Large-Eddy Simulation of the Gust Index in an Urban Area Using the Lattice Boltzmann Method

We used numerical simulations to investigate the general relationship between urban morphology and the intensity of wind gusts in built-up areas at the pedestrian level. The simulated urban boundary layer developed over a 19.2 km (length) × 4.8 km (width) × 1.0 km (height) simulation domain, with 2-m resolution in all directions, to explicitly resolve the detailed shapes of buildings and the flow at the pedestrian level. This complex computation was accomplished using the lattice Boltzmann method and by implementing a large-eddy simulation model. To generalize the results, a new parameter that expresses the intensity of gusts (the gust index, U~ _{m a x}) was defined as the local maximum wind speed divided by the freestream velocity. In addition, this parameter was decomposed into the mean wind-speed ratio, U~ and turbulent gust ratio, U~ ^′ to evaluate the qualities of gusts. These parameters were useful for quantitatively comparing the gust intensities within urban canopies at different locations or even among different experiments. In addition, the entire horizontal domain was subdivided into homogeneous square patches, in which both the simulated gust parameters and the morphological characteristics of building geometries were averaged. This procedure masked the detailed structure of individual buildings but retained the bulk characteristics of the urban morphology. At the pedestrian level, the gust index decreased with increasing building cover. Compared to U~ , the quantity U~ ^′ notably contributed to the index throughout the range of plan area index (λ_p) values. The dependences of all normalized wind-speed ratios transiently changed at λp=0.28. In cases where λ_p< 0.28 , U~ decreased with increasing λ_p, although U~ ^′ was almost constant. In cases where λ_p> 0.28 , U~ was almost constant and U~ ^′ decreased with increasing λ_p. This was explained by the change in flow regimes within the building canyon. At a higher elevation above the canopy layer, λ_p becomes less relevant to normalized wind-speed ratios, and instead the aerodynamic roughness length became important.