Flapping Insect Wings Research Articles

Computational fluid–structure interaction framework for passive feathering and cambering in flapping insect wings

AbstractIn flapping insect wings, veins support flexible wing membranes such that the wings form feathering and cambering motions passively from large elastic deformations. These motions are essentially important in unsteady aerodynamics of insect flapping flight. Hence, the underlying mechanism of this phenomenon is an important issue in studies on insect flight. Systematic parametric studies on strong coupling between a model wing describing these elastic deformations and the surrounding fluid, which is a direct formulation of this phenomenon, will be effective for solving this issue. The purpose of this study is to develop a robust numerical framework for these systematic parametric studies. The proposed framework consists of two novel numerical methods: (1) A fully parallelized solution method using both algebraic splitting and semi‐implicit scheme for monolithic fluid–structure interaction (FSI) equation systems, which is numerically stable for a wide range of properties such as solid‐to‐fluid mass ratios and large body motions, and large elastic deformations. (2) A structural mechanics model for insect flapping wings using pixel modeling (pixel model wing), which is combined with explicit node‐positioning to reduce computational costs significantly in controlling fluid meshes. The validity of the proposed framework is demonstrated for some benchmark problems and a dynamically scaled model incorporating actual insect data. Finally, from a parametric study for the pixel model wing flapped in fluid with a wide range of solid‐to‐fluid mass ratios, we find a FSI mechanism of feathering and cambering motions in flapping insect wings.

Vein-Membrane Interaction in Cambering of Flapping Insect Wings.

It is still unclear how elastic deformation of flapping insect wings caused by the aerodynamic pressure results in their significant cambering. In this study, we present that a vein-membrane interaction (VMI) can clarify this mechanical process. In order to investigate the VMI, we propose a numerical method that consists of (a) a shape simplification model wing that consists of a few beams and a rectangular shell structure as the structural essence of flapping insect wings for the VMI, and (b) a monolithic solution procedure for strongly coupled beam and shell structures with large deformation and large rotation to analyze the shape simplification model wing. We incorporate data from actual insects into the proposed numerical method for the VMI. In the numerical analysis, we demonstrate that the model wing can generate a camber equivalent to that of the actual insects. Hence, the VMI will be a mechanical basis of the cambering of flapping insect wings. Furthermore, we present the mechanical roles of the veins in cambering. The intermediate veins increase the out-of-plane deflection of the wing membrane due to the aerodynamic pressure in the central area of the wing, while they decrease it in the vicinity of the trailing edge. As a result, these veins create the significant camber. The torsional flexibility of the leading-edge veins increases the magnitude of cambering.

Ultralow-Profile Conformal Antenna With Large Axial Ratio Bandwidth for Flapping-Wing Insects

Ultralow-Profile Conformal Antenna With Large Axial Ratio Bandwidth for Flapping-Wing Insects

A near-optimal-trim-search method for six-degrees-of-freedom model insects: An application to flight in the lateral-wind condition

This paper presents an improved trim-search method for six-degrees-of-freedom (6-DOF) flapping-wing insects and its application to flight in the lateral-wind condition. A multibody-dynamics (MBD) code was developed and coupled with an extended unsteady vortex-lattice method (UVLM) to create a flapping-flight simulation framework. Using artificial neural networks (ANNs) and the genetic algorithm (GA), the trim-search method was developed to provide a near-optimal solution. The paper shows that the required power of the trim solution obtained by the present method is less than that from the conventional one. Based on the research results, the paper has revealed how a model insect can respond to sustain their hovering flight state in the lateral-wind condition. In general, the insect has to change its left- and right-wing kinematic parameters asymmetrically. The most noticeable differences between the kinematic parameters of the two wings are found in the sweep and rotation amplitudes. The research also showed an increase in the required power of the windward wing and a decrease in that of the leeward wing. However, the total required power generally grows due to the lateral wind.

Modeling and Analysis of a Simple Flexible Wing-Thorax System in Flapping-Wing Insects.

