Self-propulsion of flapping bodies in viscous fluids: Recent advances and perspectives

Flapping-powered propulsion is used by many animals to locomote through air or water. Here we review recent experimental and numerical studies on self-propelled mechanical systems powered by a flapping motion. These studies improve our understanding of the mutual interaction between actively flapping bodies and surrounding fluids. The results obtained in these works provide not only new insights into biolocomotion but also useful information for the biomimeticdesign of artificial flyers and swimmers.</div

The Frequency-Amplitude Response of a Class of Nonholonomic Systems

Nonholonomic systems have been investigated as models for locomotion due to the similarity between nonholonomic constraints and the no-slip condition of wheels or the Kutta condition of swimmers\u27 tails. The focus of most past research has been on kinematic nonholonomic systems. However, an increasing body of research points to benefits that dynamic components of these systems, such as underactuated degrees of freedom, can provide. This work investigates the effect of adding stiff degrees of freedom to the Chaplygin sleigh, a classical nonholonomic system that has been used in the past as a swimming model. Observed resonance behavior is shown to greatly increase net velocity of the sleigh at certain ranges of forcing frequency and amplitude. A change to the formulation of the nonholonomic constraint is also considered where the constraint point is defined as fixed with respect to the flow field around the body, which is defined by potential flow theory. The reduced effective inertia resulting from this change improves performance at high forcing frequency, but reduces performance for slower forcing

Design and application of a cellular, piezoelectric, artificial muscle actuator for biorobotic systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 219-227).One of the foremost challenges in robotics is the development of muscle-like actuators that have the capability to reproduce the smooth motions observed in animals. Biological muscles have a unique cellular structure that departs from traditional electromechanical actuators in several ways. A muscle consists of a vast number of muscle fibers and, more fundamentally, sarcomeres that act as cellular units or building blocks. A muscle's output force and displacement are the aggregate effect of the individual building blocks. Thus, without using gearing or transmissions, muscles can be tailored to a range of loads, satisfying specific force and displacement requirements. These natural actuators are desirable for biorobotic applications, but many of their characteristics have been difficult to reproduce artificially. This thesis develops and applies a new artificial muscle actuator based on piezoelectric technology. The essential approach is to use a subdivided, cellular architecture inspired by natural muscle. The primary contributions of this work stem from three sequential aims. The first aim is to develop the operating principles and design of the actuator cellular units. The basic operating principle of the actuator involves nested flexural amplifiers applied to piezoelectric stacks thereby creating an output length strain commensurate with natural muscle. The second aim is to further improve performance of the actuator design by imparting tunable stiffness and resonance capabilities. This work demonstrates a previously unavailable level of tunability in both stiffness and resonance. The final aim is to showcase the capabilities of the actuator design by developing an underwater biorobotic fish system that utilizes the actuators for resonance-based locomotion. Each aspect of this thesis is supported by rigorous analysis and functional prototypes that augment broadly applicable design concepts.by Thomas William Secord.Ph.D

A numerical study of fin and jet propulsions involving fluid-structure interactions

