Experimental study on flow and ventilation behaviours over idealised urban roughness
Flows in the urban boundary layer (UBL) are strongly affected by the inhomogeneous roughness elements at the bottom surface. In particular, in the near-ground region (roughness sublayer), the effect of the surface roughness dominates that complicates the behaviours of mean flow and turbulence and subsequently the near-wall transport processes. To safeguard the health of urban inhabitants, it is crucial to develop an in-depth understanding of the correlation among near-wall fluid motions, UBL turbulence and city ventilation. However, rather limited information is available. In this study, physical modelling in a laboratory wind tunnel is employed to measure the profiles of both stream-wise and vertical velocities over an array consisting of idealised two-dimensional (2D) roughness elements. Various arrangements are adopted in attempt to cover different flow regimes to examine city ventilation problems. The ventilation performance is measured by the air exchange rate (ACH). Consistent with our previous large-eddy simulation (LES) results, the current wind tunnel measurements suggest that city ventilation is dominated by the ACH turbulent component, i.e., air masses are mainly driven by atmospheric turbulence (at least 80% of the total ACH).
Keywords: air exchange rate, ACH, air pollution, atmospheric turbulence, city ventilation, street canyons, surface roughness, wind tunnel experiments, urban areas