Modeling multi-phase gases in cosmological simulations using compressible multi-fluid hydrodynamics

TL;DR

This paper introduces a novel computational method for simulating multi-phase gases in cosmological contexts, accurately modeling interactions between cold clouds and hot diffuse media using a two-fluid hydrodynamics approach.

Contribution

It presents a new moving-mesh finite-volume method to model multi-phase gases in cosmological simulations, maintaining pressure equilibrium and resolving phase interactions more accurately.

Findings

Maintains pressure equilibrium with machine precision.

Second order accuracy on smooth hydrodynamics problems.

Demonstrates improved modeling of galaxy formation processes.

Abstract

The diffuse medium in and around galaxies can exist in a multi-phase state: small, cold gas clouds contributing significantly to the total mass embedded in pressure equilibrium with a hotter, more diffuse volume-filling component. Modeling this multi-phase state in cosmological simulations poses a significant challenge due to the requirements to spatially resolve the clouds and consequently the interactions between the phases. In this paper, we present a novel method to model this gas state in cosmological hydrodynamical simulations. We solve the compressible two-fluid hydrodynamic equations using a moving-mesh finite-volume method and define mass, momentum and energy exchange terms between the phases as operator-split source terms. Using a stratified flow model, our implementation is able to maintain volume fraction discontinuities in pressure equilibrium to machine precision, allowing for the treatment of both resolved and unresolved multi-phase fluids. The solver remains second order accurate on smooth hydrodynamics problems. We use the source and sink terms of an existing two-phase model for the interstellar medium to demonstrate the value of this type of approach in simulations of galaxy formation, compare it to its effective equation of state implementation, and discuss its advantages in future large-scale simulations of galaxy formation.