# Riemann solver with internal reconstruction (RSIR) for compressible single-phase and non-equilibrium two-phase flows

## TL;DR

This paper introduces the Riemann solver with internal reconstruction (RSIR), a novel method designed to accurately and robustly solve complex compressible and two-phase flow models, improving upon existing solvers in handling discontinuities and waves.

## Contribution

The paper develops RSIR, a new Riemann solver that extends the Linde method with internal reconstruction, tailored for complex two-phase flow models with multiple waves and discontinuities.

## Key findings

- RSIR accurately handles stationary volume fraction discontinuities.
- The method demonstrates low dissipation and high accuracy on test problems.
- RSIR is versatile for complex flow models, outperforming traditional solvers.

## Abstract

A new Riemann solver is built to address numerical resolution of complex flow models. The research direction is closely linked to a variant of the Baer and Nunziato (1986) model developed in Saurel et al. (2017a). This recent model provides a link between the Marble (1963) model for two-phase dilute suspensions and dense mixtures. As in the Marble model, Saurel et al. system is weakly hyperbolic with the same 4 characteristic waves, while the system involves 7 partial differential equations. It poses serious theoretical and practical issues to built simple and accurate flow solver. To overcome related difficulties the Riemann solver of Linde (2002) is revisited. The method is first examined in the simplified context of compressible Euler equations. Physical considerations are introduced in the solver improving robustness and accuracy of the Linde method. With these modifications the flow solver appears as accurate as the HLLC solver of Toro et al. (1994). Second the two-phase flow model is considered. A locally conservative formulation is built and validated removing issues related to non-conservative terms. However, two extra major issues appear from numerical experiments: The solution appears not self-similar and multiple contact waves appear in the dispersed phase. Building HLLC-type or any other solver appears consequently challenging. The modified Linde (2002) method is thus examined for the considered flow model. Some basic properties of the equations are used, such as shock relations of the dispersed phase and jump conditions across the contact wave. Thanks to these ingredients the new Riemann solver with internal reconstruction (RSIR), modification of the Linde method, handles stationary volume fraction discontinuities, presents low dissipation for transport waves and handles shocks and expansion waves accurately. It is validated on various test problems showing method's accuracy and versatility for complex flow models. Its capabilities are illustrated on a difficult two-phase flow instability problem, unresolved before.

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Source: https://tomesphere.com/paper/1905.12086