论文标题
磁化等离子体中各向异性扩散的网格细化
Mesh Refinement for Anisotropic Diffusion in Magnetized Plasmas
论文作者
论文摘要
对于磁性限制的融合反应器的成功设计和运行,需要对等离子体传输的高度精确模拟。不幸的是,磁化等离子体中存在的极端各向异性导致薄的边界层解决方案昂贵。这项工作调查了网格改进策略如何减少费用以进行更有效的模拟。首先,一旦网格解决薄边界层,高阶离散化只会意识到适当的收敛速率,从而激发了对边界层的焦点。研究了三种网状细化策略:一种通过使用等于边界层宽度的矩形元素来聚焦整个层的完善,该元素允许在网格间距中脱离层的指数增长,并利用既定的Zienkiewicz和Zhu误差估算器利用一种自适应策略。在4个具有高各向异性的二维测试用例中,自适应网状细化策略始终达到与使用较小的自由度的均匀精炼相同的准确性。在磁场与网格对齐的测试案例中,其他改进策略也显示出效率的大幅提高。这项工作还包括将结果推广到较大的磁各向异性比率和三维问题的讨论。结果表明,各向同性网状细化需要按照层宽度(2D)或层宽度的平方(3D)的平方的顺序进行自由度,而各向异性细化需要在所有维度的层宽度的log log of log of log。还表明,在用代数多族式预处理时,共轭梯度迭代的数量会缩放为层宽度的力量,而在用ILU进行预处理时,该数字独立于层宽度。
Highly accurate simulation of plasma transport is needed for the successful design and operation of magnetically confined fusion reactors. Unfortunately, the extreme anisotropy present in magnetized plasmas results in thin boundary layers that are expensive to resolve. This work investigates how mesh refinement strategies might reduce that expense to allow for more efficient simulation. It is first verified that higher order discretization only realizes the proper rate of convergence once the mesh resolves the thin boundary layer, motivating the focusing of refinement on the boundary layer. Three mesh refinement strategies are investigated: one that focuses the refinement across the layer by using rectangular elements with a ratio equal to the boundary layer width, one that allows for exponential growth in mesh spacing away from the layer, and one adaptive strategy utilizing the established Zienkiewicz and Zhu error estimator. Across 4 two-dimensional test cases with high anisotropy, the adaptive mesh refinement strategy consistently achieves the same accuracy as uniform refinement using orders of magnitude less degrees of freedom. In the test case where the magnetic field is aligned with the mesh, the other refinement strategies also show substantial improvement in efficiency. This work also includes a discussion generalizing the results to larger magnetic anisotropy ratios and to three-dimensional problems. It is shown that isotropic mesh refinement requires degrees of freedom on the order of either the layer width (2D) or the square of the layer width (3D), whereas anisotropic refinement requires a number on the order of the log of layer width for all dimensions. It is also shown that the number of conjugate gradient iterations scales as a power of layer width when preconditioned with algebraic multigrid, whereas the number is independent of layer width when preconditioned with ILU.