Numerical Simulation Of The Ventilation And Pollutant Dispersion At High-density Urban Area Driven By Wind And Buoyancy

High-density populations are often accompanied by high-intensity pollutants and waste heat emissions,resulting in “urban heat island” and “urban polluted island”.These phenomenon could be alleviated by reasonable urban design through changing the air flow and transport of heat and pollutants in urban area.This paper firstly analyzes the ventilation of urban canopy layer driven by wind,thermal buoyancy and wind-buoyancy force.Then,the dispersion characteristics of pollution leakage in urban area was evaluated.For urban ventilation driven by wind,the effects of increasing the number of wind channels on the ventilation capacity were analyzed under two typical building densities.The ventilation capacity was evaluated by multiple ventilation parameters,including the air age and air exchange rate.It is found that increasing wind channels weakened the purification ability at pedestrian level,resulting in pollutants accumulation at pedestrian.However,the ventilation capacity can be improved effectively by reducing the building area density.Then,the effect of 3-D street length on its ventilation capacity was analyzed.There is a critical length for street canyon that the ventilation capacities of 2-D street canyon and 3-D street canyon are identical.Furthermore,there is a street length that has the lowest dilution rate for pollutants.For urban ventilation driven by thermal buoyancy force,a new model was developed,which could be used to predict urban ventilation driven by buoyancy force.An experimental research was carried out to validate the new computational model.It was found that the thermal plumes generated by buildings attracted each other and eventually merged into a single thermal plume.The converging flow at the top of the street canyon plays a similar role to the natural wind flow,forming vortices in the street canyon.Due to the existence of convergent flows,the air temperature has increased from the edge of the building cluster to the center,which explains that the largest urban heat island intensity appears in the city center.An empirical formula of the street air exchange rate under windless conditions is derived.Then,the influence of the overall characteristics of the urban buildings on the thermal environment and pollutant dispersion under windless conditions is analyzed.Both the convex and concave types were found to have lower contaminant concentrations and air temperatures in the street canyon than the uniform buildings.For urban ventilation driven by wind-buoyancy combined force,the airflow patterns,pollutant concentration distribution,air exchange rate and pollutant retention time of three typical aspect ratios are analyzed.An abnormally large pollutant retention time is found at deep street canyons due to the flow seperation,which is about 20 times higher than that of other street canyons.Increasing the surface heat flow can reduce the retention of pollutants.When Ri reaches-20,the retention time falls back to the normal value.It is found from 3-D pollutant field that the pollutant plume spread and height are significantly affected by the wind direction,the aspect ratio and the vertical movement at the releasing point.In the absence of vertical motion,the building array plays a leading role in the spread of pollutants,and the influence of wind direction is not significant.When the vertical movement was intense at the releasing point,the pollutans were transported above the roof level and its dispersion was dominated by the wind direction.Due to the deviation of the wind direction and the street direction,the pollutant plume was deflected from the main wind direction.The large-eddy simulations showed that the mechanisms of plume development within and above the urban canopy layer were different.Within the urban canopy layer,the pollutant plume direction is mainly dominated by mean wind flow.Above the urban canopy layer,the pollutant plume direction is mainly dominated by intermittent upward flow generated by the wake vortex.The pollutant concentration field shows a constant decay rate along the plume axis at positions away from the releasing point,which is consistent with the traditional Gaussian dispersion model.It indicates that the Gaussian dispersion models could have sufficient accuracy at the positions away from the releasing point on condition that the plume centerline were well reproduced..