CFD Numerical Simulation And Experimental Study On The Characteristics Of Multi-scale Urban Wind Environment

Urbanization is promoted rapidly worldwide,accommodating the more urban population and providing more social services.Featuring explosive urban expansion and increasing building density with emerging high-density urban forms,urbanization also leads to climate and environmental issues such as urban heat island(UHI)and air pollution,which threatens human comfort and health.Currently,China is marked by changes unseen in a century,and urban development has become the core and the growth driver for the rapid development of China's social economy.Considering the urban development process of ultra-fast expansion of urban land scale and populationscale in China,it is of great significance to deal well with the urban climate and environmental issues,not only for the high-quality urban development but also for the people's yearning for a better life.Urban ventilation is a proven strategy for mitigating these problems in this context.Achieving a better wind environment can enhance city breathability and reduce pollutants and heat accumulation,improving pedestrian comfort and safety.In this background,this paper focused on the urban wind environment and investigated the multi-scale characteristics of large-scale complex urban terrain,community-scale urban space,and building-scale tall buildings with Computational Fluid Dynamics(CFD).Besides,the wind flow patterns around a square building were investigated through wind tunnel testing.The study conducted in this paper has important implications for fully understanding the fundamentals of the urban wind environment,improving pedestrian comfort,and achieving high-quality urban development.The details are as follows:Firstly,the wind environment characteristics of large-scale complex terrain with an urban canopy were simulated.Hong Kong,a coastal megacity typifying hilly terrain,was used as a case study.To consider the effects of the urban canopy,a method for simplifying numerous buildings was proposed to model them explicitly in CFD simulations.The CFD model was validated against wind tunnel measurements,and good agreement was achieved.The CFD predicted overall wind fields demonstrated the effects of irregular underlying terrain,providing a technical basis for wind availability.The simulated topographical effects highlighted the immense potential of the proposed CFD model for enhancing the understanding of site wind characteristics over complex terrain.In addition,the CFD predicted site-specific wind profiles consolidated the CFD model as an essential step towards urban wind engineering,which can contribute to understanding and promoting an environmentally sustainable city.Secondly,the wind environment characteristics of community-scale urban space were simulated.A CFD model was established and validated against wind tunnel measurements.Tsuen Wan,a typical high-density urban space in Hong Kong,was selected as the case study.To fully consider the directional influence of upstream topography on incoming wind characteristics,the CFD predicted results of large-scale complex urban terrain were introduced to be the incoming wind profiles.Then the wind environment characteristics at different heights and wind directions were investigated.The wind-structure interactions in specific urban areas were analyzed,providing a better understanding of the effects of buildings on the local wind conditions,which could be helpful to the urban plan for sustainable cities.Thirdly,the wind environment characteristics of building-scale tall buildings were simulated.For single buildings,the effects of building heights,building widths,and incident wind directions were investigated.For a pair of buildings,the effects of building heights,building separations,and incident wind directions were investigated.For a row of buildings,the effects of building separations and incident wind directions were investigated.The results provided a fundamental understanding of wind-structure interactions that govern the effects of building dimensions,building separations,and incident wind directions on the wind environment for both pedestrian level and vertical wind conditions.Lastly,the wind flow patterns around a square tall building were investigated experimentally.The overall and time-averaged flow patterns were studied through wind tunnel testing with a high-speed camera and Phase Doppler Particle Analyzer(PDPA).Besides,the cycle-resolved analysis method was introduced to investigate the dynamic turbulence characteristics.The results showed that the tip vortex and base vortex in the wake could weaken the spanwise vortex shedding and thus have significant effects on the flow patterns in the near wake.The distribution characteristics of integral length scales of turbulence,Kolmogorov scale,turbulence kinetic energy,and turbulence dissipation rate under the influence of different vortices in the wake were also compared and analyzed.