Turbulent convection in stellar interiors. III. Mean-field analysis and stratification effects

TL;DR

This paper uses 3D simulations and mean-field analysis to study turbulent convection in stellar interiors, highlighting stratification effects, pressure fluctuations, and the importance of unresolved boundary physics.

Contribution

It provides a detailed mean-field analysis of turbulent convection in stellar models, emphasizing stratification effects and the role of boundary physics, with results applicable to various stellar environments.

Findings

Pressure fluctuations are more significant in stratified red giant models.

Turbulent dissipation rate is comparable to convective luminosity.

Mean-field balances are robust across different resolutions and models.

Abstract

We present 3D implicit large eddy simulations (ILES) of the turbulent convection in the envelope of a 5 Msun red giant star and in the oxygen-burning shell of a 23 Msun supernova progenitor. The numerical models are analyzed in the framework of 1D Reynolds-Averaged Navier-Stokes (RANS) equations. The effects of pressure fluctuations are more important in the red giant model, owing to larger stratification of the convective zone. We show how this impacts different terms in the mean-field equations. We clarify the driving sources of kinetic energy, and show that the rate of turbulent dissipation is comparable to the convective luminosity. Although our flows have low Mach number and are nearly adiabatic, our analysis is general and can be applied to photospheric convection as well. The robustness of our analysis of turbulent convection is supported by the insensitivity of the mean-field balances to linear mesh resolution. We find robust results for the turbulent convection zone and the stable layers in the oxygen-burning shell model, and robust results everywhere in the red giant model, but the mean fields are not well converged in the narrow boundary regions (which contain steep gradients) in the oxygen-burning shell model. This last result illustrates the importance of unresolved physics at the convective boundary, which governs the mixing there.