Preliminary study of the parameterisation of street-level ventilation in idealised two-dimensional simulations

In this paper, the flows over idealised two-dimensional (2D) urban street canyons of different building-height-to-street-width (aspect) ratios (ARs) and urban boundary layer (UBL) depths are numerically examined. We attempt to utilise the friction factor f and the air-exchange rate (ACH) to parameterise the aerodynamic resistance and the street-level ventilation performance over urban areas. The aerodynamic resistance is controlled systematically by both the AR and the UBL depth. The AR varies between 0.083 and 1 while the UBL depth between 6h and 1,200h (where h is the building height) so the three characteristic flow regimes are included. Based on the current study, it is found that atmospheric turbulence contributes most to street-level ventilation because the turbulent component of ACH (ACH″) dominates the transport process (at least 70% of the total ACH). Moreover, the collective effect of AR and UBL depth on ACH is reflected by the friction factor. A linear relation between the turbulent ACH and the square root of the friction factor (ACH″ ∝ f1/2) is revealed in which the correlation coefficient is over 0.9. Extrapolation of ACH″ on predicting the ventilation efficiency covers at least 70% of the total ACH, indicating that using friction factor alone is sufficient to describe the aerodynamic resistance over urban areas of different surface roughness and UBL depth, and to estimate the street-level ventilation performance as well.