Turbulent Convection in Stellar Interiors. II. The Velocity Field

TL;DR

This paper uses 3D hydrodynamic simulations to improve the understanding of turbulent convection in stellar interiors, challenging traditional mixing-length theory and proposing new models for stellar evolution calculations.

Contribution

It introduces the CABS method for modeling stellar convection based on simulations and develops a turbulent kinetic energy equation accounting for nonlocal and time-dependent effects.

Findings

Eddy size is approximately the depth of the convection zone.

MLT underestimates the convective velocity scale.

Dissipation length matches the mixing length in models.

Abstract

We analyze stellar convection with the aid of 3D hydrodynamic simulations, introducing the turbulent cascade into our theoretical analysis. We devise closures of the Reynolds-decomposed mean field equations by simple physical modeling of the simulations (we relate temperature and density fluctuations via coefficients); the procedure (CABS, Convection Algorithms Based on Simulations) is terrestrially testable and is amenable to systematic improvement. We develop a turbulent kinetic energy equation which contains both nonlocal and time dependent terms, and is appropriate if the convective transit time is shorter than the evolutionary time scale. The interpretation of mixing-length theory (MLT) as generally used in astrophysics is incorrect; MLT forces the mixing length to be an imposed constant. Direct tests show that the damping associated with the flow is that suggested by Kolmogorov. The eddy size is approximately the depth of the convection zone, and this dissipation length corresponds to the "mixing length". New terms involving local heating by turbulent dissipation should appear in the stellar evolution equations. The enthalpy flux ("convective luminosity") is directly connected to the buoyant acceleration, and hence the velocity scale. MLT tends to systematically underestimate this velocity scale. Quantitative comparison with a variety of 3D simulations reveals a previously recognized consistency. Examples of application to stellar evolution will be presented in subsequent papers in this series.