High fidelity modelling of neighbourhood-scale environments

Abstract

Arrays of buildings with pitched roofs are common in urban and suburban areas of European cities. Large-eddy simulations are performed to predict the boundary-layer flows over flat and pitched-roof cuboids to gain a greater understanding of the impact of pitched roofs on urban boundary layers. The simulation methodology is validated for an array of flat roof cuboids. Further simulations show that changes in the type of grid conformity have a negligible effect on the mean flow field and turbulent stresses, while having a visible, but small, effect on the dispersive stresses for a given packing density. Comparisons are made for flat and 45◦ pitched roof cuboid arrays at packing densities of 16.7% and 33.3%. The interactions between pitched-roof buildings and their effect on the urban boundary layer are considerably different to those of flat-roof buildings. The pitched roofs at a packing density of 33.3% leads to significant changes in the mean flow field, the Reynolds stresses, and the aerodynamic drag. Further work investigates the effects of changes in turbulence level and atmospheric thermal stratification in the approaching flow. Importantly, in comparison to a flat-roof array, the pitched-roof one at a packing density of 33.3% evidently increases the friction velocity and greatly reduces the effects of stable stratification conditions and changes in inflow turbulence level. iv Past work has shown that coupling can exist for urban flows between street scale (O(0.1 km)) and city scale (O(10 km)). Unfortunately, it is generally impractical at present to develop high-fidelity urban simulations capable of capturing such effects. This imposes a need to develop better parameterisations for meso-scale models but an information gap exists in that past work has generally focused on simplified urban geometries and generally assumed the buildings to be on flat ground. This study aimed to begin to address this gap in a systematic way by using the large eddy simulation method with synthetic turbulence inflow conditions to simulate atmospheric air flows over the University of Southampton campus using semi-realistic (i.e. flat terrain) and realistic geometries including significant local terrain features. The LES data were processed to obtain averaged vertical profiles of time-averaged velocities and second order turbulence statistics. The semi-realistic geometry simulation was validated against high resolution particle image velocimetry data. To address the uncertainty due to inflow turbulent intensity, the effects of different levels of inflow turbulence were assessed. The realistic geometry simulations conclusively showed that the inclusion of terrain can have a considerable effect on global quantities, such as the depth of the span wise-averaged internal boundary layer and spatially-averaged turbulent kinetic energy (TKE). The terrain effects on local time-mean velocity and TKE at the same above-ground-level height can be even more significant. These findings demonstrate the crucial importance of local terrain features (O(0.1km)), and can have significant impact on near-field dispersion and urban micro-climate.

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ORCID for Zhengtong Xie: orcid.org/0000-0002-8119-7532

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