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This paper extends a machine learning approach for moment closure models of the radiative transfer equation (RTE) to 2D2V, building upon previous work in 1D1V. By preserving the structure of the classical $P_N$ model and modifying only the highest-order block row, the authors derive algebraic conditions for a block-diagonal symmetrizer that guarantees symmetrizable hyperbolicity of the learned system. Numerical results demonstrate that the proposed framework improves upon the classical $P_N$ model while maintaining hyperbolicity.
Guaranteeing symmetrizable hyperbolicity in machine-learned radiative transfer models is now possible in 2D2V, opening the door to stable and accurate simulations in more complex geometries.
This is our fourth work in the series on machine learning (ML) moment closure models for the radiative transfer equation (RTE). In the first three papers of this series, we considered the RTE in slab geometry in 1D1V (i.e. one dimension in physical space and one dimension in angular space), and introduced a gradient-based ML moment closure [1], then enforced the hyperbolicity through a symmetrizer [2], or together with physical characteristic speeds by learning the eigenvalues of the Jacobian matrix [3]. Here, we extend our framework to the RTE in 2D2V (i.e. two dimensions in physical space and two dimensions in angular space). The main idea is to preserve the leading part of the classical $P_N$ model and modify only the highest-order block row. By analyzing the structural properties of the $P_N$ model, we show that its coefficient matrices are symmetric and admit a block-tridiagonal structure. Then we use this property to introduce a block-diagonal symmetrizer for the ML moment model and derive explicit algebraic conditions on the closure blocks which guarantee the symmetrizable hyperbolicity of the resulting ML system. These conditions lead to a natural parametrization of the closure in terms of a symmetric positive definite matrix together with symmetric closure blocks, which can be learned from data while automatically enforcing symmetrizable hyperbolicity by construction. The numerical results show that the proposed framework improves upon the classical $P_N$ model while maintaining hyperbolicity.
