CFD sample

In this article, we hypothesize that droplet transport in air-assisted spraying systems is related to the different diameter-dependent regime change, when aerodynamic drag, particle inertia, and canopy resistance interact. We also hypothesize that this porous canopy substantially affects how droplets fly and how they are made, so how they dispersed, transported and deposited. To verify this hypothesis, a 3D transient CFD model has been developed based on dynamic mesh strategy and a Lagrangian Discrete Phase model (DPM). The model is an air-assisted sprayer, based on a real fruit orchard setup with open-field and porous canopy for example. The canopy was modeled as porous to account for vegetation resistance and droplet injection was performed with air-blast atomization, and droplet movement dynamics, for example with their diameter distribution, velocity and turbulence was studied. Diameter and movement regimes of a droplet. The thin droplets (d < 80 µm) have strong air coupling and a high transport potential and drive the spray drift. Intermediate droplets, with 80-120 µm (Dv50) in diameter represent a critical phase in its transport. Droplets with large diameters are influenced more by inertia and present a good quality of deposition in the canopy. The porous canopy greatly reduces the flow of air and distorts turbulence patterns thus leading to reduced transport of droplets and hence reduced deposition efficiency. The effect of porous canopy is even more pronounced for intermediate droplet sizes which is where canopy resistance reduces acceleration and interception of jet-driven droplets. In general, our results show that droplet transport and deposition depend upon the aerodynamic composition of the droplets and canopy interaction. These results are useful for the development of spray application plans to control droplet distribution and airflow during orchard applications to ensure that the number of droplets does not increase while still delivering high deposition and drift results.本文提出两项假说：其一，风助式喷雾系统内的雾滴输运，与空气动力阻力、粒子惯性及冠层阻力共同作用下，依赖粒径的流态转变存在关联；其二，该多孔冠层会显著影响雾滴的飞行过程、生成方式，进而决定其扩散、输运与沉积过程。 为验证上述假说，本研究基于动网格策略与拉格朗日离散相模型（DPM），构建了三维瞬态计算流体动力学（CFD）模型。该模型以真实果园场景为原型，涵盖露地与多孔冠层两种场景；冠层采用多孔介质建模以表征植被阻力，雾滴通过气流雾化方式注入，并对雾滴的运动动力学特征——包括粒径分布、运动速度与湍流特性展开了研究。 雾滴的粒径与运动流态特征如下：细粒径雾滴（粒径d<80μm）与空气耦合作用强，输运潜力高，是引发喷雾飘移的主要组分；中粒径雾滴（粒径80~120μm，Dv50）是雾滴输运过程中的关键阶段；大粒径雾滴则更多受惯性主导，在冠层内可实现优质沉积。 多孔冠层会大幅削弱气流流速并扭曲湍流流场，进而降低雾滴输运效率与沉积效果。对于中粒径雾滴而言，多孔冠层的影响更为显著：冠层阻力会抑制射流驱动雾滴的加速过程，并增加其被拦截的概率。 总体而言，本研究结果表明，雾滴的输运与沉积过程，取决于雾滴自身的空气动力学特性与冠层的相互作用。上述研究结果可为果园喷雾作业方案的优化提供支撑：通过调控雾滴分布与气流状态，在不额外增加雾滴总量的前提下，实现优质沉积并有效控制喷雾飘移。