On the Turbulence Structure over Different Surface Roughness: A Perspective from Wind Tunnel Measurements

In densely urbanized cities, narrow streets are flanked by closely packed, high-rise buildings. The complex urban geometry affects the atmospheric surface layer (ASL), which in turn complicates the wind flows and pollutant dispersion over urban areas. An improved understanding of the flow structure is needed to unveil the essential mechanism of ventilation and pollutant removal over various configurations of surface roughness. In this paper, a series of wind tunnel experiments were carried out to characterize the turbulence structure over different types of surface roughness. Hypothetical urban areas were assembled in the form of idealized street canyons fabricated by square ribs and building obstacles by LEGO bricks. The velocities (mean and fluctuating components) were measured by hot-wire anemometry (HWA) with X-wire probes. Our previous numerical studies had shown that the air exchange rate (ACH), as a measure of ventilation ability, is closely related to the friction factor. It is thus proposed that streetlevel ventilation is induced by rough surfaces to be measured by the aerodynamic resistance. Based on the wind tunnel results, detailed analysis of the velocity profiles, the relation between ACH and friction factor, as well as quadrant analysis and frequency spectrum of turbulence will be reported in the conference in attempt to further illustrate the ventilation process in ASL