The Study On The Temperature Drop Coefficient And Temperature Field Of Vegetation Canopy

Urban heat island has been a difficult problem that puzzles people deeply,and green vegetation is an effective means to solve this problem.The canopy alleviate the radiation heat in the urban by transpiration,and through shade leaves to block the direct solar radiation and the reflected heat from the pavement,walls,then adjust the air temperature and humidity.It has special and important significance to alleviate the urban heat island and dry is land effect,and to improve the living comfort and quality of life.However,there lack enough systemic studies for a long term,including that how plants affect the thermal environment around them,how to describe and simulate these effects,and what the reasonable evaluation indexes are.In order to solve these problems,some studies are carried out in this thesis.First,through a lot of experiments,study the structure characteristics of a variety of campus common evergreen canopy.According to the characteristics of canopy structure,the related structural parameters are proposed: the leaf area index LLAI and the volume porosity Vp.Second,through a lot of wind tunnel experiments,the influence factors of single plant canopy on the scale of branches on the surrounding thermal environment were studied,analyzed the influence of structural characteristics of canopy in different wind speed and ambient temperature.Through regression analysis of the influence of canopy structure parameters on the thermal environment,the equation of the relation between the canopy temperature drop and wind speed,leaf area index and ambient temperature is given.To verify the adaptability of the correlation,a variety of tree species were taken into the environment.The results show that the canopy has signif icant cooling effect on the surrounding thermal environment,the changes of canopy temperature drop coefficient with the structural parameters,air flow and ambient temperature show a certain regular ity.The cooling effect is different for the different size of the canopy leaves,and the canopy temperature drop correlation has certain applicability,it is necessary to make some corrections to predict the cooling effect of different canopy.Third,According to the actual height of the canopy,the canopy shape and the leaf cluster size,several two-dimensional simplif ied canopy model was established.According to the results of wind tunnel experiment of different canopy,the specific distribution and variation of temperature field around the canopy were studied.By comparing the experiment and simulation results,analysis of the causes of errors and error degree.The results showed that only the model 1 with lower canopy height,the error is small,and the wind tunnel test results are in good agreement,only in a certain circumstances has some deviations.while the model 2 and model 3 because of its higher canopy height than the wind tunnel.its large difference in size result in the large deviation of simulation results and the experiment results.Using mode l and the blade cluster size invariant to simulate the other canopy of wind tunnel experiment again.It turned out that the simulation results agree with experimental results well,that is really the size of model is too large to the wind tunnel due to the large errors.it is explained that the canopy temperature drop correlation measured by wind tunnel test is only applicable to the canopy scale and wind tunnel is smaller,the height is about 3m.If it is to be used in the actual tall crown,it is needed to do some modification or further experiment and simulation.Fourth,using the concept of the fractal dimension to describe two-dimensional canopy structure.The influence of two dimensional canopy on the thermal environment was studied,and the relationship between the temperature and the fractal dimension,the leaf area index and the wind speed were analyzed.