| 摘要: |
| 城市街谷内热不稳定流动是促进污染物扩散的重要影响因素之一。本文基于街谷内热平衡分析，结合大涡模拟方法，研究了一个南北走向的城市街谷内温度、风场的日变化特征，并分析了壁面对流换热及长波辐射对街谷内环境的影响。结果显示：壁面对流换热是影响街谷内温度、风场的主要因素，而长波辐射的影响非常小，长波辐射引起街谷内空气温度升高不足对流换热影响的10%，而其对平均风速和脉动量的影响更是在2%和1%以内；街谷内空气温度从早上开始逐渐增加，到15∶00的时候达到最大，可达38℃；上午时段，迎风面壁面热浮力减弱街谷内风速，街谷底部和迎风墙侧的脉动量根均方值较大，而下午时段街谷顶部的脉动量根均方值达到最大。街谷内不同位置和不同时段内，通过建筑材料选择和表面结构设计，适当调控建筑壁面的温度，可以促进街谷内温度分布和空气流通改善。 |
| 关键词: 城市街谷 热平衡模型 日变化过程 数值模拟 |
| DOI：10.7515/JEE202010 |
| CSTR：32259.14.JEE202010 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41977182，41861144021）；黄土与第四纪地质国家重点实验室开放基金（SKLLQG1844）；陕西省自然科学基础研究计划（2019JM-387） |
| 英文基金项目:National Natural Science Foundation of China (41977182, 41861144021); Open Foundation of the State Key Laboratory of Loess and Quaternary Geology (SKLLQG1844); Basic Research Plan of Natural Science of Shaanxi Province (2019JM-387) |
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| Simulation on diurnal variation of thermal unsteady flow in a north — south orientated street canyon |
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ZHANG Yunwei, XU Wen, WANG Qingru, SU Junwei, HUANG Yu, GU Zhaolin
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1. Department of Earth and Environmental Sciences, Xi’an Jiaotong University, Xi’an 710049, China
2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope Thermal unsteady flow in urban street canyon is one of the important factors to promote the dispersion of pollutants. As the diurnal variation of solar radiation, wall heating patterns inside urban street canyons changed, and hence influenced the air flow and pollutant dispersion characteristics. Better understanding the diurnal variation of thermal unsteady flow inside urban street canyons would be helpful for air pollution mitigation by, e.g., reasonable urban planning. Materials and methods Based on the heat balance model and the large eddy simulation method, numerical simulations were conducted to investigate the air flow and pollutant dispersion characteristics and diurnal variation features in a north— south orientated street canyon. Results The wall heat transfer was shown to be the main factor affecting air temperature and wind field in the street canyon, and the effects of long wave radiation was very weak. The air temperature increase caused by long wave radiation was less than 10% of that induced by convective heat transfer, while the effects of long wave radiation on average wind velocities and the turbulent fluctuation were less than 2% and 1%, respectively. Air temperature increased from morning to 15∶00, could up to 38℃, and then decreased. In the morning, the windward wall thermal buoyancy reduced the wind speed in the street canyon, and turbulence near the road and building walls was larger; while in the afternoon, turbulence at the top of the canyon of maximum value. Discussion In the morning, the dispersion of vehicle exhausted inside a north—south orientated street canyon would be efficient in the lower part of the canyon, as the turbulence near the road and building walls were large. While in the afternoon, the escaping coefficient of pollutant on top of the canyon would be larger, but the inside canyon dispersion of pollutant was worse. Conclusions Appropriately adjusting the wall temperature from designing the materials and structures of building wall would improve the air temperature and air ventilation in urban street canyons. In the morning, as the increasing of air temperature, the thermal unsteady flow inside the urban street canyon would promote the dispersion of vehicle exhausts. Recommendations and perspectives Tree plantings in city avenues were generally expected to improve the thermal comfort conditions. And then, the influence of tree plantings on air flow and the diurnal variation characteristics should be investigated in our future works. |
| Key words: urban street canyon heat balance model diurnal variation numerical simulation |
