Direct Numerical Simulation of Flexible Patches

Canopy flows are of significant interest across various natural and industrial applications, since several elongated slender objects clamped on a wall or a surface can be addressed to as canopies. Modern world issues, such as river pollution mitigation and urban heat cooling, can benefit from a deeper understanding of the flow behavior around such structures. Although extensive research has already been conducted on rigid canopies through experimental and numerical studies, flexible canopies are more commonly encountered in many fields, highlighting the need to investigate not only the fluid behavior around flexible structures, but also the dynamics of these flexible elements and their interactions with the surrounding flow. In their recent study of Direct Numerical Simulation (DNS) of flexible canopies, Foggi Rota et al. (2024) focused on the fluid-structure interactions within and above a distribution of filaments covering the whole wall, paving the way for studies on more complex configurations. In this thesis, we expand on these findings by examining the fluid and filament dynamics in a canopy where flexible filaments cover only one half of the wall-surface, thus creating an edge aligned with the flow direction. This scenario reminds of roads cutting through forests or rivers where submerged seagrass grows along the banks. We take into account two limit flexibilities, one with almost-rigid filaments, and another where the filaments deflect significantly when hit by the flow. The mean flow characteristics in the patch case fall well in between those of the two limit scenarios - the open channel and the full canopy. Moreover, the mean flow shows the appearance of mean vortex structures, that brings high energy flow into the canopy through the lateral edge of the patch, and that creates an updraft of the flow, transporting low-energy flow outside of the canopy through the canopy top. The analysis of the shear balance equation indicates that, unlike in full canopies and open channels, the mean flow plays a key role in balancing the forcing pressure gradient over time in canopy patches, although the turbulent shear still represents the major contributor for all the three scenarios investigated. Velocity fluctuation reveals that ejections dominate over sweeps on horizontal planes above vegetated regions, regardless of the vegetation distribution, while sweeps are predominant along the vertical planes separating vegetated and non-vegetated regions. Finally, the filament dynamics show that the average tip positions are compliant with the mean vortex structures in the domain. Overall, although the dynamics of the filaments in the patch scenario are similar to those in the full canopy, their swinging behavior varies with the spanwise position: indeed, filaments closer to the non-vegetated regions exhibit greater deflection and tend to oscillate in a wider range of frequencies if compared to those deeper within the canopy.