【摘要】：魚群或鳥群保持規(guī)律隊(duì)形的遷徙運(yùn)動(dòng)是自然界中常見的現(xiàn)象,探究其中的流體力學(xué)原理,不僅有利于進(jìn)一步了解魚群和鳥類集群運(yùn)動(dòng)的內(nèi)在機(jī)制,而且有利于仿生飛行器和水下航行器的結(jié)構(gòu)優(yōu)化設(shè)計(jì)和隊(duì)形優(yōu)化設(shè)計(jì)。本文利用浸入邊界方法,數(shù)值分析了魚群或鳥群遷徙運(yùn)動(dòng)中的流體力學(xué)機(jī)理。本文提出了側(cè)向自由的彈性細(xì)絲模型,將其置于撲翼下游,探究了細(xì)絲在不同渦街(卡門渦街、平直渦街、反卡門渦街)中的運(yùn)動(dòng)特性及其流場結(jié)構(gòu),發(fā)現(xiàn)了細(xì)絲存在四種運(yùn)動(dòng)模式:細(xì)絲在渦街的外側(cè)往復(fù)振動(dòng),細(xì)絲穿梭于渦核之間往復(fù)振動(dòng),細(xì)絲被夾在渦帶之間往復(fù)振動(dòng),細(xì)絲微弱振動(dòng)。在反卡門渦街中,細(xì)絲總是會(huì)在渦街的外側(cè)振動(dòng),這是因?yàn)榱鲌鰝?cè)向誘導(dǎo)速度太大所致,此時(shí),細(xì)絲受到的阻力明顯小于細(xì)絲在渦街里面時(shí)的阻力,這種運(yùn)動(dòng)模式對應(yīng)的斯特勞哈爾數(shù)為St=0.3~0.4,在魚群具有最佳推進(jìn)效能所對應(yīng)的斯特勞哈爾數(shù)St=0.2~0.4范圍內(nèi);在大部分的卡門渦街中,細(xì)絲穿梭于渦核之間往復(fù)運(yùn)動(dòng),這種運(yùn)動(dòng)模式與游魚躲在障礙物下游的行為相似;在含有渦帶的渦街中,細(xì)絲會(huì)延長渦街中原有的渦帶,且細(xì)絲被兩側(cè)渦帶夾在中間上下擺動(dòng),上游撲翼的尾渦脫落被延遲在細(xì)絲尾部脫落,此時(shí)上游撲翼的阻力減小;在密集的卡門渦街(相鄰渦核間距小)中,細(xì)絲只能產(chǎn)生微弱振動(dòng)(振幅小于細(xì)絲長度5%),彈性細(xì)絲沒有渦街脫落,流場中原有渦街被破壞,此時(shí)上游撲翼阻力明顯減小(最高減阻達(dá)14%)。本文構(gòu)建了傾斜布置(后者位于前者斜后方)和三角形布置的多個(gè)擺動(dòng)水翼模型,數(shù)值分析了縱向和側(cè)向間距(D_x、Dy)對魚群推進(jìn)特性及流場結(jié)構(gòu)的影響,得到了總體推進(jìn)效能最佳的隊(duì)形。研究表明,總體推進(jìn)效能最優(yōu)的隊(duì)形布置方式與個(gè)體的運(yùn)動(dòng)有關(guān):兩條魚同步運(yùn)動(dòng)時(shí),D_x=0.5且Dy=0.3的傾斜隊(duì)形具有最佳的總體推進(jìn)效率;三條魚同步運(yùn)動(dòng)時(shí),D_x=0.75的倒三角隊(duì)形具有最佳總體推進(jìn)效能;異步運(yùn)動(dòng)時(shí),并列隊(duì)形(D_x=0.0)具有最佳的推進(jìn)效能。另外,當(dāng)縱向間距小于個(gè)體身長時(shí),后面游魚的推力和推進(jìn)效率優(yōu)于前面游魚;當(dāng)縱向間距大于個(gè)體身長時(shí),前面游魚的推力和推進(jìn)效率會(huì)優(yōu)于后面游魚。本文構(gòu)建了并列布置的雙撲翼模型,數(shù)值研究了雙翼對稱振動(dòng)的振幅A、頻率f、間距L和攻角α等參數(shù)對其推進(jìn)特性和流場結(jié)構(gòu)的影響。研究表明,在相同的參數(shù)空間下,相對于單翼振動(dòng),雙翼對稱振動(dòng)中單個(gè)機(jī)翼會(huì)產(chǎn)生更大的推力、更強(qiáng)的渦和射流,具有更好的推進(jìn)性能,并且振幅或頻率越大,間距越小,雙翼的推進(jìn)優(yōu)勢越明顯。這是因?yàn)閷ΨQ振動(dòng)的雙翼能夠產(chǎn)生更大的上下翼面壓力差,進(jìn)而產(chǎn)生更大的推力和推進(jìn)效率。此外,當(dāng)斯特勞哈爾數(shù)St≥1.0時(shí)(St=fA/UL),雙翼會(huì)產(chǎn)生混亂的尾渦結(jié)構(gòu),這是因?yàn)槌霈F(xiàn)了尾渦倒吸現(xiàn)象,即已經(jīng)在合攏運(yùn)動(dòng)末期脫落的漩渦會(huì)在打開運(yùn)動(dòng)中被吸入雙翼間,并與新形成的漩渦融合,從而產(chǎn)生了混亂的渦街結(jié)構(gòu)。本文利用浸入邊界方法,數(shù)值模擬了魚群和鳥群遷徙運(yùn)動(dòng)中的不可壓粘性流動(dòng),發(fā)現(xiàn)了彈性細(xì)絲的運(yùn)動(dòng)與周圍流場之間的相互作用關(guān)系,得到了隊(duì)形參數(shù)和運(yùn)動(dòng)參數(shù)與群體推進(jìn)效能之間的關(guān)系,并根據(jù)流場結(jié)構(gòu)的變化解釋了個(gè)體之間的流—固耦合作用機(jī)理,為進(jìn)一步理解魚群和鳥群遷徙運(yùn)動(dòng)中的流體力學(xué)機(jī)理提供了一些參考。
[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.
【學(xué)位授予單位】：南昌航空大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：V211

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