Fish swimming is elegant and efficient, which inspires humans to learn from them to design high-performance artificial underwater vehicles. Research on aquatic locomotion has made extensive progress towards a better understanding of how aquatic animals control their flexible body and fin for propulsion. Although the structural flexibility and deformation of the body and fin are believed to be important features to achieve optimal swimming performance, studies on high-fidelity deformable body and fin with complex material behavior, such as non-uniform stiffness distributions, are rare. In this thesis, a fully coupled three-dimensional high-fidelity fluid-structure interaction (FSI) solver is developed to investigate the flow field evolution and propulsion performance of caudal fin and jet propulsion involving body and/or fin deformation. Within this FSI solver, the fluid is resolved by solving unsteady and viscous Navier-Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics are solved by a nonlinear finite element method. The coupling between the two solvers is achieved in a partitioned approach in which convergence check and sub-iteration are implemented to ensure numerical stability and accuracy. Validations are conducted by comparing the simulation results of classical benchmarks with previous data in the literature, and good agreements between them are obtained. The developed FSI solver is then applied to study the bio-inspired fin and jet propulsion involving body deformation. Specifically, the effect of non-uniform stiffness distributions of fish body and/or fin, key features of fish swimming which have been excluded in most previous studies, on the propulsive performance is first investigated. Simulation results of a sunfish-like caudal fin model and a tuna-inspired swimmer model both show that larger thrust and propulsion efficiency can be achieved by a non-uniform stiffness distribution (e.g., increased by 11.2% and 9.9%, respectively, for the sunfish-like model) compared with a uniform stiffness profile. Despite the improved propulsive e performance, a bionic variable fish body stiffness does not yield fish-like midline kinematics observed in real fish, suggesting that fish movement involves significant active control that cannot be replicated purely by passive deformations. Subsequent studies focus on the jet propulsion inspired by squid locomotion using the developed numerical solver. Simulation results of a two-dimensional inflation-deflation jet propulsion system, whose inflation is actuated by an added external force that mimics the muscle constriction of the mantle and deflation is caused by the release of elastic energy of the structure, suggest larger mean thrust production and higher efficiency in high Reynolds number scenarios compared with the cases in laminar flow. A unique symmetry-breaking instability in turbulent flow is found to stem from irregular internal body vortices, which cause symmetry breaking in the wake. Besides, a three-dimensional squid-like jet propulsion system in the presence of background flow is studied by prescribing the body deformation and jet velocity profiles. The effect of the background flow on the leading vortex ring formation and jet propulsion is investigated, and the thrust sources of the overall pulsed jet are revealed as well. Finally, FSI analysis on motion control of a self-propelled flexible swimmer in front of a cylinder utilizing proportional-derivative (PD) control is conducted. The amplitude of the actuation force, which is applied to the swimmer to bend it to produce thrust, is dynamically tuned by a feedback PD controller to instruct the swimmer to swim the desired distance from an initial position to a target location and then hold the station there. Despite the same swimming distance, a swimmer whose departure location is closer to the cylinder requires less energy consumption to reach the target and hold the position there.Fish swimming is elegant and efficient, which inspires humans to learn from them to design high-performance artificial underwater vehicles. Research on aquatic locomotion has made extensive progress towards a better understanding of how aquatic animals control their flexible body and fin for propulsion. Although the structural flexibility and deformation of the body and fin are believed to be important features to achieve optimal swimming performance, studies on high-fidelity deformable body and fin with complex material behavior, such as non-uniform stiffness distributions, are rare. In this thesis, a fully coupled three-dimensional high-fidelity fluid-structure interaction (FSI) solver is developed to investigate the flow field evolution and propulsion performance of caudal fin and jet propulsion involving body and/or fin deformation. Within this FSI solver, the fluid is resolved by solving unsteady and viscous Navier-Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics are solved by a nonlinear finite element method. The coupling between the two solvers is achieved in a partitioned approach in which convergence check and sub-iteration are implemented to ensure numerical stability and accuracy. Validations are conducted by comparing the simulation results of classical benchmarks with previous data in the literature, and good agreements between them are obtained. The developed FSI solver is then applied to study the bio-inspired fin and jet propulsion involving body deformation. Specifically, the effect of non-uniform stiffness distributions of fish body and/or fin, key features of fish swimming which have been excluded in most previous studies, on the propulsive performance is first investigated. Simulation results of a sunfish-like caudal fin model and a tuna-inspired swimmer model both show that larger thrust and propulsion efficiency can be achieved by a non-uniform stiffness distribution (e.g., increased by 11.2% and 9.9%, respectively, for the sunfish-like model) compared with a uniform stiffness profile. Despite the improved propulsive e performance, a bionic variable fish body stiffness does not yield fish-like midline kinematics observed in real fish, suggesting that fish movement involves significant active control that cannot be replicated purely by passive deformations. Subsequent studies focus on the jet propulsion inspired by squid locomotion using the developed numerical solver. Simulation results of a two-dimensional inflation-deflation jet propulsion system, whose inflation is actuated by an added external force that mimics the muscle constriction of the mantle and deflation is caused by the release of elastic energy of the structure, suggest larger mean thrust production and higher efficiency in high Reynolds number scenarios compared with the cases in laminar flow. A unique symmetry-breaking instability in turbulent flow is found to stem from irregular internal body vortices, which cause symmetry breaking in the wake. Besides, a three-dimensional squid-like jet propulsion system in the presence of background flow is studied by prescribing the body deformation and jet velocity profiles. The effect of the background flow on the leading vortex ring formation and jet propulsion is investigated, and the thrust sources of the overall pulsed jet are revealed as well. Finally, FSI analysis on motion control of a self-propelled flexible swimmer in front of a cylinder utilizing proportional-derivative (PD) control is conducted. The amplitude of the actuation force, which is applied to the swimmer to bend it to produce thrust, is dynamically tuned by a feedback PD controller to instruct the swimmer to swim the desired distance from an initial position to a target location and then hold the station there. Despite the same swimming distance, a swimmer whose departure location is closer to the cylinder requires less energy consumption to reach the target and hold the position there

