TURBULENT DYNAMOS IN SPHERICAL SHELL SEGMENTS OF VARYING GEOMETRICAL EXTENT
TURBULENT DYNAMOS IN SPHERICAL SHELL SEGMENTS OF VARYING GEOMETRICAL EXTENT
复制标题
不同几何范围的球壳段中的湍流动力学
DOI：
10.1088/0004-637x/697/1/923
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发表时间：
2008
期刊：
影响因子：
--
通讯作者：
D. Moss
中科院分区：
文献类型：
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作者：
D. Mitra;R. Tavakol;A. Brandenburg;D. Moss
本文采用三维直接数值模拟的方法对球形壳段的螺旋强迫磁流体动力学方程进行了数值模拟，研究了区域几何形状和大小的变化对大尺度磁场增长和饱和的影响。我们利用Chandrasekhar-Kendall函数通过螺旋强迫在球域中注入动能和动能螺旋度。我们对磁场采用了完美的导体边界条件，以确保没有磁螺旋度脱离域边界。我们发现发电机作用产生的磁场比力的特征尺度更大。在耗散时间尺度上，磁能超过了动能，类似于之前在周期盒中的笛卡尔模拟。当我们在方位角方向上增加畴的尺寸时，我们发现非线性饱和磁场组织成长寿命的细胞结构，其长径比接近于统一。这些结构沿着方位角方向铺平磁畴，从而导致大磁畴尺寸时纵向平均磁场非常小。这些结构的尺度是由区域的最小尺度决定的，在我们的模拟中通常是径向尺度。我们还发现，增加域的经向范围几乎没有产生质变，除了大尺度场的边际增加。我们在具有相似长宽比的笛卡尔域中获得了质量上相似的结果。
We use three-dimensional direct numerical simulations of the helically forced magnetohydrodynamic equations in spherical shell segments in order to study the effects of changes in the geometrical shape and size of the domain on the growth and saturation of large-scale magnetic fields. We inject kinetic energy along with kinetic helicity in spherical domains via helical forcing using Chandrasekhar–Kendall functions. We take perfect conductor boundary conditions for the magnetic field to ensure that no magnetic helicity escapes the domain boundaries. We find dynamo action giving rise to magnetic fields at scales larger than the characteristic scale of the forcing. The magnetic energy exceeds the kinetic energy over dissipative timescales, similar to that seen earlier in Cartesian simulations in periodic boxes. As we increase the size of the domain in the azimuthal direction, we find that the nonlinearly saturated magnetic field organizes itself in long-lived cellular structures with aspect ratios close to unity. These structures tile the domain along the azimuthal direction, thus resulting in very small longitudinally averaged magnetic fields for large domain sizes. The scales of these structures are determined by the smallest scales of the domain, which in our simulations is usually the radial scale. We also find that increasing the meridional extent of the domains produces little qualitative change, except a marginal increase in the large-scale field. We obtain qualitatively similar results in Cartesian domains with similar aspect ratios.