风向改变对非平角冠毛结构周围流场和拖曳力系数影响的数值研究

Flow pattern change and drag coefficient enhancement for non-flattened pappus models due to reversed wind direction

蒲公英种子的良好飞行能力归功于其冠毛结构特征.以往研究主要集中于风从蒲公英种子的冠毛结构底部通过至顶部(简称为“一型风”)时,空气流动与种子的相互作用.然而,由于大气流和种子结构特征的多样性,蒲公英种子从花序头部脱落或空中飞行过程中,风从蒲公英冠毛结构顶部通过至底部(简称为“二型风”)的情况也很常见.但是,目前尚缺乏二型风情况下空气流动与种子相互作用的相关研究.本文中,我们通过数值模拟,考虑风速、冠毛结构角度和孔隙率的变化,研究和比较了在两种风型下冠毛结构模型的拖曳力系数和周围流场分布特征。结果表明:由于风向的反转,流场分布特征和拖曳力系数会发生显著变化.当风从第一型转为第二型时,冠毛丝对气流的干扰从背风侧移前至迎风侧,导致在第二型风时涡环大多数情况下附着在非扁平冠毛结构模型的中心圆盘上,而在第一型风时则发生涡环分离。当风从第一型变为第二型时,压力诱导的拖曳力对总拖曳力的贡献显著增加,从而增强了非扁平冠毛结构模型的拖曳力系数.然而,随着风速、冠毛结构角度和孔隙率的增加,这种增强作用会逐渐减弱.这些发现将有助于种子风驱扩散的定量预测,并为微型仿生飞行器的设计提供一定指导.

The architectural features of dandelion seeds contribute to their excellent flight ability.Previous studies have primarily focused on the interaction between air flow and seeds when wind passes through dandelion seeds from pappus bottom to pappus top(called‘type-I wind'hereafter).However,it is also common that wind passes through dandelion seeds from pappus top to pappus bottom(called‘type-II wind'hereafter)during abscission on the inflorescence head or flight in the air,due to the atmospheric turbulence and the diversity in architectural features.Nevertheless,the interaction between airflow and seeds in the type-II wind has not yet been investigated.In this study,we conducted numerical investigations on the variations in drag coefficients of dandelion pappus models and flow patterns around them in both wind types,considering changes in wind speed,pappus angle,and pappus porosity.Generally,flow patterns and drag coefficients could be significantly altered due to reversing wind direction.The disturbance of pappus'filaments to air flow is moved from the leeward side to the windward side when the wind changes from type-I to type-II,leading to that vortex rings in the type-II wind to be more likely to attach to the central disk of non-flattened dandelion pappus models while separation vortex rings occur in the type-I wind.The contribution of the pressure-induced drag force to the total drag force is largely promoted when the wind changes from type I to type II,resulting in the enhancement of the drag coefficients of non-flattened dandelion pappus models.However,the enhancement role may be weakened as wind speed,pappus angle,and pappus porosity increase.These findings will be helpful for further quantitative prediction of seed dispersal by wind and guiding the design of micro bionic air vehicles.