Unsteady Boundary Layer Flow in Urban Canopies

In atmospheric boundary layer (ABL) flows, non-stationarity is a rule rather than an exception. Under this condition, the flow may experience departures from equilibrium with the underlying surface stress, misalignment of shear and strain rates, and three-dimensionality of turbulence statistics. Existing ABL flow theories are largely established for stationary flow conditions, and cannot predict such behaviors. Here, we analyze the impact of time-varying pressure gradients on mean flow and turbulence over and within urban-like surfaces. To that end, a series of large-eddy simulations of pulsatile flow over cuboid arrays are performed with varying oscillation amplitude and forcing frequency. The flow response is then examined in terms of long-time averaged and phase-dependent flow statistics. The analysis reveals that the aerodynamic roughness length characterizing the surface increases with both frequency and amplitude of the imposed oscillation. In this presentation, we discuss fundamental mechanisms responsible for this behavior and present a phenomenological surface drag model that captures the impact of flow unsteadiness on surface drag.