微小柔性体摆动自推进机理的数值和实验研究

Some small animals in nature, such as tadpole and spermatozoa, propel themselves by beating their tails or flagella. Some bionic artificial swimmer with small scale is to be created for special purpose such as medication transport. For these soft swimmers, the interaction between the tail/flagellum and the ambient fluid is the only source of propulsion. In order to control the self-propulsion by beating the swimmer's tail/flagellum, it is necessary to understand its fluidic mechanism. For this purpose, the studies about experimental measurement of tadpole and of spermatozoa and numerical simulation of a deformability model with body and tails are suggested in the present project..For the animals, they can beat their tail/flagellum actively via inner neural control and muscle mechanism. At the same time, their soft tail/flagellum maybe deform passively by the interaction of the surrounding fluid. Both active beating and passive deformation affect their motion. However, for the self-propulsion, the point is not why the motion is, but what it is. Thus, in the present project, the swimmer motion is divided into two parts: beating, which refers to a change in the tail/flagellum shape relative to the body, and swimming, which refers to the movement of the whole body due to the resultant force and moment. .The 3-dimensional beating motion is to be observed by high speed CCD camera and described in a series of time for tadpole and spermatozoa. Then, the kinematics is restructured from the seriate images and the beating pattern is to be provided. The swimming motion and the fluid flow will be analyzed experimentally and numerically. The flow induced by tadpole and spermatozoa in vivo is measured by particle image velocimetry (PIV) technique. Based on these animals, a deformability body with a bendable tail is used to study the relationship between the beating motion and the swimming motion numerically. The interaction between body surface and the fluid is calculated by the immersed boundary method (IBM) and the fluid flow is solved by fractional step method in 2-dimension and by Lattice Boltzmann method (LBM) in 3-dimension. The effects of external conditions such as the fluid properties, body properties, boundary conditions and the interaction of multi-bodies will be analyzed in detail..Based on the results of this project, it is possible to improve or impair the animal's motility by choosing the proper external conditions. And it has very important value in development of bionic devices with very small scales.

自然界很多微、小生物通过身体变形或尾/鞭毛的摆动推进自身游动，以此为原型，人们试图开发具备特定功能如药物微颗粒输送的微小游动器，但对控制其自推进能力、效率等问题的认识仍非常有限。对这类微小可变形柔性体低速游动机理的研究是流体力学的前沿问题，在临床医学、仿生设备研制等生产实践活动中具有重要的实用价值。本项目以蝌蚪和精子为主要实验对象，以高速摄像系统观测其尾部/鞭毛的空间三维摆动运动，重构典型摆动模式；采用粒子成像测速仪等设备测量其诱导的流场，探索其游动的基本特征；以微小生物为基础，建立可摆动推进的柔性体模型，采用浸没边界法处理运动边界与流体的相互作用，数值研究典型尾部/鞭毛的摆动模式与柔性体自推进能力的关系，分析液体属性、边界条件、柔性体间相互作用等的影响，考察相位锁、群体运动的特征与机理；探索控制或调节微小柔性体自推进能力的有效方法，为微、小型仿生游动器的研制提供技术积累。

自然界很多生物，大如刀鱼，小如精子、细菌，均可通过鳍/尾/鞭毛波动来推进自身游动，进而实现各种生物功能；另一方面，人们也试图以这些自然生物为原型，开发具备各种功能的、灵活稳定的仿生推进游动器。当前不论是在生物自推进机理如精子前向运动的导向机制，还是仿生推进的机动性、稳定性、高效率等方面，均还存在诸多不足。深入认识波动推进机理和特征，在生物、医学、仿生等各领域均有重要意义。本项目以精子、蝌蚪等微、小尺度波动推进生物为原型，以数值模拟为主要手段，辅助于试验观测，研究了微小可变形柔性体低速推进机理及其影响因素。针对所讨论问题具有大变形、运动边界等特征，采用反馈力模型、速度修正模型等浸没边界法处理流固交界面相互作用，将流体流动与柔性体游动解耦。流动求解在二维时采用分裂步法、三维时则采用并行性能更佳的格子Boltzmann方法。柔性体的运动分解为尾部/鞭毛摆动及整体游动：摆动可以根据实验观测预先给定，也可简化为梁模型求解其被动变形；游动则简化为自由刚体运动，应用四元数法求解以避免转换矩阵的奇异性。为了建立鞭毛摆动模型，本项目尝试进行了微米尺度三维运动的显微观测，初步搭建了基于微透镜阵列的光场显微观测平台。编制了相应的数值计算程序，系列验证表明程序是可靠的。对三维程序实现了GPU并行化，测算表明单机计算时间花费可减低一个量级。以此为基础，讨论了鞭毛/尾部摆动方式、不同壁面特别是运动壁面、生物体尺度(Re数)、多体/多鞭毛相互作用等对波动推进性能的影响及作用机理。本项目有别于通常以低Re数流动理论为基础的方法，为小、微尺度柔性游动体的低速运动建立了一套完整的数值研究方法及微米尺度三维运动的光场显微平台，研究结果为后续微小生物动力学一体化研究及微小尺度仿生推进技术开发奠定了有益的基础。