Three-dimensional effects and turbulence characteristics in heated urban canyons

Ground heating in urban environments generates buoyancy that interacts with wind-driven shear and modifies street-canyon ventilation and heat removal. Although thermally affected flows in two-dimensional (2D) canyons have been widely investigated, the response of three-dimensional (3D) canyons remains unclear as lateral exchange, tip-corner eddies, and buoyant uplift occur simultaneously. This study presents a wind tunnel comparison of 2D and 3D street canyons under similar oncoming flow conditions and controlled floor heating, at floor temperatures from ambient to 150 °C for the 2D canyon and to 190 °C for the 3D canyon. Time-resolved and scanning stereo particle image velocimetry and thermocouple measurements quantify mean flow structures, turbulence statistics, turbulent momentum-transfer events, and thermal stratification. Results show that lateral channel flow and tip-corner vortices weaken the mid-span primary recirculation in the 3D canyon compared to the 2D canyon. Ground heating strengthens the buoyancy-assisted updraft along the leeward wall and enhances roof-level exchange. In the 2D canyon, increasing thermal forcing can reorganize the flow toward a more coherent buoyancy-driven regime. In the 3D canyon, lateral exchange persists and maintains stronger spatial variability and event intermittency. Reynolds-stress and quadrant analyses show that moderate thermal forcing amplifies ejection and sweep events near the canyon top, whereas stronger forcing can reduce the intermittency of shear-driven motions in the 2D canyon. Temperature and gradient Richardson number profiles indicate that 3D lateral mixing smoothens local stability gradients while allowing buoyancy-driven vertical heat removal. These findings clarify how three-dimensionality modulates the balance between shear-driven exchange, buoyant uplift, and lateral redistribution in heated urban canyons.