Modelling Air Flow and Temperature in Urban Area

Abstract

In this thesis, a model for air flow and temperature in urban areas is studied. In particular, the influence of vegetation in a street canyon is investigated. This includes the effect of vegetation on air flow and the cooling property of vegetation to lower temperature in street canyons. Computational Fluid Dynamics simulations with vegetation in a street canyon (green facades and trees) are performed. To model this, the Reynolds Averaged Navier Stokes equations in 3D are closed using the k-e turbulence model with source and sink terms to account for the effects of vegetation on air flow. In the thermal equation, the Simple Gradient Diffusion Hypothesis is used to close the equations and a cooling power term is introduced to account for the transpirational cooling property of vegetation. The results regarding the vertical velocities in the street canyon are compared with earlier simulations by Gromke et al. The simulations involving temperature showed significant effects of the vegetation in the street canyon. At street level, the green facades yielded a stronger cooling effect than trees. However, the cooling effect of trees is stronger halfway the canyon and just above the canyon in comparison with green facades. An important note is that the temperature drop inside the tree canopy strongly suggests that the cooling power in the simulation is modelled stronger than one would expect in reality. Another part of this research is the implementation of a Higher Order Scheme. This is done with the Van Leer limiter. The simulations with this new numerical method yielded very similar results compared with the UDS simulations. One outlier is noticed in the velocity in the 푦-direction for the empty street canyon. A possible explanation could be the sensitivity of the method near the ground. The study of the effect of vegetation in a street canyon could be extended in the future by comparing combinations of vegetation and adjusting the parameters of the green facades. Furthermore, the Van Leer limiter could be extended to the temperature part of the simulations in the future, since sharper gradients are expected here, so the improvement with respect to UDS and QUDS could be more significant. Finally, also other flux limiters, such as the Koren scheme, could be tested.