EHI meaning in Governmental ?
Answer: What is Energy Helicity Index mean? This page is about helicity in fluid dynamics. For helicity of magnetic fields, see magnetic helicity. For helicity in particle physics, see helicity (particle physics).
In fluid dynamics, helicity is, under appropriate conditions, an invariant of the Euler equations of fluid flow, having a topological interpretation as a measure of linkage and/or knottedness of vortex lines in the flow. This was first proved by Jean-Jacques Moreau in 1961 and Moffatt derived it in 1969 without the knowledge of Moreau's paper. This helicity invariant is an extension of Woltjer's theorem for magnetic helicity.
Let u ( x , t ) {\displaystyle \mathbf {u} (x,t)} be the velocity field and ∇ × u {\displaystyle \nabla \times \mathbf {u} } the corresponding vorticity field. Under the following three conditions, the vortex lines are transported with (or 'frozen in') the flow: (i) the fluid is inviscid; (ii) either the flow is incompressible ( ∇ ⋅ u = 0 {\displaystyle \nabla \cdot \mathbf {u} =0} ), or it is compressible with a barotropic relation p = p ( ρ ) {\displaystyle p=p(\rho )} between pressure p {\displaystyle p} and density ρ {\displaystyle \rho } ; and (iii) any body forces acting on the fluid are conservative. Under these conditions, any closed surface S {\displaystyle S} on which n ⋅ ( ∇ × u ) = 0 {\displaystyle n\cdot (\nabla \times \mathbf {u} )=0} reference
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