Abstract Vegetation in the urban environment has various impacts on microclimate. While optimal strategies for investigating these impacts are the subject of ongoing research, most approach rely on Computational Fluid Dynamics (CFD) simulations. We evaluate mean wind and turbulence fields simulated using the fast-running Quick Urban & Industrial Complex (QUIC) Dispersion Modeling System, a simplified CFD tool that resolves buildings and vegetation. We use QUIC to investigate the role of deciduous trees in modifying the airflow of a real neighbourhood in Bologna, Italy by running large ensembles of simulations per case study, obtained by varying the input wind direction. This approach can minimise intrinsic model uncertainty as well as uncertainties associated with real environments. Model validation is performed using measurements from an experimental field campaign focused on a vegetated urban street canyon in Bologna. Ensemble simulation results show good agreement with the experimental data for various conditions (i.e., simulation ensembles overlap experimental variability in most cases). The role of trees is investigated by comparing simulations with and without trees. Trees are found to reduce airflow by constraining local circulation and reducing turbulence intensity. Finally, the combined effect of building morphology and vegetation is investigated by adopting a formalism to represent the presence of vegetation using area-fraction coefficients. An area-fraction coefficient threshold of 0.225 has been identified that separates flow behaviours and is present in cases with and without vegetation. Below this threshold, a constant wind and turbulence regime exists, while above the threshold, winds and turbulence vary with area-fraction coefficient.

中文翻译：

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植被对欧洲城市社区平均和湍流场影响的数值研究

摘要 城市环境中的植被对小气候有多种影响。虽然调查这些影响的最佳策略是正在进行的研究的主题，但大多数方法依赖于计算流体动力学 (CFD) 模拟。我们使用快速运行的 Quick Urban & Industrial Complex (QUIC) Dispersion Modeling System（一种用于解析建筑物和植被的简化 CFD 工具）来评估模拟的平均风场和湍流场。我们使用 QUIC 来研究落叶树在改变意大利博洛尼亚一个真实社区的气流中的作用，方法是运行每个案例研究的大型模拟集合，通过改变输入风向获得。这种方法可以最大限度地减少固有的模型不确定性以及与真实环境相关的不确定性。模型验证是使用集中在博洛尼亚一个植被茂盛的城市街道峡谷的实验现场活动的测量结果进行的。集合模拟结果与各种条件下的实验数据非常吻合（即，在大多数情况下，模拟集合与实验变异性重叠）。通过比较有树和没有树的模拟来研究树的作用。发现树木通过限制局部循环和降低湍流强度来减少气流。最后，通过使用面积分数系数来表示植被存在的形式主义，研究了建筑形态和植被的组合效应。已经确定了 0.225 的面积分数系数阈值，它将流动行为分开，并且存在于有和没有植被的情况下。低于这个门槛，