Wind Conditions in Idealized Building Clusters: Macroscopic Simulations Using a Porous Turbulence Model

Abstract

Simulating turbulent flows in a city of many thousands of buildings using general high-resolution microscopic simulations requires a grid number that is beyond present computer resources. We thus regard a city as porous media and divide the whole hybrid domain into a porous city region and a clear fluid region, which are represented by a macroscopic k-ε model. Some microscopic information is neglected by the volume-averaging technique in the porous city to reduce the calculation load. A single domain approach is used to account for the interface conditions. We investigated the turbulent airflow through aligned cube arrays (with 7, 14 or 21 rows). The building height H, the street width W, and the building width B are the same (0.15 m), and the fraction of the volume occupied by fluid (i.e. the porosity) is 0.75; the approaching flow is parallel to the main streets. There are both microscopic and macroscopic simulations, with microscopic simulations being well validated by experimental data. We analysed microscopic wind conditions and the ventilation capacity in such cube arrays, and then calculated macroscopic time-averaged properties to provide a comparison for macroscopic simulations. We found that the macroscopic k-ε turbulence model predicted the macroscopic flow reduction through porous cube clusters relatively well, but under-predicted the macroscopic turbulent kinetic energy (TKE) near the windward edge of the porous region. For a sufficiently long porous cube array, macroscopic flow quantities maintain constant conditions in a fully developed region.

Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method

“…Compared to wind tunnel data, the standard k-ε model with present grid arrangements can provide results of mean flows u(z) in good agreement with wind tunnel data, but does a little worse at predicting k(z) and can only predict the shape of vertical profile well. Such findings are similar with the same CFD validation studies the literature [23,[51][52][53]. In addition, CFD results with the fine grid are only slightly different from those with the present medium grid.…”

“…At the domain inlet, the measured power-law profile of time-averaged (or mean) velocity U 0 (z) in the upstream free flow is adopted (see Equation (1a)) (Brown et al [51]). Moreover, the vertical profiles of turbulent kinetic energy k(z) and its dissipation rate (ε) at the domain inlet are calculated by Equations (1b) and (1c) [23,[51][52][53]: 11-column cubic building array with a parallel approaching wind. Building width (B), building height (H), and street width (W) are the same (B = H = W = 15 cm, H/W = 1, building packing densities λp = λf = 0.25, Lx = 13H, Ly = 21H).…”

“…In the CFD validation case, the full-scale seven-row building array is numerically investigated (B = H = W = 30 m, λp = λf = 0.25, H/W = 1; see Figure 1b). According to the literature [23,[51][52][53], the lateral urban boundaries hardly affect airflows in the middle column as the wind tunnel model is sufficiently wide in the lateral (y) direction (Ly = 21H). Thus, it is effective to only consider half of this middle column (shaded area in Figure 1a) to reduce the computational requirement.…”

City breathability in medium density urban-like geometries evaluated through the pollutant transport rate and the net escape velocity

“…To reduce the computational load, only half of this column was considered (see Fig. 1a) [44,45]. Fig.…”

Seasonal Changing Effect on Airflow and Pollutant Dispersion Characteristics in Urban Street Canyons

“…Influential factors such as building morphology, canyon aspect ratio, ambient wind speed and direction were intensively studied. Hang et al [6][7][8] investigated the effect of geometry morphology on street canyon ventilation by varying aspect ratio, length and building packing density. They found that lowering aspect ratios or increasing street lengths may enhance the pollutant removal.…”