  The root systems of mangroves, a tree species found in intertidal tropical
and sub-tropical coastal zones, provide a natural barrier that dissipates wave
energy effectively and reduces sediment erosion. In this work, we use a
combination of experiments and numerical simulations to examine the wake and
drag characteristics of porous arrays of cylinders, which serve as simplified
models of mangrove root networks. Optimal arrangements of the arrays are
obtained by coupling Navier-Stokes simulations with a multi-objective
optimization algorithm, which seeks configurations that minimize wake enstrophy
and maximize drag acting on the porous structure. These optimal configurations
are investigated using Particle Image Velocimetry, and the internal and
external flow around the porous arrays are analyzed using a combination of
Proper Orthogonal Decomposition and Lagrangian particle tracking. Large
variations in drag and enstrophy are observed by varying the relative positions
of the cylinders, which indicates that the geometrical arrangement of porous
arrays plays a prominent role in determining wake and drag characteristics. A
sensitivity analysis suggests that enstrophy is more sensitive than drag to
specific cylinder placement, and depends on distinctive flow patterns that
develop in the interior due to interactions among individual cylinders. Arrays
with higher drag are found to have higher average flow speeds in the wake and
give rise to time-varying Lagrangian Coherent Structures, making them
unfavorable for particle deposition and erosion. Overall, the results indicate
that high-level metrics such as array porosity may not be sufficient on their
own for predicting wake and drag characteristics.
