Simulations of radiative turbulent mixing layers
Simulations of radiative turbulent mixing layers
复制标题
辐射湍流混合层的模拟
DOI：
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复制
发表时间：
2018
影响因子：
4.8
通讯作者：
P. Masterson
中科院分区：
文献类型：
--
作者：
Suoqing Ji;Suoqing Ji;S. Oh;P. Masterson
辐射湍流混合层在具有剪切流的多相气体中应该是普遍存在的。对于高速云和星系的环河介质（CGM）中看到的 O vi 等高离子，它们是一个潜在有吸引力的解释。我们使用非平衡 (NEI) 和光电离建模进行 3D 磁流体力学 (MHD) 模拟，着眼于测试简单的分析模型。即使是纯粹的流体动力学碰撞电离平衡 (CIE) 计算，其柱密度也远低于观测值。特征流入速度和湍流速度远小于剪切速度，并且层宽度 $h propto t_{mathrm{cool}}^{1/2}$ 而不是 h ∝ tcool。正如分析比例预测的那样，柱密度并不独立于密度或金属丰度，并且对剪切速度和密度对比的依赖性令人惊讶地弱。在确定饱和状态时，辐射冷却而不是开尔文-亥姆霍兹不稳定性显得至关重要。由于快速冷却而产生低压，既引发湍流，又设定热气体的夹带速率，热气体的焓通量以及湍流耗散为该层提供能量。无论初始几何形状如何，磁场都会被放大并通过磁张力稳定混合层，产生几乎层流并降低柱密度。 NEI 效应可以将柱密度提高几倍。原则上，通过 NEI 或光电离抑制冷却也可以增加 O vi 柱密度，但实际上，对于 CGM 条件来说并不重要。为了解释观测结果，视线必须穿透数百或数千个混合层，如果 CGM 以微小云朵的“雾”形式存在，这可能是合理的。
Radiative turbulent mixing layers should be ubiquitous in multi-phase gas with shear flow. They are a potentially attractive explanation for the high ions such as O vi seen in high-velocity clouds and the circumgalactic medium (CGM) of galaxies. We perform 3D magnetohydrohynamics (MHD) simulations with non-equilibrium (NEI) and photoionization modelling, with an eye towards testing simple analytic models. Even purely hydrodynamic collisional ionization equilibrium (CIE) calculations have column densities much lower than observations. Characteristic inflow and turbulent velocities are much less than the shear velocity, and the layer width $h propto t_{mathrm{cool}}^{1/2}$ rather than h ∝ tcool. Column densities are not independent of density or metallicity as analytic scalings predict, and show surprisingly weak dependence on shear velocity and density contrast. Radiative cooling, rather than Kelvin–Helmholtz instability, appears paramount in determining the saturated state. Low pressure due to fast cooling both seeds turbulence and sets the entrainment rate of hot gas, whose enthalpy flux, along with turbulent dissipation, energizes the layer. Regardless of initial geometry, magnetic fields are amplified and stabilize the mixing layer via magnetic tension, producing almost laminar flow and depressing column densities. NEI effects can boost column densities by factors of a few. Suppression of cooling by NEI or photoionization can, in principle, also increase O vi column densities, but, in practice, is unimportant for CGM conditions. To explain observations, sightlines must pierce hundreds or thousands of mixing layers, which may be plausible if the CGM exists as a ‘fog’ of tiny cloudlets.
DOI：
10.3847/1538-4357/aac884
发表时间：
2018-03
期刊：
The Astrophysical Journal
影响因子：
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作者：
J. Stern;C. Faucher-Giguère;J. Hennawi;Zachary Hafen;S. Johnson;D. Fielding
通讯作者：
J. Stern;C. Faucher-Giguère;J. Hennawi;Zachary Hafen;S. Johnson;D. Fielding