| Eels and sea snakes,known for long migration and high endurance cruising,can be a source of bio-inspiration for underwater robots.These kinds of slender swimmers,having high swimming efficiency,strong maneuverability,and high stability,carry many favorable characteristics of that are desirable for the snake-like robot,and thus,these creatures may be used to inspire a new direction for the underwater robot.However,the mechanisms of the efficient propulsion of slender swimmers,their high stability,and excellent maneuverability are still unclear.Therefore,the swimming performance of current underwater snake robot is still far from the performance of its biological inspiration,leaving the technology not yet ready for practical application.The fins play a crucial role in fish swimming,which provides an essential reference for designing underwater robots.There are many kinds of fins,such as pectoral fin,dorsal fin,caudal fin,and each type of fin plays an essential role in swimming performance.Optimizing the design of underwater snake-like robot by understanding the mechanism of each fin’s impact on the propulsion performance is key for the next generation robot design.In this thesis,the fin design of the snake-like robot is mainly inspired by fish fins.The impacts of fin shapes,tandem patterns,and motion parameters on the swimming performance of snake-like robots were systematically studied through robotic prototype experiments and computational fluid dynamic simulations,and the influence mechanism of fins on the swimming performance of robots has been revealed.Firstly,we take inspiration from the caudal fin,which is vital for propulsion in fish swimming,and to study the impact of caudal fin shape on the swimming performance of the snake-like robot.Based on the morphological analysis to obtain the critical shape variables of the caudal fin,and by systematically varying the caudal fin geometry by changing the leading edge and trailing edge angles,25 kinds of typical caudal fins have been considered.This study employs computational and experimental techniques to explore the impact of caudal fin geometry design on the swimming performance of a snake-like robot.The results show that the snake-like robot achieves the best swimming speed and efficiency when the caudal fin with an 85° leading edge angle and a 120°convex trailing edge angle.In addition,the numerical simulation results show that the wake structure formed by the snake-like robot consists of two parallel disconnected verse van Karman vortex rings.When the leading edge angle is slightly below 90°,the body vortex could well merge and interacts with the caudal fin’s vortex.This interaction contributes strongly to wake construction and significantly enhances swimming speed and efficiency.Secondly,taking inspiration from the dual dorsal fins structure from the lamprey,an underwater snake-like robot with dual dorsal fins was designed.We performed an experimental and computational study to understand the impact of the dual dorsal fin design on an underwater snake-like robot’s swimming speed and efficiency.Four spacings ranging from 0 to 3 link lengths were tested at three different amplitudes(15°,30°,and 45°)and angular velocities(2,3,and 4 rad/s).It is found that when the space between the dorsal fins is one joint length,the swimming speed and efficiency of the snake-like robot are improved most significantly.When the robot swam at the amplitude of 30°,compared with the dual dorsal fin without space,the dual dorsal fin with one space increased the swimming speed of the snake-like robot by 2.14 times.Meanwhile,the changes in the spacing would significantly affect the interactions of the vortices shedding from the fore and hind fins.This interaction,in turn,affects the vortex formation on the rear fin,resulting in changes in wake strength and formation direction,which would significantly affect the swimming speed and efficiency of the underwater snake-like robot.Thirdly,inspired by the special function of fishes’ pectoral fin in stabilizing and steering,the pectoral fin of the snake-like has been designed.The impact of initial pectoral fin configuration,motion pattern,and motion parameters on snake-like robots’ swimming speed,efficiency,maneuverability,and stability was studied,and its mechanism was also analyzed.It was found that the initial configuration of parallel pectoral fins can effectively improve the swimming speed and efficiency of the robot.On the other hand,the lateral force shows that this configuration can also reduce the amplitude of the lateral force generated by the robot,which could reduce the center of mass oscillation,thus improving the stability of the robot.Besides,the left and right turn of the robot can also be easily realized by changing the initial configuration of the pectoral fin.In addition,the effects of three typical pectoral fin motion modes and motion frequencies on the swimming performance of the robot were studied.It was found that the rowing motion could significantly improve the swimming performance of the snake-like robot at high frequency,and the feathering motion and flapping motion would reduce the swimming performance of the snake-like robot regardless of the frequency.Finally,we further explore the impact of multi-fin configuration on the snake-like robot’s swimming performance by coupling the previous study of caudal,dorsal,and pectoral fins.It was found that the specific multi-fin configuration can significantly improve the swimming performance of the snake-like robot.For anguilliform gait,the swimming speed of the robot with dorsal fin rearward multi-fin configuration increased by 34% compared with that of the snake-like robot with dorsal fin forward multi-fin configuration.But for serpentine gait,when the snake robot is swimming at small amplitude,the dorsal fin forward configuration can help the snake-like robot swim faster and more efficiently.Analyzing the flow field of the snake-like robot found that the multi-fin system plays a role in passing the wake.Specific fin configuration would be conducive to the flow field interaction,eventually improving the snake-like robot’s swimming speed and efficiency.In this thesis,the swimming performance optimization of the snake-like robot by caudal fin morphology,dual dorsal fin tandem pattern,pectoral fin initial configuration,and the multi-fin interaction have been studied by robotic prototype experiment and computational fluid dynamic simulation.The vortex ring theory has been used to analyze the corresponding mechanism.The multi-fin underwater snake-like robot with high speed,high efficiency,high stability,and increased mobility has been designed,making it a more practical application prospect.These studies may also help explain how fishes control their fins and modulate their multi-fin configuration based on the requirements. |
PDF Full Text Request