Numerical investigation of solar impacts on canyon vortices and its dynamical generation mechanism

Abstract With the booming concerns over urban heat island phenomenon, impacts of solar radiation on flow and temperature in street canyons have been revealed by some experimental and numerical investigations, where amplified single-vortex structures are found in shallow canyons in high-Re scenarios (Re ~ 107). Canyon surfaces are heated mainly by absorbing solar radiation, and street air is warmed primarily through thermal exchange with canyon surfaces. Most previous publications focus on simulations of flow regimes, pollutant dispersion and thermal comfort in canyon's microenvironments, while dynamical investigations on vortex genesis are vague and mostly emphasize analysis of dimensionless numbers. This research will theoretically explore the impacts of solar radiation on vortex genesis over idealized two-dimensional urban canyons by coupling k-e turbulence model and P-1 radiation model. A possible dynamical mechanism of canyon-vortex generation will be introduced based on baroclinic vortex generation theory. Result shows that, the activity of wind shear (momentum field) and buoyancy imbalance (thermal field) are mutual, and their activation have a prerequisite of low vertical wind-speed environment. Signal of disturbance wind shear is critical for main vortex maintenance and the genesis of secondary or corner vortices. This mechanism has been proved both by our steady- and unsteady-RANS simulation.