jupiter’s cloud bands. It is these velocity differences that create the shear forces which cause Kelvin-Helmholtz instabilities–the series of overturning eddies–seen between the bands. Earth also has bands of winds moving in opposite directions, but there are fewer of them and the composition of our atmosphere is such that they do not make for such a dramatic naked eye view of large-scale fluid dynamics. (Video credit: NASA/JPL/B. Jónsson/I. Regan)

  • Distorted Rings

    Distorted Rings

    The Marangoni effect is generated by variations in surface tension at an interface. Such variations can be temperature-driven, concentration-driven, or simply due to the mixing between fluids of differing surface tensions as is the case here. The pattern in the image above formed after a dyed water droplet impacted a layer of glycerin. The initial impact of the drop formed an inner circle and outer ring. This image is from 30 seconds or so after impact, after the Marangoni instability has taken over. The higher surface tension of the water pulls the glycerin toward it, resulting in a flower-like pattern. (Photo credit: E. Tan and S. Thoroddsen)

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    “Chromatic Mushrooms”

    Chemical Bouillon’s art often mixes chemistry and fluid dynamics. Here dense UV dyes falling through a less dense fluid form long strings with mushroom-like caps or tree-like branches. (For reference, gravity is pointing up relative to the video frame in most clips.) This behavior is related to the Rayleigh-Taylor instability that deforms interfaces and causes mixing between unstably stratified fluids.  (Video credit: Chemical Bouillon)

  • Wing-Warping

    Wing-Warping

    This replica of the Wright brothers’ 1902 glider demonstrates one of the important innovations the brothers contributed toward powered heavier-than-air flight. To control an aircraft in roll, the Wright brothers developed the idea of wing-warping. The pilot would lie in the cradle (center of image) and shift his body to one side. A system of wires and pulleys would then twist the wings from their rear edge, pulling one side down and the other up. This deflection is akin to changing the wing’s angle of attack. Deflecting the right wingtip downward increased the lift on the right side of the glider, while simultaneously the upward deflection on the left decreased the lift on that side. This causes the glider to bank, or roll, with the right wing up, thereby generating a leftward turn. The lift differential also caused a drag differential, though, with increased drag on the lifted (right, in this case) wing. That extra drag tended to pull the aircraft’s nose rightward, a condition known as adverse yaw. To counter it, the Wright brothers installed a steerable rudder and linked it to the wing-warping mechanism, allowing them to turn with much less effort than other conventional craft. Although wing-warping has been replaced with ailerons, the control principles remain the same. For more, watch this demo of the wing warping mechanism on a 1903 Wright Flyer replica. (Image credit: C. Devers)

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    Stirring Up

    When a viscoelastic non-Newtonian fluid is stirred, it climbs up the stirring rod. This behavior is known as the Weissenberg effect and results from the polymers in the fluid getting tangled and bunched due to the stirring. You may have noticed this effect in the kitchen when beating egg whites. In this video, researchers explore the effect using rodless stirring. The first example in the video shows a viscous Newtonian fluid being stirred. The stirring action creates a concave shape in the glycerin-air interface, and dye injection shows a toroidal vortex formed over the stirrer. Fluid near the center of the vortex is pulled downward and circulates out to the sides. In contrast, the viscoelastic fluid bulges outward when stirred. Dye visualization reveals fluid being pulled up the center into the bulge. It then travels outward, forming a mushroom-cap-like shape before sinking down the outside. This is also a toroidal vortex, but it rotates opposite the direction of the Newtonian one. Exactly how the polymers create this change in flow behavior is a matter of active research. (Video credit: E. Soto et al.)

  • Brazuca

    Brazuca

    Since 2006, Adidas has unveiled a new football design for each FIFA World Cup. This year’s ball, the Brazuca, is the first 6-panel ball and features glued panels instead of stitched ones. It also has a grippy surface covered in tiny nubs. Wind tunnel tests indicate the Brazuca experiences less drag than other recent low-panel-number footballs as well as less drag than a conventional 32-panel ball. Its stability and trajectory in flight are also more similar to a conventional ball than other recent World Cup balls, particularly the infamous Jabulani of the 2010 World Cup. The Brazuca’s similar flight performance relative to a conventional ball is likely due to its rough surface. Like the many stitched seams of a conventional football, the nubs on the Brazuca help trip flow around the ball to turbulence, much like dimples on a golf ball. Because the roughness is uniformly distributed, this transition is likely to happen simultaneously on all sides of the ball. Contrast this with a smooth, 8-panel football like the Jabulani; with fewer seams to trip flow on the ball, transition is uneven, causing a pressure imbalance across the ball that makes it change its trajectory. For more, be sure to check out the Brazuca articles at National Geographic and Popular Mechanics, as well as the original research article. (Photo credit: D. Karmann; research credit: S. Hong and T. Asai)

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    Going Supercritical

    Supercritical fluids exist at temperatures and pressures above the critical point, in a region of the phase diagram where there is no clear boundary between the liquid and gaseous state. Supercritical fluids have some of the properties of each state: they can move as freely as a gas, but they are still capable of dissolving materials like a liquid does. They also have no surface tension because there is no interface between liquid and solid. These properties make supercritical fluids very useful in industrial applications, including decaffeination and chemical deposition. Interestingly, the temperatures and pressures on Venus are so high that scientists think the atmosphere at the surface is a supercritical fluid. (Video credit: SCFED Project)

  • Mach Diamonds

    Mach Diamonds

    Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)