Testing turbulent closure models with convection simulations

TL;DR

This study evaluates simple analytical turbulence closure models against 3D convection simulations across various stellar rotation rates and latitudes, finding that the models perform well at slow and fast rotations but less so at intermediate rates.

Contribution

It introduces a calibration of closure model parameters using simulation data and assesses their validity across different rotation rates and latitudes in stellar convection.

Findings

Closure parameters depend on rotation rate and latitude.

Closure models perform well at slow and fast rotations.

Model accuracy decreases at intermediate rotation rates.

Abstract

We compare simple analytical closure models of homogeneous turbulent Boussinesq convection for stellar applications with three-dimensional simulations. We use simple analytical closure models to compute the fluxes of angular momentum and heat as a function of rotation rate measured by the Taylor number. We also investigate cases with varying angles between the angular velocity and gravity vectors, corresponding to locating the computational domain at different latitudes ranging from the pole to the equator of the star. We perform three-dimensional numerical simulations in the same parameter regimes for comparison. The free parameters appearing in the closure models are calibrated by two fitting methods using simulation data. Unique determination of the closure parameters is possible only in the non-rotating case or when the system is placed at the pole. In the other cases the fit procedures yield somewhat differing results. The quality of the closure is tested by substituting the resulting coefficients back into the closure model and comparing with the simulation results. To eliminate the possibilities that the results obtained depend on the aspect ratio of the simulation domain or suffer from too small Rayleigh numbers we performed runs varying these parameters. The simulation data for the Reynolds stress and heat fluxes broadly agree with previous compressible simulations. The closure works fairly well with slow and fast rotation but its quality degrades for intermediate rotation rates. We find that the closure parameters depend not only on rotation rate but also on latitude. The weak dependence on Rayleigh number and the aspect ratio of the domain indicates that our results are generally valid