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GATE 2025 ME – Question 44

Fluid Mechanics · Viscous flow, boundary layer and elementary turbulent flow · 2 marks · Multiple select

For a fully-developed pipe flow, which of the following options is/are correct?

  1. For the same maximum velocity, the average velocity is higher in the turbulent regime than that of the laminar regime
  2. Compressibility effects are important if Mach number is less than 0.3
  3. For laminar flow, the friction factor is independent of surface roughness
  4. For laminar flow, friction factor decreases with decrease in Reynolds number

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Show answer and explanation

Correct answer: (A) For the same maximum velocity, the average velocity is higher in the turbulent regime than that of the laminar regime; (C) For laminar flow, the friction factor is independent of surface roughness

Explanation

In turbulent flow the velocity profile is flatter, so the average velocity is closer to the maximum than in laminar flow, where the average is half the maximum. So A is true. Compressibility matters when the Mach number is above about 0.3, so B is false. In laminar flow the friction factor is $\frac{64}{Re}$, which does not depend on roughness, so C is true. This also means the friction factor increases as the Reynolds number decreases, so D is false.