Flow and Turbulence Characteristics in the Roughness Sublayer over Real Urban Morphology of Hong Kong

Mean flow and turbulence in roughness sublayers (RSLs) over urban areas are complicated because of the diversified building configurations (such as size, shape and orientation, etc.). This study investigates the RSL flows over (part of) the real urban morphology of Hong Kong downtown by wind tunnel measurements. The urban models are fabricated by 3D printing using high-resolution digital maps of building morphology and topography. Vertical profiles of mean and turbulent components in three parallel transects are measured by a constant-temperature hot-wire anemometer (CTA). The wind tunnel results reveal that individual (vertical) profiles of streamwise fluctuating velocity u’’, vertical fluctuating velocity w’’ and vertical momentum flux u’’w’’ show noticeable variations in the RSL. It is largely attributed to the wakes and recirculations after the upstream high-rise buildings. A new analytical solution is proposed to predict the mean wind profiles in the RSL as well as the inertial sublayer (ISL) that is more accurate than the conventional logarithmic law of the wall (log-law). The turbulence in the RSL and ISL are examined in terms of quadrant analysis. The ejection Q2 and the sweep Q4 are stronger in the RSL than those in the ISL, collectively improving street-level air entrainment and pollutant removal.