Using spanwise flexibility of caudal fin to improve swimming performance for small fishlike robots

This paper examines the beneficial effects of the spanwise flexibility of the caudal fin for the improvement of the swimming performance for small fishlike robots. A virtual swimmer is adopted for controlled numerical experiments by varying the spanwise flexible trajectories and the spanwise flexible size of the caudal fin while keeping the body kinematics fixed. 3-D Navier-Stokes equations are used to compute the viscous flow over the robot. Elliptical, parabolic and hyperbola trajectories are chosen to describe the spanwise flexible profile of the caudal fin. According to the sign（positive or negative） of the phase difference of the swinging motion, the spanwise flexibility can be divided into the fin surface of ＂bow＂ and the fin surface of ＂scoop＂. It is observed that for both the fin surface of ＂bow＂ and the fin surface of ＂scoop＂, the spanwise elliptical trajectory has the optimal swimming velocity, thrust, lateral force, and efficiency. With comparisons, using the flexible caudal fin with the fin surface of ＂bow＂, the lateral force and the power consumption can be reduced effectively and the swimming stability can be increased while reducing little the swimming velocity and thrust. Meanwhile, using the flexible caudal fin with the fin surface of ＂scoop＂ can greatly improve the swimming velocity, thrust, and efficiency while increasing part of the lateral force and the power consumption. Three-dimensional flow structures clearly indicate the evolution process around the swimming robot. It is suggested that the fish, the dolphin, and other aquatic animals may benefit their hydrodynamic performance by the spanwise flexibility of the caudal fin.

Divergence and constraint in the thermal sensitivity of aquatic insect swimming performance

Environmental temperature variation may play a significant role in the adaptive evolutionary divergence of ectotherm thermal performance curves(TPCs).However,divergence in TPCs may also be constrained due to various causes.Here,we measured TPCs for swimming velocity of temperate and tropical mayflies(Family:Baetidae)and their stonefly predators(Family:Perlidae)from different elevations.We predicted that differences in seasonal climatic regimes would drive divergence in TPCs between temperate and tropical species.Stable tropical temperatures should favor the evolution of"specialists"that perform well across a narrow range of temperatures.Seasonally,variable temperatures in temperate zones,however,should favor"generalists"that perform well across a broad range of temperatures.In phylogenetically paired comparisons of mayflies and stoneflies,swimming speed was generally unaffected by experimental temperature and did not differ among populations between latitudes,suggesting a maintenance of performance breadth across elevation and latitude.An exception was found between temperate and tropical mayflies at low elevation where climatic differences between latitudes are large.In addition,TPCs did not differ between mayflies and their stonefly predators,except at tropical low elevation.Our results indicate that divergence in TPCs may be con strai ned in aquatic in sects except under the most differe nt ther・mal regimes,perhaps because of trade-offs that reduce thermal sensitivity and increase performance breadth.

SWIMMING PERFORMANCE ANALYSIS FOR ANGUILLIFORM PROPULSION

This paper investigates the swimming performance of fish undulatory motion which is the basic form in locomotion of aquatical animal from a hydrodynamics point of view.In particular,the propulsive characteristics is discussed.The three-dimensional potential flow over a model rectangular flexible plate performing the motion which consists of a progressive wave of a given wave length and phase velocity along the chord is treated.Vortex ring method is used to calculate the thrust,the power required and the hydrodynamic propulsive efficiency,etc.The dependence of these energetics on certain physical parameters,such as the aspect ratio,the reduced frequency and the wave number,is discussed.It is found that as the wave tength gets close to the body length,propulsive performance is no longer sensitive to the aspect ratio.Some qualitative explanation of the fish swimming phenomena is also given.

Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish

Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish,to increase the swimming efficiency and the swimming speed and control the motion direction more easily,is investigated by combining optimization algorithms,unsteady computational fluid dynamics and dynamic control in this study.The 3D computational fluid dynamics package contains the immersed boundary method,volume of fluid method,the adaptive multi-grid finite volume method and the control strategy of fish swimming.Through shape optimizations of various swimming speeds,the results show that the optimal caudal fins of different swimming modes are not exactly the same shape.However,the optimal fish of high swimming speed,whose caudal fin shape is similar to the crescent,also have higher efficiency and better maneuverability than the other optimal bionic fish at low and moderate swimming speeds.Finally,the mechanisms of vorticity creation of different optimal bionic fish are studied by using boundary vorticity-flux theory,and three-dimensional wake structures of self-propelled swimming of these fish are comparatively analyzed.The study of vortex dynamics reveals the nature of efficient swimming of the 3D bionic fish with the lunate caudal fin.

