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The drag crisis on a sphere or cylinder is the sudden fall in drag coefficient caused by

  • A.A reduction in the free-stream velocity
  • B.Transition to turbulence, which delays separation
  • C.The onset of compressibility
  • D.The boundary layer becoming laminar

Answer: B

Once the layer goes turbulent before separating, its extra near-wall momentum carries it further round the body, so the wake narrows sharply and the pressure drag collapses. The drag coefficient of a smooth sphere drops from about 0.5 to near 0.1 across the transition.

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