吸积盘三维全球分层MHD模拟中的Turbulence和Steady Flows
Turbulence and Steady Flows in 3D Global Stratified MHD Simulations of Accretion Disks
摘要：
本文对吸积盘进行了全二维π整体三维分层MHD模拟.我们解释我们的研究结果的背景下，原行星盘。我们使用PLUTO Goddom程序在球坐标系中使用精确和鲁棒的HLLD Riemann求解器来研究由磁旋转不稳定性（MRI）驱动的湍流。我们遵循超过1500个轨道的湍流在最内半径的域来测量湍流运动的整体强度和详细的吸积流模式。我们发现，在两个刻度高度的中平面内的区域有一个约0.1的湍流马赫数和磁压力两到三个数量级小于气体压力，而以外的三个刻度高度的磁压力等于或超过气体压力和湍流是跨音速的，导致大的密度波动。最强的大尺度密度扰动是螺旋密度波，这些波中最强的有m=5。计算中没有出现明显的纬向环流，因为波动的径向压力梯度会导致轨道频率的变化，其重要性与粘性模型中驱动纬向流的应力梯度相当。净质量流率通过使用从MHD计算得到的平均应力分布的粘性模型很好地再现。平均湍流磁场的强度与半径成反比，因此在最大尺度上，磁场近似是无力的。因此，吸积应力随半径的平方反比而减小。
英文摘要：
We present full 2 Pi global 3-D stratified MHD simulations of accretion disks. We interpret our results in the context of proto-planetary disks. We investigate the turbulence driven by the magneto-rotational instability (MRI) using the PLUTO Godunov code in spherical coordinates with the accurate and robust HLLD Riemann solver. We follow the turbulence for more than 1500 orbits at the innermost radius of the domain to measure the overall strength of turbulent motions and the detailed accretion flow pattern. We find that regions within two scale heights of the midplane have a turbulent Mach number of about 0.1 and a magnetic pressure two to three orders of magnitude less than the gas pressure, while outside three scale heights the magnetic pressure equals or exceeds the gas pressure and the turbulence is transonic, leading to large density fluctuations. The strongest large-scale density disturbances are spiral density waves, and the strongest of these waves has m=5. No clear meridional circulation appears in the calculations because fluctuating radial pressure gradients lead to changes in the orbital frequency, comparable in importance to the stress gradients that drive the meridional flows in viscous models. The net mass flow rate is well-reproduced by a viscous model using the mean stress distribution taken from the MHD calculation. The strength of the mean turbulent magnetic field is inversely proportional to the radius, so the fields are approximately force-free on the largest scales. Consequently the accretion stress falls off as the inverse square of the radius.
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