A large-eddy simulation study of bottom-heating effects on scalar dispersion in and above a cubical building array

Thermal effects on scalar dispersion 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 scalar dispersion patterns in the two cases are compared. In the no-heating case, scalar ejections in the low-speed flow structures play an important role in transporting scalar upward above the building array. In the bottom-heating case, streamwise elongated and isolated scalar ejections appear below upper low-speed and upper high-speed regions above the building array. In both cases, bottom-emitted scalar flux is balanced by streamwise scalar advection and vertical turbulent scalar flux at the rooftop height. The vertical turbulent scalar flux at the rooftop height is mainly composed of scalar ejections and scalar sweeps that are related to low- and high-speed flow structures, respectively. Furthermore, the low- and high-speed flow structures at the rooftop height induce spanwise converging and spanwise diverging flow in the building array in both the no-heating and bottom-heating cases. Thus, the mean scalar concentration in the building array is high below the low-speed flow structures (above the building array) in both cases. Dominant scalar dispersion patterns in the building array are found to be spanwise scalar transport events that are composed of negative scalar concentration perturbation and spanwise flow therein. In the bottom-heating case, a large-scale secondary circular flow develops, causing stronger spanwise scalar dispersion patterns in the building array.