Small-scale flapping-wing micro air vehicles (FWMAVs) are an emerging robotic technology with many applications in areas including infrastructure monitoring and remote sensing. However, challenges such as inefficient energetics and decreased payload capacity preclude the useful implementation of FWMAVs. Insects serve as inspiration to FWMAV design owing to their energy efficiency, maneuverability, and capacity to hover. Still, the biomechanics of insects remain challenging to model, thereby limiting the translational design insights we can gather from their flight. In particular, it is not well-understood how wing flexibility impacts the energy requirements of flapping flight. In this work, we developed a simple model of an insect drive train consisting of a compliant thorax coupled to a flexible wing flapping with single-degree-of-freedom rotation in a fluid environment. We applied this model to quantify the energy required to actuate a flapping wing system with parameters based off a hawkmoth Manduca sexta. Despite its simplifications, the model predicts thorax displacement, wingtip deflection and peak aerodynamic force in proximity to what has been measured experimentally in flying moths. We found a flapping system with flexible wings requires 20% less energy than a flapping system with rigid wings while maintaining similar aerodynamic performance. Passive wing deformation increases the effective angle of rotation of the flexible wing, thereby reducing the maximum rotation angle at the base of the wing. We investigated the sensitivity of these results to parameter deviations and found that the energetic savings conferred by the flexible wing are robust over a wide range of parameters.

Aerodynamic modelling of flapping insect: A review

A review of the aerodynamic modelling of insect flight is presented. This paper consists of two sections covering various aspects related to the development of flapping wing and research on the aerodynamic modelling of insect flight. First, we provide an overview of the nature of the flyers, including current issues and challenges related to the evolution of nature-inspired design. The second section discusses some of the techniques required to develop aerodynamic models, seeks to address issues and to identify appropriate aerodynamic modelling methods for the development of insect flight models. Experimental models of flapping wing insect are discussed, followed by numerical models. Finally, we highlight the development of prototypes in insect flight, on the design, sizing and classification of the flying insect prototypes.

Aerodynamic explanation of flight speed limits in hawkmoth-like flapping-wing insects

It has long been unknown why the hawkmoth's maximum forward flying speed is much lower than the theoretical prediction based on its body mass. Our computational fluid dynamics study revealed that as a hawkmoth's flight speed increases, its wings inevitably generate a significant amount of negative lift during the upstroke, which renders the hawkmoth incapable of sustaining steady forward flight. A similar trend has also been observed for other insects, including fruit flies and bumblebees. However, birds and other flying vertebrates are able to overcome this limitation by flexing their wings during the upstroke.

The Motion Analysis of Attacus Atlas Rigid Wing

The remarkable aerodynamic efficiency of flapping insect wings has fascinated researchers for many years. Butterfly wings are distinguished by a much larger lifting surface, and thus, a different style of flapping flight. The considerations described in the article are an introduction to a fully flexible analysis of the wing in motion. The study of the rigid wing gives the basic knowledge necessary in the further process. The yaw, pitch and roll angles obtained from the footage have been used to analyze the rigid wing. The data has been adapted to CFD calculations in Ansys Fluent software along with the geometry of the joined wings (one surface on one side). The wing deformations have not been taken into account during the analysis. The obtained results make it possible to specify clearly the aerodynamic forces in three directions and the pressure distributions on the wing surface. For a rigid wing, there is positive drag and negative lift. The negative lift indicates the exceptional importance of wing deformation, which is the only guarantee of the insect's ability to fly. On the other hand, a positive drag is evidence of the influence of the shape of the bearing surface on the flow aerodynamics.

Very low Reynolds number causes a monotonic force enhancement trend for a three-dimensional hovering wing in ground effect

This research reports the numerical results of the ground effect trend for a three-dimensional flapping insect wing at a very low Reynolds number (Re = 10). It demonstrates that the ground effect trend at this Re has a ‘single force regime,’ i.e. the forces only enhance as the ground distance decreases. This phenomenon is unlike the widely expected non-monotonic trend publicized in previous studies for higher Reynolds numbers, that shows ‘three force regimes,’ i.e. the forces reduce, recover, and also enhance as the ground distance decreases. The force trend in the ground effect correlates to a similar trend in wing–wake interaction or the downwash strength on the wing’s head. At very low Re (10), the very large viscosity causes diffused vortices and less advected vortex wake, while at relatively high Re, the vortices are easily separated from the wing and then advected downwards. This different development of the vortex wake caused different force trends for the flapping wing in the ground effect. Furthermore, by examining only the first stroke when there is no vortex wake, we found that the ‘ramming effect’ enhances the forces on the wing. This effect increases the pressure of the lower wing surface due to the squeezed air between the wing and the ground. The ‘ramming effect’, combined with the reduced downwash (or wing–wake interaction) effect, causes the force enhancement of the wing near the ground’s vicinity. It is further comprehended that the trend is dependent on Re. As the Re is increased, the trend becomes non-monotonic. The effect of varying angles of attack, flapping amplitude and wing planform at very low Re does not change this trend. This ground effect might help insects by enhancing their lift while they hover above the surface. This finding might prove beneficial for developing micro air vehicles.

