Investigations On The Coupling Exchange Between Outdoor Wind Flow In Street Canyon And Indoor Air Flow Inside Buildings

Natural ventilation is the simplest and most economic way to provide sufficient fresh air andimprove thermal comfort for room ventilation. In temperate climate time periods in a year,building occupants are likely to open windows on both sides of a room for natural ventilation.However, when the outdoor air pollution is severe, natural ventilation may cause unacceptablelevels of indoor pollutants in a building. In order to explore the coupling exchanges betweenindoor and outdoor air when windows are opened in the roadside buildings, three-dimensionalCFD simulations and field measurements were conducted to analyze and discuss the air flow andpollutant distribution in street canyons, and also the coupling effects between indoor and outdoorair environments.Numerical simulations were carried out to study the flow fields and pollutant (CO) distributionsin street canyons and buildings with different window opening percentages (WOP). The numericalresults show that, more amount of fresh air entering the canyon through the upstream buildingwindows with increasing the WOP, this leads to a new flow pattern and pollutant distribution.When the WOP is increased from0%to10%, the averaged concentration level is decreased byabout23%and23%~27%for regular and staggered canyons, respectively. It is also shown thatthe flow structure and the averaged pollutant concentration within the canyon are not influencedby the WOP when the canyon width to building height ratio W/H is increased to4.Meanwhile, the numerical results also show that the WOP of the buildings has no evidentinfluence on the pressure distribution in the region near the upstream building, but it would resultin obvious changes of the pressure distribution around the downstream building. Besides, thepressure differences between the two sides (i.e., the facades) of the upstream buildings are always positive, the indoor air environment of the upstream building can be considered free of thepollutants released from the canyon. For this reason, the indoor air quality of the downstreambuilding will be greatly influenced by the WOP. The statistical results show that the naturalventilation performance in the downstream building and pollutant concentrations outdoors varieswith locations. For most of the rooms in the downstream building, the ventilation flow is suppliedby the outdoor air from the wake region of the downstream building, which is decreased withincreasing the WOP. On the other hand, if the indoor air is taken in from the canyon, the trend ofthe ventilation flux is found to be opposite. The pollutant concentrations both near the windwardand leeward wall of the downstream building increase with increasing WOP. In order to get aquantitative analysis of the potential contribution of outdoor traffic pollution on the indoor airquality in a naturally ventilated room, the dimensionless effective intensity of outdoor sources isproposed in the present study. The numerical results show that the averaged effective intensity ofthe downstream buidling is decreased with increasing the WOP for regular and staggered canyons,and the effective intensity in the staggered canyon is higher than that in the regular canyon.The asymmetric characteristics of the street canyon is one of the most important parameters thathave strong effect on the flow and pollutant distributions in the street canyon. The flow field andpollutant distribution in the canyon with different building height ratio (i.e. H1/H2, ratio of theupstream building height to the downstream building height) were simulated for both regular andstaggered building arrangements. Three indices for evaluating the indoor ventilation process areapplied to assess the air quality in the pedestrian domain. The numerical results show that thepollutant distributions in the canyon and the wake region of the downstream building are greatlyinfluenced by the asymmetric characteristics of the canyon. The concentration in the wake regionincrease with decreasing the downstream building height. The highest values of the dimensionlessaverage concentration occur in the pedestrian domain when the height ratios, i.e., H1/H2, are7/3and7/2for regular and staggered canyons, respectively. The air quality is better in the step-upcanyons than that of the step-down canyons. Furthermore, the impact of the asymmetriccharacteristics of the canyon on the indoor air quality was evaluated by the effective intensity. Thestatistical results show that in the step-up canyons, the ventilation flows in the rooms in thedownstream building are all driven in from the canyon. In the step-down canyons, the ventilationflow in some rooms is supplied by the rear region of the downstream building, while the air in theother rooms is coming from the canyon. Pollutant concentrations near the windward and leewardwall of the downstream building increase with increasing H1/H2. Wind directions are always at anarbitrary incidence to the canyons in realistic urban areas, it is shown that regular canyons withH1/H2=1/7and7/1yield the maximum effective intensity for the downstream building. For staggered arrangements, canyons with small height difference are favorable for the indoor airquality. The favorable height ratios H1/H2for staggered arrangements are developed in the rangesbetween5/7~7/5. Between the two arrangements without the unfavorable height ratios, staggeredcanyons are shown to have lower effective intensity than the regular canyons.To fully understand the street canyon geometric configuration on pollutant dispersionmechanism, the effect of ratio L/W (the building length L and the street width W) on the airflowrate and pollutant transport rate was evaluated by CFD simulations. The numerical results showthat with small L/W ratio, most of the wind and pollutants in the street domain are exchangedacross the street opening, and the pollutant concentrations are higher near the street opening thanthat in the middle part along the canyon length. For the canyon with large L/W ratio, however, thecanyon roof is almost entirely responsible for the transport of airflow and pollutants. Most of thepollutants are accumulated in the middle part of the canyon.The threshold value of the transitionbetween these dispersion processes lies in the range of L/W=2~2.5. The pollutant dispersionmechanism is changed due to different L/W ratio, but the averaged pollutant concentrations in thecanyon always show an increase with increasing the ratio L/W.The frontal buildings located in the upwind region of the upstream building can block theincoming air flow, and hence influence the air ventilation and pollutant dispersion in the canyon.Numerical simulations were conducted to study the impact of the frontal building height on theflow and concentration fields in both regular and staggered street canyons with different layouts ofthe frontal building. The numerical results show that the vortex structure and the airflow rate inthe target street canyon are mainly influenced by the layout of the frontal building and theupstream building, and the height of the frontal building (Hz), regardless of the layout of the targetstreet canyon. When the frontal building is aligned with the upstream building, the airflow rate inthe canyon exhibits an reduction as Hzincreases, then it has an increment when Hz=2H. If thefrontal building is staggered with the upstream building, the airflow rate in the canyon decreasesgradually with increasing Hz. The pollutant dispersion is govered by both the wind velocity andthe vortex structure. When the frontal building is lower than1.5H, the maximum pollutantconcentration occurs in the leeward region of the canyon. For Hz=2H, the maximumconcentration is shown in the windward reigon.The time series of indoor and outdoor CO, PM2.5and PM10concentrations in natural ventilatedrooms were monitored continuously to investigate the relationship between indoor and outdoorpollutant concentrations. It is shown that the variations of the indoor and outdoor COconcentrations of weekdays during summer are steady in the building in the vicinity of the traffcroad and the building far away from the street, i.e., there are two obvious CO concentration peaks for both of the outdoor and indoor concontration, one during the morning traffic peak and theother one during the evening traffic peak. By analyzing the indoor and outdoor PM2.5and PM10concentration relationship, the test data show that I/O ratios of the pollutant concentrations are lessthan but close to unity, and the test data show that there exists a significant positive correlationbetween indoor and outdoor pollutant concentrations. There is no time delay effect between indoorand outdoor CO, PM2.5and PM10concentrations in natural ventilated rooms. A mathematicalcorrelation of the indoor concentration is obtained by the outdoor concentrations for weekdays.The time series of outdoor CO concentrations is regressed by using the Fourier series techniques.Good agreement can be obtained between the test and calculating data by eliminating thetime-delay effect which is introduced by the Fourier series from the calculated curves.