Shock Diamond - Mechanism

Mechanism

Shock diamonds form when the supersonic exhaust from a nozzle is slightly over or under-expanded, meaning that the pressure 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 were ideal and frictionless, however, turbulent shear at the contact discontinuity causes the wave pattern to dissipate with distance.