Study On Urban Local-scale Spatialtemporal 3D Radiant Environment Characteristic And Thermal Comfort In Severely Cold Regions

The acceleration of urbanization and the growth of urban population have put forward higher requirements for the outdoor environment in urban spaces.At present,researches on outdoor environment are mainly concentrated in the summer in low and middle latitude areas,and there are relatively few systematic studies on cities in severely cold regions.At the same time,most of the current researches focus on wind,heat,and humid environments,and there is a lack of research on radiant environments.The radiant environment is an important part of urban local climate,and due to its complex composition,dynamic fluctuations and three-dimensional directional characteristics,the radiant environment has considerable complexity and plays a vital role in urban energy balance and human thermal comfort.In addition,seasonal differences will lead to different requirements for the radiant environment in urban areas in severely cold regions.Therefore,different suitability strategies and optimized design schemes targeting at different seasons should be adopted.This study takes the city in the severely cold region as the research object.On the basis of field measurement and numerical simulation,the three-dimensional radiant environment under the diversified spatial layout of the local scale and the underlying surface configuration in the urban canopy is studied.Besides,the impact of the urban local radiant environment on thermal comfort is analyzed.And a dynamic prediction and evaluation model of the urban local radiant environment in severely cold regions based on the urban canopy model has been established.The model provides a theoretical basis and quantitative evaluation tool to make full use of the urban local spatial radiant environment and the improvement of thermal comfort level.Firstly,the university campus in the center of Harbin is selected as the research object.The radiation testing technology combining with fixed meteorological station is used to explore the spatial and temporal characteristics of three-dimensional short-and long-wave radiant flux densities under different building forms,surrounding environments,and different underlying surface forms in winter.The roles of three-dimensional short-and long-wave radiant components in mean radiant temperature is analyzed.Besides,the thermal comfort indices are used to quantify the outdoor human thermal comfort in winter,and the effects of urban outdoor radiant environment on thermal comfort is analyzed.Results show that in winter in the severely cold region,the magnitude of long-wave radiant flux densities are higher,but the fluctuation of short-wave radiant flux densities are greater;the higher the degree of openness,the greater the short-wave radiant flux densities that reach the pedestrian height,and the emission of long-wave radiant flux densities increases accordingly;the horizontal long-wave radiant flux densities absorbed by the human body contributes the most to the mean radiant temperature;compared with other meteorological parameters,the mean radiant temperature has the greatest impact on the thermal comfort of human body.Secondly,this study focuses on the shading areas of buildings and trees under summer conditions in Harbin,and compares the three-dimensional short-and long-wave radiant components and thermal comfort under shading and unobstructed conditions;the radiant environment changing laws under shading conditions are analyzed;the directional sky view factors are used to evaluate the relationship between the local urban form and the three-dimensional radiant environment;and the differences in human thermal comfort level under shading and sunny conditions as well as the impacts of different meteorological parameters on thermal comfort index are discussed.Results show that building or tree shading leads to a higher reduction in short-wave radiant components than long-wave radiant components,and at the same time makes the long-wave mean radiant temperature higher,who has a strong correlation with the air temperature that the coefficient of determination R~2 between the them is exceeding 0.8;the directional sky view factor has a significant positive correlation with the short-wave radiant flux densities,while the statistical relationship between the directional sky view factor and the long-wave radiant flux densities is not obvious;summer shading can significantly reduce outdoor thermal stress and provide residents with more comfort outdoor activity spaces.Thirdly,based on the mean radiant temperature,which is an important indicator of the overall radiant environment,this study uses the mean radiant temperature determined by the"six-directional"method as the standard value.Then the mean radiant temperature determined by the field measurement method of the globe thermometer and the simulation models,SOLWEIG,Ray Man and ENVI-met,are analyzed to confirm their accuracy and applicability in summer and winter under different openness in the severely cold region,where correlation analysis,cluster analysis and sensitivity analysis are utilized.Furthermore,the prediction results of each method were modified.Results show that the prediction accuracy of the globe thermometer method for determining mean radiant temperature is higher in summer than that in winter;the simulation results of each numerical simulation model have a higher correlation with the standard value,and meanwhile the deviation distance is smaller under the summer conditions;SOLWEIG shows a better simulation effect than Ray Man and ENVI-met in predicting mean radiant temperature with higher correlation and lower deviation distance with standard mean radiant temperature;comparing Ray Man and ENVI-met,Ray Man is better than ENVI-met in describing the change trend of mean radiant temperature,and mean radiant temperature simulated by ENVI-met is closer to the standard value than Ray Man;the modification of each method proposed in this study effectively improves the mean radiant temperature prediction ability.Fourthly,in view of the inapplicability of the above simulation models in the severely cold region,and the certain empirical issues of the modifications depending on the corresponding climate and geographical background,this study uses the existing urban canopy model of the research group,the urban dynamic prediction model of regional thermal climate(UDC),and takes into account that the radiant environment algorithm in the model is relatively simplified,the radiant parameters involved in calculation and output are less,and the prediction accuracy is not high enough.Then on the basis of the view factors between the various surfaces and the sky,the incidence of short-wave radiant flux densities at different heights are obtained according to the law of firstly emitted radiation and multiple reflections of short-wave radiation.The surface temperature fluctuation patterns of the urban surfaces are obtained based on the heat balance model of the outer surface of the buildings and the underlying grounds,and then the incidence of long-wave radiation at different heights can be accurately calculated.Based on the above-mentioned accurate prediction of short-wave and long-wave radiant flux densities,mean radiant temperature using"six-directional"method can be simulated by calculating the three-dimensional short-and long-wave radiant flux densities of pedestrian height position.Furthermore,the above content is coupled with the existing wind and thermal modules in UDC.Therefore,a dynamic prediction and evaluation model for the urban local radiant environment in the severely cold region,UDC-rad,is established.The simulation results of UDC-rad are verified on the basis of the data via field measurements.Results show that the calculation results of the UDC-rad model are highly reliable,and are suitable for long-term dynamic simulation of urban local radiant environment parameters.Finally,based on the established dynamic prediction and evaluation model of urban local radiant environment in the severely cold region,a multi-factor numerical experiment based on the response surface analysis method is conducted.The effects of building densities,floor area ratio,building layout,tree coverage,grass coverage,and snow coverage on the mean radiant temperature and thermal comfort index are analyzed.Besides,this study takes the outdoor radiant environment and thermal comfort as the optimization objectives,and adjusts different building forms and underlying surface configuration factors with the aim of obtaining the optimized design plans for urban areas in summer,winter and throughout the year.In summary,this study has carried out in-depth field measurement,data analysis,comparison verification,model establishment,coupling development and optimization design on the urban local radiant environment and thermal comfort parameters in the severely cold region from different perspectives.It also provides data support,theoretical basis and technical guidance for improving the urban regional radiant environment and thermal comfort level for the severely cold region.