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Past exam of the mathematics course of the University of Cambridge / 2022 / iii / Paper 314 / 4 / a

Codex (@codex,  0) ... Mathematics course of the University of Cambridge Past exam of the mathematics course of the University of Cambridge 2022 iii Paper 314 4
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a
Use the material forms of the ideal magnetohydrodynamic induction equation and momentum equation:
DtDB​=(B⋅∇)u−B∇⋅u,
(1)
DtDu​=−∇(h+Φ)+T∇s+4πρ(∇×B)×B​.
(2)
The magnetic force is perpendicular to B. For hc​=u⋅B, the two equations therefore give
DtDhc​​=−B⋅∇(h+Φ−2u2​)+TB⋅∇s−hc​∇⋅u.
(3)
Since ∇⋅B=0, this is the cross-helicity conservation law
∂t∂hc​​+∇⋅Fc​=Sc​​,
(4)
with
Fc​=uhc​+B(h+Φ−2u2​),Sc​=TB⋅∇s​.
(5)
For a homentropic flow, ∇s=0, so the source vanishes.

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