Showing papers on "Flapping published in 2019"
TL;DR: Results show how flapping kinematics can be used to control locomotion within wakes, and that flow interactions provide a mechanism which promotes group cohesion.
Abstract: Many species of fish and birds travel in groups, yet the role of fluid-mediated interactions in schools and flocks is not fully understood. Previous fluid-dynamical models of these collective behaviors assume that all individuals flap identically, whereas animal groups involve variations across members as well as active modifications of wing or fin motions. To study the roles of flapping kinematics and flow interactions, we design a minimal robotic “school” of two hydrofoils swimming in tandem. The flapping kinematics of each foil are independently prescribed and systematically varied, while the forward swimming motions are free and result from the fluid forces. Surprisingly, a pair of uncoordinated foils with dissimilar kinematics can swim together cohesively—without separating or colliding—due to the interaction of the follower with the wake left by the leader. For equal flapping frequencies, the follower experiences stable positions in the leader’s wake, with locations that can be controlled by flapping amplitude and phase. Further, a follower with lower flapping speed can defy expectation and keep up with the leader, whereas a faster-flapping follower can be buffered from collision and oscillate in the leader’s wake. We formulate a reduced-order model which produces remarkable agreement with all experimentally observed modes by relating the follower’s thrust to its flapping speed relative to the wake flow. These results show how flapping kinematics can be used to control locomotion within wakes, and that flow interactions provide a mechanism which promotes group cohesion.
47 citations
TL;DR: In this article, a two-dimensional forward-backward-facing step submerged in a deep turbulent boundary layer is investigated using a time-resolved particle image velocimetry, and the results indicate that the lowfrequency flapping motion of the separation bubble over the step is induced by the oncoming large-scale alternating low and high-velocity streaky structures.
Abstract: Turbulent separation bubbles over and behind a two-dimensional forward–backward-facing step submerged in a deep turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the step height and free-stream velocity is 12 300, and the ratio of the streamwise length to the height of the step is 2.36. The upstream turbulent boundary layer thickness is 4.8 times the step height to ensure a strong interaction of the upstream turbulence structures with the separated shear layers over and behind the step. The velocity measurements were performed in streamwise–vertical planes at the channel mid-span and streamwise–spanwise planes at various vertical distances from the wall. The unsteady characteristics of the separation bubbles and their associated turbulence structures are studied using a variety of techniques including linear stochastic estimation, proper orthogonal decomposition and variable-interval time averaging. The results indicate that the low-frequency flapping motion of the separation bubble over the step is induced by the oncoming large-scale alternating low- and high-velocity streaky structures. Dual separation bubbles appear periodically over the step at a higher frequency than the flapping motion, and are attributed to the inherent instability in the rear part of the mean separation bubble. The separation bubble behind the step exhibits a flapping motion at the same frequency as the separation bubble over the step, but with a distinct phase delay. At instances when an enlarged separation bubble is formed in front of the step, a pair of vertical counter-rotating vortices is formed in the immediate vicinity of the leading edge.
41 citations
TL;DR: The wing kinematics for miniature-insect species of different sizes are measured and the aerodynamic forces are computed to produce the required vertical force for flapping as larger insects.
Abstract: Miniature insects fly at very low Reynolds number (Re); low Re means large viscous effect. If flapping as larger insects, sufficient vertical force cannot be produced. We measure the wing kinematics for miniature-insect species of different sizes and compute the aerodynamic forces. The planar upstroke commonly used by larger insects changes to a U-shaped upstroke, which becomes deeper as size or Re decreases. For relatively large miniature insects, the U-shaped upstroke produces a larger vertical force than a planar upstroke by having a larger wing velocity and, for very small ones, the deep U-shaped upstroke produces a large transient drag directed upwards, providing the required vertical force.
37 citations
TL;DR: A bio-inspired flapping-wing-assisted jumping robot, named Jump-flapper, which mimics jumping insects' locomotion strategy and is capable of jumping to a height of approximately 0.9 m, showing about 30% improvement in the jumping height compared to that of the robot without assistance of the flaps.
