Shock diamond
Encyclopedia
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
exhaust plume of an aerospace propulsion system, such as a supersonic jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
, rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
, ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
, or scramjet
Scramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
, when it is operated in an atmosphere. The diamonds form from a complex flow field and are visible due to the ignition of excess fuel. Mach diamonds (or disks) are named for Ernst Mach
Ernst Mach
Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...
, the physicist who first described them.
Mechanism
Shock diamonds form when the supersonic exhaust from a nozzle is slightly over or under-expanded, meaning that the pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
of the gases exiting the nozzle is different from the ambient air pressure. The exhaust is generally over-expanded at low altitudes where air pressure is higher, and under-expanded at higher altitudes.
As the flow exits the nozzle, ambient air pressure will either expand or compress the flow; over-expanded flow is compressed while under-expanded flow expands. The compression or expansion is caused by oblique shock waves inclined at an angle to the flow. When the compressed flow becomes parallel to the center line, a shock wave perpendicular to the flow forms, called a normal shock wave. The first shock diamond is located here and the space between it and the nozzle is called the "zone of silence". The distance from the nozzle to the first shock diamond can be approximated by:
where x is the distance, D0 is the nozzle diameter, P0 is atmospheric pressure and P1 is flow pressure.
As the exhaust passes through the normal shock wave, its temperature increases, igniting excess fuel and causing the glow that makes the shock diamonds visible. The illuminated regions either appear as disks or diamonds, giving them their name.
At each shock diamond, the flow becomes compressed enough that it expands outward in a set of expansion waves called the expansion fan. Eventually the flow expands enough so that its pressure is again below ambient, at which point the expansion fan reflects off the contact discontinuity (the outer edge of the flow). The reflected waves, called the compression fan, cause the flow to compress. If the compression fan is strong enough, another oblique shock wave will form, creating a second shock diamond. The pattern of disks would repeat indefinitely if the gases where ideal and frictionless, however, turbulent shear at the contact discontinuity causes the wave pattern to dissipate with distance.
Alternative sources
Shock diamonds are most commonly associated with jet and rocket propulsion, but they can form in other systems.Artillery
When artillery pieces are fired, gas exits the cannon muzzle at supersonic speeds and produces a series of shock diamonds. The diamonds cause a bright "gun flash" which can expose the location of gun emplacements to the enemy. It was found that when the ratio between the flow pressure and atmospheric pressure is close to one, the shock diamonds were greatly minimized. Adding a flare to the end of the muzzle balances the pressures and prevents shock diamonds.Volcanism
Some volcanoVolcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es have been shown to produce jets containing shock diamonds. The ash-gas mixture output by a volcano has a speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
on the order of 100 m/s, and many eruptions are known to have exit velocities well in excess. This makes the base of the eruption supersonic, leading to the same dynamics that produce shock diamonds. These highly destructive jets have occurred in gas-rich volcanoes such as Mount St. Helens
Mount St. Helens
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It is south of Seattle, Washington and northeast of Portland, Oregon. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a...
and Krakatoa
Krakatoa
Krakatoa is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island , and the volcano as a whole. The island exploded in 1883, killing approximately 40,000 people, although some estimates...
.
Radio jets
Some radio jets, powerful jets of plasma that emanate from quasarQuasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s and radio galaxies, are observed to have regularly spaced knots of enhanced radio emissions. The jets travel at supersonic speed through a thin "atmosphere" of gas in space, so it is hypothesized that these knots are shock diamonds.
See also
- afterburnerAfterBurnerThe AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs.Originally, portablemonopoly.net was a website created to petition Nintendo to put some kind of light in their Game Boy Advance system...
- exhaust gasExhaust gasExhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
- plume (hydrodynamics)Plume (hydrodynamics)In hydrodynamics, a plume is a column of one fluid or gas moving through another. Several effects control the motion of the fluid, including momentum, diffusion, and buoyancy...
- rocket engine nozzle
- shock waveShock waveA shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
External links
- "Methane blast" - shock diamonds forming in NASA's methane engine built by XCOR AerospaceXCOR AerospaceXCOR Aerospace is an American private rocket engine and spaceflight development company based at the Mojave Spaceport in Mojave, California. XCOR was formed by former members of the Rotary Rocket rocket engine development team in September, 1999...
, NASA website, 4 May 2007