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Which decomposition splits the velocity gradient into four parts?

  • A.Rotational and divergent parts, obtained by applying the Helmholtz theorem to the horizontal wind field
  • B.Divergence, vorticity, stretching deformation and shearing deformation
  • C.Advection, stretching, tilting and the solenoidal term, which are the members of the vorticity budget
  • D.Zonal and meridional components

Answer: B

The two by two horizontal velocity gradient tensor has exactly four independent components, conventionally taken as divergence, vertical vorticity and the two deformations. Every local distortion of a fluid element is a combination of these four. The vorticity equation terms describe how vorticity changes rather than how the flow is decomposed, and the Helmholtz split is a different and coarser division into two parts. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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