Dimensional analysis of spring-wing systems reveals performance metrics for resonant flapping-wing flight.

Flapping-wing insects, birds and robots are thought to offset the high power cost of oscillatory wing motion by using elastic elements for energy storage and return. Insects possess highly resilient elastic regions in their flight anatomy that may enable high dynamic efficiency. However, recent experiments highlight losses due to damping in the insect thorax that could reduce the benefit of those elastic elements. We performed experiments on, and simulations of, a dynamically scaled robophysical flapping model with an elastic element and biologically relevant structural damping to elucidate the roles of body mechanics, aerodynamics and actuation in spring-wing energetics. We measured oscillatory flapping-wing dynamics and energetics subject to a range of actuation parameters, system inertia and spring elasticity. To generalize these results, we derive the non-dimensional spring-wing equation of motion and present variables that describe the resonance properties of flapping systems: N, a measure of the relative influence of inertia and aerodynamics, and , the reduced stiffness. We show that internal damping scales with N, revealing that dynamic efficiency monotonically decreases with increasing N. Based on these results, we introduce a general framework for understanding the roles of internal damping, aerodynamic and inertial forces, and elastic structures within all spring-wing systems.

Reconstructing full-field flapping wing dynamics from sparse measurements

Flapping insect wings deform during flight. This deformation benefits the insect’s aerodynamic force production as well as energetic efficiency. However, it is challenging to measure wing displacement field in flying insects. Many points must be tracked over the wing’s surface to resolve its instantaneous shape. To reduce the number of points one is required to track, we propose a physics-based reconstruction method called system equivalent reduction expansion processes to estimate wing deformation and strain from sparse measurements. Measurement locations are determined using a weighted normalized modal displacement method. We experimentally validate the reconstruction technique by flapping a paper wing from 5–9 Hz with 45° and measuring strain at three locations. Two measurements are used for the reconstruction and the third for validation. Strain reconstructions had a maximal error of 30% in amplitude. We extend this methodology to a more realistic insect wing through numerical simulation. We show that wing displacement can be estimated from sparse displacement or strain measurements, and that additional sensors spatially average measurement noise to improve reconstruction accuracy. This research helps overcome some of the challenges of measuring full-field dynamics in flying insects and provides a framework for strain-based sensing in insect-inspired flapping robots.

Influence of wing flexibility on the aerodynamic performance of a tethered flapping bumblebee

The flight of insects has enlightened the flying dream of human beings for centuries. Wing flexibility is often used by insects to increase their flight efficiencies. However, the mechanism of the increased efficiencies still remains mysterious. Prof. Kai Schneider's group studies the aerodynamics of a tethered flapping bumblebee using a mass-spring fluid-structure interaction numerical solver. It indicates that a higher flight efficiency or a larger lift-to-power ratio can be achieved by flapping insects with optimal mechanical properties of the flexible wings. The novel understanding of insects’ body structure and flying behavior will benefit the design of micro-air vehicles (MAVs). The sophisticated structures of flapping insect wings make it challenging to study the role of wing flexibility in insect flight. In this study, a mass-spring system is used to model wing structural dynamics as a thin, flexible membrane supported by a network of veins. The vein mechanical properties can be estimated based on their diameters and the Young's modulus of cuticle. In order to analyze the effect of wing flexibility, the Young's modulus is varied to make a comparison between two different wing models that we refer to as flexible and highly flexible. The wing models are coupled with a pseudo-spectral code solving the incompressible Navier–Stokes equations, allowing us to investigate the influence of wing deformation on the aerodynamic efficiency of a tethered flapping bumblebee. Compared to the bumblebee model with rigid wings, the one with flexible wings flies more efficiently, characterized by a larger lift-to-power ratio.

