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Riblets, the fine streamwise grooves trialled on transport aircraft, reduce drag by

  • A.Reducing the wetted area of the surface
  • B.Eliminating the laminar sublayer
  • C.Impeding near-wall turbulent structures
  • D.Making the boundary layer laminar

Answer: C

Grooves of the right size impede the lateral movement of the streamwise vortices that dominate the near-wall region, cutting turbulent momentum transport and with it the shear stress. Reductions of a few per cent have been measured, though keeping the surface clean and intact in service is the obstacle.

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