Variational Principles for Shock Dynamics in Compressible Euler Flows

TL;DR

This paper develops a novel variational framework that extends Hamilton's principle to include shock discontinuities in compressible Euler flows, enabling direct derivation of shock conditions from variations.

Contribution

It introduces a modified action principle with localized contributions at shocks, unifying shock dynamics in compressible fluids within a variational setting.

Findings

Derives Rankine--Hugoniot conditions from unrestricted variations.

Links the additional term to a dissipation potential and energy loss.

Extends the framework to full compressible Euler equations with thermodynamic considerations.

Abstract

Hamilton's principle plays a central role in fluid mechanics as a fundamental tool for deriving governing equations, analyzing conservation laws, and designing structure-preserving numerical schemes. However, its classical formulation is restricted to smooth solutions and does not directly accommodate shock discontinuities. Addressing this limitation within a variational framework has long been a challenging issue. In this paper, we develop a variational framework that extends Hamilton's principle to shock solutions in compressible fluid dynamics. For the barotropic Euler equations, we introduce a modified action principle that incorporates additional contributions localized at discontinuities. This allows the Rankine--Hugoniot conditions for mass and momentum to emerge directly from unrestricted variations, without imposing continuity across shocks. The additional term admits a natural interpretation as a dissipation potential, linking the variational structure to energy loss at shocks. We then extend the approach to the full compressible Euler equations. Using a variational formulation of nonequilibrium thermodynamics together with suitable variational and phenomenological constraints, we recover the Rankine--Hugoniot relations for mass, momentum, and energy. This yields a unified variational description of shock dynamics in compressible fluids and highlights a fundamental distinction between barotropic and full compressible models in the treatment of energy and entropy at discontinuities.