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A variational principle for extended irreversible thermodynamics: heat conducting viscous fluids

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Published/Copyright: September 2, 2025

Abstract

Extended irreversible thermodynamics is a theory that expands the classical framework of nonequilibrium thermodynamics by going beyond the local-equilibrium assumption. A notable example of this is the Maxwell–Cattaneo heat flux model, which introduces a time lag in the heat flux response to temperature gradients. In this paper, we develop a variational formulation of the equations of extended irreversible thermodynamics by introducing an action principle for a nonequilibrium Lagrangian that treats thermodynamic fluxes as independent variables. A key feature of this approach is that it naturally extends both Hamilton’s principle of reversible continuum mechanics and the earlier variational formulation of classical irreversible thermodynamics. The variational principle is initially formulated in the material (Lagrangian) description, from which the Eulerian form is derived using material covariance (or relabeling symmetries). The tensorial structure of the thermodynamic fluxes dictates the choice of objective rate in the Eulerian description, and plays a central role in the emergence of nonequilibrium stresses – arising from both viscous and thermal effects – that are essential to ensure thermodynamic consistency. This framework naturally results in the Cattaneo–Christov model for heat flux. We also investigate the extension of the approach to accommodate higher-order fluxes and the general form of entropy fluxes. The variational framework presented in this paper has promising applications in the development of structure-preserving and thermodynamically consistent numerical methods. It is particularly relevant for modeling systems where entropy production is a delicate issue that requires careful treatment to ensure consistency with the laws of thermodynamics.


Corresponding author: François Gay-Balmaz, Division of Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore, E-mail: 

Sponsored by Fülöp, Tamas.


  1. Research ethics: Not applicable.

  2. Informed consent: Not applicable.

  3. Author contributions: The author has accepted responsibility for the entire content of this manuscript and approved its submission.

  4. Use of Large Language Models, AI and Machine Learning Tools: None declared.

  5. Conflict of interest: The author states no conflict of interest.

  6. Research funding: This work was supported by a Start-up grant from the Nanyang Technological University and by the Ministry of Education, Singapore, under Tier 1 Grant.

  7. Data availability: Not applicable.

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Received: 2025-02-26
Accepted: 2025-08-11
Published Online: 2025-09-02
Published in Print: 2025-10-27

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