Properties of mixing length and dispersive stress in airflows over urban-like roughness obstacles with variable height

Abstract

Effects of obstacle-height variability on mixing length and dispersive stress are investigated by conducting large-eddy simulations of airflows over arrays of roughness obstacles with variable height. We evaluate differences among three simulations of flows over obstacles with no, moderate, and high obstacle-height variability. Within the canopies, effective mixing length shows one local maximum and minimum in the simulation with no obstacleheight variability but two maxima and minima in the simulations with obstacle-height variability. The number of the local maxima and minima corresponds to that of the shear layers seen at the heights of obstacle tops. Enhanced dispersive stress appears within the canopy between the heights of the lower- and higher-obstacle tops in the simulations with obstacle-height variability. Particularly in the simulations with high obstacle-height variability, the magnitude of dispersive stress becomes comparable to that of the Reynolds stress at the height of the lower-obstacle top. These results suggest that actual urban areas with high building-height variability should significantly affect properties of mixing length and dispersive stress.