Prediction And Evaluation Of Wind And Thermal Environment Of Urban Residential Areas In Severe Cold Regions

With rapid urbanization,urban local climate is qutie different from that in suburban and rural areas and mainly shows the urban effects due to the artificial landscapes replace the original natural surface.A residential area is one of the basic component and most important living spaces in an urban area.The outdoor environment of a residential area is closely related to the health,comfort and safety of residents.However,most studies on urban residential areas pay more attentions on the problems of thermal comofort and safety caused by overheating in the southern climate zone.The outdoor environment of residential areas in severe cold regions is usually neglected in the urban planning stage.There is a lack of studies on resuidential areas in severe cold regions.Therefore,researhces on microclimate of residential areas in severe cold cites is necessary.In this study,field measurements,wind tunnel experiments and numerical simulations were combined to study the wind and thermal environment of residential areas in severe cold cites.An original dynamic prediction model on wind and thermal environment of urban residential areas in severe cold regions was proposed based on the urban canopy model.Using the dynamic prediction model,the influence factors of wind and thermal environment of residential areas in severe cold cites were analysed.Firstly,taking Harbin as the representative severe cold city,the development history of residential areas in Harbin was summarized.Then the typical types of building layouts for residential areas in Harbin were investigated and summarized from the forms and characteristics of bulidng layouts of existing residential areas in Harbin.Wind tunnel experiments and computational fluid dynamics(CFD)simulations were performed to study the wind environement in residential areas with typical builidng layouts summarized above.The pedestrian-level wind conditions at these representative residential areas were assessed from the perspective of wind mechanical comfort and wind chill by the wind mechanical comfort criterion of NEN 8100 and the wind chill criterion proposed in this study.The results showed that the hybrid-type and the enclosed-type layouts can provide better pedestrian wind environment and comfort in residential areas in severe cold cities than parallel-type layouts.Besides,a strict control of building height in residential areas is important to improve the pedestrian-level wind comfort.Secondly,thermal environment over urban residential areas in severe cold region were measured using the eddy covariance method combined with fixed meteorological observation.Based on the measurement data,dynamic changes of the atmospheric parameters,radiation fluxes,and turublent fluxes throughout the year were analysed.Special emphasis was placed on the distinctive energy balances in severe cold regions under different snow conditions during winter.The measurement results indicated that the thermal and humidity parameters,and turbulent fluxes showed obvious seasonal changes.Energy balances and partitioning showed different characteristics under different snow conditions.The daytime net radiaion was mostly dissipated as turbulent fluxes during the fresh snow periods but wasmostly stored by the urban surfaxes for old snow periods.The fraction of sensible heat flux decreased and the fraction of latent heat flux increased due to the snowmelt process consumed a considerable amount of energy during the melting snow periods.Due to the lack of moisture source of snow,the fraction of latent heat flux during the snow-free periods was lower than that during the snow periods.Thirdly,considering that the snow cover was not included in the existing urban canopy model by our research group,a dynamic snow model was developed based on the heat and mass balances to account for the energy and mass exchange between the snow cover,underlying surface and atmosphere.Then the dynamic snow model was coupled with the urban canopy model.In addtion,the drag coeffiencet in the urban canopy model was revised based on the drag force results of different layouts of residential areas by CFD simulations.Therefore,the wind and thermal environment dynamic prediction model for urban residential areas in severe cold region(called UDC-cold model)was establised based on the revision work mensioned above.The accuracy of UDC-cold model was validated by the field measurement results.The validation results showed that the UDC-cold model can provide reliable simulated results of urban thermal climate in severe cold reiongs under snow and snow-free periods.Finally,based on the establised wind and thermal environment dynamic prediction model for urban residential areas in severe cold region,the influences of building density,building height,building layout,coverage raito of grassland,forestland,and snow cover in winter on wind and thermal environment,and human comfort under different weather conditions of different seasons were investigated by the single factor numerical test and orthogonal test.The test results indicated that the influece of building density was most significant in summer,while the factors of builidng density and building height were the significant factors in winter.The thermal comfort in residential areas in summer can be improved by increasing the building density and building heitht to a certain extent,increasing the fraction of parallel-type layout and greening rate.The thermal comfort in residential areas in winter can be improved by increasing the building density and building heitht to a certain extent,decreasing the fraction of the parallel-type layout and coverage of snow.In conclusion,the features of wind and thermal environment in residential areas of cities in severe cold reiongs were investigated using wind tunnel experiments,numerical simulations and field measurements in this study.Besides,the influences of different factors on human comfort in residential areas of severe cold cities based on the establised urban climate dynamic prediction model for severe cold region.The conclusions in this paper can provide theoretical guidance and technical support for improvement of the outdoor environemnt and livable degree of urban residential areas in severe cold reiongs and the quality of urban dwellers' daily life through effective urban planning methods.