径向伸展盘状风自相似MHD模型的三维数值模拟
3D simulations of disc-winds extending radially self-similar MHD models
摘要：
起源于吸积盘内部的盘风被认为是磁准直外流的基本成分。唯一可用的解析MHD解决方案来描述盘驱动射流是那些径向自相似对称的特点。然而，径向自相似MHD喷流模型，在一般情况下，有三个几何缺陷，（i）在喷流轴的奇异性，（ii）轴对称的必要假设，和（iii）不存在一个内在的径向尺度，即喷流形式上延伸到径向无穷大。因此，数值模拟是必要的扩展的解析解向轴，通过求解完整的三维方程的MHD和施加一个终止半径在有限的径向距离。在这里，我们专注于研究放松的影响（ii）假设轴对称，即进行全三维数值模拟的盘风穿越所有磁流体动力学临界表面。我们比较这些运行的结果与以前的轴对称2.5D模拟。所有模拟中的流动结构显示出很强的相似性。除了极向磁场和环向速度略微偏离轴对称外，3D运行达到稳定状态，并且对于大多数物理量保持接近轴对称。后者的数量显示出不稳定的迹象，然而，这是局限于快速磁声分界面内的区域。在2.5D和3D运行中，存在于流中的力（准直和加速性质）都很好地一致。我们的结论是，解析解的行为也很好，放松轴对称的基本假设。
英文摘要：
Disc-winds originating from the inner parts of accretion discs are considered as the basic component of magnetically collimated outflows. The only available analytical MHD solutions to describe disc-driven jets are those characterized by the symmetry of radial self-similarity. However, radially self-similar MHD jet models, in general, have three geometrical shortcomings, (i) a singularity at the jet axis, (ii) the necessary assumption of axisymmetry, and (iii) the non-existence of an intrinsic radial scale, i.e. the jets formally extend to radial infinity. Hence, numerical simulations are necessary to extend the analytical solutions towards the axis, by solving the full three-dimensional equations of MHD and impose a termination radius at finite radial distance. We focus here on studying the effects of relaxing the (ii) assumption of axisymmetry, i.e. of performing full 3D numerical simulations of a disc-wind crossing all magnetohydrodynamic critical surfaces. We compare the results of these runs with previous axisymmetric 2.5D simulations. The structure of the flow in all simulations shows strong similarities. The 3D runs reach a steady state and stay close to axisymmetry for most of the physical quantities, except for the poloidal magnetic field and the toroidal velocity which slightly deviate from axisymmetry. The latter quantities show signs of instabilities, which, however, are confined to the region inside the fast magnetosonic separatrix surface. The forces present in the flow, both of collimating and accelerating nature, are in good agreement in both the 2.5D and the 3D runs. We conclude that the analytical solution behaves well also after relaxing the basic assumption of axisymmetry.
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