General-Purpose Object-Oriented Framework for Vorticity-Dominated Flow Simulation

This paper describes the development of a computational framework for aerodynamic simulations based on the unsteady vortex lattice method under the object-oriented programming paradigm. The effort aims to fulfill the fluid-dynamics portion of the fluid–structure interaction problem in the multiphysics field. The construction and implementation of an object-oriented model is introduced along with its analysis and design stages, focusing on the generality and versatility. The overall class organization is presented, and the main classes’ characteristics are discussed. The software validation indicates that the transient and the steady-state aerodynamics for the impulsively started motion of a body are accurately captured by the simulations. In addition, comparisons against experimental results show that rotors in hover flight are acceptably represented for small angles of attack of the blades, within the limitations of the flat-plate representation and the inviscid hypothesis of the model. Regarding applications, six different models are analyzed: 1) a single propeller rotor, 2) a small-scale quadcopter drone, 3) a flying-wing drone, 4) a joined-wing aircraft, 5) a drag-reduction system for cars, and, 6) a flapping-wing insect. In all cases, the results indicate that a well-tuned simulation based on vorticity-dominated flows is widely suitable for the aerodynamic study of several cases. All simulations expose the main advantages of a modular object-oriented program: the ability to change the case of analysis with minimal reprogramming, and the flexibility to link with different models in a co-simulation strategy.

Effects of flapping-motion profiles on insect-wing aerodynamics

Flapping wings of insects can follow various complex-motion waveforms, influencing the flow structures over a wing and consequently the aerodynamic performance. However, most studies of insect-wing models incorporate either simple harmonic or robofly-like motion waveforms. The effects of other waveforms appear to be under-explored. Motivated by this, the present study investigates the individual and combined effects of the sweep- and pitch-motion waveforms for fixed flapping frequency and amplitude of a fruit-fly wing planform. Physical experiments are conducted to directly measure the forces and torques acting on the wing. Interestingly, the sweep waveform is observed to influence the overall variation in the lift coefficient ( ), whereas the pitch waveform is observed to influence only the instantaneous during stroke reversal. Carefully validated three-dimensional numerical simulations reveal that a change in the strength of the large-scale vortex over the wing as the sweep profile parameter is varied is responsible for the observed variations in . An exploration over wide ranges of the sweep and the pitch profile parameters shows that the waveforms maximising the mean lift coefficient are different from those maximising the power economy. Consistent with some previous experiments on robotic insects, the possibility of passive pitch motion is observed at slower pitching rates. Contours of the mean lift coefficient and power economy mapped on the planes of the sweep and the pitch profile parameters can help designers of flapping-wing micro air vehicles in selecting the waveforms appropriate for their design criteria.

An investigation of kinematic parameters and stroke function on stroke reversal for three-dimensional vortex structures around a flapping insect wing

Abstract Complex flows or vortical structures are observed around flapping wings. In particular, as observed in various experiments and numerical simulations, these flows are affected when kinematic parameters and translational velocity functions are changed. In the present study, numerical simulations of three-dimensional flows around a flapping wing are conducted to investigate parameters and translational velocity functions on the stroke reversal stage using immersed boundary lattice Boltzmann method. We consider a flapping Drosophila wing without pitching motion, and the effects of the stroke amplitude and stroke reversal duration are investigated. First, at high stroke amplitude, we found that a hairpin-like vortex loop is obviously observed. Second, the stroke reversal duration affects remnant vortex structures in the wake. In addition, this parameter affects a time instant which the wake capture occurs. Third, it is shown that the translational velocity function has a significant effect on drag force. If the translational velocity function is modeled with trigonometric function, the drag force has a discrepancy with well-known experimental data. In order to solve the problem, we suggest a higher order polynomial. As a result, our function shows a better agreement with the experimental data.

A Novel Fluid–Structure Interaction Framework for Flapping, Flexible Wings

Fluid–structure interaction (FSI) plays a significant role in the deformation of flapping insect wings. However, many current FSI models are high-order and rely on direct computational methods, thereby limiting parametric studies as well as insights into the physics governing wing dynamics. We develop a novel flapping wing FSI framework that accommodates general wing geometry and fluid loading. We use this framework to study the unilaterally coupled FSI of an idealized hawkmoth forewing considering two fluid models: Reynolds-averaged Navier–Stokes computational fluid dynamics (RANS CFD) and blade element theory (BET). We first compare aerodynamic modal forces estimated by the low-order BET model to those calculated via high fidelity RANS CFD. We find that for realistic flapping kinematics, BET estimates modal forces five orders of magnitude faster than CFD within reasonable accuracy. Over the range flapping kinematics considered, BET and CFD estimated modal forces vary maximally by 350% in magnitude and approximately π/2 radians in phase. The large reduction in computational time offered by BET facilitates high-dimensional parametric design of flapping-wing-based technologies. Next, we compare the contributions of aerodynamic and inertial forces to wing deformation. Under the unilateral coupling assumption, aerodynamic and inertial-elastic forces are on the same order of magnitude—however, inertial-elastic forces primarily excite the wing’s bending mode whereas aerodynamic forces primarily excite the wing’s torsional mode. This suggests that, via conscientious sensor placement and orientation, biological wings may be able to sense independently inertial and aerodynamic forces.

