Street canyon

Predicting solid particle transport in the lowest parts of the atmosphere is a major issue for man-made obstacles in semi-arid regions.
Here, we investigate the effects on solid particle saltation, of square obstacles on the ground with different spacings.
The aerodynamic field is determined by large eddy simulations coupled with an immersed boundary method for the obstacles.
Solid particles are tracked by a Lagrangian approach.
Take-off and rebound models are introduced for the interaction of particles with the wall.
Without particles, fluid velocity profiles are first compared with experiments showing good agreement.
Special focus is put on the recirculation zone that plays an important role in solid particle entrapment.

Particle concentration fields are presented. Accumulation zones are studied regarding the different obstacle spacings as an extension of the aerodynamic scheme by One (1988) to solid particle transport. A deposition peak appears before the first obstacle. When the spacing between the two obstacles is large enough, some particles are trapped within the recirculation and a second deposition peak arises. The streamwise evolution of the horizontal saltation flux shows that the lowest flux downstream of the obstacles is obtained for the highest separation. The deposition rate or the streamwise saltation flux are estimated globally as a function of obstacle spacing. These results illustrate how the numerical tool developed here can be used for assessing air quality in terms of solid particle concentration.

针对半干旱区域中的人工构筑物，预测近地层固体颗粒输运是一项关键课题。
本研究针对不同间距的地面方形构筑物，探究其对固体颗粒跃移的影响。
针对构筑物的空气动力场，采用耦合浸入边界法(immersed boundary method)的大涡模拟(large eddy simulations)进行求解。
固体颗粒的运动轨迹采用拉格朗日方法进行追踪。
针对颗粒与壁面的相互作用，引入了颗粒起跳与反弹模型。
首先在无颗粒工况下，将模拟得到的流体速度剖面与实验数据进行对比，二者吻合良好。
研究重点关注回流区(recirculation zone)，其在固体颗粒捕获过程中发挥着关键作用。

本文给出了颗粒浓度场的分布结果。针对不同构筑物间距下的堆积区(accumulation zones)，本研究将One(1988)提出的空气动力方案拓展至固体颗粒输运领域，对其展开分析。在首个构筑物前方会出现一处沉积峰值。当两个构筑物之间的间距足够大时，部分颗粒会被捕获于回流区内，进而出现第二处沉积峰值。沿流向的水平跃移通量演化规律表明，当构筑物间距最大时，构筑物下游区域的跃移通量最低。沉积速率与沿流向跃移通量均可通过构筑物间距的全局函数进行估算。上述研究结果证明，本文开发的数值模拟工具可用于基于固体颗粒浓度的空气质量评估。