Compressible Flow Regimes – Hypersonic Definition by Mach Number A flow is termed hypersonic when the Mach number is significantly greater than supersonic values; in engineering practice hypersonic usually means Mach number greater than 5.

Introduction:
Compressible flow regimes are categorized by the Mach number M = V / a, where V is flow speed and a is local speed of sound. Hypersonic flows exhibit strong compressibility effects, high kinetic temperature rise, and additional phenomena such as real-gas and high-temperature chemistry effects.

Given Data / Assumptions:

• Standard atmosphere and ideal-gas baseline for defining sound speed.
• Regime boundaries are conventional engineering thresholds.
• Focus on external aerodynamics parlance.

Concept / Approach:

Typical regime names: subsonic (M < 0.8), transonic (about 0.8 to 1.2), supersonic (M > 1), and hypersonic (commonly M > 5). As M increases, shock waves strengthen, boundary layers heat up, and dissociation or ionization may occur at very high speeds.

Step-by-Step Solution:

Verification / Alternative check:

Standard aerospace references and design guides for reentry vehicles and scramjet concepts adopt M > 5 as the working definition for hypersonic, distinguishing it from ordinary supersonic flight.

Why Other Options Are Wrong:

Less than 1: Subsonic. Equal to 1: Sonic. Between 1 and 5: Supersonic but not hypersonic. Between 0.3 and 0.8: Incompressible to subsonic range.

Common Pitfalls:

Assuming a sharp universal boundary without context; forgetting that local Mach numbers on a body can differ from freestream values due to acceleration over surfaces.

Final Answer:

Mach number greater than 5