Why is the Lax entropy condition $${\lambda _i}({{\mathbf{u}}_R}) \leqslant {\sigma _i} \leqslant {\lambda _i}({{\mathbf{u}}_L}),$$ where $i = 1,2$, a sufficient condition for uniqueness of the solution of the shallow water equations
$$\begin{split}
{h_t} + {\left( {hu} \right)_x} = 0, \hfill \\
{\left( {hu} \right)_t} + {\left( {h{u^2} + \tfrac{1}{2}g{h^2}} \right)_x} = 0 \hfill \\
\end{split} $$
?
Here $\sigma_i$ is the i-th shock speed and $\lambda_i$ is the i-th characteristic speed of the shallow water equations.
Furthermore, ${{\mathbf{u}}_L}$ and ${{\mathbf{u}}_R}$ are the states to the left and right of the shocks respectively, so ${\lambda _1}({{\mathbf{u}}_L}) = {u_L} – \sqrt {g{h_L}} ,\;\;\;{\lambda _2}({{\mathbf{u}}_L}) = {u_L} + \sqrt {g{h_L}} ,\;\;\;{\lambda _1}({{\mathbf{u}}_R}) = {u_R} – \sqrt {g{h_R}} ,\;\;\;{\lambda _2}({{\mathbf{u}}_R}) = {u_R} + \sqrt {g{h_R}} $ where $u_L, u_R$ are the fluid speeds to the left and right of the shocks, and $h_L, h_R$ are the heights of the fluid columns to the left and right of the shocks. Also, ${\sigma _i} = {u_L} \pm {h_R}\sqrt {\frac{g}{2}\left( {\frac{1}{{{h_R}}} + \frac{1}{{{h_L}}}} \right)} = {u_R} \pm {h_L}\sqrt {\frac{g}{2}\left( {\frac{1}{{{h_R}}} + \frac{1}{{{h_L}}}} \right)}$ where $-$ corresponds to $i=1$ and $+$ corresponds to $i=2$ are the shock speeds.
I'm reading some lecture notes (not in English, so there's no point in uploading them here) where it just says that it's easy to show that the Lax entropy conditions are sufficient for uniqueness of solution of the shallow water equations (no proof), but I don't see why this is true.
Are there any books where I can read more about this?
Best Answer
From (1) p. 274
Existence and uniqueness results need specifications. Are we speaking of classical solutions or weak solutions? Are we speaking of the Cauchy problem (general IVP) or of the Riemann problem (particular IVP)? In the latter case, there is the following result ((2) p. 84):
Here, "admissible" means satisfying the Lax entropy condition.
More results of this king can be found in (3).
(1) R.J. LeVeque, Finite Volume Methods for Hyperbolic Problems, Cambridge University Press, 2002 doi:10.1017/CBO9780511791253
(2) E. Godlewski, P.-A. Raviart, Numerical Approximation of Hyperbolic Systems of Conservation Laws, Springer, 1996 doi:10.1007/978-1-4612-0713-9
(3) C.M. Dafermos, Hyperbolic Conservation Laws in Continuum Physics, 4th ed., Springer, 2016 doi:10.1007/978-3-662-49451-6