2025.06.23

[日時]　2025年 7月24日（木）14:00〜17:00

　14:00-　Laura Currie（ダラム大学；本年度客員研究員）
　15:30-　政田 洋平（福岡大学）

[場所]　核融合科学研究所 研究I期棟8階801号室 ＋ オンライン併用

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[講師]　Prof. Laura K. Currie (Department of Mathematical Sciences, Durham University, UK)

[講演題目]　Towards a diffusion-free regime in models of stellar and planetary convection: theory and simulations

[講演概要]
The evolution of stars and planets is governed by turbulent flows in their interiors, which transport heat and generate magnetic fields. These flows are often convective, and their dynamics are strongly influenced by rotation and density stratification. Modelling such convection remains a central challenge in astrophysics due to the extreme parameter regimes involved: microscopic viscosities and thermal diffusivities in stars and planets are many orders of magnitude smaller than those accessible in simulations or experiments. In the astrophysical limit of vanishing diffusivities, it is generally assumed that the turbulent flow becomes “diffusion-free”, meaning its large-scale properties are independent of the microscopic transport coefficients. This assumption underlies Mixing Length Theory (MLT), which is widely used to model stellar and planetary convection. However, standard MLT does not incorporate key physical effects such as rotation. In this talk, I will present an extension of MLT that includes rotation and compare its predictions with results from numerical simulations of rotating convection. I will also describe an idealised simulation framework that incorporates internal heating and cooling, designed to more directly approach the diffusion-free regime by bypassing traditional boundary-driven setups. If time permits, I will also discuss how knowledge of dissipation in stratified convective flows can help constrain other properties of the convection, offering further insight into the dynamics of stellar and planetary interiors.

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[講師]　Prof. Yohei Masada (Department of Physics, Fukuoka University)

[講演題目]　Modeling of Convection and Transport in the Sun

[講演概要]
Turbulent convection plays a vital role in transporting energy from the solar interior to its surface, yet its physical nature remains poorly understood. The conventional picture, based on the gradient-diffusion and mixing-length theories, assumes that a super-adiabatic entropy gradient drives multi-scale convection throughout the entire convection zone (CZ), from small-scale granules to large-scale giant cells. However, recent observations challenge this view, revealing a significant discrepancy between predicted and observed large-scale convective motions—a puzzle known as the “convection conundrum.” As an alternative, a surface cooling-driven model has been proposed, in which entropy is lost at the photosphere, generating plume-like downflows without requiring super-adiabaticity in the deep CZ.
In this study, we investigate the physical differences between these two paradigms — entropy-grad-driven and cooling-driven convection models — through a series of direct numerical simulations (DNSs). Despite having nearly identical input energy, the two models exhibit strikingly different characteristics in turbulent energy transport. By applying Fourier filtering and double-averaging techniques, we isolate non-equilibrium effects and identify key features unique to plume-driven convection. Building on this analysis, we construct a theoretical model that captures the essential transport mechanisms in cooling-driven convection.
In this talk, I will discuss in detail the contrasting characteristics of the two convection models —entropy-gradient-driven and cooling-driven— and explore how they shape the nature of turbulent energy transport in the solar CZ.

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