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Air ventilation assessment under unstable atmospheric stratification - A comparative study for Hong Kong

机译：不稳定大气分层下的空气通风评估-香港的比较研究

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摘要

In most current air ventilation assessment (AVA) studies, a simple neutral assumption that does not consider thermal effects is adopted, particularly for numerical simulation practices. With statistics of daytime observations during summer in Hong Kong as an example, this study demonstrates that neutral atmospheric boundary conditions occur with a very low probability, which implies that current practices are indeed far away from reality. This study is devoted to addressing this knowledge gap by cross-comparisons of field measurements, wind tunnel tests, and large-eddy simulations (LES) under neutral and unstable conditions. It is found that LES-computed velocity ratios under unstable conditions are in line with field measurements, while results of simulations under neutral conditions are close to those of wind tunnel tests. Enhanced vertical mixing due to surface heating produces improved ventilation performance in the unstable case. The neutral assumption tends to underestimate pedestrian-level velocity ratios compared to a diabatic condition; hence it is deemed conservative when it is adopted in AVA practices. Moreover, stronger wind direction variance under unstable conditions results in weaker correlation between velocity ratios and frontal area indices than neutral conditions, which implies that street orientations become less important in ventilation under unstable conditions.
机译：在当前的大多数空气通风评估（AVA）研究中，都采用了一个不考虑热效应的简单中性假设，尤其是对于数值模拟实践。以香港夏季白天的观测数据为例，该研究表明中性大气边界条件发生的可能性非常低，这意味着当前的做法确实与现实相距甚远。这项研究致力于通过在中性和不稳定条件下进行现场测量，风洞测试和大涡模拟（LES）的交叉比较来解决这一知识鸿沟。发现在不稳定条件下LES计算的速度比与现场测量结果一致，而在中性条件下的模拟结果与风洞试验的结果接近。在不稳定的情况下，由于表面加热而增强的垂直混合可改善通风性能。与非绝热条件相比，中性假设往往会低估行人水平速度比。因此，在AVA惯例中采用时，它被认为是保守的。此外，在不稳定条件下较强的风向变化会导致速度比与额叶面积指数之间的相关性比中性条件弱，这意味着在不稳定条件下街道方向在通风中的重要性降低。

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