Effect of flow unsteadiness on the mean wind flow pattern in an idealized urban environment

We investigate the effect of flow unsteadiness on the mean wind flow pattern in an idealized urban environment. To that extent, steady RANS and unsteady LES simulations of the flow around a city-block are conducted for various ambient wind directions. Our simulations reveal that the selected modeling approach can have a significant effect on (i) the mechanisms causing air exchange between the regions above and below roof level, (ii) the spatial distribution of the time-averaged exchange fluxes, and (iii) the total amount of air exchange. For some specific cases, the modeling approach even has a larger impact on the predicted air exchange rate than the ambient wind direction. These results are of key-importance to researchers and engineers involved in simulations of flow in the urban neighborhood.