Compressible-Flow Basics – Definition of Subsonic Regime A flow is called subsonic if its Mach number falls in which range?

Introduction:
Mach number M is the ratio of the local flow speed to the local speed of sound. Classifying regimes by M is fundamental in compressible flow and gas dynamics. This question checks the textbook definition of subsonic flow.

Given Data / Assumptions:

• Definition uses Mach number M = V / a.
• Standard regime splits: subsonic, transonic, supersonic, hypersonic.

Concept / Approach:

By convention: subsonic M < 1, transonic roughly 0.8 to 1.2, supersonic M > 1 up to about 5, and hypersonic typically M > 5 or 6. The neat definition for subsonic is therefore M less than unity.

Step-by-Step Solution:

Verification / Alternative check:

Any aerodynamics text will define subsonic with M < 1, and reserve “unity” for sonic and “greater than 1” for supersonic/hypersonic, confirming the choice.

Why Other Options Are Wrong:

Unity: M = 1 corresponds to sonic flow, not subsonic.Between 1 and 6: This spans supersonic to hypersonic ranges.More than 6: Hypersonic; far from subsonic.Between 0.8 and 1 only: That is a subset of transonic, not the entire subsonic regime.

Common Pitfalls:

Confusing transonic margins with the exact definition, or assuming a single cutoff like 0.8. Always use the strict inequality for definitions.

Final Answer:

less than unity