Mach Number

Definition

The Mach Number ($M$) is a dimensionless quantity representing the ratio of flow velocity to the local speed of sound.

Formula

$M = \frac{u}{a}$ where $u$ is the flow velocity and $a$ is the speed of sound.

Regimes

• Subsonic: $M < 1$
• Supersonic: $M > 1$

Relationships

• Related: Speed of Sound, Compressible Flow (Topic)