Velocity and scalar concentrations with low occurrence frequencies within urban canopy regions in a neutrally stable shear flow over simplified urban arrays

The unsteadiness of urban airflow influences rare events of high wind speeds or high scalar concentrations. Therefore, this study uses large-eddy simulations to investigate the geometrical impact of generic block arrays on the statistical features of wind speeds and scalar concentrations within urban canopy regions. Six types of urban-like arrays with uniform and non-uniform block heights are considered, and probability density functions of wind speeds and scalar concentrations are derived based on the flow and concentration distributions within urban canopy regions. Exceeding wind speeds and scalar concentrations are determined through cumulative probability densities. The non-uniform spatial distribution of scalar concentrations is found to be correlated with the velocity magnitude distribution. In particular, airflows with strong wind speeds near tall blocks contribute to a reduction in the scalar concentration in that region. The probability density functions of velocity magnitude for arrays with height variations become long-tailed as the horizontally averaged wind speeds increase. Accordingly, the probability density at high scalar concentrations is lower for arrays with height variations. Finally, the exceeding wind speeds and scalar concentrations for arrays with height variation can be expressed as linear functions of the mean wind speed.