= Supersonic flow
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= Supersonic
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A supersonic flow has <Mach number> greater than one, so the flow speed exceeds the local <sound speed>.
For uniform <gas> moving at $U\mathbf e_x$ past a resting body, even an <acoustic wave> directed against the <fluid flow> has body-frame <velocity> $(U-c)\mathbf e_x$ downstream. More generally, an <acoustic wave> travelling along a <unit vector> $\mathbf n$ has body-frame <velocity> $U\mathbf e_x+c\mathbf n$; its $x$ component is at least $U-c>0$. All such weak signals move downstream and their possible directions fit inside a <Mach cone>.
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