Aerodynamics of multiple freely falling plates

Numerical simulations of the complex fluid structure interaction between multiple freely falling plates in an otherwise quiescent medium are carried out in this work. The non-vertical descending motion of the plates shows a wide variety of dynamical behavior that depends not only on the shape of the plates but also on the relative initial orientation of release. The collision between the plates and subsequent movement through a varied vorticity field cause significant deviations in the trajectory from the single plate behavior. The interaction of vortices shed from closely moving plates is seen to either support, in flutter, or oppose, during the initial transient of tumble, rotational motion. In the case of parallel fluttering, a combination of the forces exerted by the surrounding fluid and the wake of the leading plate causes the trailing plate to emerge faster at the later stage of its descent. In comparison to the single plate case, a stable tumbling motion can be achieved at a higher non-dimensional moment of inertia when the plate moves in the proximity of another one. While in some case a denser vorticity field exhibits vortex merging with a range of scales present near the plates, scattered large scale flow is mostly observed away from the plates. Initial collision and vortex interaction combinedly decide the height at which the plates settle for steady gliding motion in the case of tumble. This preferential selection of the level height for tumbling is found to be the key for subsequent similar stable trajectories. The rate of energy transfer to the surrounding fluid decides the duration and direction of flight of the plates. Even a small contribution in energy transfer from the surrounding fluid to the plates amplifies and causes significant changes in energy interaction.