Abstract
The ventilation efficiency of three periodic building patterns of equal total volume and packing density is investigated: street canyons bounded by H uniform and 0.5H/1.5H variable height buildings, via wind tunnel measurements and Large Eddy Simulation. The numerical model utilizes the Transient Wind Forcing method to take the effect of eddies larger than the domain size into account, and a special Lagrangian dispersion model, which allows for the calculation of particle trajectories exceeding the periodic boundaries of the LES domain. The spatial and temporal characteristics of the concentration responses of both pulse-like and steady point sources located at the street surfaces are analyzed. It is shown that the variable building height has a favorable effect on urban ventilation in densely built areas: the average near-ground concentration can be reduced by up to 70% for the variable height buildings in a staggered arrangement. In terms of velocity, turbulence and concentration distributions, the model results are consistent with the experiments, verifying the applicability of the model for comparative air quality studies. Since the presented dispersion model is capable of handling dynamic changes in wind direction and magnitude, the accuracy of the TWF model can potentially overcome the limitations of wind tunnel tests.
Highlights
As urbanization advances, access to healthy air is becoming increasingly important in the lives of city dwellers
A further advantage of this method is that it does not require the exact definition of inlet boundary conditions, which is beneficial for CFD models utilizing Large Eddy Simulation (LES) – a turbulence model that can produce more accurate results in urban flow and dispersion compared to the conventional RANS approaches (Tominaga and Stathopoulos, 2011; Tominaga et al, 2013)
It must be noted that the variation of the numerical results obtained on different meshes falls into the range of the difference between the experimental profiles obtained in different canyons
Summary
Access to healthy air is becoming increasingly important in the lives of city dwellers. Repetitive geometric patterns are preferred subjects of field experiments (Biltolft, 2001; Kanda, 2009; Chen et al, 2020), wind tunnel measurements (Leitl et al, 2007; Yee and Blitolft, 2004) and numerical models (Eichhorn and Balczo, 2008; Santiago et al, 2010; Dejoan et al, 2010; Rakai and Kristof, 2010) For the latter, the repetitive surface patterns can be constructed by applying periodic boundary conditions at the lateral sides of the simulation domain, taking into account an infinitely large repetitive structure in a small modeling box. Instead of prescribing the inlet velocity and turbulence profiles, in the periodic models, the flow can be created by a spatially distributed driving force, the volume integral of which balances the drag force of the surface objects, such as buildings and vegetation
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