Periodic reversal of mean flow within the transitional roughness regime in atmospheric surface layer flow

Large-eddy simulation is used to model turbulent channel flow where the channel bottom is mounted with arrangements of tree canopy elements. These elements are arranged in rows such that rows are parallel to the mean flow direction with a domain scale spanwise gap, $\delta_2/\delta$, where $\delta$ is the channel height. Further, canopies in individual rows are separated by a streamwise gap, $\delta_1/h$, where h is the canopy height, which regulates the streamwise roughness regimes, d-type, and k-type. Such an arrangement of canopy elements results in a spanwise heterogeneity in the aerodynamic roughness length and introduces undulations in the mean flow along the transverse direction. In the present work, we used a set of simulations with a scaled spanwise gap of 1.25, and 1.65 and attempted to understand concurrent effect by varying $\delta_1/h$ on the mean and secondary flows. We collected evidence of a periodic reversal in the mean flow followed by secondary flow reversal. We noticed a periodic sign reversal in the streamwise velocity fluctuations along with reversal in secondary cells. By that, we mean a temporal periodic upwelling and downwelling of low and high momentum fluid packets respectively.