Study On The Synergistic Effect Of Unsteady Wind And Thermal Disturbance On Mixed Convection In Urban Street Canyons

Urban street canyon is the basic component of modern city,and its environmental conditions have important impact on the indoor environment of street-facing buildings,cityscale mixed convection and pollutant distribution.The study of mixed convection of urban street canyons is helpful to enhance the understanding of the formation mechanism and process of outdoor semi-enclosed space environment of building groups,which is conducive to optimizing the layout of urban street canyons and street-facing buildings,thereby providing assistance for the creation of low-energy-consumption and high-quality urban air environment,and ultimately providing protection for the health and comfort of urban residents.In order to explore the distribution characteristics and evolution of urban street canyon air environment under the synergistic effect of unsteady wind and thermal disturbance,this paper used a combination of numerical simulation and experimental research to discuss the distribution and evolution characteristics of street canyon mixed convection field in various superposition modes of unsteady wind and thermal disturbance by combining numerical simulation and experimental research.In this study,based on the principle of similarity,a model bench of mixed convection flow field in the street canyon was designed and built,and the unsteady flow field of the street canyon under the coupling of incoming flow and ground heating was displayed and analyzed with the help of the PIV experimental system of Chang’ an University.It was found that with the increase of the intensity of the thermal disturbance,the intensity of the circulation in the lower part of the canyon and the fluid displacement efficiency in the near-ground area increased,which showed that thermal disturbance had an enhanced effect on the internal circulation of the canyon.Secondly,based on computational fluid dynamics methods,the steady-state and unsteady mixed convection prediction models of urban street canyons were established,and the wind and thermal environment of urban street canyons were simulated and analyzed by using the numerical method of separated vortex(DES)and ANSYS Fluent software with the measured data as boundary conditions.The results show that the steady-state model cannot accurately describe the hourly cumulative effect of thermal disturbance and the hourly wind pressure change of wind disturbance.Compared with the traditional steady-state analysis,the use of unsteady wind and thermal disturbance boundary conditions based on actual environmental characteristics is conducive to the accurate prediction of the vortex morphology of the flow field,which is conducive to the capture of mixed convective characteristics of urban street canyons.In the study of air environment in urban street canyons,it is necessary to conduct unsteady state analysis of wind and thermal disturbances.Finally,based on the measured data of large samples of street canyon temperature and wind speed,the actual evolution law of street canyon surface temperature and incoming wind speed were analyzed,and a variety of mathematical expressions of unsteady wind and thermal external disturbance were constructed by statistical methods.Furthermore,with the help of UDF(user-defined function),the mixed convection characteristics of urban streets and canyons under the synergistic effect of the above types of unsteady state wind and thermal external disturbance were obtained by numerical methods.The results show that there is a significant correlation between the morphology of the mixed convective flow field in the canyon and the Fr number,and most of the area of the canyon flow field is controlled by a major vortex at0.05< Fr.When the Fr<0.05,the external disturbance effect of wind weakened,and complex multi-vortex flow field morphology is prone to appear in the street canyon.At the same time,a variety of unsteady distribution forms of wind and thermal disturbances were obtained by fitting in this study,and it was proposed that when the thermal disturbance effect is significant,the average pulsating wind can be used to simplify the boundary conditions of unsteady wind disturbance.This study helps to strengthen the understanding of the formation mechanism of mixed convection in urban street canyons,and provides support for the prediction and optimization of unsteady wind thermal environment in urban street canyons.