Numerical study of turbulence temporal characteristics in the urban sublayer

The vertical transition from roughness sublayer to inertial layer is a well-established concept in atmospheric surface layer flows over complex canopies, where urban topographies and vegetative canopies continue to receive broad attention from the community. This transition occurs at approximately three times the canopy height, below which the turbulence statistics resemble a turbulent mixing layer. Turbulent momentum fluxes in `canopy turbulence' or `obstructed shear flows' (Ghisalberti, 2008 JFM 641, 51—61) are typically dominated by turbulent sweep events (downward excursions of high momentum fluid), owing to the presence of coherent motions that occupy the region of fluid above the canopy. We have used large-eddy simulation with an immersed boundary method to study turbulent flows over a distribution of uniform height, staggered cubes. The computational domain has been designed such that both the roughness sublayer and a region of the inertial layer are resolved. With this, we record vertical profiles of time series of fluctuations of streamwise velocity, $\tilde{u}^\prime = \tilde{u} - \langle \tilde{u} \rangle_t$, and vertical velocity, $\tilde{w}^\prime = \tilde{w} - \langle \tilde{w} \rangle_t$. Contour images of fluctuating velocity component (where fluctuation is computed as a quantity's deviation from its time-averaged value during a time period over which the simulation exhibits statistical stationarity) shown relative to vertical position and time suggest a time-lag between the passage of a high- or low-momentum region in the aloft inertial layer and excitation or relaxation of cube-scale coherent vortices in the sublayer. We show results of an ongoing study investigating the temporal characteristics of turbulent mixing in the urban canopy.