# Entropy-Preserving Coupling Conditions for One-dimensional Euler Systems at Junctions

## TL;DR

This paper introduces new entropy-preserving coupling conditions for the 1D Euler system at pipe junctions, ensuring energy and entropy conservation, applicable to complex junctions with multiple pipes and flow directions.

## Contribution

The paper proposes novel coupling conditions that preserve energy and entropy at pipe junctions, extending applicability beyond previous pressure-based methods and ensuring well-posedness of solutions.

## Key findings

- Proved existence and uniqueness of solutions near stationary states.
- Established well-posedness of the Cauchy problem for small total variation data.
- Ensured physically consistent solutions for complex pipe junctions.

## Abstract

This paper is concerned with a set of novel coupling conditions for the $3\times 3$ one-dimensional Euler system with source terms at a junction of pipes with possibly different cross-sectional areas. Beside conservation of mass, we require the equality of the total enthalpy at the junction and that the specific entropy for pipes with outgoing flow equals the convex combination of all entropies that belong to pipes with incoming flow. Previously used coupling conditions include equality of pressure or dynamic pressure. They are restricted to the special case of a junction having only one pipe with outgoing flow direction. Recently, Reigstad [SIAM J. Appl. Math., 75:679--702, 2015] showed that such pressure-based coupling conditions can produce non-physical solutions for isothermal flows through the production of mechanical energy. Our new coupling conditions ensure energy as well as entropy conservation and also apply to junctions connecting an arbitrary number of pipes with flexible flow directions. We prove the existence and uniqueness of solutions to the generalised Riemann problem at a junction in the neighbourhood of constant stationary states which belong to the subsonic region. This provides the basis for the well-posedness of the homogeneous and inhomogeneous Cauchy problems for initial data with sufficiently small total variation.