動態地面效應之近似方法應用於仿生水下航行器之研究

Abstract

在自然界中，動態地面效應能使魚類在靠近固體邊界（例如海洋底部或水槽壁面）游動的情況下，提高其推進效率並改善其流體動力性能。因此，動態地面效應為模仿魚類所設計之仿生型自主式水下載具(BAUV)，提供了設計靈感來源。然而，要設計一個可以充分利用動態地面效應的BAUV，首先必須了解動態地面效應對於提升魚類游動效能之機制。因此本文提出一套理論方法，藉以描述和量化動態地面效應對魚類游動時流體動力之影響，並探討此方法對於未來設計高性能之BAUV的潛在應用。 本文提出二種基於二維勢流理論之近似方法。首先為了獲得初步的概念模型，我們以一顆振動偶極子在平面壁旁移動之模型，來模擬和比較一受到動態地面效應影響之魚類。此方法用於探索鄰近壁面時，偶極子振盪所產生的壓力場會受到何種程度的影響。而將此模型所得出之模擬和實驗數據，與利用一BAUV貼近壁面游動所得之實驗數據進行比較，以確認偶極子模型與BAUV所造成之流場壓力變化趨勢是否一致。 本文第二部分提出另一種近似方法，以描述魚類受到動態地面效應影響時之流體動力變化。此方法擴展了T. Y. Wu 在二維勢流中以一柔性波動板描述一扁平魚之流體動力模型，額外在流場中加入平面壁之影響。從本方法可獲得波動板在鄰近壁面游動時所產生之推力、輸入功率，動能損失和推進效率之解析式，進而理解動態地面效應如何影響與提升魚類游動性能。模擬結果表示在動態地面效應的影響下，波動板可以較少的輸入功率產生幾乎同等之推力，從而顯示波動板游動效能獲得提升，而提升程度取決於自身之運動條件。此方法亦通過在臺灣大學工科海洋所的海工水槽中，以機器魚實驗進行驗證。實驗結果顯示機器魚在靠近水槽底部游動時，推進所需輸入之能量減少而推進效率亦獲提升。此現象與使用波動板模型所預測之模擬結果相似。

In nature the dynamic ground effect allows fish to improve their hydrodynamic performance by increasing their propulsive efficiency when swimming in close proximity to a substrate such as the bottom of the ocean or the walls of water tanks. Therefore the dynamic ground effect provides a source of inspiration for the development of BUV’s (Biomimetic Underwater Vehicles). However for designing a BUV that can fully benefit from the dynamic ground effect it is necessary to know how the dynamic ground effect enhances the swimming of fish. Therefore this thesis presents approximation methods for understanding and quantifying how the dynamic ground effect enhances the hydrodynamic performance of swimming fish and explores potential applications of these methods for the design of underwater vehicles that could benefit from the dynamic ground effect. The approximation methods are based on two-dimensional potential flow theory. These methods are presented in two parts, for the first part we develop a toy model which uses a moving dipole swimming next to a planar wall to represent a fish swimming with dynamic ground effect. This dipole approach is used to explore how the presence of a nearby wall influences the pressure field produced from the dipole oscillations. Then this toy model is tested experimentally with a BAUV in order to see if the dipole approach is a suitable theory to represent in a simplified way the motion of a robotic fish; and also to confirm if there is an increase in the magnitude of the pressure field which is generated from the perturbations in the flow field caused by the swimming motion of the BAUV when swimming near to the wall, as was predicted by the toy model. The second part consists of another approximation model to describe the hydrodynamics involved when a fish is swimming with dynamic ground effect. It is an extension of T. Y. Wu’s theory for the hydrodynamics of a flat fish swimming in a two-dimensional potential flow, the flat fish is represented by a flexible flat plate, and this extension consists in the addition of a planar wall into the flow field. From this model general expressions are obtained for the thrust, power input, kinetic energy loss and propulsive efficiency of the swimming plate. These expressions allows for an understanding of how the dynamic ground effect influences the swimming of fish such that their hydrodynamic performance is enhanced. It was found that the dynamic ground effect improves the swimming performance of the plate by increasing its efficiency of propulsion due to a reduction in the power input while maintaining an almost constant thrust, this degree of hydrodynamic performance improvement depends on the kinematic conditions of the plate. Experimentally, the approximation model was tested with a robotic fish swimming near the substrate of NTU ESOE’s water tank. From the experiments it was found that the swimming performance of the robotic fish improved when the robotic fish was swimming in close proximity to the substrate of the water tank. The enhancement in the swimming performance of the robotic fish was obtained due to a power input reduction and a propulsive efficiency increase which is in agreement with the flexible plate model approximation.