Understanding Flow Regimes Based on Mach Number

The flow regime of a fluid, particularly in aerodynamics, is often classified based on the Mach number. The Mach number is a dimensionless quantity representing the ratio of the speed of a flow past a boundary to the local speed of sound.

Mathematically, the Mach number ($\text{M}$) is defined as:

$\text{M} = \frac{\text{v}}{\text{c}}$

Where:

• $\text{v}$ is the local flow velocity
• $\text{c}$ is the speed of sound in the medium

The different flow regimes are determined by the value of the Mach number:

• Subsonic Flow: When the Mach number is less than unity ($\text{M} < 1$). In this regime, the flow velocity is less than the speed of sound. Disturbances can propagate upstream.
• Sonic Flow: When the Mach number is equal to unity ($\text{M} = 1$). This occurs when the flow velocity is exactly equal to the speed of sound.
• Supersonic Flow: When the Mach number is greater than unity ($\text{M} > 1$). In this regime, the flow velocity is greater than the speed of sound. Disturbances cannot propagate upstream.
• Hypersonic Flow: A subset of supersonic flow, typically considered when the Mach number is significantly greater than unity (often taken as $\text{M} > 5$). In this regime, compressibility effects and high temperatures become very significant.

The question asks what a Mach number greater than unity implies about the flow. Based on the definitions above, a Mach number greater than unity ($\text{M} > 1$) specifically defines the supersonic flow regime.

Therefore, a Mach number greater than unity implies that the flow is supersonic.