Hoyle-Lyttleton吸积的触发器不稳定性
Revisiting the Flip-Flop Instability of Hoyle-Lyttleton Accretion
摘要：
我们重新使用高保真数值模拟的二维平面吸积的触发器不稳定性。通过从初始稳态轴对称解出发，我们能够从小振幅跟踪这种超稳定性的增长。在小振幅极限下，在任何瞬态吸积盘形成之前，吸积激波的振荡周期与Hoyle-Lyttleton吸积半径（Ra）处的开普勒周期相当，与吸积体的大小无关。随着吸积体尺寸的减小，超稳定性的增长率显著增加，但对上游流动的马赫数相对不敏感。我们确认，触发器不需要在上游流的任何梯度。事实上，在发现模拟中使用的小密度梯度实际上对过稳定性的增长率没有影响。比热比确实影响过稳定性，较小的γ导致不稳定性的更快增长。对于一个相对较大的吸积体（半径为0.037Ra），平面吸积在γ = 4/3时是不稳定的，但在γ> 1.6时是稳定的。如果吸积体的半径小于0.0025Ra，则平面吸积即使在γ = 5/3时也是不稳定的。我们还证实，当吸积体是足够小的，长期的演变是描述的状态与反旋转的准开普勒吸积盘之间的突然跳跃。
英文摘要：
We revisit the flip-flop instability of two-dimensional planar accretion using high-fidelity numerical simulations. By starting from an initially steady-state axisymmetric solution, we are able to follow the growth of this overstability from small amplitudes. In the small-amplitude limit, before any transient accretion disk is formed, the oscillation period of the accretion shock is comparable to the Keplerian period at the Hoyle-Lyttleton accretion radius (R_a), independent of the size of the accreting object. The growth rate of the overstability increases dramatically with decreasing size of the accretor, but is relatively insensitive to the upstream Mach number of the flow. We confirm that the flip-flop does not require any gradient in the upstream flow. Indeed, a small density gradient as used in the discovery simulations has virtually no influence on the growth rate of the overstability. The ratio of specific heats does influence the overstability, with smaller gamma leading to faster growth of the instability. For a relatively large accretor (a radius of 0.037R_a) planar accretion is unstable for gamma = 4/3, but stable for gamma > 1.6. Planar accretion is unstable even for gamma = 5/3 provided the accretor has a radius of < 0.0025R_a. We also confirm that when the accretor is sufficiently small, the secular evolution is described by sudden jumps between states with counter-rotating quasi-Keplerian accretion disks.
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