Urban street canyon flows under combined wind forcing and thermal buoyancy

Abstract

In this work, we investigate buoyancy-driven flows within urban street canyon cavities of three aspect ratios under simultaneous inertial wind forcing. The main aim of this work is to enhance the understanding of induced urban airflow patterns under non-isothermal conditions through experimental investigation, which to date are relatively scarce. The experimental results can be used for corresponding computational fluid dynamics simulations. Scaled-down models of typical street-canyon cavity geometries were deployed inside a water channel, where different ambient atmospheric conditions were simulated using dimensional analysis and similarity criteria. Three model street-canyon cavities were examined with height-to-width (aspect) ratios of 2/3, 1, and 2. The thermal buoyancy forcing was applied by means of differential heating between the two canyon side antagonistic walls for a given background flow velocity well-above the canyon height. The non-dimensional parameter B was used to quantify the influences of buoyancy and inertial forcing on the urban-canyon flow, as well as factoring in the geometrical aspect of the street canyon. The particle image velocimetry technique was used to acquire velocity vector fields across the middle vertical planar cross section of the urban street canyon. The results showed that the canyon aspect ratio affects the resulting flow field; however, a main vortical structure is present in all the visualized flow patterns with flow direction always being consistent with that of an uprising flow along the canyon heated wall.