Research On The Characteristics Of Mesoscale Flow Field In Valley City Under Calm And Stable Weather Conditions

For valley cities,the high frequency calm and stable weather conditions caused by terrain barrier and temperature inversion will result in very unfavorable diffusion conditions and limited environmental capacity,leading to frequent haze and heat waves.At this time,the coupling effect of the two mesoscale flows of slope flow and urban heat island circulation is the basic mode of urban ventilation in the valley,which dominates the dissipation of heat and pollutants in urban areas.Domestic and foreign scholars have conducted a lot of research on slope wind and urban heat island circulation,while the coupling of the two is rarely seen.A calm and stable weather will be formed when the calm wind weather encounters a relatively stable atmospheric stratification.At this time,the ventilation mechanism of valley cities is not clear,so it is necessary to conduct in-depth research on it.The relevant research content will provide a scientific theoretical basis for how to explain and reduce the haze and warming phenomenon in valley cities,and also have important guiding significance for the planning and siting of cities and industrial parks.Based on the similarity theory,a water tank experimental platform for mesoscale flow fluid dynamics in valley cities was designed and built,and a water tank experiment and numerical simulation construction method of atmospheric stable stratification in valley city were established.Two linear density stratification construction methods:"Oster two-pot method",which is suitable for constant section water tank,and "Hill two-pot method",which are suitable for variable section water tank,were summarized.The result show that the construction of density stratification depends only on the initial density difference between the salt water and the pure water and the initial volume of the salt water after the intensity of density stratification and the horizontal cross-sectional area change with height are determined.With the help of user-defined functions,the density stratification was built,and the SST K-? model was selected as the turbulence model and the treatment method near the wall,and the numerical simulation of the water tank experiment was realized.By summarizing and analyzing the theoretical models of slope wind and urban heat island circulation under different hypothetical conditions,the main influencing factors of mesoscale flow in valley cities under calm and stable weather conditions are identified.Based on the plain city isothermal stratification conditions,and gradually considering the influence of stable stratification,valley and its derived slope wind,seven representative conditions were finally formed,and the corresponding flow patterns were revealed by laser induced fluorescence(LIF)technology.This study lays a theoretical foundation for further study on the flow pattern variation of urban plume and the influence of valley urban form on mesoscale flow field characteristics.The water tank experiment and "numerical water tank" simulation are combined to study the flow characteristics of urban plume under the role of slope or slope wind,with background stratification intensity as the control variable.With regard to the evolution of urban plume with high and low aspect ratios,the quantitative comparison of urban center mixing height under different conditions is realized by using the relationship between the ratio of urban center mixing height to urban diameter(zic/D)and Froude number(Fr),zic/D=AFr.The critical Froude number(Frc)of the heat plume transition between high and low aspect ratios under the action of downslope wind is also obtained.As for the breakup of the finite thickness inversion,two kinds of flow characteristics are found,namely,double plume structure and single plume structure.As the pre-order of the breakup of the inversion,"the inversion layer" across the upper and lower plumes still plays a limiting role in the dissipation of heat.The results show that the upslope wind is beneficial to the breakup of inversion,while the downslope wind is opposite.The breakup of inversion layer can increase the heat transfer coefficient of urban canopy surface by 35.44%.Using the "numerical water tank" simulation method,the relative size of valley bottom width to city diameter(R=W/D)and the city geometry as the control variables,the different influences of different valley city forms on mesoscale flow field characteristics were investigated.It is found that the urban expansion(reduction of R value)will lead to the weakening of the urban regional wind speed and the decrease of the heat transfer coefficient of the urban canopy surface,but the above conclusion is no longer valid when the downslope wind acts.If the downslope wind is to be effectively utilized to improve the thermal environment of urban areas,the diameter of the city should reach more than 1/3 of the width of the valley.The mixing height of urban center and the change law of wind profile of urban edge are compared and analyzed for the four urban layouts of strip,square,rhombus and circle.The strip and square urban layouts are the most favorable for atmospheric diffusion.The study also found that the downslope wind has an excellent performance in regulating the urban thermal environment under various urban layouts.