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Which Richardson number threshold indicates possible shear instability?

  • A.Ten
  • B.Zero, since any non-zero stratification is sufficient to suppress the growth of a shear instability
  • C.One quarter
  • D.One, below which the shear becomes comparable with the stratification and turbulence begins to develop

Answer: C

Miles and Howard showed that a Richardson number below one quarter somewhere in the flow is a necessary condition for instability of a stratified shear layer. Above that value the stratification suppresses the growth. The criterion is necessary rather than sufficient, so a flow may satisfy it and still remain stable, which is a distinction worth keeping in mind.

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