Supersonic flow

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.