Thermodynamically consistent modelling and simulation of two-fluid magnetohydrodynamic equations

Abstract: In this paper, we proposes a thermodynamically consistent two-fluid magnetohydrodynamic model based on the Helmholtz free energy framework, which strictly satisfies both the principles of energy conservation and entropy increase in the two-fluid model. By constructing the convexe-concave characteristics of the free energy density (convexity with respect to plasma number density and concavity with respect to temperature), the model self-consistently derives key thermodynamic quantities such as chemical potential, entropy density, and internal energy. Based on the proposed modeling framework and the convex-concave properties of the Helmholtz free energy density, we develop a temporally discrete numerical scheme with thermodynamic consistency. We rigorously prove that the proposed method satisfies the first law of thermodynamics (global energy conservation) and the second law (non-decreasing total entropy). Additionally, we provide spatiotemporal error estimates for the 2D degenerate system used in practical computations. Numerical experiments validate the effectiveness of the proposed method in capturing key plasma phenomena.