alt Hacker NewsAnyone know the physics of mach diamonds purely from compressed gas? I'm not a physicist, but I thought one of the key aspects of compressed gas is that it can't expand faster than the speed of sound in that gas -- the speed of sound is literally the limit to how fast it can expand?
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Zee2 • yesterday at 5:26 AM
Gas expanding faster than the speed of sound is called a shockwave - and the Mach diamonds are essentially the expanding gas shockwave “bouncing” back and forth as it hits the atmosphere, forming a standing wave of high and low compression zones.
Cold gas thrusters used in space applications typically produce 60-70 kgf of thrust for 1 kg/s of gas flow, which means the average axial velocity of the jet is just under 600-700 m/s. It is a little more when the gas expands into vacuum and a little less when it expands into ambient air at the sea level.
How fast the gas expands through an opening depends on the pressure ratio between entrance and exit, and for ordinary air the flow reaches the speed of sound at the narrowest part when the pressure ratio exceeds about a factor of two. The flow never becomes supersonic in the constriction itself, but after the constriction the flow can continue to accelerate to supersonic speeds, especially if the constriction is followed by an expanding nozzle (converging-diverging, or de Laval nozzle), although this is not strictly necessary. For example, one can observe shock diamonds in the air coming out of an ordinary plastic bottle filled with slightly compressed gas: https://www.youtube.com/watch?v=61tsXGUc6nU