Quantitative Study Of Pollutant Concentration Field In Valley City Based On Water Tank Experiment

Under the conditions of static wind and temperature inversion,the diffusion of atmospheric pollutants in the valley city mainly relies on the internal local thermal circulation,that is,the slope flow and the urban heat island circulation.As a typical pollution source in urban areas,the diffusion of unorganized emissions from industrial parks driven by local geothermal circulation in valley cities is very representative,and the corresponding regularities are still unknown.If this can be revealed,it will be of great significance to the scientific and reasonable explanation and management of smog phenomena and problems.For the study of pollutant diffusion on complex terrain,most scholars use two research methods: field measurement and numerical simulation.However,because the data obtained by the former is relatively one-sided and discrete,it is difficult to obtain regular results,while the latter will be restricted by Many influencing factors such as grid accuracy and turbulence model are also difficult to obtain reliable calculation results,and both have considerable limitations.The laboratory water tank experiment has many advantages,such as precise control of experimental conditions,easy reproducibility,and short experimental period,and has proved to be an effective method for studying urbanscale flow.Using the water tank experiment to simulate the diffusion of pollutants under the effect of local circulation in valley cities,the trajectory of pollutants can be observed.If further quantitative measurement methods can be used,quantitative analysis of the concentration field can be achieved.This paper is based on the local circulation water tank experimental platform of the valley city,and adopts the concentration quantification method combining the Planar Laser-induced Fluorescence technology and the MATLAB image processing technology to quantitatively analyze the diffusion law of pollutants under the effect of the local circulation in the unilateral mountainous city.The salt water stratification is used to simulate the stable atmosphere stratification state,and the rhodamine 6G solution is used as a tracer substance to simulate pollutants and is continuously released in the quasisteady state flow field.Under the irradiation of a specific wavelength laser,the tracer substance Rhodamine 6G is induced to emit fluorescence on the plane to be measured in the flow field,and the s CMOS digital camera can be used to record the movement trajectory of the tracer substance.According to the conversion relationship between fluorescence intensity(grayscale)and concentration,the concentration distribution on the two-dimensional plane can be obtained with the help of MATLAB image processing program.This paper conducts a quantitative study on the pollutant diffusion law in plain cities and unilateral hillside cities of valley cities under the atmospheric conditions of neutral stratification and stable stratification.After analyzing and comparing 10 groups of working conditions,it is found that the existence of the temperature inversion layer greatly inhibits the diffusion of pollutants in the vertical direction,and the highest position that the pollutants can reach is significantly lower than the neutral stratification.This height is called the mixing height,It largely determines the atmospheric environmental capacity.The hillside topography has an obstructive effect on the horizontal development of the urban heat island circulation,which makes the center of the urban heat island circulation move 1/8 of the city’s diameter to the hillside.The upslope flow during the day increases the mixing height,which is beneficial to the diffusion of urban pollutants in the vertical direction.At the same time,the upslope flow causes the center of the urban heat island circulation to move 1/4 of the city’s diameter to the side of the hillside.The mixing height under the action of the upslope flow alone is 1.2 times that under the combined action of the urban heat island circulation.The existence of the urban heat island circulation reduces the mixing height of the upslope flow.Under the action of the local circulation in the valley city during the day,there is a "stagnation zone" in the outflow of the urban heat island circulation and the backflow of the upslope flow,which makes it difficult for pollutants to accumulate in this area and spread.