Kinematic and dynamic collision statistics of cloud droplets from high-resolution simulations
Kinematic and dynamic collision statistics of cloud droplets from high-resolution simulations
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DOI：
10.1088/1367-2630/15/4/045032
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发表时间：
2013-04
影响因子：
3.3
通讯作者：
B. Rosa;H. Parishani;O. Ayala;W. Grabowski;Lian-Ping Wang
中科院分区：
文献类型：
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作者：
B. Rosa;H. Parishani;O. Ayala;W. Grabowski;Lian-Ping Wang
我们研究的动态和运动学的碰撞统计的云滴的流动泰勒微尺度雷诺数（高达500）的范围内，使用一个高度可扩展的混合直接数值模拟方法。利用径向相对速度（RRV）和径向分布函数（RDF）在短间距下的幂律标度特性，得到了接触处径向相对速度和径向分布函数的精确结果。在典型的云湍流（耗散率为400 cm 2 s−3）中，以系统的方式讨论了相同大小的液滴（半径从10 μ m到60 μm）的几何碰撞的三个具体但相互关联的问题。首先，采用确定性和随机性两种强迫方案对模拟结果对大尺度驱动机制的敏感性进行了检验。我们发现，在一般情况下，结果是定量相似的，与确定性强迫给出一个稍大的RDF和碰撞内核。然而，对于半径小于30 μm的液滴，这种差异可以忽略不计。其次，我们已经表明，对统计对流动雷诺数Rλ或更大规模的流体运动的依赖性是次要的，与这种影响的趋势饱和在足够高的Rλ导致Rλ无关的结果。DNS结果和理论论证都表明，对于较小的液滴，饱和发生在较小的Rλ处。最后，由于大多数以前的研究的湍流碰撞的惯性粒子有关的非沉降粒子，我们专门讨论了碰撞统计的重力的作用，同时模拟碰撞统计和没有重力。结果表明，当a < ac时，碰撞统计不受重力的影响，RRV的临界液滴半径ac约为30 μm，RDF的临界液滴半径ac约为20 μm。对于较大的液滴，重力改变颗粒涡流相互作用的时间，并显着降低RRV。重力对RDF的影响相当复杂：重力降低了中等大小液滴的RDF，但提高了较大液滴的RDF。此外，我们已经研究了标度指数的RDF和RRV，并发现重力修改的RDF标度指数的中型和大型颗粒，在一种方式非常类似的重力对RDF在接触。重力导致的标度指数RDF和RRV平稳的大液滴，相反，非沉降颗粒的指数递减。
We study the dynamic and kinematic collision statistics of cloud droplets for a range of flow Taylor microscale Reynolds numbers (up to 500), using a highly scalable hybrid direct numerical simulation approach. Accurate results of radial relative velocity (RRV) and radial distribution function (RDF) at contact have been obtained by taking advantage of their power-law scaling at short separation distances. Three specific but inter-related questions have been addressed in a systematic manner for geometric collisions of same-size droplets (of radius from 10 to 60 μm) in a typical cloud turbulence (dissipation rate at 400 cm2 s−3). Firstly, both deterministic and stochastic forcing schemes were employed to test the sensitivity of the simulation results on the large-scale driving mechanism. We found that, in general, the results are quantitatively similar, with the deterministic forcing giving a slightly larger RDF and collision kernel. This difference, however, is negligible for droplets of radius less than 30 μm. Secondly, we have shown that the dependence of pair statistics on the flow Reynolds number Rλ or larger scale fluid motion is of secondary importance, with a tendency for this effect to saturate at high enough Rλ leading to Rλ-independent results. Both DNS results and theoretical arguments show that the saturation happens at a smaller Rλ for smaller droplets. Finally, since most previous studies of turbulent collision of inertial particles concerned non-sedimenting particles, we have specifically addressed the role of gravity on collision statistics, by simultaneously simulating collision statistics with and without gravity. It is shown that the collision statistics is not affected by gravity when a < ac, where the critical droplet radius ac is found to be around 30 μm for the RRV, and around 20 μm for the RDF. For larger droplets, gravity alters the particle–eddy interaction time and significantly reduces the RRV. The effect of gravity on the RDF is rather complex: gravity reduces the RDF for intermediate-sized droplets but enhances the RDF for larger droplets. In addition, we have studied the scaling exponents of both RDF and RRV, and found that gravity modifies the RDF scaling exponents for both intermediate and large particles, in a manner very similar to the effect of gravity on the RDF at contact. Gravity is shown to cause the scaling exponents for RDF and RRV to level off for large droplets, in contrast to diminishing exponents for non-sedimenting particles.