Optimising cycle frequency: the effects of imposed cycle frequency training on the coordination and performance of skilled age-group swimmers

PURPOSE: Underwater undulatory swimming (UUS) is a fundamental skill incorporated during the starts and turns of three of the four competitive swimming strokes. Significant competitive advantage can be gained if UUS performance is optimised. The cycle frequency adopted during UUS in both animal and human swimmers have been extensively studied and it has been shown to have a strong relationship with the UUS velocity (U) achieved. The purpose of this thesis was to investigate the changes in performance and coordination in UUS which occur as a consequence of training at an imposed cycle frequency (identical to preferred) in skilled age -group swimmers (Study 3). To achieve the stated purpose, the reliability (systematic bias, within -subject variation and test -retest reliability) of the kinematic variables commonly used to describe and analyse UUS were established (Study 1). Once reliability was determined, the key kinematic performance and coordination variables in relation to the production of maximum U were identified (Study 2) to enable the key measures of performance and coordination to be monitored in response to a training perturbation (imposition of a cycle frequency) in the final study. METHODS: Measures of systematic bias, within- subject (WS) variation and inter -class correlation (ICC) of nineteen kinematic variables were determined over four sessions. This was undertaken to establish the requirement of any familiarisation training, number of cycles of data required to provide an accurate representation of each variable when reporting a mean value, and the related variability associated when reporting mean values based on a set number of data cycles (Study 1). Backward elimination ANCOVA statistical models with participant as a fixed -factor were employed to establish which of the performance and coordination variables were best in explaining the variance of cycle frequency, cycle length (CL) and ultimately U (Study 2). In the final study (Study 3) the performance and coordination variables identified from study 2 were analysed in sixteen skilled age -group swimmers which participated in a randomised controlled study. An experimental group of eight participants completed a four week imposed frequency (matched to their own preferred frequency) training programme, while a control group of eight participants completed a four week programme training at a self selected preferred cycle frequency. The UUS kinematics for both preferred cycle frequency UUS and imposed cycle frequency UUS were measured at weekly intervals throughout the training period. An additional retest (RT) was conducted 2 weeks after the cessation of the training period. RESULTS: Systematic bias was identified between the 1st and the remaining 3 testing sessions for cycle frequency, CL and U. The minimum number of data cycles required to achieve an acceptable measure of retest reliability (ICC >0.85) across all kinematic variables was 6 cycles. At 6 cycles WS variation ranged from 0.86 to 8.92 %CV. A total of 10 kinematic variables were identified as key to explaining the variance in cycle frequency and CL. A final parsimonious ANCOVA model revealed that 2 variables (maximum knee angle velocity and wave velocity between knee and ankle) explained a large proportion (Adj. r² = 0.944) of the variance in maximal U. However, when the participant was removed as a fixed factor the explained variance reduced (Adj. r²= 0.535). No significant difference in maximal U was found over the training or RT period. No variables were found to differ significantly by Session x Frequency Tested x Training Group (p <0.01). However, several discrete kinematic variables and measures of coordination showed statistically significant changes, either between Frequency Tested or across testing sessions. Discussion: After determining the systematic bias and establishing the requirement for a familiarization session, 6 cycles of data were found to be sufficient to provide high levels of reliability for each of the UUS kinematic variables. The identified key determinants of the variance in cycle frequency, CL and maximal U, revealed that the successful transmission of the propulsive waveform along the caudal aspects of the swimming body (specifically the kinematics/coordination at or around the knee) and the control of the shedding of the vortices and simultaneous recapture /reuse of previous shed rotational energy are key discriminating factors between the faster and slower UUS in skilled age-group swimmers. The 4-week training period did not result in changes in maximal U for either of the training groups. However, there were significant differences in the magnitude and process of adaptation between preferred and imposed frequency training groups' kinematics and measures of coordination over the training and testing period. The importance of each individual's own solution to the maximal UUS problem was highlighted, with coordination constrained by an individual's own idiosyncratic constraints. Further research is required to establish the efficacy of the imposition of a cycle frequency identical to an individual's own preferred frequency as an appropriate training modality for maximal U. In conclusion, the present research provides valuable insight into the effects of the simple act of cycle frequency imposition, providing a baseline for future cycle frequency interventions which take place at higher/lower cycle frequency or over longer training periods

