本文以數值方法研究牛鼻魟魚(Rhinoptera bonasus)游動之拍撲動作模式對於推進力與三維流場結構之影響。全文分為三大部分,第一部分為在週期推力方向合力為零的條件下建立完整魟魚數值模型,揭示週期定速游動之魟魚拍撲流場結構。第二部分闡述水中拍撲運動之翼尖渦流之特性與其對推力表現的影響,藉由調控順流向翼拍撲波動振幅與翼展向翼拍撲振幅,模擬不同的魟魚拍撲動作。翼尖渦流之傳遞途徑直接受到胸鰭表面波動運動軌跡的影響,在拍撲頻率與翼展向振幅不變的情況下,翼前緣的拍撲波動振幅降低,亦即雙鰭抓水動作變小順流向質量傳遞比例降低使側向速度影響加大,翼尖渦流所夾帶的低壓區被側向速度帶到魚體正後方,增加的魚體前後壓差直接降低推力表現。在翼尖渦流向後側靠攏後,推力降幅達到35%。因此微調翼前緣振幅為一種在不改變史卓荷數的情況下調控推力的策略。 第三部分為功率與魟魚節能方法之討論,在魟魚穩態拍撲前進時,不同動作策略下翼前緣渦流吸附翼面上未流逸的時間點不同,造成低壓區的存在強度與存在對擺動的表現影響不同。本研究發現魟魚會利用下衝程造成的翼前緣渦流低壓來節省上拍的能量,此翼前緣渦流吸附的節能狀況與昆蟲在翼面上的翼前緣渦流低壓區有異曲同工之妙,可增加升力節省拍動能量,但魟魚之擺動節能對拍撲運動的貢獻在於推力上的表現。 魟魚有別於普通用尾鰭擺尾推進(body and caudal fin, BCF)的魚類,使用寬闊身體兩側之一對胸鰭(pectoral fin)推進,屬於中央鰭與對鰭推進模式(median and paired fin, MPF)。因此,在構造上魟魚寬闊的身體不但適合酬載貨物,更有海空兩棲拍撲載具的發展空間。在空中拍撲翼能使用提供升力的動作軌跡,而在水中能使用魟魚的推進模式來前進。前人之研究多注重於擺尾式推進與其三維流場、空氣中的拍撲實驗與數值模擬、以及單片固定翼(有限翼或無限翼)的流場分析,鮮少有水中拍撲推進的翼與身體交互作用之流場分析,在此黏滯度下的魟魚穩態游動流場未呈現,其動作軌跡與推進力之關係也尚未明朗。因此,本文利用數值方法,建立合理的物理模型並提出假設,調整游動速度以及動作軌跡以求完整流場與推力係數。魟魚拍撲翼之流場可視化、調控推力與節能之策略之原理可應用於未來水下載具,為其提供更優良的操控性。
This work presents a numerical three-dimension wake-structure of a quasi-steady swimming batoid fish(cownose ray, Rhinoptera bonasus) with constant swimming speed. The three-dimensional, unsteady, viscous and incompressible Navier-Stokes equations were solved with a finite-volume method. A realistic cownose ray body is modeled. Under the condition of zero net forces for an undulation cycle, the performance of thrust coefficient and the transformation of three-dimension flow pattern are reported through alternating the flapping modes of the pectoral fin. Alteration of the body-undulation kinematics was established by varying the undulation amplitudes in both stream-wise and span-wise aspects. Thrust production is found to decrease significantly by 35% when flapping amplitude of leading edge is diminished without changing the span-wise undulation amplitude, fin-tip flapping amplitude and flapping frequency. The fin-tip vortices were found capable of pushing the leading edge vortices back and forth in the span-wise aspect. As the size of the leading edge vortices is diminished by reducing the stream-wise undulation amplitude along the leading edge, the wake-structure formation behind the fish body enforced the pressure difference between snout and tail, this effect further caused the variation of thrust production. This study contained wake-structure visualization and strategy of adjusting thrust performance, it could provide a new maneuvering concept to future underwater vehicle.