Gust Response of Free-Falling Porous Disks*

Studies on the dispersal of plant seeds have recently revealed new fluid mechanics features that aid diaspores to remain airborne and travel long distances. Diaspores are the biological compounds comprising the seed and other tissues dispersed with it. Understanding the aerodynamics underpinning their dispersal can inspire the design of novel unpowered flying devices with high endurance and range. Dandelion diaspores, for example, employ a bundle of bristles called pappus, which gives way to the formation of a steady Separated Vortex Ring (SVR) in their wake. Because the formation of the SVR is due to the low pressure in the near wake of the pappus, it has been associated with drag enhancement. Here, we model the flight of dandelion diaspores as a free-falling porous disk and study how the topology of the SVR changes during flight for different governing parameters. Furthermore, the changes in the flight behaviour under the influence of wind gusts of varying intensity are studied to reveal the underlying mechanisms behind the high endurance of dandelion seeds. High-fidelity simulations are performed by solving the incompressible Navier-Stokes equation at a Reynolds number of the order of 100. We solve the Darcy-Brinkman-Forchheimer equation in the porous disk, and the incompressible Navier-Stokes equations in the clear fluid region around the disk. The fluid governing equations are weakly coupled with the Newton-Euler equations of motion using a partitioned approach. The results of this work reveal that the morphology of the dandelion diaspore is optimal not only for a steady fall in quiescent flow, but also in gusty flow conditions. This research will directly benefit the design of potential control strategies for insect-scale drones, having multi-fold applications including environmental monitoring.