  Urban areas are known for their complex atmospheric environments, with the
building morphology having a significant impact on local climate patterns, air
quality, and overall urban microclimate. Understanding the heat transport and
fluid flow in complex urban environments is crucial for improving urban climate
resilience, which remains an open frontier in the field of urban studies. To
gain a more profound insight into the physical processes occurring in urban
areas, particularly within street canyons, we conducted an experimental
investigation in a large-scale water tunnel. This study involved the
simultaneous examination of heat and flow fields, carried out at high spatial
and temporal resolutions, utilizing Laser-induced Fluorescence (LIF) for heat
analysis and Particle Image Velocimetry (PIV) for flow analysis. Our results of
heat and flow in different street canyons indicate that the flow is
significantly influenced by a combination of factors, including canyon
configuration, the presence of buoyant force, and the magnitude of the
approaching flow. The ventilation rate and heat flux from the street canyon,
which are key factors shaping the urban microclimate, are found dominated
significantly by the street canyon morphology. For instance, changing the
aspect ratio of a street canyon results in a significant change of air
ventilation rate, ranging from as low as 0.02 to as high as 1.5 under the same
flow conditions. Additionally, canyons with high air ventilation rates exhibit
significant heat flux removal at the canyon roof level, which is accurately
described by the local Richardson number.
