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.)
Tag: supersonic
Supersonic Flow Around a Cylinder
This numerical simulation shows unsteady supersonic flow (Mach 2) around a circular cylinder. On the right are contours of density, and on the left is entropy viscosity, used for stability in the computations. After the flow starts, the bow shock in front of the cylinder and its reflections off the walls and the shock waves in the cylinder’s wake relax into a steady-state condition. About halfway through the video, you will notice the von Karman vortex street of alternating vortices shed from the cylinder, much like one sees at low speeds. The simulation is inviscid to simplify the equations, which are solved using tools from the FEniCS project. (Video credit: M. Nazarov)
Supersonic Stellar Jets
Astronomers studying stellar jets–massive outflows of gases and particles pouring from the poles of newborn stars–are finding reasons to turn to fluid dynamicists to understand the timelapse videos they’ve stitched together from multiple exposures from the Hubble telescope. Usually astronomical events unfold on such a slow timescale that our only view of them is as a snapshot frozen in time. Stellar jets can move relatively quickly, though, with portions of the jet flowing at supersonic speeds. Over the course of Hubble’s lifetime, these jets have been imaged multiple times, allowing astronomers to create movies that reveal swirling eddies and shock wave motion previously unseen. (submitted by sakalgirl)
How Shock Waves Form
Most people are familiar with the Doppler effect–in which the frequency of a wave changes depending on the motion of the observer relative to the wave source–from the shifting pitch of sirens as they pass. But the effect is important for pressure waves in addition to acoustic waves. When an object moves through air, its motion disturbs the surrounding air via pressure waves, which travel at the speed of sound. If an object moves slower than the speed of sound (top right), then those pressure waves extend in front of the object, carrying information about the object and allowing the air to shift and move smoothly around it.
If the object is moving at the speed of sound (bottom left), then it arrives at the same time as the pressure waves. In essence, the object is striking a stationary wall of air–this is what was meant by “breaking the sound barrier”. At Mach 1, the physics of the problem have fundamentally shifted. Now the only way for air to deflect to allow the object’s passing is by the sudden compression of a shock wave.
Moving even faster than the speed of sound (bottom right) the pressure and sound waves created by the object’s motion stretch in a cone behind it. The cone, known as a Mach cone, is the shock wave that deflects air around the moving object. The result is that the object will actually pass an observer before the observer will hear it. This is because no information can travel forward of the Mach cone’s leading edge. That’s why the area outside of the Mach cone is sometimes called the Zone of Silence. When the Mach cone passes an observer, the shock wave will register as a boom, like when the space shuttle passes overhead while landing. (via fyeahchemistry)
Bow Shock over a Perforated Plate
This schlieren image shows a sphere traveling at Mach 3 over a perforated plate. The bow shock in front of the sphere is clearly visible, as is its reflection off the plate. The pressure caused by the bow shock produces a series of spherical acoustic waves below the plate. A tiny vortex ring moves downward from each hole, followed at the right by a secondary ring moving upward from the holes in the plate. (Photo credit: U.S. Army Ballistic Research Laboratory; reprinted in Van Dyke’s An Album of Fluid Motion)
Voyager Explores the Edge of the Solar System
Though unconventional by our terrestrial concepts of fluids, the solar wind and its interaction with objects in and around our solar system can be considered a form of fluid dynamics. This NASA video discusses discoveries made by the Voyager spacecrafts as they leave our solar system and pass into interstellar space. The solar wind, a rarefied stream of charged particles, streams outward from the Sun at supersonic speeds. Eventually, the pressure from the interstellar medium surrounding the solar system is sufficient to slow the solar wind to subsonic speeds, causing a termination shock much like the hydraulic jump that forms in a kitchen sink when you turn the faucet on.
How Scramjets Work
The scramjet–supersonic combustion ramjet–engine has been a holy grail of aerospace engineering for 50 years. It is an air-breathing engine with no moving parts capable of propelling crafts at hypersonic speeds beyond Mach 5. As indicated in the name, combustion in the scramjet occurs at supersonic speeds, where the heating due to air compression is sufficient to ignite fuel when injected into the engine. At present the record for the highest speed attained in scramjet flight is held by the NASA X-43A, which reached Mach 9.8 in 2004 after about 10 seconds of scramjet free-flight. The longest scramjet flight belongs to the Boeing X-51 Waverider with 140 seconds of burn time in a 2010 test flight. Few tests of these experimental hypersonic crafts have been completely successful; they represent the frontier of aerospace technology.
Computational Shock Compression
[original media no longer available]
Computational modeling can help verify and visualize experimental results, as in this video of supersonic flow. Oak Ridge National Laboratory produced the work as part of a project using shock compression and turbines to capture carbon dioxide gas. Shock waves and velocity profiles are shown throughout the computational field, and velocity isosurfaces paint a telling portrait of the complicated flow pattern. Wired Science features other award-winning simulation videos, many of which also feature fluid dynamics. #
Rocket Diamonds
The exhaust of a Pratt and Whitney J58 shines with Mach diamonds, a series of shock waves and expansion fans that form to equalize the exhaust and ambient pressures. This pattern can occur any time an engine nozzle operates at its non-ideal altitude.
Tank Shock Waves
High-speed video of a tank firing at 18000 fps shows shock waves made visible due to light distortion. When the air density changes (due to temperature or compression), it’s index of refraction changes, causing the background to appear distorted. Most of the video shows the subsonic development of the turbulent exhaust plume. Note the speed at which the exhaust moves relative to the airborne shrapnel. (submitted by Stephan)
