鱼群和鸟群迁徙运动中的流体力学机理研究

发布时间：2018-09-12 13:54

【摘要】：鱼群或鸟群保持规律队形的迁徙运动是自然界中常见的现象,探究其中的流体力学原理,不仅有利于进一步了解鱼群和鸟类集群运动的内在机制,而且有利于仿生飞行器和水下航行器的结构优化设计和队形优化设计。本文利用浸入边界方法,数值分析了鱼群或鸟群迁徙运动中的流体力学机理。本文提出了侧向自由的弹性细丝模型,将其置于扑翼下游,探究了细丝在不同涡街(卡门涡街、平直涡街、反卡门涡街)中的运动特性及其流场结构,发现了细丝存在四种运动模式:细丝在涡街的外侧往复振动,细丝穿梭于涡核之间往复振动,细丝被夹在涡带之间往复振动,细丝微弱振动。在反卡门涡街中,细丝总是会在涡街的外侧振动,这是因为流场侧向诱导速度太大所致,此时,细丝受到的阻力明显小于细丝在涡街里面时的阻力,这种运动模式对应的斯特劳哈尔数为St=0.3~0.4,在鱼群具有最佳推进效能所对应的斯特劳哈尔数St=0.2~0.4范围内;在大部分的卡门涡街中,细丝穿梭于涡核之间往复运动,这种运动模式与游鱼躲在障碍物下游的行为相似;在含有涡带的涡街中,细丝会延长涡街中原有的涡带,且细丝被两侧涡带夹在中间上下摆动,上游扑翼的尾涡脱落被延迟在细丝尾部脱落,此时上游扑翼的阻力减小;在密集的卡门涡街(相邻涡核间距小)中,细丝只能产生微弱振动(振幅小于细丝长度5%),弹性细丝没有涡街脱落,流场中原有涡街被破坏,此时上游扑翼阻力明显减小(最高减阻达14%)。本文构建了倾斜布置(后者位于前者斜后方)和三角形布置的多个摆动水翼模型,数值分析了纵向和侧向间距(D_x、Dy)对鱼群推进特性及流场结构的影响,得到了总体推进效能最佳的队形。研究表明,总体推进效能最优的队形布置方式与个体的运动有关:两条鱼同步运动时,D_x=0.5且Dy=0.3的倾斜队形具有最佳的总体推进效率;三条鱼同步运动时,D_x=0.75的倒三角队形具有最佳总体推进效能;异步运动时,并列队形(D_x=0.0)具有最佳的推进效能。另外,当纵向间距小于个体身长时,后面游鱼的推力和推进效率优于前面游鱼;当纵向间距大于个体身长时,前面游鱼的推力和推进效率会优于后面游鱼。本文构建了并列布置的双扑翼模型,数值研究了双翼对称振动的振幅A、频率f、间距L和攻角α等参数对其推进特性和流场结构的影响。研究表明,在相同的参数空间下,相对于单翼振动,双翼对称振动中单个机翼会产生更大的推力、更强的涡和射流,具有更好的推进性能,并且振幅或频率越大,间距越小,双翼的推进优势越明显。这是因为对称振动的双翼能够产生更大的上下翼面压力差,进而产生更大的推力和推进效率。此外,当斯特劳哈尔数St≥1.0时(St=fA/UL),双翼会产生混乱的尾涡结构,这是因为出现了尾涡倒吸现象,即已经在合拢运动末期脱落的漩涡会在打开运动中被吸入双翼间,并与新形成的漩涡融合,从而产生了混乱的涡街结构。本文利用浸入边界方法,数值模拟了鱼群和鸟群迁徙运动中的不可压粘性流动,发现了弹性细丝的运动与周围流场之间的相互作用关系,得到了队形参数和运动参数与群体推进效能之间的关系,并根据流场结构的变化解释了个体之间的流—固耦合作用机理,为进一步理解鱼群和鸟群迁徙运动中的流体力学机理提供了一些参考。
[Abstract]:The migration of fish or birds in regular formation is a common phenomenon in nature. Exploring the principle of hydrodynamics is not only helpful to understand the inherent mechanism of the movement of fish and birds, but also beneficial to the structural and formation optimization design of bionic vehicles and underwater vehicles. In this paper, a lateral free elastic filament model is proposed and placed downstream of flapping wing. The motion characteristics and flow field structures of filaments in different vortex streets (Carmen vortex street, straight vortex street and anti-Carmen vortex street) are investigated. Four motion modes are found. Formula 1: The filament vibrates in the outer side of the vortex street, the filament shuttles between the vortex cores, the filament is clamped between the vortex bands, and the filament vibrates in the outer side of the vortex street. The drag in vortex street corresponds to the Straughal number St = 0.3-0.4 for this motion pattern, and to the Straughal number St = 0.2-0.4 for the fish with the best propulsion efficiency; in most Karmen vortex streets, the filaments move back and forth between the vortex cores, which is related to the behavior of swimming fish hiding downstream of obstacles. Similarly, in the vortex street with vortex bands, the filament lengthens the original vortex band in the vortex street, and the filament is swinging up and down between the two sides of the vortex bands. The wake shedding of the upstream flapping wing is delayed at the tail of the filament, and the drag of the upstream flapping wing decreases. In the dense Karman vortex street (the distance between adjacent vortex cores is small), the filament can only produce weak vibration. The amplitude is less than 5% of the filament length, the elastic filament has no vortex shedding, and the original vortex street is destroyed in the flow field. At this time, the resistance of the flapping wing on the upstream reaches a significant reduction (up to 14%). The results show that the optimal configuration of the overall propulsion efficiency is related to the individual motion: the tilted formation with D_x=0.5 and Dy=0.3 has the best overall propulsion efficiency when two fish move synchronously, and the inverted three with D_x=0.75 when three fish move synchronously. In addition, when the longitudinal spacing is less than the individual length, the thrust and propulsion efficiency of the back swimming fish is better than that of the front swimming fish; when the longitudinal spacing is greater than the individual length, the thrust and propulsion efficiency of the front swimming fish will be better than that of the back swimming fish. In this paper, a two-flapping-wing model with parallel arrangement is constructed, and the effects of amplitude A, frequency f, spacing L and angle of attack on the propulsion characteristics and flow field structure are numerically studied. The results show that, compared with the single wing vibration, the single wing in the symmetrical vibration of the two wings produces greater thrust and stronger flow field structure under the same parameter space. Vortex and jet have better propulsion performance, and the larger the amplitude or frequency, the smaller the spacing, the more obvious the advancing advantage of the two wings. This is because the symmetrical vibration of the two wings can produce greater pressure difference between the upper and lower wing surface, thus resulting in greater thrust and propulsion efficiency. The chaotic vortex structure is due to the backward suction phenomenon, that is, the vortex which has fallen off at the end of the closure movement will be sucked between the wings in the open motion and merged with the newly formed vortex, resulting in the chaotic vortex street structure. In compressible viscous flow, the interaction between the motion of elastic filaments and the flow field around them is discovered, and the relationship between the formation parameters and the motion parameters and the group propulsion efficiency is obtained. The fluid-solid coupling mechanism between individuals is explained according to the variation of the flow field structure, so as to further understand the migration of fish and birds. The mechanism of fluid mechanics provides some references.
【学位授予单位】：南昌航空大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：V211

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