Mach Discs

A pattern of evenly spaced rings, sometimes visible in the exhaust of an engine, is usually called impact diamonds or Mach discs or even Mach Rings.

The Mach discs become visible when the fuel is burned outside the engine. In a turbojet engine, this is possible when the afterburner is turned on.

Mach disks are formed in a supersonic jet of outgoing gases, the pressure of which at the nozzle section is slightly greater or less than atmospheric. At the outlet of the nozzle, atmospheric pressure expands /compresses the under-expanded /over-expanded gas jet, which leads to a change in the direction of movement of the outer layers of gas from axial to radial. The change in the direction of the supersonic gas flow occurs on oblique shock waves. Where the gas flow direction again becomes parallel to the central flow line, the first direct shock wave is located. A jet of gases passing through a direct compression jump heats up and ignites the unburned fuel, making the first Mach disk visible. The glowing area can have the shape of a disk or a diamond. When passing through a direct compression jump in the Mach disk, the gas pressure may become higher than atmospheric. Then the expansion process is repeated with the final formation of the next Mach disk. This periodic process of compression/expansion would be endless if it were not for the influence of the viscosity of the gas on the damping of pressure fluctuations.