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arXiv 2209.02404physics.comp-ph
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多群中子输运模拟的Krylov线性求解器准Monte Carlo方法

A Quasi-Monte Carlo Method with Krylov Linear Solvers for Multigroup Neutron Transport Simulations

Sam Pasmann, Ilham Variansyah, C. T. Kelley, Ryan McClarren

摘要:

在这项工作中,我们研究取代标准的正交技术中使用的确定性线性求解器与固定种子准蒙特卡罗计算,以获得更准确和有效的解决方案的中子输运方程(NTE)。准蒙特卡罗(Quasi-Monte Carlo,简称QMC)是利用低偏差序列对相空间进行采样,代替传统蒙特卡罗(Monte Carlo,简称MC)所使用的伪随机数发生器。QMC技术减少了随机输运扫描的方差,因此增加了迭代方法的精度。从历史上看,QMC在很大程度上被粒子输运界所忽视,因为它打破了模拟MC粒子模拟中建模散射所需的马尔可夫假设。然而,通过使用迭代方法,NTE可以建模为纯吸收问题。这消除了明确建模粒子散射的需要,并提供了非常适合QMC的应用程序。为了获得解决方案,我们实验了三个独立的迭代求解器:标准源迭代(SI)和两个线性Krylov求解器,GMRES和BiCGSTAB。由此产生的混合迭代QMC(iQMC)求解器进行了评估三个一维板坯几何问题。在每个样本问题中,Krylov求解器实现收敛的迭代次数(最多8次)比源迭代少得多。无论使用的线性求解器,混合方法实现了$O(N^{-1})$的近似收敛速度相比,传统MC模拟的预期$O(N^{-1/2})$,在所有的测试问题。

英文摘要:

In this work we investigate replacing standard quadrature techniques used in deterministic linear solvers with a fixed-seed Quasi-Monte Carlo calculation to obtain more accurate and efficient solutions to the neutron transport equation (NTE). Quasi-Monte Carlo (QMC) is the use of low-discrepancy sequences to sample the phase space in place of pseudo-random number generators used by traditional Monte Carlo (MC). QMC techniques decrease the variance in the stochastic transport sweep and therefore increase the accuracy of the iterative method. Historically, QMC has largely been ignored by the particle transport community because it breaks the Markovian assumption needed to model scattering in analog MC particle simulations. However, by using iterative methods the NTE can be modeled as a pure-absorption problem. This removes the need to explicitly model particle scattering and provides an application well-suited for QMC. To obtain solutions we experimented with three separate iterative solvers: the standard Source Iteration (SI) and two linear Krylov Solvers, GMRES and BiCGSTAB. The resulting hybrid iterative-QMC (iQMC) solver was assessed on three one-dimensional slab geometry problems. In each sample problem the Krylov Solvers achieve convergence with far fewer iterations (up to 8x) than the Source Iteration. Regardless of the linear solver used, the hybrid method achieved an approximate convergence rate of $O(N^{-1})$, as compared to the expected $O(N^{-1/2})$ of traditional MC simulation, across all test problems.

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