The Structure of Turbulence in Pulsatile Flow over Urban Canopies

TL;DR

This study investigates how non-stationary pulsatile flows affect turbulence structures and momentum transport in the atmospheric boundary layer, revealing phase-dependent shear effects on hairpin vortices and turbulence patterns.

Contribution

It provides new insights into the topological changes in turbulence structures caused by non-stationarity in ABL flows, especially regarding hairpin vortices and ejection-sweep patterns.

Findings

Pulsation affects ejection-sweep turbulence patterns.

Shear rate influences hairpin vortex intensity and frequency.

Non-stationarity significantly alters turbulence structure and momentum flux.

Abstract

The transport of energy, mass, and momentum in the atmospheric boundary layer (ABL) is regulated by coherent structures. Although past studies have primarily focused on stationary ABL flows, the majority of real-world ABL flows are non-stationary, and a thorough examination of coherent structures under such conditions is lacking. To fill this gap, this study examines the topological changes in ABL turbulence induced by non-stationarity and their effects on momentum transport. Results from a large-eddy simulation of pulsatile open channel flow over an array of surface-mounted cuboids are examined with a focus on the inertial sublayer, and contrasted to those from a corresponding constant pressure gradient case. The analysis reveals that flow pulsation primarily affects the ejection-sweep pattern. Inspection of the instantaneous turbulence structures, two-point autocorrelations, and conditionally-averaged flow fields shows that such a pattern is primarily influenced by the phase-dependent shear rate. From a turbulence structure perspective, this influence is attributed to the changes in the geometry of hairpin vortices. An increase (decrease) in the shear rate intensifies (relaxes) the hairpin vortices, leading to an increase (decrease) in the frequency of ejections and an amplification (reduction) of their percentage contribution to the total momentum flux. Moreover, the size of the hairpin vortex packets changes according to the hairpin vortices comprising them, while the packet inclination remains unaltered during the pulsatile cycle. Findings underscore the important impact of non-stationarity on the structure of ABL turbulence and associated mechanisms supporting momentum transport.