Dynamics of wingtip vortex in natural samaras

Natural samaras are winged seeds that fall with very low descent velocity due to thrust forces generated by the autorotation of the seed. Engineering bio-inspired samaras has potential applications in the air-dropping of impact-sensitive payloads, which brings about the need to understand and model the flow field around it comprehensively. Previous studies focused on the local flow around the samara and explained the formation of the leading edge vortex, which is responsible for much of its thrust generation. Nonetheless, limited focus has been given to the dynamics of wingtip vortex, which are essential for accurately modeling induced velocity at the rotational plane. This study addresses this lacuna through an experimental investigation of the wake flow of natural mahogany samaras at steady-state, using particle image velocimetry (PIV) in a seeded glass chamber. By capturing PIV planes slicing through helical wingtip vortex, we accurately identify the vortex centers, circulation strengths, and demarcate the upstream region, wake core, vortex regions, and outer wake regions. These data allow for the first detailed measurement of induced velocity at the rotational plane of samara, with streamlines revealing that the samara operates in a windmill brake state, analogous to helicopter autorotation. This comprehensive wake analysis not only provides novel insights into the thrust mechanics of bio-inspired samaras but also lays the groundwork for a simplified mathematical model for rapid geometry estimation of bio-inspired samaras.