A Large-Eddy Simulation Study of Thermal Effects on Turbulence Coherent Structures in and above a Building Array

Thermal effects on turbulent flow in and above a cubical building array are numerically investigated using the parallelized large-eddy simulation model (PALM). Two cases (no heating and bottom heating) are simulated and are compared with each other, focusing on thermal effects on turbulence coherent structures. In the no-heating case, the streaky or streamwise-elongated structures of low-speed regions appear above the building array and ejections in the low-speed regions play an important role in transporting momentum downward. In the bottom-heating case, plume-shaped structures appear with streamwise-elongated structures and the magnitude of vertical turbulent momentum flux averaged over the low-speed regions increases. Elliptical structures of negative streamwise velocity perturbation and vortical structures similar to hairpin vortices appear above the building array in the conditionally averaged fields in both cases, and the coherent structures expand more vertically when the bottom is heated. At or just above the rooftop height, high-speed streaks are distinct and sweeps induced by the streaks or shear instability are important for momentum transport in both cases. In the bottom-heating case, the magnitude of vertical turbulent momentum flux at the tops of cavity spaces increases, partly owing to the strengthened ejections. Below the rooftop height, the highspeed streaks occasionally enter intersection spaces and induce spanwise diverging flow there in both cases. When the bottom is heated, intensified updrafts induce more organized secondary circular flow and the spanwise flow in the building array is strengthened by the secondary flow.