辐射混合层：湍流燃烧的研究
Radiative Mixing Layers: Insights from Turbulent Combustion
摘要：
辐射混合层出现在多相气体、剪切和辐射冷却存在的地方。模拟表明，在稳态下，来自热相的热平流平衡了辐射冷却。然而，许多特征令人困惑。例如，热气体卷吸似乎是数值收敛，尽管无标度，分形结构的这样的前线尚未解决。此外，热气体热通量具有特征速度$v_{\rm in} \approx c_{\rm s,cold} (t_{\rm cool}/t_{\rm sc,cold})^{-1/4}$，其强度和标度不是直观的。我们在1D和3D流体动力学模拟中重新审视这些问题。我们发现，过冷只发生在数值扩散占主导地位的热传输;收敛仍然是可能的，即使当字段长度是未解决的。利用混合层与湍流燃烧之间的相似性可以对辐射锋有更深的物理理解，这一相似性具有成熟的理论和丰富的实验数据。一个关键参数是达姆科勒数${\rm Da} = τ_{\rm turb}/t_{\rm cool}$，即外部涡流周转时间与冷却时间之比。一旦形成${\rm Da} > 1$，前缘就分裂成多相介质。正如标量混合一样，涡的周转时间决定了混合速率，与小尺度扩散无关。因此，热传导的影响通常有限。我们表明，$v_{\rm in}$和有效的发射率可以通过调整燃烧理论缩放详细了解。平均密度和温度分布也可以很好地再现混合长度理论。这些结果对许多环境中冷气体的结构和生存以及大尺度星系模拟的分辨率要求都有影响。
英文摘要：
Radiative mixing layers arise wherever multiphase gas, shear, and radiative cooling are present. Simulations show that in steady state, thermal advection from the hot phase balances radiative cooling. However, many features are puzzling. For instance, hot gas entrainment appears to be numerically converged despite the scale-free, fractal structure of such fronts being unresolved. Additionally, the hot gas heat flux has a characteristic velocity $v_{\rm in} \approx c_{\rm s,cold} (t_{\rm cool}/t_{\rm sc,cold})^{-1/4}$ whose strength and scaling are not intuitive. We revisit these issues in 1D and 3D hydrodynamic simulations. We find that over-cooling only happens if numerical diffusion dominates thermal transport; convergence is still possible even when the Field length is unresolved. A deeper physical understanding of radiative fronts can be obtained by exploiting parallels between mixing layers and turbulent combustion, which has well-developed theory and abundant experimental data. A key parameter is the Damköhler number ${\rm Da} = τ_{\rm turb}/t_{\rm cool}$, the ratio of the outer eddy turnover time to the cooling time. Once ${\rm Da} > 1$, the front fragments into a multiphase medium. Just as for scalar mixing, the eddy turnover time sets the mixing rate, independent of small scale diffusion. For this reason, thermal conduction often has limited impact. We show that $v_{\rm in}$ and the effective emissivity can be understood in detail by adapting combustion theory scalings. Mean density and temperature profiles can also be reproduced remarkably well by mixing length theory. These results have implications for the structure and survival of cold gas in many settings, and resolution requirements for large scale galaxy simulations.
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