|Alternative Title||Fluid-Structure Interaction Simulation of Bio-inspired Flexible Structures under Active and Passive Actuation|
|Thesis Advisor||何国威 ; 张星|
|Place of Conferral||北京|
|Keyword||流固耦合 浸入边界方法 主动和被动驱动 仿生推进 流致振动|
Creatures in nature always utilize the deformation and motion under active or passive actuations for survival, such as the high performance locomotion strategies used by flying and swimming animals, and the bending deformation of leaves among aquatic or terrestrial plants when facing the flow-induced loads. These strategies are also considered as the source of inspiration for the design of man-made biomimic devices. Motions of creature always involve the interaction of the flat structures (such as wings, fins and leaves) and the surrounding fluid medium. The in-depth study of such fluid-structure interaction (FSI) can provide some theoretical evidences to the biomimic design of corresponding devices. In this dissertation, three typical FSI problems involving the deformation and movement of bio-inspired flexible plates under active and passive actuation are studied numerically by using the direct-forcing immersed boundary method based on the exact projection formulation. The dynamic behaviours and the underlying mechanisms of the corresponding FSI systems are explored in detail.
The main innovative works of this dissertation are summarized as follows:
1. Self propulsion of a bio-inspired flyer powered by one pair of pitching foils
The hovering performance and postural stability of a bio-inspired flyer composed of two thin foils which are forced to pitch in antiphase fashion is studied systematically. We first consider the constrained-flying condition where the model is only allowed to move in the vertical direction. The influences of the control parameters on the hovering performance are discussed, and the scaling laws between the driven frequency and amplitude are obtained. With the variations in parameter values, three different locomotion states, i.e., ascending, descending, and approximate hovering, are identified. The wake structures corresponding to these three locomotion states are explored. It is found that the approximate hovering state cannot persist due to the occurrence of wake symmetry breaking after long-time simulation. The occurrence of the wake symmetry breaking can be attributed to the vorticity accumulation near the flyer due to the lack of a translation velocity. Then we consider the free-flying condition where the motions in three degrees of freedom are allowed. The postural stability behaviours of the flyer are studied numerically. The results show that the orientation angle of ascending and approximate hovering state is recoverable after the physical perturbation. But the hovering state is irrecoverable after the physical or numerical perturbations due to the occurrence of the wake symmetry breaking.
2. Passive locomotion of a flexible dual-wing flyer in a vertically oscillatory flow
In this work, we numerically investigate the passive locomotion of a flexible $\Lambda$-shaped flyer in a vertically oscillatory flow with zero mean stream. In the simplified model, the flexibility of the flyer is introduced by a torsional spring installed at the hinged joint of the two foils. First, we study the effects of structural and driven parameters on the passive locomotion, angular oscillation patterns and wake structures. The results suggest that the occurrence of resonance between the flexible flyer and the oscillating flow can result in significantly different performances in flexible and rigid flyers. It is found that flexibility can have two opposing effects, reducing or increasing the actuation efforts for hovering, depending on the range of driving frequency. This result is explained by the modulation of relative velocity between the flyer and the imposed background flow due to the drastic change on both the amplitude and phase difference of the angular oscillation. The significant difference of the wake symmetry properties in different range of driven frequency can also be interpreted by the modulation of relative motion. The responses of both the flexible and rigid flyer at different driven condition to initial perturbations are examined. The stable, bistable and unstable states are identified, and the differences on the postural stability behaviour of the flexible and rigid flyer are also analysed. The results suggest that the modulation of the relative motion can also have two opposite influences on the postural stability. Besides, the geometric asymmetry of the flexible flyer is reduced when the driven frequency is high enough. The geometric effect is unfavorable to the hovering performance, but is beneficial to the postural stability.
3. Dynamic behaviours of wall-mounted flexible plates in the laminar boundary layer
The flow-induced vibration of both a single and multiple wall-mounted flexible plates in the laminar boundary layer is investigated numerically in this work. For the single-plate system, the influences of various control parameters, including bending rigidity, density ratio and Reynolds number on the dynamic behaviours of the system were explored systematically. Three distinct modes, that is, lodging, regular vortex induced vibration (VIV), and static reconfiguration, are identified. And the second natural frequency lock-in phenomenon in the regular VIV mode is illustrated. For the multi-plate system, the cavity oscillation mode due to the coupling of the adjacent plates is also observed. The occurrence of the cavity oscillation mode is directly related to the bending rigidity and the interspace between the nerghbors. Furthermore, two different frequency lock-in phemonema correspond to the regular VIV mode and the cavity oscillation mode are elucidated.
|张翔. 主动和被动驱动下仿生柔性结构的流固耦合研究[D]. 北京. 中国科学院大学,2019.|
|Files in This Item:|
|主动和被动驱动下仿生柔性结构的流固耦合研（18143KB）||学位论文||开放获取||CC BY-NC-SA||Application Full Text|
|Recommend this item|
|Export to Endnote|
|Similar articles in Google Scholar|
|Similar articles in Baidu academic|
|Similar articles in Bing Scholar|
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.