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The Importance of Flapping Kinematic Parameters in the Facilitation of the Different Flight Modes of Dragonflies
Abstract
To better understand dragonflies’ remarkable flapping wing aerodynamic performance, we measured the kinematic parameters of the wings in two different flight modes (Normal Flight Mode (NFM) and Escape Flight Mode (EFM)). When the specimens switched from normal to escape mode the flapping frequency was invariant, but the stroke plane of the wings was more horizontally inclined. The flapping of both wings was adjusted to be more ventral with a change of the pitching angle that resulted in a larger angle of attack during downstroke and smaller during upstroke to affect the flow directions and the added mass effect. Noticeably, the phasing between the fore and hind pair of wings varies between two flight modes, which affects the wing-wing interaction as well as body oscillations. It is found that the momentum stream in the wake of EFM is qualitatively different from that in NFM. The change of the stroke plane angle and the varied pitching angle of the wings diverts the momentum downwards, while the smaller flapping amplitude and less phase difference between the wings compresses the momentum stream. It seems that in order to achieve greater flight maneuverability a flight vehicle needs to actively control positional angle as well as the pitching angle of the flapping wings.
... The wings towards the axis of the turn sweep slower with a higher angle of attack during the downstroke while sweeping faster with a lower angle of attack during the upstroke than the outer wings resulting in higher drag on the inner side especially during downstroke and a thrust boost on the outer side during upstroke (Li & Dong 2017). Recently Liu et al. (2021b) studied the change of the flapping kinematics of a live tethered dragonfly resulting in two distinct flight modes: a normal (forward) flight mode and an escape-like manoeuvring flight mode. Escape flight mode, or escape flapping kinematics, is primarily a manoeuvring flight behaviour for an attempted quick take-off or 'mid-air jump' characterized by large transient lift peaks and a downward diverted compressed wake (Somps & Luttges 1986;Yates 1986; Reavis & Luttges 1988;Liu et al. 2021b). ...
... Recently Liu et al. (2021b) studied the change of the flapping kinematics of a live tethered dragonfly resulting in two distinct flight modes: a normal (forward) flight mode and an escape-like manoeuvring flight mode. Escape flight mode, or escape flapping kinematics, is primarily a manoeuvring flight behaviour for an attempted quick take-off or 'mid-air jump' characterized by large transient lift peaks and a downward diverted compressed wake (Somps & Luttges 1986;Yates 1986; Reavis & Luttges 1988;Liu et al. 2021b). Liu et al. (2021b) show that the dragonfly does not alter its flapping frequency substantially for facilitating the escape flight but controls the angle of attack of its wings by the change of the stroke plane angle and the pitching, while adjusting the stroke amplitude of the wings and their phasing are hypothesized to increase the propulsive efficiency (Liu et al. 2021b). ...
... Escape flight mode, or escape flapping kinematics, is primarily a manoeuvring flight behaviour for an attempted quick take-off or 'mid-air jump' characterized by large transient lift peaks and a downward diverted compressed wake (Somps & Luttges 1986;Yates 1986; Reavis & Luttges 1988;Liu et al. 2021b). Liu et al. (2021b) show that the dragonfly does not alter its flapping frequency substantially for facilitating the escape flight but controls the angle of attack of its wings by the change of the stroke plane angle and the pitching, while adjusting the stroke amplitude of the wings and their phasing are hypothesized to increase the propulsive efficiency (Liu et al. 2021b). The experimental methodology, however, does not allow the independent variation of these kinematic parameters, so it is not yet well understood as to how each of these changes take part in flight control or whether there is any synergistic effect between them. ...
This paper presents the effects of wing kinematics on both normal forward flight and escape flight of a dragonfly. A Navier-Stokes-based numerical model has been adopted, and results have been substantiated by experimental data. The wing kinematics of tethered specimens and the prescribed wing morphology of a free-flying dragonfly were used in the simulation. To shed light on the interplay between kinematics and aerodynamics, a parametric study of the kinematics has been conducted. It is found that in escape flight, the dragonfly generates additional lift while the thrust reduces and the overall efficiency drops. Compared with normal forward flight, the escape mode produces larger lift force peaks. When the kinematics change to facilitate escape flight, the aerodynamic forces are affected by not only the flapping kinematics but, in the case of the hindwing, the varied wing-wing vortex interactions. The direction of the resultant force on each wing changes according to the change of the mean of pitching angle and stroke plane angle. We found that in the studied configurations, the varied phasing of the wings has a marginal effect on the aerodynamics of the dragonfly. It reduces lift and increases thrust, and this force modulation is slightly more efficient when the local angle of attack also changes. On the other hand, the change of angle of attack played a major role in leading-edge vortex formations and directing the resultant forces of the wings. The results can be useful in developing flight control strategies for micro air vehicle design.
... Through the evolution over millions of years, remarkable flight abilities have developed with complex modes of locomotion [2]. The evolution of aerial locomotion was accompanied by the acquisition of anatomical and physiological adaptations in flapping flying [3][4][5]. The adaptions in birds include the fusion of parts of the skeleton, the pneumaticity of bones, and equipment of wings with strong yet lightweight feathers [6]. ...
