Gradient and GENERIC evolution towards reduced dynamics

TL;DR

This paper explores how reduced models of macroscopic systems, including open systems with external forcing, can be analyzed using thermodynamic concepts like entropy, extending classical thermodynamics to flux-thermodynamics.

Contribution

It introduces a framework connecting macroscopic and less microscopic models through entropy functions, generalizing thermodynamics to non-equilibrium and open systems.

Findings

Reduction provides a form of thermodynamics for open systems.

Lower entropy's time derivative relates to the approach of the system to its reduced model.

Classical thermodynamics emerges as a special case when the reduced model is static.

Abstract

Let (M,J) be a dynamical model of macroscopic systems and (N,K) a less microscopic model (i.e. a model involving less details) of the same macroscopic systems; M and N are manifolds, J are vector fields on M, and K are vector fields on N. Let P be the phase portrait corresponding to (M,J) (i.e. P is the set of all trajectories in M generated by a family of vector fields in J), and R the phase portrait corresponding to (N,K). Thermodynamics in its general sense is a pattern recognition process in which R is recognized as a pattern in P. In particular, the classical (both equilibrium and nonequilibrium) thermodynamics arises in the investigation of relations between models (M,J) and models without time evolution, i.e. models with K= 0. In such case R is a submanifold of M composed of fixed points. Let Su mapping M to R be a potential, called an upper entropy, generating the vector field J. The equilibrium thermodynamic relation in N is the lower entropy Sd(y) defined by Sd(y)=Su(x=y), where x is in M,y in N, and y is given by a final destination (i.e. when the time goes to infinity) of x in the time evolution generated by the vector field J. In this paper we show that if K is not zero (e.g. in externally forced or, in other words, open systems), then the reduction also provides thermodynamics (we call it flux-thermodynamics). If certain conditions are satisfied, then the lower entropy Ss, that arises in the investigation of the approach of J to K, is the time derivative of the lower entropy Sd arising in the investigation of the approach of M to N as t goes to infinity.