Observation And Simulation Study Of Atmospheric Dispersion In Semi-enclosed And Open Space

Non-traditional Atmospheric Environmental Pollution(NAEP)events(e.g.,hazardous gas leakage from chemical plants,toxic gas attack by terrorists,etc.),which are caused by accidental(or intentional)release of hazardous materials,often lead to serious casualties and huge economic losses.Compared to the traditional air pollution events,relatively less attention has been paid on the NAEP events.Therefore,two field tracer experiments are conducted in a factory building to study the NAEP events occurred in semi-enclosed space(e.g.,industrial factory building,lounge,and subway station)and in a real city to study the events occurred in open space(e.g.,downtown business district,and chemical plant district),respectively.Based on the data collected from the field experiments,this paper carried out the following three aspects of work.Firstly,the characteristics of flow field,turbulence and tracer pollutant dispersion in the above two different spaces(i.e.,factory building,and real city)are revealed.Secondly,the computational fluid dynamics(CFD)model that used for predicting heavy gas dispersion in a large indoor environment is validated.Finally,a one-way coupled WRFFluent model that can simulate the flow and dispersion in urban area was constructed.The main conclusions are summarized as follows:(1)Our observational results show that the indoor wind velocity and turbulence are very weak under natural atmospheric conditions,while forced ventilation can significantly strengthen the horizontal wind velocity,the vertical velocity variance,and the turbulent kinetic energy.It is noticeable that a strong inversion was observed in the building at daytime with the largest temperature lapse rate of-0.68 ?/m at 2:00 p.m.And the smoke dispersion test shows that the vertical mixing of the pollutants can be inhibited by the inversion.The stratification is weakly unstable at nighttime.Additionally,the stratification effect of heavy gas is apparent under natural atmospheric conditions as we expected and the average SF6 concentration at the upper-layer is only about 18% of that at the lower-layer.The vertical distribution of the light gas concentration is mainly determined by the combined effect of temperature stratification and forced ventilation.(2)The performance of a set of commonly used RANS turbulence models for predicting heavy gas dispersion in a large indoor environment was evaluated.The results suggest that the standard k-? model and the SST k-? model show the best performance(FAC2=78%)in the breathing zone,producing 78% of predictions(paired in time and space)within a factor of two of the observations;followed by the Realizable k-? model(FAC2=72%);while the standard k-? model has the worst performance(FAC2=43%).However,none of the RANS models are satisfactory in predicting the spatial-temporal variation of tracer clouds above the breathing zone.Additionally,the SST k-? model was employed to reproduce the heavy gas dispersion in the building and to analyze the relationship between ventilation rate and concentration in the breathing zone.(3)The wind velocity in the downtown area is 2 m/s smaller than that in the urban fringe,and there is a large difference in wind velocity and direction among adjacent observation sites in urban area.The atmospheric stratification is moderate unstable at daytime,while it is close to neutral at night.Also,some simple parametric relations are given to estimate the turbulence variables.The diurnal variation of surface albedo has an asymmetric “U”-shaped pattern,and the albedo at sunrise and sunset is larger than that at daytime when the albedo is close to the constant value of 0.14.The energy transfer between the urban surface and atmosphere is dominated by sensible heat flux,the ratio of the sensible(latent)heat flux to the net radiation is 23%(2%)and the Bowen ratio is 11 in the daytime.It is particularly noticeable that the ratio of the storage flux to the net radiation is 75% in the daytime,while the ratio is 1 in the night.Additionally,tracer concentrations match well with the wind field above urban canopy layer,with high tracer concentration appearing downwind of the emission source within a sector of ±20? around the main wind direction and approximate normal distribution in crosswind direction.The ground distribution of tracer concentrations significantly deviate from the normal one.(4)A one-way coupled WRF-Fluent model was constructed by combining the mesoscale model WRF with the micro-scale model Fluent,and the WRF-Fluent model was used to simulate the wind field and pollutant diffusion in Yuzhong County.The WRF-Fluent model with time-varying boundary conditions can well describe the complex flow patterns(e.g.,separation,confluence,vortex,and channeling)in urban area.Moreover,the general evolution of the polluted plume with time under the elevated source emission scenarios can be captured by the WRF-Fluent model,but there is a certain deviation in the position and intensity of concentration center between the simulated and the measured results.