在多云和晴朗空气环境之间的界面层内水滴种群动力学的间歇性加速
Intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments
摘要：
我们使用直接的数值模拟来研究湍流层上的扰动的时间演化，湍流层通常将云与周围的晴朗空气分开。穿过这个无剪切层，自然形成湍流动能梯度。在这里，一个有限的扰动形式的局部初始温度波动被施加到模拟的流体动力学不稳定性内的背景湍流气流。然后求解了两种完全相反类型的液滴总体分布的数值初值问题。具体地，我们考虑半径为15 $μ$ m的单分散液滴群体和半径在0.6 - 30 $μ$ m范围内的多分散分布。对于这两种分布，据观察，蒸发和冷凝有一个显着不同的重量内的均匀的多云区域和界面各向异性混合区域。据观察，液滴碰撞的动力学是高度影响的主机区域的湍流结构。这两个群体在其能量衰减瞬变过程中表现出一个共同的方面。也就是说，界面层中碰撞的概率增加，界面层中存在强烈的各向异性速度波动。事实上，该层引起液滴动能和尺寸的增强的差异。多分散和单分散初始颗粒分布都含有$10^7$液滴，与初始液态水含量$0.8 g/m^3$匹配。给出了粒子数平衡方程中几何碰撞的湍流碰撞核的估计。本文对在云界面区得到的两个非定常非遍历碰撞核的结构作了初步讨论。
英文摘要：
We use direct numerical simulation to study the temporal evolution of a perturbation localized on the turbulent layer that typically separates a cloud from the surrounding clear air. Across this shearless layer, a turbulent kinetic energy gradient naturally forms. Here, a finite perturbation in the form of a local initial temperature fluctuation is applied to simulate a hydrodynamic instability inside the background turbulent air flow. A numerical initial value problem for two diametrically opposite types of drop population distributions is then solved. Specifically, we consider a mono-disperse population of droplets of 15 $μ$m of radius and a poly-disperse distribution with radii in the range 0.6 - 30 $μ$m. For both distributions, it is observed that the evaporation and condensation have a dramatically different weight inside the homogeneous cloudy region and the interfacial anisotropic mixing region. It is observed that the dynamics of drop collisions is highly effected by the turbulence structure of the host region. The two populations show a common aspect during their energy decay transient. That is the increased probability of collisions in the interfacial layer hat houses intense anisotropic velocity fluctuations. This layer, in fact, induces an enhanced differentiation on droplets kinetic energy and sizes. Both polydisperse and monodisperse initial particle distributions contain $10^7$ droplets, matching an initial liquid water content of $0.8 g/m^3$. An estimate of the turbulent collision kernel for geometric collisions used in the population balance equations is given. A preliminary discussion is presented on the structure of the two unsteady non ergodic collision kernels obtained inside the cloud interface region.
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