LES analysis of turbulent fluctuation in cross-ventilation flow in highly-dense urban areas

Cross-ventilation flow in buildings is dominantly impacted by the characteristics of their surrounding built environment, specifically in highly-dense urban areas. Cross-ventilation in highly-dense urban configurations is studied using validated computational fluid dynamics models using the large eddy simulation approach. Mean flow properties, turbulent statistics, wind pressure, and crossing airflow rate are compared against different planar area ratios of 0 <= lambda(p) <= 0:6 and approaching wind angles of 0 degrees <= alpha <= 90 degrees for a generic building model surrounded by nine identical building models. The transient airflow rate is analyzed considering different airflow definitions based on the time-averaged and fluctuating velocity decomposition. It is found that the difference between the time-averaged and instantaneous airflow rate is not negligible in a highly-dense urban configuration where the velocity is generally weak, but the flow direction is highly fluctuating. Furthermore, it is shown that time averaged airflow rate calculations are not suitable for accurate prediction of cross-ventilation in sheltered buildings in medium and highly-dense urban configurations; this is because of the averaging procedure cancels out the velocity fluctuation and periodic flows through the openings. Moreover, it is proposed that the accumulative instantaneous ventilation rate can be more suitable for airflow rate calculations as it includes the instantaneous airflow exchanges.