Multi-Symplectic Lagrangian, One-Dimensional Gas Dynamics

TL;DR

This paper formulates 1D ideal gas dynamics in a multi-symplectic Lagrangian framework, deriving conservation laws and connecting Hamiltonian equations to the de Donder-Weyl formulation, with novel nonlocal conservation laws involving Clebsch variables.

Contribution

It introduces a multi-symplectic Lagrangian formulation for 1D gas dynamics, deriving new conservation laws and relating Hamiltonian equations to the de Donder-Weyl framework.

Findings

Derived multi-symplectic form for 1D gas dynamics.

Established conservation laws from symmetries, including a novel nonlocal law.

Connected Lagrangian Hamiltonian equations to de Donder-Weyl multi-momentum formulation.

Abstract

The equations of Lagrangian, ideal, one-dimensional (1D), compressible gas dynamics are written in a multi-symplectic form using the Lagrangian mass coordinate $m$ and time $t$ as independent variables, and in which the Eulerian position of the fluid element $x=x(m,t)$ is one of the dependent variables. This approach differs from the Eulerian, multi-symplectic approach using Clebsch variables. Lagrangian constraints are used to specify equations for $x_m$, $x_t$ and $S_t$ consistent with the Lagrangian map, where $S$ is the entropy of the gas. We require $S_t=0$ corresponding to advection of the entropy $S$ with the flow. We show that the Lagrangian Hamiltonian equations are related to the de Donder-Weyl multi-momentum formulation. The pullback conservation laws and the symplecticity conservation laws are discussed. The pullback conservation laws correspond to invariance of the action with respect to translations in time (energy conservation) and translations in $m$ in Noether's theorem. The conservation law due to $m$-translation invariance gives rise to a novel nonlocal conservation law involving the Clebsch variable $r$ used to impose $\partial S(m,t)/\partial t=0$. Translation invariance with respect to $x$ in Noether's theorem is associated with momentum conservation. We obtain the Cartan-Poincar\'e form for the system, and use it to obtain a closed ideal of two-forms representing the equation system.