Effects of Gravity on the Acceleration and Pair Statistics of Inertial Particles in Homogeneous Isotropic Turbulence
Effects of Gravity on the Acceleration and Pair Statistics of Inertial Particles in Homogeneous Isotropic Turbulence
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DOI：
10.1063/1.4915121
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发表时间：
2015-03
影响因子：
4.6
通讯作者：
H. Parishani;O. Ayala;B. Rosa;Lian-Ping Wang;W. Grabowski
中科院分区：
文献类型：
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作者：
H. Parishani;O. Ayala;B. Rosa;Lian-Ping Wang;W. Grabowski
本文以重粒子悬浮在湍流气流中为背景，研究了重力对惯性粒子的加速度统计和径向相对速度的影响。湍流流场采用直接数值模拟（DNS）方法在256 3网格上模拟，O（10 6）惯性粒子动力学采用点粒子法。对于半径为10 ~ 60 μm的颗粒/液滴，我们发现重力对颗粒加速度统计有重要影响：（a）在颗粒Stokes数约为1.2时，颗粒水平加速度方差在水平和垂直方向均出现峰值，此时颗粒水平加速度明显超过流体元加速度;（B）重力不断地破坏液滴对局部湍流运动的响应的准平衡，并放大垂直和水平方向上的极端加速度事件，从而有效地减小惯性过滤机制。通过将颗粒的RRV分解为三个部分：（1）差异沉降，（2）局部流动剪切，和（3）颗粒差异加速，我们评估和比较了它们各自的贡献。对于单分散颗粒，我们表明，重力的存在并没有一个显着的效果上的剪切项。另一方面，重力抑制的概率分布函数（pdf）的尾巴，由于较低的粒子涡相互作用的时间在重力的存在下，不同的加速度项。对于双分散的情况下，我们发现，重力可以减少剪切项略有分散粒子到涡流体剪切力相对较低。结果表明，碰撞加速度项与重力项呈正相关，且这种相关性随碰撞粒子半径差的减小而增强.最后，本文提出了一个理论来解释在小Stokes数下重力和湍流对惯性粒子水平和垂直加速度方差的影响，分析表明重力影响水平和垂直方向上的粒子加速度方差，导致两个方向上的粒子加速度方差增加。此外，重力对水平方向加速度方差的影响大于垂直方向，这与我们的DNS结果一致。C 2015 AIP Publishing LLC。[http：//dx.doi.org/10.1063/1.4915121]
Within the context of heavy particles suspended in a turbulent airflow, we study the eff ects of gravity on acceleration statistics and radial relative velocity (RRV) of inertial particles. The turbulent flow is simulated by direct numerical simulation (DNS) on a 256 3 grid and the dynamics of O(10 6 ) inertial particles by the point-particle approach. For particles/droplets with radius from 10 to 60 µm, we found that the gravity plays an important role in particle acceleration statistics: (a) a peak value of particle acceleration variance appears in both the horizontal and vertical directions at a particle Stokes number of about 1.2, at which the particle horizontal acceleration clearly exceeds the fluid-element acceleration; (b) gravity constantly disrupts quasi-equilibrium of a droplet’s response to local turbulent motion and amplifies extreme acceleration events both in the vertical and horizontal directions and thus eff ectively reduces the inertial filtering mechanism. By decomposing the RRV of the particles into three parts: (1) diff erential sedimentation, (2) local flow shear, and (3) particle diff erential acceleration, we evaluate and compare their separate contributions. For monodisperse particles, we show that the presence of gravity does not have a significant e ff ect on the shear term. On the other hand, gravity suppresses the probability distribution function (pdf) tails of the diff erential acceleration term due to a lower particle-eddy interaction time in presence of gravity. For bidisperse cases, we find that gravity can decrease the shear term slightly by dispersing particles into vortices where fluid shear is relatively low. The di ff erential acceleration term is found to be positively correlated with the gravity term, and this correlation is stronger when the diff erence in colliding particle radii becomes smaller. Finally, a theory is developed to explain the eff ects of gravity and turbulence on the horizontal and vertical acceleration variances of inertial particles at small Stokes numbers, showing analytically that gravity aff ects particle acceleration variance both in horizontal and vertical directions, resulting in an increase in particle acceleration variance in both directions. Furthermore, the eff ect of gravity on the horizontal acceleration variance is predicted to be stronger than that in the vertical direction, in agreement with our DNS results. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4915121]