Abstract: Jumping insects such as fleas, froghoppers, grasshoppers, and locusts take off from the ground using a catapult mechanism to push their legs against the surface of the ground while using their pairs of flapping wings to propel them into the air. Such combination of jumping and flapping is expected as an efficient way to overcome unspecified terrain or avoid large obstacles. In this work, we present the conceptual design and verification of a bio-inspired flapping-wing-assisted jumping robot, named Jump-flapper, which mimics jumping insects' locomotion strategy. The robot, which is powered by only one miniature DC motor to implement the functions of jumping and flapping, is an integration of an inverted slider-crank mechanism for the structure of the legs, a dog-clutch mechanism for the winching system, and a rack-pinion mechanism for the flapping-wing system. A prototype of the robot is fabricated and experimentally tested to evaluate the integration and performance of the Jump-flapper. This 23 g robot with assisted flapping wings operating at approximately 19 Hz is capable of jumping to a height of approximately 0.9 m, showing about 30% improvement in the jumping height compared to that of the robot without assistance of the flapping wings. The benefits of the flapping-wing-assisted jumping system are also discussed throughout the study.
36 citations
23 Jan 2019
TL;DR: In this work a double cone DEA configuration is presented, a mathematic model is developed to characterize its quasi-static and dynamic performance, and the high frequency performance of two most common dielectric elastomers: silicone elastomer and polyacrylate tape VHB is compared.
Abstract: In the last two decades, insect-inspired flapping wing micro air vehicles (MAVs) have attracted great attention for their potential for highly agile flight. Insects flap their wings at the resonant frequencies of their flapping mechanisms. Resonant actuation is highly advantageous as it amplifies the flapping amplitude and reduces the inertial power demand. Emerging soft actuators such as dielectric elastomer actuators (DEAs) have large actuation strains and thanks to their inherent elasticity, DEAs have been shown a promising candidate for resonant actuation. In this work a double cone DEA configuration is presented, a mathematic model is developed to characterize its quasi-static and dynamic performance. We compare the high frequency performance of two most common dielectric elastomers: silicone elastomer and polyacrylate tape VHB. The mechanical power output of the DEA is experimentally analyzed as a DEA-mass oscillator. A flapping wing mechanism actuated by this elastic actuator and this design is able to provide a peak flapping amplitude of 63° at the frequency of 18 Hz, which is the highest reported flapping performance of a DEA flapping mechanism. While not yet sufficient for hovering flight, the mass-specific power density of the DEA was found to be 100 W/kg at resonance, which demonstrates the viability of this technology. Limiting factors in the current transmission mechanism, such as the transient variability of speed ratio are analyzed towards DEA-driven MAVs.
35 citations
TL;DR: In this article, the effects of added mass and fluid damping on a flapping membrane are quantified using a simple damped oscillator model, and an analytic model based on thin airfoil theory coupled with a membrane equation is developed to characterize the steady and unsteady aeroelastic behavior of compliant membrane wings under different conditions.
Abstract: We present a theoretical framework to characterize the steady and unsteady aeroelastic behaviour of compliant membrane wings under different conditions. We develop an analytic model based on thin airfoil theory coupled with a membrane equation. Adopting a numerical solution to the model equations, we study the effects of wing compliance, inertia and flapping kinematics on aerodynamic performance. The effects of added mass and fluid damping on a flapping membrane are quantified using a simple damped oscillator model. As the flapping frequency is increased, membranes go through a transition from thrust to drag around the resonant frequency, and this transition is earlier for more compliant membranes. The wake also undergoes a transition from a reverse von Karman wake to a traditional von Karman wake. The wake transition frequency is predicted to be higher than the thrust–drag transition frequency for highly compliant wings.
34 citations
17 May 2019
TL;DR: In this article, the collective locomotion of two tandem autopropelled flapping foils is greatly affected by the phase difference, and two distinct vortex interactions are observed, merging interaction and broken interaction, which respectively result in the highest efficiency for the follower and the leader.
Abstract: The collective locomotion of two tandem autopropelled flapping foils is greatly affected by the phase difference. Two distinct vortex interactions are observed---merging interaction and broken interaction---which respectively result in the highest efficiency for the follower and the leader.
