Coherent Structures in Unsteady Flow over Urban Canopies

This study examines structural changes in coherent structures populating the boundary layer within and above an array of surface-mounted cuboids induced by flow unsteadiness, and the impact of such changes on momentum transport. We focus on the non-stationarity induced by a pulsatile pressure gradient. A large-eddy simulation of flow over urban-like roughness subjected to a pulsatile pressure gradient is carried out, and results are analyzed and compared against those from a constant pressure gradient case in terms of first- and second-order statistics, the relative importance of each quadrant, two-point velocity correlations, and the instantaneous flow structures. Additionally, the technique of conditional averaging is employed to extract the characteristic eddy structures during the pulsatile cycle. Throughout the pulsatile cycle, hairpin packets, comprising organized hairpin vortices aligned in the streamwise direction, maintain their orientation but experience a substantial change in their extent. When the local shear rate increases, the hairpin vortex structures are intensified, and shrink in size due to the self-induction. As a consequence, the amount of ejection events between the hairpin legs reduces, but the strength of each event becomes stronger. Such a process is reversed with the decreasing shear rate.