The 'Mach number' becomes important in fluid mechanics when:
Choose the correct answer
In all flow of water regardless of velocity
When V approaches speed of sound (Ma > 0.3) — density changes significant; not usually relevant in water flow or low-speed air
Only for laminar flow
When Reynolds number exceeds 10⁶
Correct Answer
B. When V approaches speed of sound (Ma > 0.3) — density changes significant; not usually relevant in water flow or low-speed air
Why is this the answer?
Mach number Ma = V/c where c = speed of sound = √(γRT) for ideal gas. Ma < 0.3: incompressible flow (density changes < 5%, can ignore). Ma 0.3–0.8: subsonic compressible. Ma ~ 1.0: transonic. Ma > 1.0: supersonic (shock waves). Ma > 5: hypersonic. Civil engineering: almost all applications use water or slow air → Ma << 0.3 → incompressible → Mach number irrelevant. Relevant in: pneumatic conveying, gas pipelines at high velocity, aerodynamic forces on tall buildings in special cases.
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