34 citations
TL;DR: In this paper, a baseline NACA0012 airfoil is modified using a short flap on its upper surface at a Reynolds number of Re = 1000, and the impact of the flap configuration, described by length, attachment position, deployment angle, and material properties, on the aerodynamic performance of the air-foil, measured by mean and fluctuating forces, is investigated.
Abstract: The incorporation of nature-inspired techniques to control or reduce boundary layer separation, to bring about performance enhancements on air/water vehicles, has been an active research area for many years. In this paper, a baseline NACA0012 airfoil is modified using a short flap on its upper surface at a Reynolds number of Re = 1000. The impact of the flap configuration—described by length, attachment position, deployment angle, and material properties, on the aerodynamic performance of the airfoil—quantified by mean and fluctuating forces, is investigated, and the flow field is analyzed. Inspired by the observation of pop-up feathers on a bird’s wing, the flap is first set to be rigid for a range of location, size, and inclination angles. After the optimal location of a rigid flap has been established, the flap is then allowed to be flexible, its motion is coupled to the encircling flow field, and it is tested for a range of mass ratios and bending stiffness values. The fluid motion is obtained by solving the lattice Boltzmann equation, while the dynamics of the flexible flap are calculated using the finite element method and the coupling between the flow and flap handled by the immersed boundary method. For the flexible flap, two flapping patterns are observed and the mechanism of separation control via rigid/flexible flap is explained. Compared to the flapless NACA0012 airfoil case, in the case with a flap of optimal configuration, the mean lift coefficient is improved by 13.51%, the mean drag coefficient is decreased by 3.67%, the mean lift-drag ratio is improved by 17.84%, the maximum lift fluctuation is decreased by 40.90%, and the maximum drag fluctuation is decreased by 56.90%.
33 citations
TL;DR: This study explained the 3D flapping wing thrust performance variation with different wing shapes, aspect ratios and pitch-bias angles of attack and indicated that the obtuse wing has the best thrust performance compared with other sharp wing shapes.
33 citations
TL;DR: In this paper, the cycle-averaged swept trajectory of the foil chordline was replaced with a cycle-wise swept trajectory for the wake transition in a phase space of dimensionless, showing that the drag-to-thrust wake transition of all tested modes occurs for a modified Strouhal.
Abstract: Reversed von Karman streets are responsible for a velocity surplus in the wake of flapping foils, indicating the onset of thrust generation. However, the wake pattern cannot be predicted based solely on the flapping peak-to-peak amplitude and frequency because the transition also depends sensitively on other details of the kinematics. In this work we replace with the cycle-averaged swept trajectory of the foil chordline. Two-dimensional simulations are performed for pure heave, pure pitch and a variety of heave-to-pitch coupling. In a phase space of dimensionless we show that the drag-to-thrust wake transition of all tested modes occurs for a modified Strouhal . Physically, the product expresses the induced velocity of the foil and indicates that propulsive jets occur when this velocity exceeds . The new metric offers a unique insight into the thrust-producing strategies of biological swimmers and flyers alike, as it directly connects the wake development to the chosen kinematics, enabling a self-similar characterisation of flapping foil propulsion.
30 citations
TL;DR: This paper introduces the design and control of a biomimetic robot fish for multimodal swimming, an improved Central Pattern Generator with the high-level control command that synthesizes symmetrical flapping in cruising and asymmetry of the offset in turning for the robot fish.
TL;DR: In this article, the authors investigated the influence of different schooling configurations and individual distances on the thrust performance of three-dimensional multiflapping wings and found that the closer the individual distance, the better the overall thrust performance obtained for triangle and diamond schooling.
