Construction, Validation And Application Of An Approach For Flow And Dispersion Modeling In Urban Block Scale

With the explosion of urban population, the economic activities have causedthe dramatic emission of toxic substances, such as sulfur dioxide, nitrogen oxidesand carbon monoxide. This situation has brought many health problems. More-over, the rising global terrorism increases the risk of releasing Chemical, Biologicaland Radiological agents in a heavily populated urban environment. Due to thesereasons, more attention is being focused on predicting and evaluating the dispersionin urban areas by all over the world. However, the fow, turbulence and dispersionfelds on city block scale are directly infuenced by the heterogeneous fne geometricstructures. This heterogeneity makes dispersion in city hard to simulate. So, the keyto predict the dispersion in city is to evaluate these infuences precisely. Based oncurrent technology, the Computational Fluid Dynamics (CFD), which can directlysimulate the infuences of these fne urban geometric structures on fow, turbulenceand dispersion becomes an efective method.Due to this background, the Boundary Layer Meteorology Group of PekingUniversity has developed a numerical model (Peking University Block Scale Model,PKU-BSM) to simulate the wind, turbulence and dispersion felds in urban areas onbuilding to city block scale. The model includes two sub-models: one applies Com-putational Fluid Dynamics (CFD) approach to simulate the fow in city; the otheris a Eulerian-based dispersion model. The CFD approach is based on the steady-state Reynolds-Averaged Navier-Stokes (RANS) equations with the standard turbulence model upon control volumes of non-uniform cuboid shapes. The disper-sion model computes dispersion feld by solving an unsteady transport equation ofpassive scalar using the pre-computed velocity and turbulence felds.In this thesis, the numerical model is explained and the experimental datafrom the wind tunnel are used to validate the numerical results. Compilation ofExperimental Data for Validation of Microscale Dispersion Models (CEDVAL) andThompson datasets have been used here. A method based on Gaussian plume modelis also used in CEDVAL case to correct the concentration for the frst time.The validation for CEDVAL dataset shows:i) In general, the simulated results of wind, turbulence kinetic energy (TKE)and concentration felds show a reasonable agreement with the observations. Theprediction of wind profles is the best, especially above the building. The TKEvalues are not predicted as good as those in wind velocity prediction, but the TKEof simulation also corresponds to the observed one in shapes and locations of thepeak.ii) Without the correction of turbulent Schmidt number, the spread of simu-lated concentration is underestimated by the model. After the correction by usingthe theory of Gaussian plume model, the prediction has been improved, while theprediction of isolated building also underestimates the concentration values far fromthe leeward side of building. The imperfect point sources caused by recirculationregion, heterogeneous turbulent difusivity in y-direction and constant turbulentSchmidt applied in PKU-BSM may be responsible for the underestimation.The validation for Thompson dataset shows:i) The reverse area in front of the windward side and recirculation region behindthe leeward side extend with increase of the width of building. The TKE valuesalso increase with the increaseing width;ii) PKU-BSM predicts concentration well in centerplane. Especially in theregion far from the leeward side, the simulated concentration and the observation arealmost the same. However, around the building, there are some diference betweenthe peak values of simulation and observation; iii) Compared to AMS/EPA Regulatory Model (AERMOD) which is a Gaussianplume model, PKU-BSM predicts the concentration much better than AERMOD,especially in the location and value of concentration peak.Moreover, the PKU-BSM is applied to the real urban surface, in order to eval-uate the infuence on the dispersion in diferent sources features in real urban en-vironment. Zhongguancun has been selected as the case. The simulated resultsshow:i) The wind velocity feld and TKE feld show complex features in buildingregion. Inside the building region, the wind velocity decreases by.The TKE values also decrease. Moreover, many vortices are generated among thebuildings and result in a bad condition for pollutant dispersion;ii) The position and direction of the source have large infuences on the concen-tration. The peak of concentration around the source increases dramatically whenthe source is through the building region. In some region insulated from the aroundarea by the buildings, the average concentration level will increase more than thatin the other building region.