Abstract
Outdoor urban ventilation in a real complex urban area is investigated by introducing a new ventilation indicator – the “air delay”. Computational Fluid Dynamics (CFD) simulations are performed using the 3D steady Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. The up-to-date literature shows the lack of detailed evaluations of the two approaches for real compact urban areas. This study further presents a systematic evaluation of steady RANS and LES for the assessment of the ventilation conditions in a dense district in Nicosia, Cyprus. The ventilation conditions within the urban area are investigated by calculating the distribution of the age of air. To better assess the outdoor ventilation, a new indicator, the “air delay” is introduced as the difference between the local mean age of air at an urban area and that in an empty domain with the same computational settings, allowing the comparison of the results in different parts of the domain, without impact of the boundary conditions. CFD results are validated using wind-tunnel measurements of mean wind speed and turbulence intensity performed for the same urban area. The results show that LES can accurately predict the mean wind speed and turbulence intensity with the average deviations of about 6% and 14%, respectively, from the wind-tunnel measurements while for the steady RANS, these are 8% and 31%, respectively. The steady RANS simulations overestimate the local mean air delay. The deviation between the two approaches is 52% at pedestrian level (2 m).
Highlights
More than 80% of the population living in urban areas is exposed to poor air quality levels [1]
To gain insights into the performance of steady Reynolds-Averaged Navier-Stokes (RANS) compared to Large Eddy Simulation (LES) for predicting urban ventilation in a real compact heterogeneous urban area, this paper presents steady RANS and LES Computational Fluid Dynamics (CFD) simulations of the wind flow and the local mean age of air distribution in a dense district in the old city of Nicosia, Cyprus
In order to check the horizontal homogeneity of the approaching flow, the appearance of streamwise gradients of mean wind speed and turbulence intensity between the inlet and the upstream measuring locations is analyzed by comparing the CFD results of the normalized streamwise velocity, U/Uref, and streamwise turbulence intensity, Iu, with the measured data along the line indicated by the point with label “2” upstream of the urban area (Fig. 4a)
Summary
More than 80% of the population living in urban areas is exposed to poor air quality levels [1]. The new contribution of this paper is to provide an investigation of the outdoor ventilation conditions for real complex urban areas and provide a quantitative evaluation of the accuracy of LES and RANS (through the wind-tunnel study validation) for relevant metrics for such outdoor ventilation. This motivation arises from the interest that within large long canyons in highly complex real urban areas local ventilation processes are bound to occur in a non-uniform way To cover this void, and to better assess the outdoor ventilation for the urban area, a new indicator called air delay (τd) is introduced, which is the difference between the local mean age of air inside the area of interest, and the local mean age of air in an empty domain (when the buildings are not present) with the same computational settings and parameters.
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