# Sensitivity of urban flows and dispersion to time-dependent inflow perturbations

Guangdong DUAN

^{1}, Keith NGAN^{1}, Ka Wai LO^{1}^{1}

*School of Energy and Environment, City University of Hong Kong*

Steady inflow boundary conditions are commonly used in numerical simulations of urban flows and dispersion. However, small-scale urban flows are inevitably perturbed by unsteady mesoscale meteorological fluctuations. This work investigates the sensitivity of turbulent statistics and pollutant dispersion to time-periodic perturbations. Using large-eddy simulation of a single street canyon, sinusoidal disturbances in the streamwise velocity are introduced in the vicinity of the inflow. Vertical profiles of the turbulent kinetic energy show relatively weak sensitivity to the perturbation frequency, but RMS averages reveal a complicated structure. The pollutant dispersion, which is analysed using the mean tracer age, exhibits a resonance phenomenon at perturbation periods close to the dominant frequency of the streamwise velocity. Connections with stability theory and the predictability of turbulent boundary layers are discussed. The effect of more realistic time dependence is

**Keywords:**Urban boundary layer, Turbulence, Computational methods

**Figure 1:**Spanwise averaged mean tracer age of the street canyon. The sinusoidal perturbation is incorporated into the streamwise velocity near the inflow region. \(T\) is the perturbation period; \(\hat{\tau }_{a}\) is the canyon averaged mean tracer