Vorticity and Symplecticity in Multi-Symplectic Lagrangian Gas Dynamics

TL;DR

This paper formulates multi-dimensional ideal compressible gas dynamics in a multi-symplectic framework, revealing new conservation laws related to vorticity, symplecticity, and fluid relabeling symmetries.

Contribution

It introduces a multi-symplectic formulation of gas dynamics incorporating constraints via Lagrange multipliers, and derives novel vorticity and symplecticity conservation laws using geometric methods.

Findings

Derivation of vorticity and potential vorticity conservation laws.

Establishment of symplecticity laws related to energy derivatives.

Application of Lie dragging and Noether's theorem to conservation laws.

Abstract

The Lagrangian, multi-dimensional, ideal, compressible gasdynamic equations are written in a multi-symplectic form, in which the Lagrangian fluid labels, $m^i$ (the Lagrangian mass coordinates) and time $t$ are the independent variables, and in which the Eulerian position of the fluid element ${\bf x}={\bf x}({\bf m},t)$ and the entropy $S=S({\bf m},t)$ are the dependent variables. Constraints in the variational principle are incorporated by means of Lagrange multipliers. The constraints are: the entropy advection equation $S_t=0$, the Lagrangian map equation ${\bf x}_t={\bf u}$ where ${\bf u}$ is the fluid velocity, and the mass continuity equation which has the form $J=\tau$ where $J=\det(x_{ij})$ is the Jacobian of the Lagrangian map in which $x_{ij}=\partial x^i/\partial m^j$ and $\tau=1/\rho$ is the specific volume of the gas. The internal energy per unit volume of the gas $\varepsilon=\varepsilon(\rho,S)$ corresponds to a non-barotropic gas. The Lagrangian is used to define multi-momenta, and to develop de-Donder Weyl Hamiltonian equations. The de Donder Weyl equations are cast in a multi-symplectic form. The pullback conservation laws and the symplecticity conservation laws are obtained. One class of symplecticity conservation laws give rise to vorticity and potential vorticity type conservation laws, and another class of symplecticity laws are related to derivatives of the Lagrangian energy conservation law with respect to the Lagrangian mass coordinates $m^i$. We show that the vorticity-symplecticity laws can be derived by a Lie dragging method, and also by using Noether's second theorem and a fluid relabelling symmetry which is a divergence symmetry of the action. We obtain the Cartan-Poincar\'e form describing the equations and we discuss a set of differential forms representing the equation system.