Why does air behave differently at supersonic speeds?

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I was watching a concorde documentary that explained how the behaviour of air changes when passing through a nozzle (and in other situations). I thought the sound barrier (and sonic boom) was purely a function of how sound travels through air, and not a property of the air itself. So what is happening here?

In: Physics

Anonymous 0 Comments

The sound barrier is how fast a pressure wave can move through air.

If something pushes on one bit of air, that bit of air will push on the next bit of air, which will push on the next bit of air, and so on.

That pushing is what sound is – a pressure wave moving through air.

Normally when something moves through air it pushes on the air in front of it, and that air pushes forwards as well (on the next bit of air), so you get this bow wave of air pushed and being pushed out of the way. The air a bit further ahead gets some warning of what is coming from the air itself, so can start moving out of the way.

But at supersonic speeds the object is moving too fast for this. It hits the second bit of air before the first bit of air can push onto it. The air doesn’t have time to move forwards and start clearing a path, it can only get pushed sideways or squished. And that makes things messy.