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Comment by venning | Hacker News Reader

> [When] at Mach 2.2, the air feels like Jello.

I know that's a metaphor, but it got me thinking. Can someone more learned help me out: does air behave in a non-Newtonian manner at supersonic speed?

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You can think of it this way: at subsonic speeds, air ahead of the aircraft "knows" that the plane is coming, and can flow around it. At supersonic speeds, this information doesn't propagate forwards, and the plane smashes into the air ahead of it, creating a shock wave.

I'm anthropomorphizing and oversimplifying (airflow over a wing can be locally supersonic even in subsonic flight) but I don't care because I enjoy thinking of this in terms of information flow.

I am delighted by the idea of a little anthropomorphic chunk of air shouting "Move, there's a plane coming!" and then face palming upon realizing too late that it is using sound to warn other bits of air about something moving at supersonic speeds.

The speed of sound through air is effectively the speed of shockwaves through air. The way that the air responds to being pushed around at speeds faster than its shockwaves can respond results in something that behaves more like a liquid than a gas.

Not an engineer, but from my understanding air is a fluid that can be compressed. At extreme rates of speed in general atmospheric conditions, that results in a compression that the object is punching through. Often I learn about this stuff from high speed auto experiments (Bugatti, Toyota Land Speed Cruiser).

By non-Newtonian I think you mean relativistic. Boom is going Mach 2.2 or about 1500 mph. Light is going 186,000 mile per second.

Not even close.

https://en.wikipedia.org/wiki/Non-Newtonian_fluid

Thanks. They didn't cover viscosity in 7ABC at Cal. I just locked onto non Newtonian.

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