Droplet dynamics and fine-scale structure in a shearless turbulent mixing layer with phase changes
Droplet dynamics and fine-scale structure in a shearless turbulent mixing layer with phase changes
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DOI：
10.1017/jfm.2017.23
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发表时间：
2017-02
影响因子：
3.7
通讯作者：
P. Goetzfried;B. Kumar;R. Shaw;J. Schumacher
中科院分区：
文献类型：
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作者：
P. Goetzfried;B. Kumar;R. Shaw;J. Schumacher
本文利用三维直接数值模拟的方法，研究了云-晴空界面上一小部分无剪切混合层对晴空湍流卷吸云丝的响应。这项工作的主要目标是了解多云和晴朗空气的混合如何演变为湍流和热力学通过相变相互作用，以及云滴如何响应。在主要的模拟情况下，混合在多云细丝中的较高水平的湍流和晴朗空气环境中的较低水平的湍流之间进行-典型的无剪切混合层设置。流体湍流仅由浮力驱动，浮力结合了来自温度、蒸汽含量和液态水含量场的反馈。两种不同的变化的核心集的无剪切混合层模拟进行了讨论，模拟在一个更大的域和模拟与相同的湍流水平内的灯丝和它的环境。总体而言，它被发现，作为蒸发发生的液滴，进入欠饱和晴朗的空气区域，浮力来主导随后的混合层的演变。浮力反馈导致最初的下沉气流在多云，明确的空气界面和上升气流在散装地区。初始瞬变后的湍流强度取决于域的大小，表明尺度的范围是无剪切混合层设置中的一个重要参数。与此相反，在晴朗的空气中的湍流水平被发现有混合过程的演变的影响不大。云水液滴的大小，过饱和度在液滴的位置和垂直速度的分布更敏感的域的大小比湍流剖面的细节，这表明云微物理的演变是更敏感的大尺度，而不是小尺度的流量特性。
Three-dimensional direct numerical simulations of a shearless mixing layer in a small fraction of the cloud–clear air interface are performed to study the response of an ensemble of cloud water droplets to the turbulent entrainment of clear air into a cloud filament. The main goal of this work is to understand how mixing of cloudy and clear air evolves as turbulence and thermodynamics interact through phase changes, and how the cloud droplets respond. In the main simulation case, mixing proceeds between a higher level of turbulence in the cloudy filament and a lower level of turbulence in the clear air environment – the typical shearless mixing layer set-up. Fluid turbulence is driven solely by buoyancy, which incorporates feedbacks from the temperature, the vapour content and the liquid water content fields. Two different variations on the core set of shearless mixing layer simulations are discussed, a simulation in a larger domain and a simulation with the same turbulence level inside the filament and its environment. Overall, it is found that, as evaporation occurs for the droplets that enter subsaturated clear air regions, buoyancy comes to dominate the subsequent evolution of the mixing layer. The buoyancy feedback leads initially to downdraughts at the cloudy–clear air interface and to updraughts in the bulk regions. The strength of the turbulence after initial transients depends on the domain size, showing that the range of scales is an important parameter in the shearless mixing layer set-up. In contrast, the level of turbulence in the clear air is found to have little effect on the evolution of the mixing process. The distributions of cloud water droplet size, supersaturation at the droplet positions and vertical velocity are more sensitive to domain size than to the details of the turbulence profile, suggesting that the evolution of cloud microphysics is more sensitive to large-scale as opposed to small-scale properties of the flow.