Heat And Wind Environment Observation Analysis And Numeric Simulation In Urban Areas

Microclimate in cities has a strong connection with human activities,so improving the wind and heat environment in cities means great importance to citizens' daily life.This paper has finished the following task:1.Temperature and Urban Heat Island changes in Nanjing and Shanghai in the summer of 2013 were analyzed.Meteorology stations in the 2 cities were classified with building density,city planning map,Google map et al.The effects of water,plants covered surfaces on microclimate in warm days were discussed.2.The capability of PALM(Parallelized LES Model)in simulating wind fields was evaluated with CEDVAL wind tunnel data.Wind fields in several idealized building cases,as well as in typical CBD(Central Business District)Xinjiekou and RD(Residential District)Dangxiao areas in Nanjing,.were simulated with PALM and evaluated by ventilation indexes.Based on the work above,main conclusions in this paper are as follows:1)Different cities in Yangtze River Delta shows different features in hot summer days.The overall daily and monthly changes of temperature in Shanghai and Nanjing in Aug 2013 showed the same trend.However,it is obviously that urban heat island characters in these 2 cities are different.Daily heat island in Nanjing shows V distribution while it shows W distribution in Shanghai.The main reason is supposed to be the different character of city surface.In the city center of Shanghai,the building density is much higher than the sub-urban areas.During daytime,buildings absorbed a lot of sun radiation and anthropologic heat improved also,making the day temperature in city center increase obviously.Besides,the sub-urban or rural areas of Shanghai,influenced by great water bodies like Long River and sea,further decreased the area temperature in daytime,leading to the same urban heat island intensity(UHII)in daytime as that in nighttime.However,sea breeze does not affect Nanjing and the building density in Nanjing is much lower,leading to relatively less temperature difference between urban and rural in daytime when UHII valley appeared.2)Negative relationship exist between urban wind speed and UHII in hot summer days.Usually,high UHII will correspond to low surface wind speed,vice versa.During heat wave,UHII shows strong negative correlation with mean wind speed,and the correlation coefficient is 0.872.Improving the ventilation of cities may be helpful to decrease the urban heat intensity effect in warm periods.3)Buildings have direct effects on heat and wind environment.For the urban heat fields,densely built districts may be easily to get higher temperature and stronger UHII.Meanwhile,temperature change might delay in high-density areas compared with low-density areas.In high-density districts,temperature increases later in the morning and decreases later in the afternoon.Besides,different shelter of sky in high-density areas may greatly change the local heat environment.During daytime,higher S VF(Sky View Factor)(more shelter)means less sun radiation absorbed and lower local UHII.During nighttime,higher SVF areas contains a lot of building heat accumulation,which means higher local temperature and stronger UHII.For the wind fields,buildings structure is the determining factor of the whole wind fields,regardless of other obstacle in cities.The wind fields in buildings may be highly relative to the buildings' mean height,height variance and building arrays.Results shows that,when building groups made the lower wind fields damp larger,the upper wind speed might be intensified to be larger too.The building groups with higher mean height and more complex arrangements may have weaker wind among the buildings.Cases with chessboard arrays and larger height variance can hinder the horizontal air ventilation and improve the vertical ventilation.4)Typical CBD and residential districts show obviously different features in wind fields.In typical CBD and RD,wind fields differ both in horizontally and vertically aspects due to their difference in building distribution and height variance.In typical CBD areas,the overall wind speed is low.The maximum values always appear around tall buildings and wind speed in big squares will be relatively gentle.In the typical RD areas,the overall inner wind speed is larger and maximum values may appear around building tubes as well as those flat surfaces.Vertically,a simple exponential shape of the spatially-averaged mean velocity profile cannot be expected in real building areas.The vertical wind profile of typical CBD presents a three-level stratified distribution according to the variation rule of building height.Significant wind shear occurred near 25m and 109m(approximately 0.82h and 3.9h corresponding to normalized height),and weak wind shear occurred at 220m(7.9h).The vertical wind profile of the residential area is divided into two floors with the height of the building group,and wind shear appears only around 23m(about 1.5H).The wind speed rises in power exponent of different shapes in each height of the above stratification respectively.The higher the height of each layer,the smaller the corresponding fitting power exponent is.