The influence of aspect ratios and solar heating on flow and ventilation in 2D street canyons by scaled outdoor experiments

Abstract

Wind-driven dynamic force and buoyancy force are two main driving forces in urban ventilation. As a novelty, this paper performs scaled outdoor measurements to examine the relative role of these two driving forces by non-dimensional buoyancy parameter B within two-dimensional (2D) street canyons in SOMUCH (Scaled Outdoor Measurement of Urban Climate and Health). Four aspect ratios (building height/street width, H/W = 1, 2, 3, 6; H = 1.2 m) were considered to mimic different morphologies. The temperature difference between east and west walls (ΔTeast−west) was emphasized to reveal the flow characteristics affected by such thermal effects.The results show that the wall temperature varies with street-wall orientation. ΔTeast−west becomes negative in the morning but positive in the afternoon, and maximum |ΔTeast−west| could reach 12.7 °C. Street aspect ratio largely determines the thermal structure and flow characteristics inside street canyons. As aspect ratio increases, ΔTeast−west and normalized horizontal velocity U0.25H/U2H (i.e. ratio of horizontal velocity at height of z = 0.25H to that at z = 2H) decrease. Besides, two flow regimes are observed: For H/W = 1, 2, 3, U0.25H/U2H is approximately constant (i.e. 0.26, 0.17, 0.12) when B < Bc (critical value, Bc ≈ 0.006, 0.002, 0.0008) and dynamic force dominates urban airflow; However, when B > Bc (i.e. buoyancy force cannot be negligible), U0.25H/U2H rises non-linearly with B, and such increase is faster as H/W = 2, 3 than H/W = 1. It indicates that narrower streets attain more evident impact of buoyancy effect on street ventilation. Overall, this study directly investigates the influence of solar heating on turbulent flow and provides high-quality experimental data for validation of further numerical simulations.