The Use of Flexible Biomimetic Fins in Propulsion

This thesis documents a series of investigations exploring the role of stiffness profile in propulsion using pitching flexible fins. Stiffness profile is defined as the variation in local bending stiffness along the chord of a fin, from leading to trailing edge. An unmanned robotic submarine was created, using simple pitching flexible fins for propulsion. Its design and performance prompted a review of literature covering many aspects of oscillating fin propulsion, paying special attention to the studies of pitching flexible fins, of the type used in the submarine. In the body of previous work, fin stiffness profile was a consequence of the external shape profile of a fin; fins had not thus far been designed with stiffness profile specifically in mind. A hypothesis was proposed: “Use of a biomimetic fin stiffness profile can improve the effectiveness of a flexible oscillating fin, over that of a standard NACA designated fin shape.” Rectangular planform flexible fins of standard NACA 0012 design and 1:1 aspect ratio were tested alongside similar fins with a stiffness profile mimicking that of a pumpkinseed sunfish (Lepomis gibbosus). The fins were oscillated with a pitching-only sinusoidal motion over a range of frequencies and amplitudes, while torque, lateral force and static thrust were measured. Over the range of oscillation parameters tested, it was shown that the fin with a biomimetic stiffness profile offered a significant improvement in static thrust over a fin of similar dimensions with a standard NACA 0012 aerofoil shape, and produced thrust more consistently over each oscillation cycle. A comparison of different moulding materials showed that the improvement was due to the stiffness profile itself, and was not simply an effect of altering the overall stiffness of the fin, or changing its natural frequency. Within the range of stiffnesses and oscillation conditions tested, fins of the same stiffness profile were found to follow similar thrust-power curves, independently of their moulding material. Biomimetic fins were shown to produce between 10% and 25% more thrust per watt of mechanical input power.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

How flexibility affects the wake symmetry properties of a self-propelled plunging foil

The wake symmetry properties of a flapping-foil system are closely associated with its propulsive performance. In the present work, the effect of the foil flexibility on the wake symmetry properties of a self-propelled plunging foil is studied numerically. We compare the wakes of a flexible foil and a rigid foil at a low flapping Reynolds number of 200. The two foils are of the same dimensions, flapping frequency, leading-edge amplitude and cruising velocity but different bending rigidities. The results indicate that flexibility can either inhibit or trigger the symmetry breaking of the wake. We find that there exists a threshold value of vortex circulation above which symmetry breaking occurs. The modification of vortex circulation is found to be the pivotal factor in the influence of the foil flexibility on the wake symmetry properties. An increase in flexibility can result in a reduction in the vorticity production at the leading edge because ofthe decrease in the effective angle of attack, but it also enhances vorticity production at the trailing edge because of the increase in the trailing-edge flapping velocity. The competition between these two opposing effects eventually determines the strength of vortex circulation, which, in turn, governs the wake symmetry properties. Further investigation indicates that the former effect is related to the streamlined shape of the deformed foil while the latter effect is associated with structural resonance. The results of this work provide new insights into the functional role of passive flexibility inflapping-based biolocomotion

