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Which of these does NOT reduce the drag of a bluff body?

  • A.Fitting a boat-tail to ease the pressure recovery
  • B.Polishing it below the critical Reynolds number
  • C.Adding a streamlined tail fairing
  • D.Roughening the surface to trip the layer turbulent

Answer: B

Below the critical Reynolds number a polished surface keeps the layer laminar, so it separates early and the wake stays broad. Roughening it, counter-intuitively, reduces the drag. The other options all narrow the wake by easing or delaying separation.

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