Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypothetical urban areas in the form of identical surface-mounted ribs in crossflows (two-dimensional scenarios) to enrich our fundamental understanding of the street-level ventilation mechanism. We critically compare the flow behaviors over rough surfaces with different aerodynamic resistance. It is found that the friction velocity u τ is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds ( U ∞ ) over rough surfaces suggest that the drag coefficient C d (= 2 u τ 2 / U ∞ 2 ) is able to characterize the turbulent transport processes over hypothetical urban areas. Linear regression shows that street-level ventilation, which is dominated by the turbulent component of the air change rate (ACH), is proportional to the square root of drag coefficient ACH ″ ∝ C d 1 / 2 . This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) w ^ RSL ″ , for breathability assessment over urban areas with diversified building height. Quadrant analyses and frequency spectra demonstrate that the turbulence is more inhomogeneous and the scales of vertical turbulence intensity w ″ w ″ ˉ 1 / 2 are larger over rougher surfaces, resulting in more efficient street-level ventilation.
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机译：周围高层建筑经常削弱街道通风。全面了解城市地区的流动和湍流有助于改善城市空气质量以及实现环境管理。在本文中，采用风洞中的缩减物理模型以横流（二维方案）中相同的表面安装肋骨的形式检查假设城市区域中的动力学，以丰富我们对街道水平的基本理解通风机制。我们严格比较具有不同气动阻力的粗糙表面上的流动行为。发现摩擦速度uτ适合于缩放近壁区域而不是外层的动力学。粗糙表面上不同的自由流风速（U∞）表明，阻力系数C d（= 2 uτ2 / U∞2）能够表征假设城市地区的湍流运输过程。线性回归表明，以空气变化率（ACH）的湍流成分为主的街道通风与阻力系数ACH C d 1/2的平方根成比例。然后，该概念框架得以扩展以制定新的指标，即粗糙度子层（RSL）w ^ RSL”中的垂直波动速度标度，用于评估具有不同建筑物高度的城市区域的透气性。象限分析和频谱分析表明，在较粗糙的表面上，湍流更加不均匀，垂直湍流强度的尺度w” w”ˉ1/2更大，从而使街道通风更为有效。
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