This stunning National Geographic photo contest winner shows an F-15 banking at an airshow and a array of great fluid dynamics. A vapor cloud has formed over the wings of the plane due to the acceleration of air over the top of the plane. The acceleration has dropped the local pressure enough that the moisture of the air condenses. Some of this condensation has been caught by the wingtip vortices, highlighting those as well. Finally, the twin exhausts have a wake full of shock diamonds, formed by a series of shock waves and expansion fans that adjust the exhaust’s pressure to match that of the ambient atmosphere. (Photo credit: Darryl Skinner/National Geographic; via In Focus; submitted by jshoer)
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 shockwave, 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)
In this static test of XCOR Aerospace’s Lynx rocket engine, Mach diamonds (shown at the top of the frame) are visible in the rocket exhaust. The distinctive pattern is a result of the over- or under-expansion of the exhaust jet with respect to the ambient air; in other words, the gases exiting the rocket are either too high or too low in pressure relative to the surrounding air. A series of shock waves and expansion fans forms in the exhaust jet until the pressure is equalized to ambient. It is these compressions and expansions that form the diamond pattern. (Video credit: XCOR Aerospace)
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.
In this still image from a video of a 2008 demonstration of a U.S. Navy railgun, the shock waves in front of the projectile are momentarily visible. When travelling faster than the speed of sound in air, information (in the form of pressure waves) is unable to travel ahead of the projectile, meaning that the air cannot deform around the object as it does at low speeds. Instead, a front known as a shock wave forms on or in front of the object, depending on its speed and shape. Across this shock wave, thermodynamic properties of the gas are discontinuous; the pressure, temperature, and density of the air rise drastically, but the air is also deformed so that it passes around the object. (See also: bullet from a gun.)