生物在流体中的自推进运动广泛存在于自然界中，如空中鸟的飞行，水中鱼的游动等。独特的几何外形、运动形态以及生活习性赋予了它们对流体高超的驾驭能力，这使得它们大多都拥有很好的运动性能。通过模仿生物的外形及运动方式，人们可以设计出性能优秀的仿生推进器服务于人类的生活。本文的研究目的是探索自然界生物推进现象中的流体力学基础问题，并认识流体动力学效应在生物推进中的作用。本文选取鱼类的游泳运动为研究对象，从模仿鱼类的运动形态和运动习性这两个方面出发，研究了若干仿鱼自推进问题，着重考察其运动性能和形成机理。其结果可以为人造水下航行器的设计提供指导性的依据，因此该研究具有重要意义。

        对于采用躯干-尾鳍（BCF mode）方式游泳的鱼类，其推进形态完全由中线决定，其具体形状只是增加了一个厚度效应。此外，在持续性稳定游泳过程中，该类鱼的变形和推进运动始终集中在侧向和流向上，且沿着深度方向不存在明显差异。因此，本文以二维柔性薄板为简化的仿鱼推进模型，采用流固双向耦合的方式对仿鱼自推进问题进行了数值模拟研究。计算中在头部施加单点驱动（其他部位自由），柔性薄板被动发生变形，然后与流体作用实现自推进运动。模仿自然界中鱼类沿身长的刚度变化，本文通过在柔性薄板上给定不同类型的空间刚度分布实现了鱼类的鳗鲡式、鲹科式推进形态。以此为基础，我们对自然界中鱼类的三大推进现象—近壁推进、集群推进以及间歇性推进—进行了系统的研究。结果显示：合适的壁面效应、编队推进和间歇性运动策略都可以提高仿生鱼的推进性能，即增加推进速度、减少推进能耗。

       本文的主要创新性工作包含如下三个部分：

（一）、柔性翼在近壁面处的自推进运动研究

        该部分通过数值模拟系统地研究了柔性翼在近壁面处的自推进运动，考察了非定常壁面效应对柔性翼推进性能的影响。通过改变其距壁面的侧向距离来模拟非定常壁面效应的强弱；同时改变了柔性翼的刚度大小，以探索壁面效应对于不同柔性程度的推进器推进性能的不同影响。结果显示，壁面的存在可以显著地提高柔性翼的推进速度，但同时所需的输入功率也明显增加，由此导致的推进效率结果较为复杂。其中，中等刚度翼越靠近壁面推进效率越高，最高可增加10.5%；极柔的翼在极度靠近壁面时推进效率会极大地提高，最高可达35.3%；而完全刚性的翼在壁面附近运动时，推进效率反而会下降。此外，由于壁面的存在，柔性翼尾迹中的涡街朝着远离壁面的一侧偏斜，相邻正、负涡之间发生配对、旋转、上升运动。

（二）、多条鲹科鱼结伴推进时的稳定构型

        该部分通过设置沿身长变化的刚度分布，使得柔性薄板呈现鲹行式推进模式，以此数值模拟多条鲹科鱼（柔性薄板）结伴推进时的情况，考察它们推进达到稳定时的队伍构型和能耗情况。本文分别考察了两、三、四条鱼结伴推进的情况，得到了每种情况下鱼队推进的稳定构型。在两条鱼中，发现了侧向并联、前后串联、前后交错分布的三大类构型；在三条鱼中，发现了侧向并联、侧向梯队、前后交错的三大类构型；在四条鱼中，发现了紧凑、松散两种矩形构型，以及稳定的菱形构型。对比D. Weihs 1973年从理论上提出的反相菱形构型，本文得到的菱形构型为同相驱动，且空间上排布得更加紧凑。以上所有稳定构型中，整体最节省能量的是两条鱼反相并联构型，最多可减少16%的能耗。而所有成员鱼中，节能最多的成员鱼为三条鱼反相并联构型的中间鱼，最多可减少22%的能耗。本文该部分工作证明：仅靠流体作用力仿生鱼群就可以实现流向上稳定的编队推进，且通过合理的空间编队鱼群可以节省自身的推进能耗。

（三）、鱼类间歇性驱动（burst-and-coast）的推进性能及尾涡结构研究

        该部分采用鲹科式推进的柔性薄板为仿生鱼模型，将连续的头部驱动改为半周期摆尾（Half tail-beat mode）的间歇式驱动，数值模拟考察了仿生鱼在该驱动模式下的能耗及其尾涡结构的演化过程。研究发现，对于鲹科式推进的仿生鱼：在巡游雷诺数较高时，相同推进速度下间歇性运动比连续性运动更节省能量，并且巡游雷诺数越高间歇性运动节省的能量就越多、节能的速度范围也更广；当巡游雷诺数较小时，相同推进速度下，间歇性运动比连续性运动消耗更多能量；临界状态对应的巡游雷诺数大概在360~900之间。另外，半周期摆尾间歇模式使得流场尾迹呈现出三列互成角度的涡街，整体关于流向对称。其中，外侧两列为主涡街，向两侧倾斜；中间的一列为较弱的涡街，与流向平行。该部分工作首次通过被动柔性与流固耦合作用，模拟了真实鱼类运动形态下的间歇性运动，得到了间歇性运动节能的巡游雷诺数范围。

     Animal locomotion in fluid is widely existed in nature, such as the flying of birds in air and the swimming of fish in water, etc. The distinct geometrical shapes, kinematic morphologies, and living habits bring about them with strong capability of controlling the fluid, which make most of them good athletes. By mimicking the shapes and kinematics of various moving animals, people can creat numbers of bionic propellers with good propulsive performance, which can be used for serving the daily life of human being. The objective of this doctoral dissertation is to explore the basic issues of fluid mechanics within various biolocomotion phenomena, and unveil the effect of fluid dynamics in them. Choosing swimming motion of fish as the object of study, this dissertation studied several issues of self-propelled fish-like locomotion from the aspects of mimicking fishes’ kinematics and habits, and focus particularly on the propulsive performance and its mechanism. These results can provide guiding gists for the design of underwater vehicles, thus hold great significance.

