Stabilizing Flows: The Role of Poro-Elasticity in Reducing Unsteady Drag Forces*

In nature, flexibility and porosity enable structures like feathers, leaves, and seeds to adapt to strong winds, making them more resilient and lightweight. However, few aerodynamic applications and experimental studies have explored the effect of poro-elasticity on the forces and coherent flow structures behind these objects. Our work investigates the effects of poro-elasticity on thin circular membrane disks under different fluid loadings. We measure the unsteady deformation, vortex-dominated flow fields, and drag forces for varying porosity levels. As dynamic pressure increases, non-porous disks deform into spherical cap shapes, leading to increased drag compared to flat disks due to higher turbulent kinetic energy in the wake. However, increasing porosity significantly reduces the average drag and drag fluctuations. Time-resolved PIV measurements reveal that poro-elasticity stabilizes the unsteady wake behind the membranes, similar to natural mechanisms. Using a frequency analysis on the unsteady membrane deformation, drag forces, and flow fields, we find that poro-elasticity suppresses the shear-layer instability associated with the high unsteady forces experienced by non-porous membranes. We present a coupled aero-elastic scaling for membrane deformation and porosity that predicts the membrane shape and drag forces experienced by the membrane. Our findings highlight the potential of poro-elastic structures, particularly for the reduction of unsteady drag forces.