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A surface is said to be hydraulically smooth when its roughness elements

  • A.Protrude well beyond the boundary layer
  • B.Are larger than the displacement thickness
  • C.Are visible to the naked eye
  • D.Lie entirely within the laminar sublayer

Answer: D

Roughness buried inside the sublayer never meets the turbulent part of the flow and so adds no drag. Once it protrudes through, it generates its own wakes and the friction rises sharply, which is why the leading edges of transport wings are kept exceptionally clean.

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