翘曲粒子的松弛：纯流体力学情形
Relaxation of Warped Disks: the Case of Pure Hydrodynamics
摘要：
轨道盘可能由于盘的内部区域和外部区域的角动量之间的不对准而呈现弯曲。我们从最简单的情况开始对翘曲圆盘的物理学进行系统的模拟研究：在纯流体动力学下，即除了雷诺应力之外没有内应力的情况下，非受迫翘曲的松弛。我们专注于非线性制度中的弯曲率是大相比，磁盘的纵横比。当翘曲是非线性的，强大的径向压力梯度驱动跨音速径向运动沿磁盘的顶部和底部表面，有效地混合角动量。由此产生的非线性衰减率的翘曲的翘曲率和翘曲宽度增加，但至少在这里研究的参数制度，是独立的声速。相关角动量通量的特征量同样随着局部翘曲率和翘曲延伸的径向范围而增加;它也随着声速的增加而增加，但比线性增加慢。角动量通量在延迟之后响应于翘曲速率，该延迟与声波穿过翘曲的径向范围的时间的平方根成比例。这些行为与分析模型中描述线性翘曲动力学时常用的一些假设不同。
英文摘要：
Orbiting disks may exhibit bends due to a misalignment between the angular momentum of the inner and outer regions of the disk. We begin a systematic simulational inquiry into the physics of warped disks with the simplest case: the relaxation of an unforced warp under pure fluid dynamics, i.e. with no internal stresses other than Reynolds stress. We focus on the nonlinear regime in which the bend rate is large compared to the disk aspect ratio. When warps are nonlinear, strong radial pressure gradients drive transonic radial motions along the disk's top and bottom surfaces that efficiently mix angular momentum. The resulting nonlinear decay rate of the warp increases with the warp rate and the warp width, but, at least in the parameter regime studied here, is independent of the sound speed. The characteristic magnitude of the associated angular momentum fluxes likewise increases with both the local warp rate and the radial range over which the warp extends; it also increases with increasing sound speed, but more slowly than linearly. The angular momentum fluxes respond to the warp rate after a delay that scales with the square-root of the time for sound waves to cross the radial extent of the warp. These behaviors are at variance with a number of the assumptions commonly used in analytic models to describe linear warp dynamics.
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