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Effect of city shape on urban wind patterns and convective heat transfer in calm and stable background conditions

机译：平稳环境下城市形态对城市风型和对流换热的影响

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摘要

Extreme heatwaves and air pollution in urban areas often occur in calm and stable background conditions. In such situations, urban heat dome flows are induced by the urban heat island (UHI) effect, which has a strong influence on the urban thermal and wind environment. Cities have various shapes, but most studies of urban heat island circulation have focused on ideal circular shape and ignored the potential effects of shape. In this study, the effects of the various ideal urban shapes on the natural convective flow over urban areas (urban dome flow) are investigated experimentally. The urban dome flow over an equilateral polygonal city is characterised as convergent inflow along the interior angle bisector at the lower level, upward flow near the urban centre and divergent side outflow perpendicular to the side at the upper level. The characteristics of the urban dome flow over a rectangular urban area depend greatly upon the aspect ratio of the long side to the short side. As the aspect ratio increases, the divergent flows of the short sides disappear, and those of the two long sides dominate. Moreover, recirculation occurs between the divergent outflow perpendicular to the long edges and the convergent inflow near the short edges. Our results also show that the Nusselt (Nu) number (Eq. (1)), which quantifies the heat transfer efficiency, is significantly lower in a stable environment than in a neutral environment. A significant correlation between the shape and Nu was not found. It indicates that strong UHIs prefer stable background conditions.
机译：城市地区的极端热浪和空气污染通常发生在平静，稳定的背景条件下。在这种情况下，城市热穹顶流动是由城市热岛效应引起的，它对城市的热风环境具有很大的影响。城市具有各种形状，但是大多数关于城市热岛环流的研究都集中在理想的圆形上，而忽略了形状的潜在影响。在这项研究中，通过实验研究了各种理想城市形状对城市区域自然对流流动（城市穹顶流动）的影响。等边多角形城市上空的城市穹顶流的特征是在较低水平处沿内角等分线汇聚流入，在城市中心附近向上流动，垂直于较高水平侧流出的侧向流出。矩形市区上的城市圆顶流的特征在很大程度上取决于长边与短边的长宽比。随着长宽比的增加，短边的发散流消失，而两个长边的发散流占主导。而且，在垂直于长边缘的发散流出和短边缘附近的会聚流入之间发生再循环。我们的结果还表明，量化传热效率的Nusselt（Nu）数（等式（1））在稳定环境中比在中性环境中要低得多。在形状和Nu之间未发现明显的相关性。这表明强大的UHI更喜欢稳定的背景条件。

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