Department of Mechanical and Materials Engineering, Queen's University, Kingston K7L 3N6, Canada.
Bioinspir Biomim. 2021 Oct 29;16(6). doi: 10.1088/1748-3190/ac2b01.
Inspired by the reproductive success of plant species that employ bristled seeds for wind-borne dispersal, this study investigates the gust response of milkweed seeds, selected for their near-spherical shape. Gust-response experiments are performed to determine whether these porous bodies offer unique aerodynamic properties. Optical motion-tracking and particle image velocimetry (PIV) are used to characterize the dynamics of milkweed seed samples as they freely respond to a flow perturbation produced in an unsteady, gust wind tunnel. The observed seed acceleration ratio was found to agree with that of similar-sized soap bubbles as well as theoretical predictions, suggesting that aerodynamic performance does not degrade with porosity. Observations of high-velocity and high-vorticity fluid deflected around the body, obtained via time-resolved PIV measurements, suggest that there is minimal flow through the porous sphere. Therefore, despite the seed's porosity, the formation of a region of fluid shear, accompanied by vorticity roll-up around the body and in its wake, is not suppressed, as would normally be expected for porous bodies. Thus, the seeds achieve instantaneous drag exceeding that of a solid sphere (e.g. bubble) over the first eight convective times of the perturbation. Therefore, while the steady-state drag produced by porous bodies is typically lower than that of a solid counterpart, an enhanced drag response is generated during the initial flow acceleration period.
受采用带刺种子进行风传播的植物物种繁殖成功的启发，本研究调查了乳草种子的阵风响应，这些种子因其近似球形而被选中。进行阵风响应实验是为了确定这些多孔体是否具有独特的空气动力学特性。使用光学运动跟踪和粒子图像测速 (PIV) 来描述乳草种子样本的动力学，因为它们自由响应在非定常阵风风洞中产生的流动扰动。观察到的种子加速度比与类似大小的肥皂泡以及理论预测一致，这表明空气动力学性能不会随孔隙率降低而降低。通过时间分辨 PIV 测量获得的高速和高涡度流体在物体周围的偏转观察表明，通过多孔球体的流量很小。因此，尽管种子具有多孔性，但在伴随物体周围和尾流中涡度卷起的流体剪切区域的形成并没有被抑制，这对于多孔体通常是预期的。因此，种子在扰动的前八个对流时间内实现的瞬时阻力超过了固体球体（例如气泡）的阻力。因此，虽然多孔体产生的稳态阻力通常低于相应的固体体，但在初始流动加速期间会产生增强的阻力响应。
