Buoyant flow in simplified 2D street canyons: The fluctuating nature revealed by LES and PIV/LIF measurements

Urban climate studies require a detailed understanding of airflow and heat transport in street canyons, where the combined effects of wind forcing and buoyancy create highly unsteady dynamics. While most previous investigations and urban climate models emphasize steady or time-averaged profiles, the fluctuating nature of velocity and heat flux remains insufficiently explored. In this study, we employ simultaneous PIV/LIF measurements in a water tunnel and high-resolution LES simulations to examine unsteady buoyant flows in simplified two-dimensional street canyons. The numerical results demonstrate conservation of mass and heat flux across canyon openings, with roof-level velocity and temperature profiles revealing distinctive corner-induced features such as saddle-shaped velocity distributions and dual temperature peaks. Importantly, both experiments and simulations capture the generation and evolution of flame-like thermal plumes, highlighting their central role in driving unsteadiness. Frequency analysis (PSD) of canyon-scale heat flux shows dominant periodic structures whose frequency increases with surface heating, alongside the emergence of higher-frequency fluctuations at elevated Richardson numbers. These findings advance the mechanistic understanding of unsteady buoyant ventilation and heat removal in urban canyons and provide a critical basis for improving urban climate modeling, building design, and heat-island mitigation strategies.