Abstract: After billions of years of natural selection, flying animals with flapping wings have superior flight and mobility capabilities. The aerodynamic characteristics and the propulsion mechanism of bionic wings have attracted a large number of researchers because they will be beneficial to novel bio-inspired micro air or underwater vehicle design. Except the single activities, for fish, birds, and insects, there is a very popular and interesting biological clustering phenomenon known as schooling. Considering the real biological movements in schooling under low Reynolds number, the study of the flow mechanisms and thrust performance of bionic multiflapping wings in different schooling configurations could be applied to the design of future bionic flapping wing aircraft formation. The unsteady flow mechanisms and the thrust performance of three-dimensional multiflapping wings in three different schooling configurations are numerically investigated using the immersed boundary-lattice Boltzmann method with the Chinese TianHe-II supercomputer. The influences of different schooling configurations and individual distances on the thrust performance of multiflapping wings are thoroughly investigated. Numerical results indicate that the individual horizontal distance has great effects on the thrust performance of multiflapping wings in schooling, and the average thrust coefficient of each flapping wing in different schooling configurations at a specific individual horizontal distance is larger than that of the single flapping wing. There is an optimum distance for different schooling configurations, where the individual interaction lead to best propulsion performance. Different from the simple tandem schooling, the closer the individual distance, the better the overall thrust performance obtained for triangle and diamond schooling.
TL;DR: In this article, the aerodynamic performance of an X-wing bio-inspired flapping wing micro air vehicle (MAV) underlying clap-and-fling motion by means of force and flow field measurements was explored.
TL;DR: A backstepping-based robust fault-tolerant control scheme is developed for attitude tracking control of a medium-scale unmanned autonomous helicopter with rotor flapping dynamics, external unknown disturbances and actuator faults and closed-loop system stability is proved via Lyapunov analysis.
Abstract: In this paper, an adaptive robust fault-tolerant control scheme is developed for attitude tracking control of a medium-scale unmanned autonomous helicopter with rotor flapping dynamics, external un...
TL;DR: In this paper, the fluid-structure-acoustics interaction of flexible flapping wings is numerically studied by using an immersed boundary method at a Mach number of 0.1.
TL;DR: In this paper, the fluid-structure interaction problem of the flapping wings of bumblebees is considered, with focus on the action of elastic joints between wings and body.
01 Nov 2019
TL;DR: A generalized approach that uses a model with direct collocation methods to plan dynamically feasible flight maneuvers and is made to be both accurate through collection of load cell force data for parameter selection and computationally inexpensive such that it can be used efficiently in a nonlinear solver.
Abstract: Planning flight trajectories is important for practical application of flying systems. This topic has been well studied for fixed and rotary winged aerial vehicles, but far fewer works have explored it for flapping systems. Bat Bot (B2) is a bio-inspired flying robot that mimics bat flight, and it possesses the ability to follow a designed trajectory with its on-board electronics and sensing. However, B2’s periodic flapping and its complex aerodynamics present major challenges in modeling and planning feasible flight paths. In this paper, we present a generalized approach that uses a model with direct collocation methods to plan dynamically feasible flight maneuvers. The model is made to be both accurate through collection of load cell force data for parameter selection and computationally inexpensive such that it can be used efficiently in a nonlinear solver. We compute the trajectory of launching B2 to a desired altitude and a banked turn maneuver, and we validate our methods with experimental flight results of tracking the launch trajectory with a PD controller.
TL;DR: In this paper, a method coupled three-dimensional CFD (Computational Fluid Dynamics) and computational structural dynamics (CSD) considering FMBD (flexible multi-body dynamics) between flexible wings and body is developed.
TL;DR: At low Reynolds numbers, revolving wings become less efficient in generating lift for hovering flight due to the increasing adverse viscous effects as discussed by the authors, and flying insects use reciprocating revolving wings.
Abstract: At low Reynolds numbers, revolving wings become less efficient in generating lift for hovering flight due to the increasing adverse viscous effects. Flying insects use reciprocating revolving wings...
TL;DR: In this paper, the role of the vortex structures particularly the counter-rotating periodic vortices generated from the leading and trailing edges of the inverted foil, and the interaction between them on the LAF was examined.
TL;DR: It was found the vortex formation, aerodynamic pressure, and pharyngeal wall force were significantly affected by the width of the pharynGEal airway, and higher order harmonics of base frequency with significant pressure amplitudes and energy intensities were revealed.
TL;DR: In this paper, a minimum-time optimal control problem to steer a FWMAV dynamical system from a hovering condition to forward flight with a prescribed forward speed using time-periodic and averaged dynamics formulations is formulated.
TL;DR: In this article, the authors investigated the heat transfer characteristics of rectangular channels equipped with a flapping flag as a vortex generator and determined the influences of channel geometry, flag material properties and flow conditions on the resulting heat transfer behavior of the system.
