Impact of different building facades and thermal stratification on the flow field in street canyons

Abstract: Urban areas are warmer than their surrounding rural areas. This phenomenon is called the urban heat island effect. To study the microclimate in the urban areas, it is important to know the flow field in the street canyons of a city.

Description: To better understand the urban microclimate in street canyons and urban neighbourhoods, modelling and simulation will be performed using computational fluid dynamics (CFD) at Empa. By means of an atmospheric boundary layer wind tunnel validation experiments will be undertaken to validate the CFD simulations. For the validation strongly simplified and scaled geometries will be used. The walls of the build-ing models will be heated in the wind tunnel to model the higher temperatures of the building facades caused by the absorption of solar radiation. Very high wall temperatures would be needed to get a similar flow field for the scaled model and the real building, what is very difficult to achieve in the wind tunnel.The aim of this project is to study how detailed the models for the wind tunnel have to be and to study the scaling for street canyon with heated walls. The first step will be to compare CFD simulations of a full-scale street canyon with CFD simulations of the scaled and simplified street canyon. The goal of this comparison is to see how the simplifications influence the flow field. A second goal is to find a way to model the omitted parts of the building facades in the simplified CFD model. For example the omitted balconies could be taken into account by model the balconies as roughness. The second step will be to study the scaling. The goal of this part is to find the wall temperatures and velocities that are needed to have a similar flow field for the scaled and the full-scale street canyon. For this purpose it has to be known which temperatures and velocities are needed to get natural, mixed or forced convection for the different scales. For this part of the project it has to be taken into account, that it will not be possible to have very high temperatures in the wind tunnel.

Tasks: Literature study; Comparison of the scaled with the full-scale street canyon; Study of the flow in a thermal stratified street canyon

Type: Bachelor, Semester or Master Thesis

Internal supervisor: Jonas Allegrini

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