Tracking maple seeds falling through wind

One of the most beautiful moments of spring is the sight of falling maple seeds. These single-winged seeds spiral gracefully to the ground and can travel long distances when blown by the wind. Understanding how these seeds — called samaras — behave both when falling and when buffeted by the wind can reveal details of their aerodynamics and inform bioinspired flying machines.

Niu et al. developed an experimental setup to track falling samaras in still air and through an applied crosswind. They revealed how the rotation of the samara works to stabilize its motion in light winds and provide lift in strong winds.

“We devised a new experimental setup that employs a laser sheet array for in-situ velocity measurements, which allowed us to quantitatively observe new descent characteristics of samaras,” said author Tian Jian Lu.

Their measurements allowed the authors to identify multiple different behaviors exhibited by falling samaras in crosswinds. In weak winds, the autorotation works to rapidly stabilize the fall. In stronger winds, the samara has a bimodal response depending on whether the winds initially impact the trailing or leading edge. In the first case, the samara rapidly drops out of the crosswind before stabilizing, whereas it experiences enhanced lift and lateral movement in the second case.

The researchers are planning additional experiments to focus on the dynamics of the samara’s leading-edge vortex.

“We aim to systematically investigate the behaviors of the leading-edge vortex when exposed to advancing and retreating conditions created by strong crosswind, as its transient response may control the bimodal response of a single-winged autorotating body like a samara,” said Lu.

Source: “Kinematic responses of an autorotating samara to concentrated crosswind,” by Pei Xing Niu, Michael David Atkins, Yan Yan Liu, Tian Jian Lu, and Tongbeum Kim, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0125421 .