Computational formulation for the evaluation of street canyon ventilation and pollutant removal performance

A computational formulation using the concept of air exchange rate (ACH), pollutant exchange rate (PCH), average pollutant concentration (Θ) and pollutant retention time (τ) is proposed to evaluate the ventilation and pollutant removal performance of street canyons. Using computational fluid dynamics (CFD), the newly developed formulation is applied to two-dimensional (2D) idealized street canyons with different building-height-to-street-width (aspect) ratios. The Reynolds-averaged Navier-Stokes (RANS) equations equipped with the Renormalization Group (RNG) k − ɛ turbulence model is adopted. The accuracy of three numerical discretizations, including the 1st-order upwind, 2nd-order upwind and 3rd-order monotone upstream-centered schemes for conservation laws (MUSCL), are compared by considering the pollutant conservation. It is found that the 1st-order upwind is not accurate enough for the pollutant transport mainly due to its over dissipative nature while the 2nd-order upwind and 3rd-order MUSCL exhibit an error of 10%. The ACH and PCH are decomposed into the mean and turbulent components in which the roof-level transport processes are dominated by the turbulent component. The spatial distributions of the vertical wind velocity and pollutant flux are also investigated to examine the ventilation and pollutant removal mechanisms of street canyons.