Hindawi Publishing Corporation Te Scientifc World Journal Volume 2013, Article ID 781748, 11 pages http://dx.doi.org/10.1155/2013/781748 Research Article Microscale Obstacle Resolving Air Quality Model Evaluation with the Michelstadt Case Anikó Rakai and Gergely Kristóf Department of Fluid Mechanics, Budapest University of Technology and Economics, Budapest 1111, Hungary Correspondence should be addressed to Anik´ o Rakai; rakai@ara.bme.hu Received 30 April 2013; Accepted 27 June 2013 Academic Editors: J. Corte-Real, M. A. Gal´ an, and J. Koch Copyright © 2013 A. Rakai and G. Krist´ of. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Modelling pollutant dispersion in cities is challenging for air quality models as the urban obstacles have an important efect on the fow feld and thus the dispersion. Computational Fluid Dynamics (CFD) models with an additional scalar dispersion transport equation are a possible way to resolve the fowfeld in the urban canopy and model dispersion taking into consideration the efect of the buildings explicitly. Tese models need detailed evaluation with the method of verifcation and validation to gain confdence in their reliability and use them as a regulatory purpose tool in complex urban geometries. Tis paper shows the performance of an open source general purpose CFD code, OpenFOAM for a complex urban geometry, Michelstadt, which has both fow feld and dispersion measurement data. Continuous release dispersion results are discussed to show the strengths and weaknesses of the modelling approach, focusing on the value of the turbulent Schmidt number, which was found to give best statistical metric results with a value of 0.7. 1. Introduction Prognostic microscale obstacle resolving meteorological models and Computational Wind Engineering (CWE) mod- els deal with the common felds of wind and pollutant dispersion modelling inside the urban canopy. Baklanov and Nuterman [1] show that these models with increasing computational capacity can be the fnal scale in a nested multiscale meteorological and dispersion model. In this paper the intersection of the two described disciplines is investigated with the assumptions of quasistationary fow and neutral meteorological conditions and the pollutant is considered a passive scalar. Stull [2] defnes microscale in meteorology as a few kilo- meters or less where the typical phenomena include mechan- ical turbulence caused by the buildings. Britter and Hanna [3] suggest the following lengthscales: regional (up to 100 or 200 km), city scale (up to 10 or 20 km), neighborhood scale (up to 1 or 2 km), and street scale (less than 100 to 200 m). Te last two correspond to the microscale defnition of Stull and are used here. Te general approach in CWE for air pollution dispersion modelling is adding a passive scalar transport equation decoupled from the solution of the fowfeld or modelling Lagrangian particle dispersion in the computed fowfeld. Although this is essentially less efort than computing the fowfeld itself, if there are errors already in the mean velocity feld, for example, reattachment length overestimation, or in the turbulent felds, for example, stagnation point anomaly, those errors are propagated in the pollutant transport mod- elling. Especially the turbulent felds which are not always vital for the fowfeld are equally important in the dispersion model as they are responsible for the turbulent difusion. A comparison of the Eularian additional transport equa- tion approach and the Lagrangian approach in a single building confguration is given in Gorl´ e et al.’s [4]. Tey fnd no great diference between the two approaches, but the additional transport equation has slightly better statistical results. However, the shape of the plume is signifcantly diferent between simulations and experiments. It must be added that the test case they are using has a pollutant source in the wake of the building, where the fow feld modelling is already problematic, and they focus on the diferent modelling approaches of the fow feld. Gorl´ e et al. [5] have investigated the efect of turbulent kinetic energy inlet boundary conditions on the dispersion results and