Compressible flows – definition of sonic flow A flow is termed “sonic” (critically choked) when the Mach number (M) is unity. Do you agree?

Introduction:
Mach number M is the ratio of local flow speed to local speed of sound. Sonic conditions play a central role in nozzle choking, shock formation, and compressible-flow similarity.

Given Data / Assumptions:

• Local speed of sound a depends on thermodynamic state (e.g., a = sqrt(gamma * R * T) for ideal gases).
• Flow speed V may vary along streamlines.
• Continuum, compressible regime applies.

Concept / Approach:
By definition, M = V / a. Sonic flow occurs when V equals a, i.e., M = 1. In ducts and nozzles, this condition marks the critical station that controls mass flow when choking occurs.

Step-by-Step Solution:
1) Determine local a from fluid properties.2) Measure or compute V at the point of interest.3) Compute M = V / a.4) If M = 1, the state is sonic; M < 1 is subsonic; M > 1 is supersonic.

Verification / Alternative check:
In converging–diverging nozzles, the throat reaches M = 1 at choked conditions; further backpressure reduction shifts acceleration to the diverging section with supersonic exit speeds.

Why Other Options Are Wrong:

• Disagree: contradicts the definition of Mach number.
• Restrictions to ideal gases, nozzles, or sea level are unnecessary; M = 1 is definitional regardless of medium.

Common Pitfalls:
Confusing “sonic” with “sound speed in still air” rather than the local medium; ignoring temperature dependence of a.

Final Answer:
Agree