FYFD

Tag: compressible flow

  • Sheep as a Compressible Flow

    Not everything that flows is a fluid. And when viewed from above traffic, crowds, and even herds of sheep flow in patterns like those of a fluid. In particular, these conglomerations move like compressible fluids – ones that allow substantial changes in density as they flow. From above, each sheep is just a few pixels of white, but you can see which areas of the herd have the highest density by how white an area looks. The highest density regions also tend to be the slowest moving – not surprising in a crowd.

    Now watch the gates. They act like choke points in the flow and, to some extent, like a nozzle in supersonic flow. As the sheep approach the gate, they’re in a dense, slow moving clump, but as they pass through it, the sheep speed up and spread out. This is exactly what happens in a supersonic nozzle. On the upstream end, flow in the nozzle is subsonic and dense. But once the flow hits the speed of sound at the narrowest point in the nozzle, the opening on the downstream side allows the flow to spread out and speed up past Mach 1.  (Video credit: MuzMuzTV*; submitted by Trent D.)

    *Editor’s Note: I do my best to credit the original producers of any media featured on FYFD, but this is especially difficult with viral videos as there can be many copies, all of which are uncredited. I’ve made my best guess on this one, but if this is your video, please let me know so that I can credit you properly. Thanks!

  • Ants Avoid Traffic Jams by Giving Up

    Ants Avoid Traffic Jams by Giving Up

    Both ants and traffic are well-connected to fluid dynamics, even if they are not, strictly speaking, fluids. As it happens, ant traffic has interesting implications not only for human transit but for avoiding clogs in crowds or when pouring granular materials. 

    Ants tend to dig narrow tunnels. This helps individual ants recover from potential slips, but it also makes clogging more likely. Researchers studying the behavior of individual ants during tunnel digging found that ants entering the tunnel often turn around without collecting a grain and carrying it away. When they encounter heavy traffic, they simply reverse direction and give up. So 70% of the work of digging was done by only 30% of the ants. This seemingly unfair division of labor actually optimizes the overall traffic flow and work output for the ants as a whole. Without this instinct to turn around and ease the jam, incoming ants would cascade the traffic and worsen the jamming. (Image and research credit: J. Aguilar et al.; see also Physics Today)

  • Meteoroids

    Meteoroids

    Meteoroids are debris from earlier eras in our solar system. They can be leftovers from planets that never formed or remains of ancient collisions. When these bits rock and metal enter our atmosphere, they become meteors. Since they travel at speeds of several kilometers per second, they create incredibly strong shock waves off their bow once they’re in the atmosphere. These shock waves are so strong that they rip the air molecules apart and create a hot plasma that can scorch the outside of the meteor. That plasma also glows, which is why meteors look like a streak of light from the ground. Any remains that make it to the ground are known as meteorites, and they have some pretty awesome features. Check out the full Brain Scoop episode below to learn some of the typical (and not so typical!) characteristics of meteorites. (Image and video credit: The Brain Scoop/Field Museum)

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    Sheep as a Fluid

    Not all fluids are, well, fluid. Traffic, flocks of birds, ants, and even sheep can behave like fluids. This video shows an aerial perspective on sheep being herded, and despite the four-legged nature of these particles, they have a lot of fluid-like characteristics. You can watch ripples and waves travel through the herd and see how disturbances propagate. The herd is actually a brilliant example of compressible flow; notice how the sheep slow down and bunch up as they near the gate then speed up and spread out once they pass the constriction. This is exactly how supersonic fluids behave! (Video credit: T. Whittaker; submitted by Simon H and John B)

    If you’re in the DC area, I’ll be speaking at the Annals of Improbable Research Show at the AAAS meeting Saturday evening. Our session is open to the public, but it’s likely to be crowded, so you may want to arrive early!

  • Shock Waves in Flight

    Shock Waves in Flight

    Schlieren photography allows visualization of density gradients, such as the sharp ones created by shock waves off this T-38 aircraft flying at Mach 1.1 around 13,000 ft. Although shock waves are relatively weak at this low supersonic Mach number, they persist, as seen in the image, at significant distances from the craft. The sonic boom associated with the passage of such a vehicle overhead is due to the pressure change across a shock wave. The higher the altitude of the supersonic craft, the less intense its shock wave, and thus sonic boom, will be by the time it reaches ground level. (Photo credit: NASA)

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    Traffic Fluid Dynamics

    What does traffic have to do with fluid dynamics? Rather a lot, actually! Many parallels exist between traffic and compressible fluid flow. One such example, the concept of a shock wave, is demonstrated in the video above. As the traffic jam develops, the cars experience sudden changes in their velocity and relative distance (in a fluid, this would be density). This change travels backward through the traffic in the form of a shockwave, just the same as discontinuous changes in a fluid.

    Road construction provides another common example of compressible-flow-like behavior in cars.  For an incompressible fluid like water, reducing the area of a pipe would increase the velocity, but just the opposite happens when a road is reduced from two lanes to one.  Traffic slows down and clumps together. When the road opens back up from one lane to two, suddenly the speed and the distance between cars increases. This is exactly what happens in a rocket nozzle–it’s the expanding bell-like shape that causes air to accelerate supersonically. (Video credit: New Scientist)

  • Blast Waves

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    Watch closely in this high-speed video of a bomb exploding and you will see the spherical blast wave moving outward as a visual distortion. The increase in temperature caused by the leading shockwave changes the index of refraction of the air, bending the light and distorting our view of the background. The mechanism is similar to schlieren photography, which has been used for more than a century to capture images of compressible flows.

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    Shock Waves

    Flow visualization really can be considered a form of art. Though we fluid mechanicians are looking for physics, we’re quite aware of the beauty of what we study. The clips in this video mostly show transient shockwave behavior, including lots of shock reflection and even a few instabilities. It’s unclear what the speeds are, aside from faster than sound; the medium is air.

  • Shock Waves From a Gun

    Shock Waves From a Gun

    Often fluid motion is invisible to the human eye. Researchers use techniques like schlieren photography to make changes in fluid density apparent. In this high-speed schlieren photo, an AK-47 is being fired. The spherical shock wave centered on the gun’s muzzle is due to the explosive discharge of gases used to fire the bullet.  At the left of the frame, the bullet also causes a shock wave, this time a conical one, as it travels supersonically out of the gun.

    Photo Source; Credit: Gary Settles, Penn State Gas Dynamics Lab