Alphabeta Math
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A contact discontinuity with equal velocity and pressure

Example

Fix γ>1, pressure p0>0, velocity u0∈R, and distinct positive densities ρL,ρR. Set EK=p0/(γ−1)+ρKu02/2 and UK=(ρK,ρKu0,EK) for K=L,R.

Facts & Assumptions

Given: Two admissible ideal-gas states with common pressure and velocity but different densities.

[F1]

Admissible states and pressure for one-dimensional ideal-gas Euler: F(U)=(ρu,ρu2+p,u(E+p)) and E=p/(γ−1)+ρu2/2.

[F2]

Eigenstructure of the admissible ideal-gas Euler system: the middle eigenvalue is λ0=u and its field is linearly degenerate.

[F3]

Rankine–Hugoniot jumps and Lax shocks: a linearly-degenerate contact has equal family speeds on its two sides and is not a strict Lax shock.

Verification

1.1givenF1algebra

Both states are admissible because ρK>0 and their internal energy is p0/(γ−1)>0. For either state, F(UK)−u0UK=(0,p0,u0p0); subtracting the two identities gives F(UR)−F(UL)=u0(UR−UL), the Rankine–Hugoniot equality at speed u0.

2.1step 1.1F2F3algebra∎

By [F2] the speed u0 is the middle characteristic speed on both sides, so the Rankine–Hugoniot jump from step 1.1 is a linearly-degenerate contact. Density, energy, and momentum may jump while pressure and velocity remain continuous; [F3] says this contact is not a strict Lax shock because its two middle characteristic speeds are equal.

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