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The transport of energy, mass, and momentum in the atmospheric boundary layer(ABL) is regulated by coherent structures. Although past studies have primarilyfocused on stationary ABL flows, the majority of real-world ABL flows arenon-stationary, and a thorough examination of coherent structures under suchconditions is lacking. To fill this gap, this study examines the topologicalchanges in ABL turbulence induced by non-stationarity and their effects onmomentum transport. Results from a large-eddy simulation of pulsatile openchannel flow over an array of surface-mounted cuboids are examined with a focuson the inertial sublayer, and contrasted to those from a corresponding constantpressure gradient case. The analysis reveals that flow pulsation primarilyaffects the ejection-sweep pattern. Inspection of the instantaneous turbulencestructures, two-point autocorrelations, and conditionally-averaged flow fieldsshows that such a pattern is primarily influenced by the phase-dependent shearrate. From a turbulence structure perspective, this influence is attributed tothe changes in the geometry of hairpin vortices. An increase (decrease) in theshear rate intensifies (relaxes) the hairpin vortices, leading to an increase(decrease) in the frequency of ejections and an amplification (reduction) oftheir percentage contribution to the total momentum flux. Moreover, the size ofthe hairpin vortex packets changes according to the hairpin vortices comprisingthem, while the packet inclination remains unaltered during the pulsatilecycle. Findings underscore the important impact of non-stationarity on thestructure of ABL turbulence and associated mechanisms supporting momentumtransport.

The Structure of Turbulence in Pulsatile Flow over Urban Canopies