Advances in urban ventilation assessments using large-eddy simulation

Abstract

Today, the majority of the world's population lives in city areas. This renders the urban climate to be the most impacting local climate to the global society. To understand and improve the urban climate, local governments demand urban ventilation assessments (UVAs). Such UVAs often simplify the highly complex urban climate in order to make an assessment possible. In order to simplify the assessed case, the general behaviour of the urban ventilation must be understood so that important impacts are not neglected. However, many interactions inside the urban atmospheric boundary layer are still unknown. In this thesis, two aspects of these unknown interactions of urban climate are studied in detail: (a) the ventilation of courtyards, particularly, the influence of lateral openings on courtyard ventilation; and (b) the interaction between neighbourhood ventilation and mean building parameters like mean building height and building density under different atmospheric stratification. These two aspects are investigated by means of large-eddy simulations.

To confirm the liability of the utilized simulation model PALM, an evaluation study was conducted prior to the investigation of the two above-mentioned aspects. The comparison of simulation results against wind-tunnel data revealed differences in mean wind speed and wind direction of 5% and 4°, respectively, on average. The maximum differences occurred within the first grid points adjacent to obstacles and rapidly decreased with distance. Turbulence parameters like turbulence intensity and the spectral energy-density distribution agreed to a similar degree. Differences were found to be well within the acceptable margins. Hence, it was concluded that the model is able to correctly simulate the urban boundary layer.

In the following part, the ventilation of courtyards through lateral openings was investigated. Various courtyard configurations were analysed in an idealized building setup. Lateral openings were found to have nearly no effect on the ventilation of wide courtyards. However, for deep courtyards, pollutant concentration and the residence time of pollutants were found to be significantly influenced by lateral openings. Most configurations showed a negative impact on air quality by lateral openings. Depending on the placement of the opening and the surrounding ventilation conditions, however, lateral openings could also positively impact the air quality by removing pollutants. It could be shown that the impact of lateral openings is complex and should not be neglected in case of building-scale ventilation assessments.

In the last part of this thesis, the effect of atmospheric stratification on the ventilation of neighbourhood areas was investigated. In a real-case building setup of Hong Kong city, the ventilation was compared for neutral and unstable stratification in a weak-wind summer scenario. It was found that the overall ventilation is higher in an unstably stratified atmosphere due to the enhanced vertical mixing. The correlation between the plan area index (building density) and the ventilation was found to be stronger under unstable conditions compared to neutral stratification. Mean building height, however, was found to have no significant impact on the ventilation which contradicted findings by other studies. It could be shown that the overall ventilation differs between neutral and unstable stratification. To get an overall estimation of the city ventilation, UVAs should therefore cover different stratification scenarios.