Wind gusts in thermally-stratified urban turbulent boundary-layer flows and the influence of surface roughness

Abstract:

Wind gusts are examined for the mean wind speed, fluctuations, turbulence intensities and fluxes for a real urban topography. Using large-eddy simulation (LES), a wide range of boundary-layer stability is considered: the bulk Richardson number, Rb∈[-0.41, 0.82]. The ratios of the gustiness statistics, G, over the conventional time-averaged turbulence statistics are maximised for z/H_ave~1 (where H_ave is the mean building height). There is a strong linear scaling of G with the plan-area building packing density, λ_p (R²~0.8), and the trend persists to stably- and unstably-stratified flows. By contrast, the normalised building-height variability (σ_H), σ_H/H_ave, and the scaled frontal-area density (λ_f), λ_fH_ave/σ_H, are found to be of more appropriateness as scaling parameters for G compared to their original forms, σ_H and λ_f. While the dependence of G on λ_p, λ_f and σ_H are inconclusive at greater heights, effects of the surface inhomogeneity do not vanish and those of thermal stratification amplify. No noticeable differences can be distinguished between the sensitivities of the zeroth- to the second-order gustiness statistics to the boundary-layer stability. It is argued that wind-gust-related disasters may be weakened through urban planning with optimised combinations of the morphometric parameters.