    For fishes of Body-Caudal-Fin type, their swimming kinematics are fully determined by their central lines, and detailed shapes only serve as the thickness effect. Besides, during the sustanined steady swimming process, the defomation and locomotion of this type of fish are limited to the lateral and streamwise directions with no obvious offset along depth direction. Therefore in this dissertation, a zero-thickness 2D plate with flexibility are used as the model of bionic fish propulsor. And two-way coupling of the fluid-structure interaction system is adopted to numerically study the fish-like self-propelled locomotion. By exerting actuations at the leading edge (other parts free), the thin plate deforms passively, and ineract with the fluid to realize its self-propelled swimming. Mimicking the bending rigidity variation of real fish, the anguilliform and canrangiform swimming kinematics are realized by prescribing different spatial distributions of flexibility in the simulations. Based on this, three issues in fish locomotion—near-wall swimming, group swimming, intermittent swimming—are explored with systematic studies. The results showed that: suitable wall effect, grouping style and intermittent strategyies can all promote the propulsive performance of bionic fish, i.e. increase the cruising speed and /or decrease the propulsive energy cost.

    The main innovative works of this dissertation are as follows:

1.    Self-propelled swimming of a flexible heaving foil near a solid wall

     Systematic numerical studies are carried out at this part to simulate the self-propulsion movement of a flexible foil near the wall, and the unsteady ground effects on its propulsive performance are investigated. The strength of the ground effect is reflected by various values of perpendicular distance between the foil head and the solid wall; meanwhile, bending rigidity is changed from case to case in order to explore the influences of the solid wall on the propulsion of foils with different degrees of flexibility. The results indicate that existence of solid wall can dramatically enhances the cruising speed of the foil, but input powers are also increased at the same time. For the foil with medium degree of flexibility, the propulsive efficiency augments monotonously as the wall distance shrinks, and the biggest augment is 10.5%. For extremely flexible foil, its efficiency has a tremendous enhancement when moves closer to the wall, and the highest enhancement can be up to 35.3%. For the totally rigid foil, its efficiency can even decreases when swimming near the solid wall. Besides, the vortex street in the wake deflects obviously upward due to the solid wall, and the positive and negative vortices form pairing, rotating and ascending motions.

2.    Stable formations of multiple schooling carangiform swimmers

     Stable formations and energy consumption of multi-fish swimming are studied here, based on the carangiform kinematics of the plate model by using a spatially varying distribution of bending rigidity. Three basic cases of two-, three-, and four-fish are considered respectively in this dissertation, and their corresponding stable formations are obtained. For two-fish swimming, three types of formations are formed: side-by-side, in-line and staggered. For three-fish swimming, also three types of formations are obtained: side-by-side, echelon and staggered. For four-fish swimming, the compact and loose rectangular formations are obtained, together with the diamond-shaped formation. Comparing with the anti-phase diamond shape theoretically proposed by D. Weihs in 1973, the one obtained here is at in-phase and spatially more compact. Among all the stable formations, the most energetically efficient one is the anti-phase side-by-side formation of two-fish, which can at most saves energy by 16%. Among all the fish menmbers of these stable formations, the most energetically efficient one is the middle fish of the anti-phase side-by-side formation of three-fish, which can at most saves energy by 22%. This work validated that: fluid dynamics alone can lead to the streamwise stable formations of fish schools, and propulsive energy can be saved by reasonable spatial configurations.

3.    Intermittent swimming of a carangiform swimmer: study of the propulsive performance and vortex wake

    A flexible thin plate with carangiform swimming kinematics are adopted as the bionic fish model, and traditional continuous actuation at the head of the plate is replaced by the intermittent (busrt-and-coast) actuation of Half-Tail beat mode. Propulsive performance and wake dynamics of the carangiform swimmer under this actuation style are probed. According to the results, it is found that: (1) at high cruising Reynolds number, intermittent swimming can save energy compared with continuous style at the same cruising speed, and the higher the Reynolds number, the larger the energy saving; (2) yet at low cruising Reynolds number, intermittent swimming cost more energy than continuous one with the speed unchanged; (3) the critical cruising Reynolds number that separate these two states is about 360~900. At the other side, fluid wake with three vortex streets are found under the intermittent swimming, and is symmetric about the streamwise axis. Among the three vortex streets, two sided ones are composed of major vortices and open up towards downstream, while the middle one is composed of minor vortices and keeps parallel. In this work, swimming kinematics of real fish is applied into the study of intermittent swimming by passive flexibility and fluid-structure interaction for the first time. Based on this, the range of cruising Reynolds number corresponding to energy-saving state are obtained for the intermittent swimming.