The time step constraint in radiation hydrodynamics. Issue 1 (3rd March 2020)

• Record Type: Journal Article
• Authors: Brandenburg, Axel; Das, Upasana
• Is Part Of: Geophysical and astrophysical fluid dynamics. Volume 114:Issue 1/2(2020)
• Page Start: 162
• Page End: 195
• Publication Date: 2020-03-03
• Subjects: Numerical stability -- radiative cooling -- convection -- accretion discs -- hot stars; Fluid dynamics -- Periodicals; Astrophysics -- Periodicals; Geophysics -- Periodicals; 551
• DOI: 10.1080/03091929.2019.1676894
• Languages: English
• ISSNs: 0309-1929

Abstract

ABSTRACT: Explicit radiation hydrodynamic simulations of the atmospheres of massive stars and of convection in accretion discs around white dwarfs suffer from prohibitively short time steps due to radiation. This constraint is related to the cooling time rather than the radiative pressure, which also becomes important in hot stars and discs. We show that the radiative time step constraint is governed by the minimum of the sum of the optically thick and thin contributions rather than the smaller one of the two. In simulations with the Pencil Code, their weighting fractions are found empirically. In three-dimensional convective accretion disc simulations, the Deardorff term is found to be the main contributor to the enthalpy flux rather than the superadiabatic gradient. We conclude with a discussion of how the radiative time step problem could be mitigated in certain types of investigations.