Wing flexibility reduces the energetic requirements of insect flight

Flapping insect wings deform under aerodynamic as well as inertial-elastic forces. This deformation is thought to improve power economy and reduce the energetic costs of flight. However, many flapping wing models employ rigid body simplifications or demand excessive computational power, and are consequently unable to identify the influence of flexibility on flight energetics. Here, we derive a reduced-order model capable of estimating the driving torques and corresponding power of flapping, flexible insect wings. We validate this model by actuating a tobacco hornworm hawkmoth Manduca sexta (L.) forewing with a custom single-degree-of-freedom mechanical flapper. Our model predicts measured torques and instantaneous power with reasonable accuracy. Moreover, the flexible wing model predicts experimental trends that rigid body models cannot, which suggests compliance should not be neglected when considering flight dynamics at this scale. Next, we use our model to investigate flight energetics with realistic flapping kinematics. We find that when the natural frequency of the wing is roughly three times that of the flapping frequency, flexibility can reduce energy expenditures by almost 25% compared to a rigid wing if negative work is stored as potential energy and subsequently released to do positive work. The wing itself can store about 30% of the 1200 J of total energy required over a wingbeat. Peak potential energy storage occurs immediately before stroke reversal. We estimate that for a moth weighing 1.5–2.5 g, the peak instantaneous power required for flight is 75–125 W kg−1. However, these peak values are likely lower in natural insect flight, where the wing is able to exchange strain energy with the compliant thorax. Our findings highlight the importance of flexibility in flapping wing micro aerial vehicle design and suggest tuned flexibility can greatly improve vehicle efficiency.

Computational and Experimental Investigation of a Flapping-Wing Micro Air Vehicle in Hover

Experimental and computational study of flapping-wing insect at low Reynolds number is conducted in this work. The paper is broadly divided into two sections: experiments and computational fluid dynamics–based numerical analysis. A dynamically scaled wing is simulated using Overset Transonic Rotor Unsteady Navier–Stokes (OVERTURNS) computational platform, a three-dimensional Navier–Stokes solver. The computational results are compared with experimental results of an isolated flapping wing performed in an oil tank. Good agreement was observed between the experiments and numerical simulations in terms of magnitude and trend of the unsteady instantaneous lift and drag forces over the flapping cycle. A parametric study was performed to observe the influence of the kinematic, flapping frequency and pitch angle, on the force production and power requirements. As a consequence of the study, lift-to-power ratio versus average lift was identified as a principal efficiency metric to assess the performance of flapping-wing vehicles for given geometry and kinematic parameters. A clear advantage for a wing pitch angle of 40° was observed compared with that of 30° or 60°.

New mechanism reveals the aerodynamics of flapping insect wings

A tornado-like force that keeps flying insects afloat may be partially stabilized by a torque arising from velocity gradients.

Design of flexible hinges in electromagnetically driven artificial flapping-wing insects for improved lift force

This paper presents a design approach to determine the bending stiffness of the flexible hinge in electromagnetically driven artificial flapping-wing insects for improved lift force. The prototype insects are fabricated by laser cutting technique and have a wingspan of 3.6 cm and a total mass of 84 mg. To induce effective wing rotational movements for improved aerodynamic performances, the geometric parameters of the flexible hinge in the wing rotation mechanism are optimized based on lift force simulations and bending stiffness tests on the enlarged flexible hinges. With an applied AC current of 420 mA (AC voltage of 0.5 V), the prototype #4 with optimized flexible hinge can produce effective wing rotational movements with a maximum rotation angle of 50° and a flapping amplitude of ±50.6° at 65 Hz. Such wing movements can generate a measured average lift force of 384.5 µN to produce a total lift force to weight ratio of 0.47. With this magnitude of lift force, the prototype can slide on a tilted guide rail with an inclined angle of 7°. Such results validate the feasibility of the design approach of the flexible hinge in the artificial insects for improved lift force.