Compressible Flow — Mach Number Classification When the Mach number M is less than 1 (M < 1), the flow regime is called:

Introduction:
Mach number M = V/a (ratio of flow speed to local speed of sound) classifies compressible flow regimes and dictates which physical effects dominate, such as compressibility and shock formation.

Given Data / Assumptions:

• Definition of Mach number M = V/a.
• Local thermodynamic state determines a (speed of sound).
• We consider standard aerodynamic regimes.

Concept / Approach:
Flow regimes: subsonic (M < 1), transonic (around M ≈ 1 with mixed zones), supersonic (M > 1 up to about 5), hypersonic (typically M ≥ 5). Sonic flow strictly refers to M = 1 at a point (e.g., throat of a choked nozzle).

Step-by-Step Solution:
Given M < 1, compressibility effects are mild to moderate; no detached shocks form in uniform external flow.Therefore the correct classification is subsonic.

Verification / Alternative check:
Textbook tables list standard ranges: subsonic M < 1; transonic ~0.8–1.2; supersonic >1; hypersonic ≥5. Our case falls clearly in subsonic.

Why Other Options Are Wrong:
Sonic: applies at M = 1, not M < 1.
Super-sonic: requires M > 1.
Hyper-sonic: extreme range M ≥ 5.

Common Pitfalls:
Confusing transonic with sonic; ignoring that local regions can reach M ≈ 1 even when freestream is below 1; using sea-level a for high-altitude cases without correction.

Final Answer:
sub-sonic flow