湍流度随高度的变化及其对城市宏观地形的依赖

Variation in Turbulence Degree with Height and its Dependence on Urban Macroscopic Topography

分别构建广州主建成区垂直比例尺为1﹕2000、1﹕1000和1﹕500的3个建筑物模型,利用大型边界层风洞,在西北和东南两风向下,基于中性流模拟分析了复杂城市地形下湍流度随高度的变化及其对宏观地形的依赖。结果表明:风廓线指数α与不同高度的湍流度之间的关系密切,利用现有模型,根据4类粗糙度边界层和不同垂直比例尺,可确定相应的湍流度随高度变化模型的主要系数,预测精度高。城市地形下最大湍流度面发育在0~0.2h之间狭窄的范围内。用湍流度形态指数β来表征湍流度随高度的变化,无论城市屋脊还是平坦地形,随着风程区的延伸,廓线的指数α升高,湍流度形态指数β降低。表明同一高度湍流度值具有由迎风区、丘顶区向背风区增高,沿风程逐渐增大的规律,对地形部位和风程的依赖性强,与来流翻越简单地形时的特征一致。

Wind speed and turbulence are two closely related indicators that measure the properties of a wind profile.Obtaining an insight into the development of turbulence over a complex urban terrain can help deepen the understanding of the performance of urban wind farms.In this research,three building models with vertical scales 1:2000,1:1000,and 1:500,respectively,were constructed.Using large boundary-layer wind tunnels and generating wind from two directions(northwest and southeast),the variation in turbulence with height over a complex urban terrain,and its dependence on the macroscopic terrain characteristics,were analyzed in a neutral flow simulation.Based on the experimental data obtained in the wind tunnel,the two model coefficients A and B were determined with respect to four types of boundary-layer roughness,under neutral flow or with turbulence varying with height at different vertical scales.In both cases,the average correlation of the proposed model was about 0.8.A close relationship between the wind profile index and the turbulence at different heights was observed.Based on the profile indexαthe turbulence at different heights could be predicted,so that the variation in turbulence with height over a complex urban terrain could also be quantified.Generally speaking,the turbulence decreases with altitude,and the maximum turbulence develops at the bottom.However,there are exceptions.It is common that the turbulence of the hole at the lowest measuring point is not the largest,which makes the shape of turbulence change with the height like a hook.The shape of turbulence varying with height can be summarized into four types.The height at which the maximum turbulence occurs is found to be concentrated in the range 0-0.2 h(where h is the dimensionless unit),which makes up more than 80%of the total number.Therefore,in the height range 0-0.2 h above the urban terrain,the wind direction and velocity of the airflow showed complex patterns,and turbulence is extremely developed,with an important impact on the diffusion of urban pollutants and the transfer of heat.Using the existing model,the main coefficient of the turbulence model corresponding to a given height could be determined,with high precision accuracy,according to the four kinds of boundary-layer roughness and the different vertical scales.The development along the height of the non-maximum turbulence intensity depended on the difference between the actual wind profile and the standard wind velocity at the same height,whereas the maximum turbulence level under the given urban topography occurred within the narrow range 0-0.2 h.The turbulence degree indexβwas used to characterize the variation of turbulence intensity with height.The exponentialβof the turbulence intensity decreased with increase in the exponential alpha of the profile,regardless of the shape of the terrain(e.g.,a ridge or a flat terrain).It was shown that the overall turbulence profile increases from the upwind and top areas to the leeward area,and increases gradually along the wind flow direction.Turbulence profile also has a strong dependence on the terrain and the wind path,and has the same flow characteristics as those over a simple terrain.At the same time,the shape of theβisolines of the three models did not show the same overlap.On the contrary,great differences were observed.This shows that in the past,when the wind tunnel simulations were carried out,the method of ensuring the number of thunderbolts simply by increasing the vertical scale was affected by a large uncertainty.