Aerodynamics and Flight Dynamics of Free-Falling Ash Seeds

Abstract

Samaras or winged seeds spread themselves by wind. Ash seed, unlike other samaras, has a high aspect ratio wing which can generate enough lift force to slow down descent by rotating about the vertical axis and spinning around its wing span axis simultaneously. This unique kinematics and inherent fluid mechanism are definitely of great interest. Detailed kinematics of free falling ash seeds were measured using high-speed cameras, then corresponding aerodynamic forces and moments were calculated employing computational fluid dynamics. The results show that both rotating and spinning directions are in the same side and the spinning angular velocity is about 6 times of rotating speed. The terminal descending velocity and cone angles are similar to other samaras. Analysis of the forces and moments shows that the lift is enough to balance the weight and the vertical rotation results from a processional motion of total angular moment because the spin-cycle-averaged aerodynamic moment is perpendicular to the total angular moment and can only change its direction but maintain its magnitude, which is very similar to a spinning top in processional motion except that the total angular moment of ash seed is not along the spin axis but almost normal to it. The flow structures show that both leading and trailing edge vortices contribute to lift generation and the spanwise spinning results in an augmentation of the lift, implying that ash seeds with high aspect ratio wing may evolve in a different way in utilizing fluid mechanisms to facilitate dispersal.

Investigation of the Free-Fall Dynamic Behavior of a Rectangular Wing with Variable Center of Mass Location and Variable Moment of Inertia

“…However, during the uncontrolled falling motion between being dropped from the carrier to the start of flight control, the UAV presents uncontrolled falling. It is well known that the free-fall motion exists widely in nature and industry, such as the falling motion of leaves, paper, feathers, dust particles, diffusion of plant seeds [15][16][17], the uncontrolled re-entry of spacecraft [18], etc. Lightweight objects such as falling leaves, pieces of paper, thin disks [19], and plant seeds exhibit complex and abundant dynamic behaviors in the falling process.…”