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A single velocity reading at the centre of a duct overstates the mean because the profile is

  • A.unrelated to the position across the section
  • B.fastest at the centre and slowest at the walls
  • C.uniform across the whole section
  • D.fastest at the walls and slowest at the centre

Answer: B

Friction at the wall makes the velocity fall to zero there, so the centre-line value exceeds the mean by a tenth or more depending on the flow regime. A traverse of several points, or a factor from a standard, is what corrects it.

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