Definition check – meaning of sonic flow When the Mach number equals unity (M = 1), the flow is called sonic flow. Is this statement true or false?

Introduction:
Terminology in compressible flow hinges on Mach number M = V / a. The label “sonic” corresponds to the special condition where the flow speed matches the local speed of sound, and it underpins critical-flow phenomena like choking in nozzles.

Given Data / Assumptions:

• M is computed with the local speed of sound a, not a reference value.
• Medium can be gas or, in principle, any compressible fluid with a defined a.
• No additional thermodynamic restrictions are required for the definition.

Concept / Approach:

By definition, sonic conditions occur when V = a, i.e., M = 1. This is independent of whether the process is isothermal or adiabatic; those assumptions matter for evaluating a, not for the naming convention. Sonic points often correspond to critical sections in nozzles where mass flow becomes maximum for given upstream conditions.

Step-by-Step Solution:

Verification / Alternative check:

Nozzle theory and shock-tube experiments consistently identify the M = 1 location as the sonic or critical condition, confirming the terminology.

Why Other Options Are Wrong:

False: Contradicts the standard definition.Conditional statements about isothermal, ideal gas, or incompressible flows are irrelevant to the naming convention.

Common Pitfalls:

Using a nonlocal value of a to compute M; always use the local thermodynamic state.

Final Answer:

True