Good representation of turbulence in urban canopy models is necessary for accurate prediction of momentum and scalar distribution in and above urban canopies. To develop and improve turbulence closure schemes for one-dimensional multi-layer urban canopy models, turbulence characteristics are investigated here by analyzing existing large-eddy simulation and direct numerical simulation data. A range of geometries and flow regimes are analyzed that span packing densities of 0.0625 to 0.44, different building array configurations (cubes and cuboids, aligned and staggered arrays, and variable building height), and different incident wind directions ($$0^\circ $$ 0 ∘ and $$45^\circ $$ 45 ∘ with regards to the building face). Momentum mixing-length profiles share similar characteristics across the range of geometries, making a first-order momentum mixing-length turbulence closure a promising approach. In vegetation canopies turbulence is dominated by mixing-layer eddies of a scale determined by the canopy-top shear length scale. No relationship was found between the depth-averaged momentum mixing length within the canopy and the canopy-top shear length scale in the present study. By careful specification of the intrinsic averaging operator in the canopy, an often-overlooked term that accounts for changes in plan area density with height is included in a first-order momentum mixing-length turbulence closure model. For an array of variable-height buildings, its omission leads to velocity overestimation of up to $$17\%$$ 17 % . Additionally, we observe that the von Kármán coefficient varies between 0.20 and 0.51 across simulations, which is the first time such a range of values has been documented. When driving flow is oblique to the building faces, the ratio of dispersive to turbulent momentum flux is larger than unity in the lower half of the canopy, and wake production becomes significant compared to shear production of turbulent momentum flux. It is probable that dispersive momentum fluxes are more significant than previously thought in real urban settings, where the wind direction is almost always oblique.

No abstract

<p>Natural ventilation is widely used for low-carbon building design. Its potential is influenced largely by the building’s micrometeorological context. Traditionally, weather data used in building energy simulation are observed at rural sites which are far from the site of interest and not representative of the area’s surroundings. Here we combine the Surface Urban Energy and Water Balance Scheme (SUEWS) and the building energy simulation tool, EnergyPlus, to predict the natural ventilation potential (NVP) in buildings located in urban areas in five representative Chinese cities in different climate zones. The meteorological data required by EnergyPlus (e.g. air temperature, relative humidity, wind speed profile) are modelled by SUEWS. The dense urban areas (building fraction <em>λ<sub>P</sub></em> = 0.6) have an overall warmer and less windy environment compared to rural areas. In summer, the urban-rural natural ventilation hour differences are -3% to -85% (cf. rural) across all climates, while in spring/autumn differences are -25% to 42%. The method is intended to improve the accuracy of NVP prediction using EnergyPlus in cities.</p>