CFD and wind-tunnel analysis of outdoor ventilation in a real compact heterogeneous urban area: Evaluation using “air delay”

摘要

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 steadyReynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. The up-to-date literatureshows the lack of detailed evaluations of the two approaches for real compact urban areas. This studyfurther presents a systematic evaluation of steady RANS and LES for the assessment of the ventilation conditionsin a dense district in Nicosia, Cyprus. The ventilation conditions within the urban area are investigated bycalculating the distribution of the age of air. To better assess the outdoor ventilation, a new indicator, the “airdelay” is introduced as the difference between the local mean age of air at an urban area and that in an emptydomain with the same computational settings, allowing the comparison of the results in different parts of thedomain, without impact of the boundary conditions. CFD results are validated using wind-tunnel measurementsof mean wind speed and turbulence intensity performed for the same urban area. The results show that LES canaccurately predict the mean wind speed and turbulence intensity with the average deviations of about 6% and14%, 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 twoapproaches is 52% at pedestrian level (2 m).