Three-equation turbulent convection models in classical variables

TL;DR

This work addresses the limitations of 1D turbulent convection models in nonlinear radial stellar pulsations and the computational burden of multidimensional simulations by extending Kuhfuss's three-equation Reynolds-stress framework to this context. The authors introduce five targeted modifications to capture nonlocal, anisotropic convection, ionization, and opacity effects in pulsating stellar envelopes, while retaining a mixing-length-based, local dissipation description. The key contributions are enhanced dissipation of turbulent kinetic energy, a local anisotropy model, ionization energy transport corrections, opacity fluctuation handling, and turbulent damping of entropy fluctuations and convective flux. By enabling more accurate mode selection and pulsation dynamics within 1D codes, this framework facilitates efficient parameter studies and provides improved inputs for multidimensional modeling of pulsating stars.

Abstract

Context. Turbulent convection models in nonlinear radial stellar pulsation models rely on an extra equation for turbulent kinetic energy and fail to adequately explain mode-selection problems. Since multidimensional calculations are computationally expensive, it is reasonable to search for generalizations of physically grounded 1D models that approximate multidimensional results with sufficient accuracy, at least in a given parameter range. A natural way of progressing from one-equation models is to use additional nonlocal equations. While these types of models also exist in the literature, they have not been adopted for this type of object. Aims. We aim to adapt the three-equation turbulent convection model from Kuhfuss to radial stellar pulsation modeling. Methods. We use a Reynolds-stress one-point closure approach to derive our extensions alongside the model, while using additional models from the literature to close the anisotropy and dissipation terms. Results. We provide five extensions to the original model. These include an enhanced dissipation correction to the mixing length, a local anisotropy model replacing eddy viscosity, a second-order correction for turbulent ion transport in the atmosphere (alongside opacity effects), and turbulent damping of entropy fluctuations and convective flux.