Shock Diamonds and Mach Disks in jet engines exhaust can be particularly evident and fascinating at night.
The video below shows RAF Lakenheath’s Propulsion Test Cell during a test of an F-15E Strike Eagle’s afterburning turbofan engine. The 4K footage is particularly interesting as it allows us to see, from close distance, the shock diamonds in the exhaust plume of the Pratt & Whitney F100.
In short, these shock diamonds are the visible effect of an abrupt change in local density and pressure in the exhaust. The exit pressure of the exhaust plume is lower than the atmospheric pressure. This is called an “overexpansion”. When a flow is overexpanded, the higher external atmosphere causes the lower pressure exhaust to compress. This compression increases the pressure of the exhaust that expands again gradually reducing the pressure again. Therefore, the flow contracts and expands until the pressure of the plume equalizes the atmospheric pressure.
A great description of how shock diamonds form and the theory behind it, can be found at the Aerospaceweb.org website.
Here’s an excerpt of the article by Jeff Scott posted there:
While exiting the nozzle, the flow near the centerline will be moving parallel to that centerline. However, the pressure of the ambient atmosphere beyond the free jet boundary is higher than that of the exhaust. This higher pressure forces the exhaust to turn inward towards the centerline. This turning is made possible through a type of wave called an oblique shock wave. Any shock wave causes a change in the pressure of a flow, an increase in this case. […]
On the other hand, a shock wave that is perpendicular to the direction of the flow is called a normal shock wave. A normal shock can be seen in the above diagram when the flow again turns parallel to the centerline. This normal shock creates a Mach disk in the exhaust flow. Passing through this normal shock wave causes the temperature of the flow to increase, igniting any excess fuel present in the exhaust making it burn. It is this burning fuel that makes the Mach disk glow and become visible to create the ring pattern.
Like the oblique shock, this normal shock wave also increases the pressure of the exhaust gases. However, the flow becomes so compressed that the pressure is now greater than that of the ambient atmosphere. As a result, the flow begins to turn outward and the exhaust expands as it tries to equalize with the external air. This turning of the flow is accomplished through a series of expansion waves that reflect off the free jet boundary and towards the centerline. These waves cause the flow to turn outward and reduce in pressure. The expansion waves then encounter their “twins” from the opposite side of the nozzle at the centerline and reflect back outward towards the free jet boundary, also called the contact discontinuity. As the flow passes through these reflected expansion waves, it is turned parallel to the centerline and again reduces in pressure. These two sets of expansion waves are collectively referred to as an expansion fan.
The expansion fan decreases the pressure of the exhaust, but it is now lower than the ambient pressure again. As the expansion waves reach the contact discontinuity, they again reflect back inward to create compression waves and a compression fan. These compression waves force the flow to turn back inward and increase in pressure. If the compression waves are strong enough, they will merge into an oblique shock wave and form a new Mach disk similar to that near the nozzle exit. This series of compression and shock waves increase the pressure above the external air causing a new expansion fan to form, and so on. This process repeats itself again and again to create the series of Mach disks that we recognize as shock diamonds in the exhaust of a jet or rocket engine.
Along with the Mach Disks the clip shows also the variable geometry of the nozzles, something I have already described in details here.
The theory behind the nozzles operations is simple: when the thrust increases, the nozzles open incrementally to adapt the exhaust section and accommodate the fuel enriched re-ignited gasses. If the nozzles did not open after selecting the afterburner, the high pressure and temperature could cause the turbine blades to overheat and fail.
Therefore, generally speaking:
- nozzles are open at rest, with the aircraft standing still at the parking slot
- when the aircraft is on the ground taxiing under idle thrust, the diameter section is decreased (the nozzles close)
- when afterburner is selected, the nozzles open
- when in flight, the nozzle position remains to minimum diameter until afterburner is selected
One last thing, the video shows a blue-colored plume. Although the color of the plume may depend on various factors, usually, the blue/purple flame color is typical of Russian engines. As explained in a previous article about the Tu-22M3 Backfire engines, when the color of the plume blue, there’s been a more complete combustion and the reaction has created enough energy to excite and ionize gas molecules in the flame, leading to a blue flame appearance. In other words the blue flame is produced by molecular radicals, especially CH and C2, which emit most of their light in the blue region of the visible spectrum. The orange color is instead caused by fuel not completely burned out before exiting the nozzle, thus creating soot (unburnt carbon particles) that glows in the heat of the flame giving the orange color.