Abstract
Some plant species (e.g., dandelions) have evolved plumed seeds made of thin filaments to assist their dispersal, while some other plants (e.g., maples) opt for winged seeds that autorotate after release, which could effectively prolong their descent. Inspired by these plants, in this paper, we designed a series of autorotating sub-1 mm three-dimensional passive microfliers comprising conic spiral filamentous wings and conducted computational fluid dynamics analysis on their autorotation kinetics. The effects of flier density (ρs = 0.1 × 103–2.7 × 103 kg m−3), wing number (n = 2–4), wing shape (Archimedean- or Fibonacci-spiral), and relative airflow velocity (V = 0.1–1 m s−1) were systematically investigated. We found that (1) at a given V, the terminal rotation speed (ST) is almost invariant with ρs; (2) during natural descent, a flier with larger n would fall faster yet spin slower, while the wing-tip speed is only marginally dependent on n; (3) an Archimedean flier would fall slower yet spin faster than its Fibonacci counterpart, resulting in a lift-to-drag ratio more than doubled. The angular momenta and kinetic energies during natural descent were also compared between the Archimedean and Fibonacci fliers, which could serve as metrics for flight stability. It was found that the Archimedean fliers outperform the Fibonacci counterparts in both stable flight and prolonged descent. Our results here could offer guidance for the design of miniaturized fluid-immersed (aerial/aquatic) vehicles and robots featuring rotary modules working passively or actively in low-Reynolds-number regime.
Subject
Condensed Matter Physics,Fluid Flow and Transfer Processes,Mechanics of Materials,Computational Mechanics,Mechanical Engineering