Wind-tunnel study of turbulent boundary layer over idealised urban roughness with application to urban ventilation problem

Abstract

The flows in the lowest part of the urban atmospheric boundary layer (ABL) are investigated in this dissertation. The flow structure is characterised by the surface morphology that has direct effect on the street-level ventilation. Numerical study using computational fluid dynamics (CFD), wind tunnel experiments and analytical derivation are performed to examine the dynamics in the inner region of a neutral urban ABL. In the first part of the dissertation, the flows over idealised two-dimensional (2D) urban street canyons of different building-height-to-street-width (aspect) ratios (ARs) and urban ABL thickness are numerically investigated from a street canyon point of view. The friction factor f and the air-exchange rate (ACH) are used to parameterise the aerodynamic resistance and the street-level ventilation performance over urban areas respectively. 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). Besides, the collective effect of AR and urban ABL thickness on ACH is reflected by the f. A linear relation between ACH’’ and the square root of friction factor (ACH’’ ∝ f^(1/2)) is revealed. An empirical parameterisation is thus proposed by extrapolating ACH’’ to predict the street-level ventilation efficiency. In the second part of the dissertation, focus is put on the urban ABL adjacent to the urban surfaces (boundary-layer point of view). The dynamics in the inner layer (roughness sublayer RSL and inertial sublayer ISL) of the urban ABLs over either idealised or real complex urban configurations are studied analytically and experimentally. An analytical solution to the mean wind profile which is a continuous function applicable to both RSL and ISL is derived based on the Monin-Obukhov similarity theory (MOST) to account for the RSL effects over urban surfaces. The RSL function is then coupled with the mixing-length model to elucidate how surface roughness alters the RSL turbulence. A series of wind tunnel experiments are performed to measure the turbulent boundary layer structure over idealised surface (in the form of 2D rib-type roughness elements) and real complex urban surfaces (an urban model of Hong Kong) using hot-wire anemometry (HWA). The analytical prediction agrees well with the experimental result that improves the estimate to mean velocity profiles and turbulence length scales in the near-ground region over urban areas. Consolidating the analytical and experimental results, it is confirmed that the inner layer flows over urban areas exhibit two distinct behaviours: 1) logarithmic properties in the ISL and 2) physical influence of individual buildings on near-surface RSL flows are demonstrated in which the RSL effects should be taken in account. Finally, an improved ventilation estimate with RSL consideration is proposed using a vertical fluctuating velocity scale 〈w ̂ 〉 which facilitates a handy parameterisation for estimating street-level air quality. The correlation of the urban surface geometry (in terms of f) with the newly proposed velocity scale provides a reliable parameterisation of street-level ventilation.