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

N. Antoniou, H. Montazeri, Hans Wigo, Marina Neophytou, B.J.E. Blocken, Mats Sandberg

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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).
Original languageEnglish
Pages (from-to)355-372
Number of pages18
JournalBuilding and Environment
Issue numberdecember 2017
Publication statusPublished - 13 Oct 2017


  • Air delay
  • City breathability
  • Computational Fluid Dynamics (CFD)
  • Large Eddy Simulations (LES)
  • Steady RANS
  • Urban microclimate


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