... Five flight feathers (Nos. [1][2][3][4][5] were tested under two deflection directions, repeated three times for each feather and each direction. Then the vanes were removed from the shaft of the feathers using a razor blade and the damping tests were repeated on the shafts without the vanes. ...
Bird feathers sustain bending and vibrations during flight. Such unwanted vibrations could potentially cause noise and flight instabilities. Damping could alter the system response, resulting in improving quiet flight, stability, and controllability. Vanes of feathers are known to be indispensable for supporting the aerodynamic function of the wings. The relationship between the hierarchical structures of vanes and the mechanical properties of the feather has been previously studied. However, still little is known about their relationship with feathers’ damping properties. Here, the role of vanes in feathers’ damping properties was quantified. The vibrations of the feathers with vanes and the bare shaft without vanes after step deflections in the plane of the vanes and perpendicular to it were measured using high-speed video recording. The presence of several main natural vibration modes was observed in the feathers with vanes. After trimming vanes, more vibration modes were observed, the fundamental frequencies increased by 51–70%, and the damping ratio decreased by 38–60%. Therefore, we suggest that vanes largely increase feather damping properties. Damping mechanisms based on the morphology of feather vanes are discussed. The aerodynamic damping is connected with the planar vane surface, the structural damping is related to the interlocking between barbules and barbs, and the material damping is caused by the foamy medulla inside barbs.
... Meanwhile, some researchers also set their sights on the flight principle of dragonflies 12 , as one of the oldest creatures on the earth, who have special tandem flapping configurations and show remarkable flight performance. Observational studies on dragonfly flight kinematics have revealed their typical features, that is, an adjustable phase difference between their forewings and hindwings 13 and the asymmetric flapping mode with an inclined stroke plane 14,15 . ...
Dragonflies show impressive flight performance due to their unique tandem flapping wing configuration. While previous studies focused on forewing-hindwing interference in dragonfly-like flapping wings, few have explored the role of asymmetric pitching angle in tandem flapping wings. This paper compares the aerodynamic performance of asymmetric dragonfly-like wings with symmetric hummingbird-like wings, both arranged in tandem. Using a three-dimensional numerical model, we analyzed wing configurations with single/tandem wings, advance ratios (J) from 0 to 0.45, and forewing-hindwing phase differences from 0 to 180 at a Reynolds number of 7000. Results show that asymmetric flapping wings exhibit higher vertical force and flight efficiency in both single and tandem wing configurations. Increasing the phase difference improves flight efficiency with minimal loss of vertical force in the asymmetric flapping mode, while the symmetrical flapping mode significantly reduces vertical force at a 180 phase difference. Additionally, symmetric tandem flapping wings unexpectedly gain extra vertical force during in-phase flapping. This study uncovers the flow characteristics of dragonfly-like tandem flapping wings, providing a theoretical basis for the design of tandem flapping wing robots.
... The transition from normal flight mode to escape flight mode can be considered by modifying the motion pattern of the pitch angle. 11 The lift-generation mechanisms of insect flight are diverse. Initially, researchers focused solely on quasi-steady aerodynamic effects. ...
During the hovering flight of dragonflies, the coupling interaction between the forewings and hindwings leads to a reduction in the lift of each wing. Numerous scholars have reached a unanimous conclusion that under the coupling effect, the lift of the hindwings is significantly decreased. Meanwhile, the coupling of the forewings and hindwings enhances the controllability of dragonfly flight. In this article, a novel hovering flight model termed the partial advanced dual-wing model (PADM) is proposed. This model is capable of increasing the lift of both the forewings and hindwings. The maximum average lift of the forewings is increased by 18.09%, and the maximum average lift of the hindwings is increased by 41.58%. In addition to the shared advantage of enhanced positive pressure on the rear half of the wing surface due to the advanced rotation, the superior performance of the hindwings compared to the forewings is attributed to the hindwings cutting off the trailing-edge vortex ring formed by the coupling of the fore and hind wings during the downstroke phase. The vertical force and energy consumption exhibit a linear relationship with the partially advanced time, independent of the coupled aerodynamic effects. The PADM model not only sustains the weight of the dragonfly but also plays a controlling role in transitioning from a hovering flight model to a vertical leap flight model. Furthermore, it enables dragonflies and micro air vehicles to maintain hovering flight while carrying additional loads.
... Indeed, the complex interactions between wing morphology and the various flight modes odonates adopt pending the context, such as foraging, escape and male-male confrontation modes, are yet to be disentangled. Moreover, flight kinematics likely play an important role, for fore-and hindwing likely perform differently [55,56]. Nevertheless, it is generally admitted that petiolate wings allow slow flight speed owing to their higher torsional compliance, making it possible to generate lift during both down-and upstroke [54,57]. ...
