Experimental Study On The Variation Law Of Mixed Layer Height Under The Local Circulation Of Valley City

Due to the ongoing city process,more and more people are exposed to high temperatures and air pollution.city ventilation is an important basis for eliminating heat and air pollution,and has received widespread attention in recent years.China is a mountainous country.A large number of cities are located in the valley due to economic or historical factors.The relatively closed topography makes the interior of the valley city calm for a long time,which makes it difficult to spread atmospheric pollutants inside the city.The only source of energy for the diffusion of regional atmospheric pollutants is the internal local thermal circulation.Therefore,understanding and quantifying the local circulation characteristics formed by the hot pressing under static wind conditions is particularly important for the ventilation of valley cities.In the study of topographic thermal circulation,most researchers use field measurement and computer numerical simulation.However,the data obtained by field measurement is relatively one-sided,while computer numerical simulation is affected by factors such as grid accuracy and turbulence model.Therefore,both have considerable limitations.Because the indoor fluid experiment is not limited by the test conditions such as natural weather conditions,the operation is convenient,the repeatability is strong,the test period is short,and it is easy to reproduce or set certain test conditions,and the field measurement is controlled by the indoor fluid experiment.The boundary conditions and the results of the numerical simulation are validated to predict the actual topographic thermal circulation.Therefore,it is of great significance to establish an indoor fluid experimental platform for geothermal circulation in the valley city.Firstly,according to the similarity theory,the similarity conditions required for the scale experiment to simulate the local circulation are obtained.Based on the simplification of the physical model of the actual valley city,an indoor water tank experimental platform for simulating the local circulation of the valley city is designed..Since the atmosphere of the actual valley city is often in a stable stratification state,the stable stratification means that the disturbed atmosphere has a tendency to return to its equilibrium position during the ascending motion.At this time,the atmospheric state is called the stable stratification state,and the stable stratification is diffused to the atmospheric pollutants.Extremely disadvantageous.In this paper,by analyzing the mechanism of the actual stable atmospheric stratification,the linear density layered brine is used to simulate the stable atmospheric stratification in the water tank.The experimental improvement is carried out on the basis of the“Oster two-tank method”for the preparation of linear density stratified salt water.The flow control equation of the peristaltic pump in the experimental system was determined when preparing different linear density stratified salt water.Finally,three different linear density layered salt water were designed,and the reliability of the above control equation was verified by experiments.The experimental results show that it can well construct the linear density layered salt water required for the experiment.The height of the mixed layer is an important parameter in the problem of the diffusion of atmospheric pollutants.It refers to the near-reaching process of the material,energy and momentum in the mixed layer of the atmosphere due to the thermal flotation or mechanical force turbulence.The height of the ground,which determines how much air is being diluted.Based on the water tank experimental platform of the local circulation in the valley city,the flow field display technology is used to visualize the development of the local circulation flow in the valley city by injecting the fluorescent solution.Using visual images,the spatio-temporal evolution of urban mixed layer height under three different stable stratifications(N=0.5s^-1,N=0.7s^1-,N=1.0s^-1),valley city day and night local circulation is analyzed.The experimental results are compared with the relevant literature research results,which proves that the fluorescence visualization method in the water tank is reliable for predicting the height of the mixed layer.At the same time,the experimental results show that the stable stratification will inhibit the development of convection in the vertical direction,and the greater the stratification stability,the smaller the height of the urban mixed layer.Under the local circulation at night,the height of the mixed layer of the valley city center and the urban edge will eventually reach a consistent height,and the mixed layer height of the valley city edge under the local local circulation will be higher than the urban center mixed layer height.Under the same size of thermal boundary conditions and stable layer stability,the height of the mixed layer in the center of the daytime valley city under local circulation will be lower than that at night,while at the edge of the city,the height of the daytime mixed layer will be higher than that at night.