生物模倣ソフト魚ロボットの研究開発

In nature, the environment varies from day to day. Through natural selection and competition law of survival of the fittest, the winning creatures survive and their species are able to retain and persist in nature. Based on this fact, creatures existent in nature have their unique features and advantages adapt to the surrounding environment. In recent years, many researches focused on the features of the creatures in nature have been done actively to clarify their morphology and functions and apply the morphology and functions to various fields. Among these researches, the development of the biomimetic robots based on mimicking the creature’s structures and functions has become an active field in robotics recently. In the research, the development of biomimetic robotic fish is focused. So far, there are many researches on biomimetic robotic fish, but improvement on motion performances and efficiency is still an important issue for robot development. Specially, on the biomimetic soft robotic fish utilizing the flexibility of fishes, the developments have been done by the trial and error approach. That is, the design and control method of soft robotic fish has not been established currently. Therefore, it motives us to investigate the design and control of soft robotic fish by numerical simulation that takes into account the interaction between flexible structure and surrounding fluid to develop the biomimetic soft robotic fish with high performance. In order to develop the biomimetic soft robotic fish with high performance, the basic design method and corresponding numerical simulation system are firstly proposed and constructed in this dissertation. Then, based on finite element method (FEM), modelling of soft robotic fish by mimicking the soft structure and driving mechanism of fishes is carried out. The propulsion motion and propulsive force of the soft robotic fish are investigated through two kinds of numerical analyses. One is the modal and transient analysis considering the surrounding fluid as acoustic fluid. The propulsion mode and amplitude of the propulsion motion of soft robotic fish corresponding directly to the propulsion mechanism and motion performance of the robotic fish can be investigated. The other is the fluid-structure interaction (FSI) analysis. The interaction between soft robot structure and surrounding fluid including the dissipation due to fluid viscosity and influence of wake performance around the soft robotic fish are taken into account. From FSI analysis, the hydrodynamic performances of the soft robotic fish can be obtained for investigating its propulsion motion. It is possible to further improve the performance of the soft robotic fish through its design and control based on FSI analysis. Besides, based on coupling analysis by using acoustic fluid, the turning motion control of the soft robotic fish is investigated by its propulsion modes in the fluid. In order to investigate the feasibility of modelling method and numerical simulation analysis on design and control of the biomimetic soft robotic fish, the performance evaluation is carried out by comparison between the simulation and experiment on an actual prototype. Finally, the optimization and improvement are performed for developing the biomimetic soft robotic fish with higher performance based on verified coupling analysis considering the fluid as acoustic fluid, and corresponding performance evaluation on new robot prototype is presented. The performance improvement of the soft robotic fish is confirmed through the new robot prototype. The dissertation consists of six chapters and the main contents are shown as follows. Chapter 1 is an introduction. The background and relative previous work about biomimetic soft robotic fish are briefly reviewed. It summarizes the current research status and problems of biomimetic soft robotic fish, and describes the purposes of this research. Chapter 2 presents the design method, procedures and numerical simulation system in the present research for developing the biomimetic soft robotic fish with high performance. Different from previous development method, our purpose is how to design and control the soft robotic fish by utilizing interaction between the flexible structure and surrounding fluid effectively based on numerical simulations. Therefore, it is necessary to model a fish-like soft robot structure including soft actuators and an enclosed fluid. Besides, by the numerical analysis considering the interaction between flexible structure and fluid, the fish-like propulsion motion should be realized and established, and then the robot structure and control inputs are needed to be optimized for performance improvement. In order to meet these requirements of designing and developing the optimal soft robotic fish, the design method based on modelling, simulation analysis and improvement is presented and the numerical simulation system for soft robotic fish is built. In the simulation system, modelling of soft robotic fish, modal and transient analysis considering the enclosed fluid as acoustic fluid are firstly described based on FEM to realize the fish-like propulsion motion with large amplitude for the soft robotic fish. Then, the FSI analysis is performed to describe and establish the hydrodynamic performances of the soft robotic fish. Based on this numerical simulation system, it is possible to develop the biomimetic soft robotic fish with high performance effectively by optimization of design and control of the soft robotic fish. Chapter 3 describes the modelling and numerical analysis of biomimetic soft robotic fish by using the method presented in Chapter 2. The soft robotic fish uses the piezoelectric fiber composite (PFC) as soft actuator. Firstly, the relationships between the input voltage and generated stress of the PFC are derived. The generated stress can be applied on soft structure to investigate the motion performance of the soft robotic fish. To support the driving model of the PFC, the corresponding experiments on simple beam model are carried out. By comparing the simulation results with experimental results, the effectiveness of the driving model is verified. Then, the modal analysis in which the fluid is considered as acoustic fluid is performed. The structural mode frequencies and mode shapes of the soft robotic fish in the fluid are calculated. By comparing these modes’ motion with those of the real fishes, the fish-like propulsion mode is identified to realize the corresponding propulsion motion of the soft robotic fish. Furthermore, based on the verified driving model of soft actuator, the amplitude of the main propulsion motion of soft robotic fish is calculated. Through FSI analysis, the relationships of driving frequencies of input signal with propulsive force and displacement of propulsion motion, and vortex distribution in the wake around the soft robotic fish are investigated for the case of fixing robot head. Besides, the motion control of soft robot is investigated to realize turning motion in the fluid. Through controlling the input voltage amplitude on soft actuators of the robot, turning right and turning left motion are identified in the swimming when the input voltage amplitudes on two actuators are in asymmetric distribution. Chapter 4 is experiment evaluation. In order to validate the results of numerical simulation analysis described in Chapter 3, the mode shapes, amplitude of propulsion motion, propulsive force and vortex distribution around soft robotic fish for the case of fixing robot head, and turning motion are measured by using actual robot prototype. The present simulation results are congruent with experiments. By the results, the effectiveness of the modelling method and numerical analysis used in the research is verified and they are useful to predict the propulsion characteristics of the soft robotic fish in the fluid for performance improvement. Chapter 5 develops a new soft robotic fish with high performance based on above modelling method and numerical analysis by optimization. Firstly, the structural parameters of the robot are allowed to vary within a range and the amplitude of the propulsion motion for the soft robot is calculated for different parameters by the numerical analysis. Then the structural parameters of the robot capable of propulsion motion with largeramplitude are chosen for improvement. Based on this result, new soft robot is designed and evaluated by experiments. From the experimental results of the new soft robot, it is confirmed that the higher swimming speed, better fish-like swimming performance and larger turning velocity are realized. It can be said that the new soft robotic fish has been developed successfully for improvement. Chapter 6 summarizes the conclusions and future works of this research.電気通信大学201

