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Increased knowledge on dispersion processes in urban environment will enhance the ability in the society to handle events where releases of toxic substances can occur. Also, the ability to increase preparedness at locations where such events potentially can emerge. 

Can Computational Fluid Dynamics (CFD) models contribute to increased knowledge and what type of models are most suitable considering dispersion in urban environment? 

CFD-models can simulate almost any scenario but urban scales are still computationally de-manding. Simplifications of the basic equations are needed. Mainly two methods to do this is feasible, namely Reynolds Averaged Navier-Stokes models (RANS) and Large Eddy Simula-tion models (LES). These methods are commonly used for hydrodynamic flow studies. In this thesis the eddy viscosity hypothesis is implemented and used in all turbulence models. 

The scenarios studied includes flow and dispersion past objects at the side of a road, flow over buildings, dispersion in urban environments and on synthetic stochastic boundary condi-tions. The basic flow around objects assume that the turbulence is realistically modelled. In RANS the flow is steady state while the turbulence is fully modelled. In LES only the smallest turbulent eddies are modelled while the flow is resolved in time. In the urban environment tur-bulent fluctuations have the dimension of the buildings and the wind speed. Thus, it is important that these fluctuations are correctly described for the purpose of the simulation with a CFD-model.

The results show that CFD can replace the real world in well specified scenarios when stud-ying certain aspects, like effects from objects in the path of the dispersion and effects of atmos-pheric stability. However, the simulations of dispersion in urban environments show that RANS and LES models can produce quite unequal results regarding hazard area estimation. When comparing LES results to data from full scale experiments, it is clear that LES-models have fundamental ability to handle effects found in real life. Here can be mentioned dispersion paths and maximum values which are important when estimating the extent of hazard areas. On the other hand specific temporal fluctuations can hardly be predicted, only statistics.

Finally, by using synthetic inflow boundary conditions, realistic representation of spectra of turbulent kinetic energy is enforced in areas with low and sparse buildings. In the high rise building area the building interaction with the flow develops a turbulent urban sub layer that is not much influenced from the inflow boundary. By studying synthetic forcing as a part of the boundary conditions together with a stably stratified boundary layer, more tools to simulate the real world events are examined.