Abstract
Dandelion seeds can stably diffuse owing to the dominant drag rather than the lift-based mechanism of the streamlined leaves of the plant, where this is known to favor their long-distance dispersal with the steady vortex attached. However, the generation mechanism of the vortex and the aerodynamic force exerted on the seeds through multiple filaments remain unknown. Clarifying these subjects may help realize the optimal performance of porous structures under different flight conditions. This study conducts numerical simulations to illuminate the influence of gaps and the Reynolds number (Re) on the wake structures and consequent drag force of dandelion seeds. We fabricate the seeds into circular disks composed of evenly distributed square cylinders placed in a vertical flow field with Re of 100 and 400, with the porosity of the pappus (ε) ranging from 0.887 to 0.964. We explain the geometric properties of the attached, steady vortex rings and clarify their generation mechanism, i.e., the base bleed and convection effects competed with vorticity generation, based on which the gaps are confirmed to delay chaotic vortices from occurring compared with the solid case. The weakened leeward pressure is critical for the increase in the drag coefficient to reach the peak level. The enhanced drag coefficient is several times higher than that in the solid case, endowing the seeds with a high loading capacity, and the porosity corresponding to its peak is beneficial for the structural design. These conclusions provide positive insights into the design of ventilated aircrafts with optimal long-distance dispersal performance.
Highlights
Research on dandelion seeds in the field of aerodynamics has gained considerable attention in recent years due to their capability of hovering at a relatively settled height in low-Reynoldsnumber updrafts, where the steady three-dimensional (3D) vortex rings downstream can be favorable for stable dispersal
The generation mechanism of the recirculating wake relies on the balance between the convection effects and vorticity generation
The accompanied base bleed and convection effects are weakened with a decrease in ε
Summary
Research on dandelion seeds in the field of aerodynamics has gained considerable attention in recent years due to their capability of hovering at a relatively settled height in low-Reynoldsnumber updrafts, where the steady three-dimensional (3D) vortex rings downstream can be favorable for stable dispersal. The generation mechanism of the vortex pair and the aerodynamic force, especially the drag, imposed on the canopy as the number of filaments varies remain unknown Clarifying these issues may extend our knowledge of the optimal aerodynamic performance of plumed seeds. Casseau et al. numerically analyzed the flow regimes and drag coefficient of plumed seeds by simplifying them into cone-shaped membranes They confirmed the positive effects of a stable recirculating wake and a reasonable drag coefficient on different flight conditions. By drawing upon two strands of research on wake structures and the drag coefficient, in comparison with a solid body, this study assesses the implications of gaps on the stable flight of plumed seeds to identify the optimal flight conditions for porous structures.
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