Hydrodynamics of pitching foils: flexibility and ground effects

En termes de propulsió la rigidesa flexural i l'efecte terra en una placa rectangular en piteig pur han estat investigats. Velocimetria per imatges per partícules, mesures de forces i moments amb una cèl·lula de carga de 6 eixos, mesures de velocitat i adquisició d'imatges de la cinemàtica de la placa han estat realitzades per estudiar els patrons de flux i les forces hidrodinàmiques en plaques de diferent flexibilitat. La presència de la paret va millorar la velocitat de creuer fins a un 25% i l'empenta fins a un 45% per angles escombrats de 160 i 240 graus. El mecanisme físic sota aquest efecte és discutit estudiant els camps de vorticitat produïts per l'estela de l'aleta bioinspirada en un rajiforme. Les forces hidrodinàmiques linkejades a les tècniques de visualització, van permetre calcular eficiències i camps de vorticitat promitjats en fase. Aquestes dades van revelar com l'angle escombrat de la placa juga un paper fonamental en la distribució de moment en l'estela d'una placa rígida per incrementar la propulsió. En termes de rigidesa flexural, l'òptima flexibilitat va ser determinada amb una placa semi-flexible amb una eficiència d'un 69% amb un angle d'atac de 72 graus.En términos de propulsión la rigidez flexural y el efecto suelo en una placa rectangular en puro picheo han sido investigados. Velocimetría de imágenes por partículas, medidas de fuerzas y momentos con una célula de carga de 6 ejes, medidas de velocidad y adquisiciones de imágenes de la cinemática de la placa han sido realizadas para estudiar los patrones de flujo y las fuerzas hidrodinámicas en placas con diferentes flexibilidad. La presencia de la pared mejoró la velocidad de crucero hasta en un 25% y el empuje hasta un 45% para ángulos barridos de 160 y 240 grados. El mecanismo físico bajo este efecto es discutido estudiando los campos de vorticidad producidos por la estela de la aleta bioinspirada en un rajiforme. Las fuerzas hidrodinámicas linkadas a las técnicas de visualización, permitieron calcular eficiencias y campos de vorticidad promediados en fase. Estos datos revelaron como el ángulo barrido de la placa juega un papel fundamental en la distribución de momento en la estela de un foil rígido para incrementar la propulsión. En términos de rigidez flexural la óptima flexibilidad fue determinada con la placa semi-flexible con una eficiencia de un 69% con un ángulo de ataque de 72 grados.The roles of the chordwise flexural stiffness and ground effect in a rectangular plate undergoing in pure pitching motion have been investigated. Digital Particle image velocimetry (DPIV), load measurement with a 6-axes balance, measurements of the swimming speed and image acquisition of the kinematics of the foil have been done to study the flow patterns and hydrodynamics forces around the flapping flexible plates. The presence of the wall enhances the cruising velocity in some cases up to 25% and the thrust by a 45% , for swept angles of 160 and 240°. The physical mechanisms underlying of this effect are discussed by studying the vorticity dynamics in the wake of the foil. Experimental data of the hydrodynamic forces and moments allowed to obtain the efficiencies of the flapping propulsion. These load measurements were linked to the wakes of the flapping foils in order to reveal configurations with higher thrust. The momentum distribution in the wake of the foil has allowed the physical explanation for the cases with highest thrust production capacity. In terms of flexural stiffness, the optimum flexibility has been determined with the semi − flexible plate up to 69% of efficiency under a swept angle of 72 degrees for Re = O(10^4) tested in the investigation