Flow Characteristics Around Step-Up Street Canyons with Various Building Aspect Ratios

Abstract

We investigate the flow characteristics around step-up street canyons with various building aspect ratios (ratio of along-canyon building length to street-canyon width, and upwind building height to downwind building height) using a computational fluid dynamics (CFD) model. Simulated results are validated against experimental wind-tunnel results, with the CFD simulations conducted under the same building configurations as those in the windtunnel experiments. The CFD model reproduces the measured in-canyon vortex, rooftop recirculation zone above the downwind building, and stagnation point position reasonably well. We analyze the flow characteristics, focusing on the structural change of the in-canyon flows and the interaction between the in-and around-canyon flows with the increase of building-length ratio. The in-canyon flows undergo development and mature stages as the building-length ratio increases. In the development stage (i.e., small building-length ratios), the position of the primary vortex wanders, and the incoming flow closely follows both the upstream and downstream building sidewalls. As a result, increasing momentum transfer from the upper layer contributes to a momentum increase in the in-canyon region, and the vorticity in the in-canyon region also increases. In the mature stage (i.e., large building-length ratios), the primary vortex stabilizes in position, and the incoming flow no longer follows the building sidewalls. This causes momentum loss through the streetcanyon lateral boundaries. As the building-length ratio increases, momentum transfer from the upper layer slightly decreases, and the reverse flow, updraft, and streamwise flow in the in-canyon region also slightly decrease, resulting in vorticity reduction. Keywords Building-length aspect ratio • Computational fluid dynamics model • Development and mature stages • Flow characteristics • Step-up street canyon

Energetics of Urban Canopies: A Meteorological Perspective

“…A more effective momentum transfer is likely to be accompanied by greater thermal energy removal from inside the canyon to the external flow. Although Park et al [37] do not approach heat transfer, it seems at a first glance that the heterogeneity might favor the cooling process in urban areas. On the other hand, the effect of buoyancy over the re-circulation patterns is apparent from wind tunnel simulation with thermal stratification.…”

“…More recently, the concern about non-uniform aspect ratios has been addressed by Park et al [37] and Zhao et al [36], who carried out laboratory and numerical experiments. These studies deployed obstacles representing buildings with different heights along them stream wise.…”

“…These studies deployed obstacles representing buildings with different heights along them stream wise. Park et al [37]'s simulations consider a flow with no thermal stratification, different aspect ratios, and different building-height ratios, the ratio between upwind and downwind building height, H up /H down (called steepness ratio by Zhao et al [36]). Changes in the circulation pattern and vorticity were observed.…”

Influence of Building Height Variation on Air Pollution Dispersion in Different Wind Directions: A Numerical Simulation Study

“…Considering the employment of a simplified ideal street canyon model in this study, the validation method encompassed the comparison of simulated data with wind tunnel experiments, following a methodology derived from established studies [7,16,[19][20][21][22][23][24]26,31,[35][36][37]. The wind tunnel experiment data were obtained from CODASC (http://www.codasc.de (accessed on 23 June 2023)).…”

“…Conversely, they are termed step-up canyons when BHR is less than 1, indicating that the upwind building is lower than the downwind building. In most studies on the impact of building height variations on the dispersion of pollutants in street canyons [13][14][15][16][17][18], the variability of building height is limited (0.33 < BHR < 2). In addition, given the background of regulating the vertical growth of residential structures, it is worth studying whether street canyons composed of low-rise or multi-story buildings can facilitate the dispersion of pollutants in the street canyon.…”