Abstract
The non-uniformity of building height has been attracting considerable attention because the wind above the urban area blows down to the ground level, thereby improving the poor ventilation in urban districts. In a previous study, (part 1 of this study), it was reported that non-uniformity of building height draws the wind above the urban district to the pedestrian-level and improves the outdoor ventilation even in highly dense urban districts. Additionally, it increases the aerodynamic resistance of buildings and decreases streamwise momentum transport of air to the leeward side of the urban district.
In this study, large-eddy simulations (LESs) were applied to the flow fields in two districts consisting of rectangular buildings with uniform and non-uniform heights. The streamwise distributions of total kinetic energy transport and the energy dissipation rates from the windward to leeward sides of the urban area in the two city models were calculated from numerical data provided by LES computations, to investigate the influence of aerodynamic resistance of buildings on the dissipation of total kinetic energy (adverse effect on the wind environment in the leeward side of the focused urban district). Consequently, in the urban area, energy dissipation occurred at an approximately constant rate, and the total kinetic energy transport decreased constantly with energy dissipation (Fig. 10). As shown in Fig. 11, the decrease in total kinetic energy transport in the case of non-uniform building height was approximately 1.5 times of that in the case of uniform building height.
To clarify the influence of all the terms in the equations of total, mean, and turbulent kinetic energy transport on the transportation and dissipation of kinetic energy when the wind above the urban area blows down to the ground level, the vertical distributions of the balances of each term in the transport equations of the total, mean, and turbulent kinetic energies were analyzed. As shown in Fig. 13, above the urban area, the transportation of total kinetic energy by advection and diffusion showed positive values, and the dissipation and pressure terms indicated negative values. The peak of transportation and dissipation of total kinetic energy occurred at the maximum height of the buildings in urban districts, and these absolute values increased due to the non-uniformity of building height. In Fig. 14, the mean kinetic energy was supplied owing to the advection and diffusion terms, which was then dissipated through energy transport from mean kinetic energy to turbulent kinetic energy, －Pk. The－Pk in Case Non-uniform was larger than that in Case Uniform. There was only a little energy dissipation of the mean kinetic energy in both cases. As shown in Fig. 15, the distribution of the dissipation term is nearly symmetrical to that of the production term of the turbulent kinetic energy, Pk. As shown in Figs. 7 and 16, the region where the dissipation term of turbulent kinetic energy is generally large corresponds to the region where Pk is large. This indicates that in the region where turbulent kinetic energy is produced, the mean kinetic energy is converted into turbulent kinetic energy, and then the turbulent kinetic energy is dissipated in the same region.
