On the Sensitivity of Urban Flow and Ventilation to Time-Dependent Inflow Perturbations

Urban turbulence and pollutant dispersion depend on unsteady mesoscale winds. Since building-resolving computational fluid dynamics models typically employ steady Dirichlet boundary conditions or prescribed steady-state forcing, the accuracy of numerical simulations could be limited by the neglect of inflow perturbations. This work investigates the effects of time-dependent perturbations on the flow and ventilation over a unit-aspect-ratio street canyon: time-periodic perturbations are incorporated into a large-eddy simulation model via the nudging technique.

Although the spatial structures of the pollutant field and turbulence are largely insensitive to the time-dependent forcing, the ventilation (mean tracer age) and differences between perturbed and unperturbed fields (raw and normalised errors) show a strong dependence on the perturbation. The response of the error kinetic energy is maximised for a perturbation period around 300 s. Spectral analysis indicates that this behaviour arises from a resonance between the inflow forcing and the mean canyon circulation. Qualitatively similar results are obtained from perturbations derived from realistic winds.