The evolution of phenotypic plasticity in fish swimming

Fish have a remarkable amount of variation in their swimming performance, from within species dif- ferences to diversity among major taxonomic groups. Fish swimming is a complex, integrative phenotype and has the ability to plastically respond to a myriad of environmental changes. The plasticity of fish swimming has been observed on whole-organismal traits such as burst speed or critical swimming speed, as well as underlying phenotypes such as muscle fiber types, kinematics, cardiovascular system, and neuronal processes. Whether the plastic responses of fish swimming are beneficial seems to depend on the environmental variable that is changing. For example, because of the effects of temperature on biochemical processes, alterations of fish swimming in response to tem- perature do not seem to be beneficial. In contrast, changes in fish swimming in response to variation in flow may benefit the fish to maintain position in the water column. In this paper, we examine how this plasticity in fish swimming might evolve, focusing on environmental variables that have received the most attention： temperature, habitat, dissolved oxygen, and carbon dioxide variation. Using examples from previous research, we highlight many of the ways fish swimming can plastic- ally respond to environmental variation and discuss potential avenues of future research aimed at understanding how plasticity of fish swimming might evolve. We consider the direct and indirect ef- fects of environmental variation on swimming performance, including changes in swimming kine- matics and suborganismal traits thought to predict swimming performance. We also discuss the role of the evolution of plasticity in shaping macroevolutionary patterns of diversity in fish swimming.

Topology Optimization of the Caudal Fin of the Three-Dimensional Self-Propelled Swimming Fish

Based on the boundary vorticity-flux theory,topology optimization of the caudal fin of the three-dimensional self-propelled swimming fish is investigated by combining unsteady computational fluid dynamics with moving boundary and topology optimization algorithms in this study.The objective functional of topology optimization is the function of swimming efficiency,swimming speed and motion direction control.The optimal caudal fin,whose topology is different from that of the natural fish caudal fin,make the 3D bionic fish achieve higher swimming efficiency,faster swimming speed and better maneuverability.The boundary vorticity-flux on the body surface of the 3D fish before and after optimization reveals the mechanism of high performance swimming of the topology optimization bionic fish.The comparative analysis between the swimming performance of the 3D topology optimization bionic fish and the 3D lunate tail bionic fish is also carried out,and the wake structures of two types of bionic fish show the physical nature that the swimming performance of the 3D topology optimization bionic fish is significantly better than the 3D lunate tail bionic fish.

Design,fabrication,and realisation of a robotic fish actuated by dielectric elastomer with a passive fin

Robotic fish actuated by smart materials has attracted extensive attention and has been widely used in many applications.In this study,a robotic fish actuated by dielectric elastomer(DE)films is proposed.The tensile behaviours of DE film VHB4905 are studied,and the Ogden constitutive equation is employed to describe the stress‐strain behaviour of the DE film.The fabrication processes of the robotic fish,including prestretching treatment of the DE films,electrode coating with carbon paste,and waterproof treatment,are illustrated in detail.The dynamic response of the fabricated DE actuators under different excitation voltages is tested based on the experimental setup.Experimental results show that the first‐order natural frequencies of the obtained DE actuator in air is 4.05 Hz.Finally,the swimming performances of the proposed robotic fish at different driving levels are demonstrated,and it achieves an average swimming speed of 20.38 mm/s,with a driving voltage of 5kV at 0.8 Hz.

Numerical study on the effects of a semi-free and non-uniform flexible filament in different vortex streets

The variable flexibility of a fish body is believed to play a significant role in improving swimming performance.To explore the effect of non-uniform flexibility on the motion performance of fish under biologically relevant conditions,we set up three different flexible distribution modes for a semi-free filament and compared the motion performance of different flexible distribution modes through numerical simulations.The filament is located in the wake of the front flapping foil;it can swing adaptively in the lateral direction according to the flow situation of the surrounding fluid and finally reach a stable position.The results show that the motion state of the filament will alter with a change in the flexibility of the filament,from moving in the vortex street to moving on the side of the vortex street.In the Bénard-von Kármán(BvK)vortex streets,the drag coefficient of the filament increases as the flexibility of the filament increases,and the value of the drag coefficient is at a minimum when the flexibility of the filament increases linearly along the length of the filament.Further investigation indicates that at 85%–90%of the filament length(starting from the leading edge),the flexibility of the filament begins to increase significantly,and the filament can obtain its best propulsion performance.The results of this work provide new insights into the role of non-uniform flexibility during the process of fish movement and provide a valuable reference for the design of bionic underwater vehicles.

Lack of functional link in the tadpole morphology induced by predators

Most studies of predator-induced plasticity have focused on documenting how prey species re- spond to predators by modifying phenotypic traits and how traits correlate with fitness. We have previously shown that Pleurodema thaul tadpoles exposed to the dragonfly Rhionaeschna varie- gata responded strongly by showing morphological changes, less activity, and better survival than non-exposed tadpoles. Here, we tested whether there is a functional link between morphological plasticity and increased survival in the presence of predators. Tadpoles that experienced predation risk were smaller, less developed, and much less active than tadpoles without this experience. Burst speed did not correlate significantly with morphological changes and predator-induced deeper tails did not act as a lure to divert predator strikes away from the head. Although we have previously found that tadpoles with predator-induced morphology survive better under a direct predator threat, our results on the functional link between morphology and fitness are not conclu- sive. Our results suggest that in P. thaultadpoles （1） burst speed is not important to evade preda- tors, （2） those exposed to predators reduce their activity, and （3） morphological changes do not divert predator attacks away from areas that compromise tadpole survivalEE. Our results show that morphological changes in P. thaul tadpoles do not explain burst speed or lure attraction, al- though there was a clear reduction of activity, which itself reduces predation. We propose that changes in tadpole activity could be further analyzed from another perspective, with morphological change as an indirect product of behavior mediated by physiological mechanisms.