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A linearly-degenerate characteristic curve is a contact wave

Statement

Let ri be linearly degenerate on a neighborhood and let Ri(σ;u0) be its local rarefaction curve. Then the states u0 and u1=Ri(σ;u0) are joined by a Rankine–Hugoniot jump of speed λi(u0), and the corresponding moving discontinuity is a weak solution. The characteristic speed is equal on the two sides; this is a contact wave, not a strict Lax shock.

Facts & Assumptions

Given: A C2 linearly-degenerate eigenpair and its local integral curve.

[F1]

Strictly hyperbolic conservation systems and characteristic families: Df(u)ri(u)=λi(u)ri(u) and Dλi(u)ri(u)=0 for a linearly-degenerate family.

[F2]

Local characteristic rarefaction curve: Ri′=ri(Ri).

[F3]

Rankine–Hugoniot jumps and Lax shocks: the jump condition is [f]=s[u].

[F4]

A moving jump is a weak solution exactly under Rankine–Hugoniot: a moving jump is weak exactly when that equality holds.

Proof

1.1givenF1F2algebra

Along the curve, the chain rule gives ddσλi(Ri(σ;u0))=Dλi(Ri)ri(Ri)=0, so λi(Ri)=λi(u0). Also ddσf(Ri)=Df(Ri)ri(Ri)=λi(u0)ri(Ri).

2.1step 1.1F3F4algebra∎

Integrating the second identity from 0 to σ yields f(u1)−f(u0)=λi(u0)(u1−u0), the Rankine–Hugoniot equality [F3]. Applying [F4] gives the weak moving contact wave; equality of the characteristic speed on both sides was proved in step 1.1.

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