Modification of modal characteristics in wakes of square cylinders with multi-scale porosity

Physics of Fluids, Volume 33, Issue 4, April 2021. Wake flows behind porous patches are complex and host several spatiotemporal features associated with multi-scale excitation. This is in stark contrast to flow past a square cylinder dominated by nonlinear energy cascade stemming from primary vortex shedding instability. In this study, we analyze wakes created by multi-scale patches containing three iterations while maintaining a constant plan porosity. The results are compared to flows obstructed by a square cylinder and a single-scale patch of uniformly distributed elements from the second iteration having the same footprint. Multi-scale porous patches show a protracted wake having a greater spanwise dimension compared to square cylinder. The characteristics are dependent on the arrangement of elements within the patch which affect the extent of bleed flow through the configuration. Besides, we use proper orthogonal decomposition and dynamic mode decomposition to elucidate instability mechanisms at different scales. Distributions of spatial modes reveal element-scale flow structures in the near-wake region with patch-scale turbulence further downstream. Our analysis confirms manipulation of characteristic scales and the associated high-energy events driven by the arrangement of elements within a given patch despite having the same plan porosity.