Subsonic flow
= Subsonic flow
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A subsonic flow has <Mach number> less than one, so its speed is below the local <sound speed>.
For uniform <gas> moving at $U\mathbf e_x$ past a resting body, an upstream-directed <acoustic wave> has body-frame <velocity> $(U-c)\mathbf e_x$. When $0\leq M<1$, this <velocity> points upstream. This is why the body can influence the upstream <fluid flow> through small <pressure> disturbances.