Aerodynamic parameters of regular arrays of rectangular blocks with various geometries

The aerodynamic effects of various configurations of an urban array were investigated in a wind-tunnel experiment. Three aerodynamic parameters characterising arrays - the drag coefficient (C_d), roughness length (z_o) and displacement height (d) - are used for analysis. C_d is based on the direct measurement of the total surface shear using a floating element, and the other two parameters are estimated by logarithmic fitting of the measured wind profile and predetermined total drag force. The configurations of 63 arrays used for measurement were designed to estimate the effects of layout, wind direction and the height variability of the blocks on these parameters for various roughness packing densities. The results are summarised as follows: (1) The estimated C_d and z_o of the staggered arrays peak against the plan area index (λ_p) and frontal area index (λ_f), in contrast with values for the square arrays, which are less sensitive to λ_p and λ_f. In addition, the square arrays with a wind direction of 45° have a considerably larger C_d, and the wind direction increases z_o/H by up to a factor of 2. (2) The effect of the non-uniformity of roughness height on z_o is more remarkable when λ_f exceeds 20%, and the discrepancy in z_o is particularly remarkable and exceeds 200%. (3) The effect of the layout of tall blocks on C_d is stronger than that of short blocks. These results indicate that the effects of both wind direction and the non-uniformity of the heights of buildings on urban aerodynamic parameters vary greatly with λ_p and λ_f; hence, these effects should be taken into account by considering the roughness packing density.