TL;DR: In this paper, the authors investigated the influence of the deformation magnitude β and the position of deformation center on the energy extraction efficiency of a deformable airfoil under a constant arc length.
Abstract: The active synchronous deformation in the arc length of an airfoil employed in a flapping wing can improve its energy extraction efficiency. The present study seeks to understand the underlying physics of this energy extraction by conducting transient numerical simulations of a novel arc-deformable flapping foil design based on dynamic mesh technology and a relative heaving motion reference system. The influence of the flapping frequency and the pitching amplitude on the energy extraction efficiency of the flapping foil modeled under a constant arc length is investigated. The effects of the deformation magnitude β and the position of the deformation center on the energy extraction efficiency are also examined at a constant flapping frequency and pitching amplitude. The results show that active synchronous arc deformation can greatly improve the energy extraction efficiency of a flapping foil compared to the efficiency of a conventional non-deformable flapping foil design. In addition, the results provide sets of optimal flapping frequencies and pitching amplitudes for the deformable flapping foil design with fixed deformation parameters and the non-deformable foil design that obtains the highest energy extraction efficiencies. A single high efficiency zone is obtained for the deformable foil design at a relatively high flapping frequency. In contrast, relatively high efficiency zones are obtained for the non-deformable foil design at both a relatively low flapping frequency and a high flapping frequency. The energy extraction efficiency of the deformable flapping foil first increases with increasing β up to a maximum value of β = 0.25 and then decreases with a further increase in β. The energy extraction efficiency of the deformable flapping foil is also demonstrated to increase as the deformation center moves from the leading edge of the foil to the trailing edge, attaining a maximum value when the deformation center coincides with the center of the pitching axis, and then decreases.
TL;DR: The robustness and reliability of the 3D flexible multibody aeroelastic framework for an anisotropic flapping wing flight involving battens and membranes with composite material is studied and the accuracy of the coupled solution is validated with the available experimental data.
TL;DR: In this paper, the stability and flapping behaviors of dual inverted flags with finite height are investigated for a side-by-side arrangement, and their noticeable characteristics are compared to those of dual conventional flags.
TL;DR: In this paper, the authors investigated the relation between a flapping motion event and the solar wind parameters and found that there is a one-to-one correspondence between temporal variations of the flapping and the ZGSM component of solar wind velocity VZ, SW.
Abstract: We report on a flapping motion event near the substorm onset on 17 June 2017 using Magnetospheric Multiscale (MMS) mission data. A strong current density with a maximum value of ~190 nA/m is observed during the flapping. The north‐to‐south (south‐to‐north) crossing of the neutral sheet corresponds to an increase (a decrease) of the ZGSM component of the solar wind VZ, SW. The periods (~8 min) of the flapping and variations of VZ, SW are almost equal. In addition, dVZ, SW/dt and dBX/dt observed byMMS exhibit a strong negative correlation. These observations suggest that the flappingmotions are triggered by the solar wind directional change via creating a motion of the current sheet in the north‐south direction. The pressure difference between the northern and southern lobes caused by the solar wind is expected to be a possible contribution to the formation of the flapping. Plain language summary Flapping motions are large‐amplitude waves of the magnetotail current sheet in the north‐south direction. These waves are believed to be excited in the magnetotail center and then propagate to its flanks. However, their generation mechanisms are not fully understood so far. In this study, we investigate the relation between a flappingmotion event and the solar wind parameters. We find that there is a one‐to‐one correspondence between temporal variations of the flapping and the ZGSM component of the solar wind velocity VZ, SW. Moreover, the vertical movement of the ion flow is also in accordance with VZ, SW. These observations suggest that the flapping can be generated by the solar wind directional change. During the flapping, a strong current density with a maximum value of ~190 nA/m is generated, much larger than the current reported by previous studies.
TL;DR: In this article, the authors investigated the energy harvesting of a hydrokinetic turbine based on tandem flapping hydrofoils and found that the highest hydrodynamic efficiency of single hydrofoil is 25.2 % at S t = 0.24 with Φ / 2 π ≈ 0.235.