Compressible-flow regimes – classify by Mach number According to standard nomenclature, when the Mach number exceeds 6, the flow is called:

Introduction:
Mach number M = V / a (flow speed divided by local speed of sound) classifies compressible-flow regimes. Recognizing regime boundaries is crucial for aerothermodynamics, nozzle design, and high-speed testing.

Given Data / Assumptions:

• Standard regime names: subsonic, transonic, supersonic, hypersonic.
• Approximate thresholds used in practice.

Concept / Approach:

Common conventions are: subsonic M < 1, transonic roughly 0.8 to 1.2, supersonic 1 to about 5, and hypersonic above about 5 or 6 depending on source. The question uses M > 6, which clearly lies in the hypersonic regime. Therefore the correct label is hypersonic (often spelled “hypersonic”; some sources write “hyper-sonic” in options).

Step-by-Step Solution:

Verification / Alternative check:

Aircraft like the reentry vehicles and certain missiles operate at hypersonic speeds (M ≫ 5), confirming the classification.

Why Other Options Are Wrong:

Subsonic / sonic / supersonic / transonic: Do not cover M greater than 6.

Common Pitfalls:

Assuming an exact single cutoff everywhere; while literature varies between 5 and 6, M > 6 is unquestionably hypersonic.

Final Answer:

hyper-sonic flow