During its 320 Myr evolution, dragon-and damselfly (Odonata) wing morphology underwent intense modifications. The resulting diversity prompted comparative analyses focusing on phylogeny. However, homoplasy proved to plague wing-related characters. Concurrently, limited benefits were obtained from considering fossil taxa, similarly impacted. Herein, we investigate two aspects particularly affected by convergence, namely the acquisition of vein-like structuring elements derived from regular cross-venation, termed conamina; and the evolution of butter knife wing shape. Conamen implementation is found to be consistently linked with vein curvature sharpening, itself generating potential breaking points. Conamina therefore likely evolved to address wing integrity issues during ever-more-demanding flight performance. Moreover, an existing conamen is likely to trigger the acquisition of further, associated conamina. As for butter knife shape, previously documented in the extinct Archizygoptera and among damselflies, we report a new, 315 Ma occurrence with the rare species Haidilaozhen cuiae gen. et sp. nov. (family Haidilaozhenidae fam. nov.), from the Xiaheyan locality (China). The repeated acquisition of butter knife-shaped wing can be related to slow speed flight and, in turn, predator avoidance. In both cases of iterated regularities, the unique 'network-and-membrane' wing design proper to insects is found to compose a strong, constraining factor.
... Passive means of wing morphing during flight could contribute to the overall flight efficiency by reducing the load on flight musculature. Pitching is also a primary tool for dragonflies to control some of their unsteady flow features [e.g., leading edge vortex (LEV), rapid pitch rotations, and wake capture], the importance of which in low Reynolds number flight cannot be overstated (Thomas et al., 2004;Liu et al., 2021a). Regarding the aerodynamic effects and importance of wing pitching, there is ample literature on this, for example, the phase of pitching to the flapping and the asymmetry (Chen et al., 2022;Wang et al., 2016;Diaz-Arriba et al., 2021) in upstroke vs downstroke as well as the duration (Park and Choi, 2012;Sudhakar and Vengadesan, 2010;Bluman and Kang, 2017) of the pitching motion at stroke reversal. ...
In this work we designed and characterized a passive structural wing actuation setup that was able to realistically mimic the flapping and pitching kinematics of dragonflies. In this setup an inelastic string limited the wing pitch that may be sufficiently simple for practical micro air vehicle (MAV) applications. In order to further evaluate the dominance of inertial passive and active muscle-controlled pitch actuation in dragonfly flight, the flow fields and pitching angle variations of the naturally actuated wing of a tethered dragonfly were compared with that of the same wing artificially actuated via a proposed passive mechanism. We found that passive rotation characterizes most of the forewing flapping cycle except the upstroke reversal where the dragonfly uses its muscle movement to accelerate its forewing rotation. The measured flow fields show that accelerated wing rotation at the upstroke reversal will result in a stronger leading edge vortex (LEV) during the downstroke, the additional force from which is estimated to account for 4.3% of the total cycle averaged force generated.
... They can hover (Ellington, 1984), turn 90°-180°in two or three wing beats (Li and Dong, 2017), glide (Wakeling and Ellington, 1997a), and produce total aerodynamic force equal to~4.3 times their own body weight (Su et al., 2020). Therefore, researchers are interested in their unique flapping characteristics and excellent flying skills, and hope that studying the aerodynamic characteristics of dragonflies can provide guidance for the optimization of MAV (Liu et al., 2021a;Liu et al., 2021b). The accurate description of flapping kinematics and the investigation of flapping aerodynamics under various kinds of dragonfly flight modes (Wakeling. ...
This study presents a detailed analysis of dragonflies’ climbing flight by integratinghigh-speed photogrammetry, three-dimensional reconstruction, and computational fluid dynamics. In this study, a dragonfly’s climbing flight is captured by two high-speed cameras with orthogonal optical axes. Through feature point matching and three-dimensional reconstruction, the body kinematics and wing kinematics of 22 dragonflies in climbing flight are accurately captured. Experimental results show that the climbing angles (η) are distributed from 10° to 80° and are concentrated within two ranges, 60°–70° (36%) and 20°–30° (32%), which are defined as large angle climb (LAC) and small angle climb (SAC), respectively. In order to study the aerodynamic mechanism of the climbing flight based on the biological observation results, the kinematic parameters of the dragonfly during LAC and SAC are selected for analysis and numerical simulation. The results show that the climbing angle η and wing kinematics are related. There are considerable differences in wing kinematics during climbing with different η, while the wing kinematics are unchanged during climbing with similar η. With the increase in η, the phase difference (λ) between the forewing and the hind wing decreases and the amplitude of the positional angle (θ mean) of the hind wing increases, while θ mean of the forewing remains almost unchanged. Through numerical simulation of LAC and SAC, it can be found that during the climb with different η, the different wing kinematics have a significant influence on aerodynamic performance. During SAC, the increase in λ and the decrease in θ mean of the hind wing weaken the aerodynamic disturbance of the forewing by the vortex wing of the hind wing, thus improving the flight efficiency.
... Although the wing kinematics of dragonflies during their free flying are already clear at present and the aerodynamic performance of dragonflies in actual flight is preliminarily understood, the effect of the cooperation among each flapping motion on the aerodynamic performance is still unclear. Recent studies have shown that the pitching motion of the tandem wings plays a significant role in weight support of the dragonfly [13] [14]. To better understand the relationship between the pitching motion and flapping motion on the tandem flapping wing, various pitching amplitudes have been numerically investigated during a three dimensional dragonfly-like wing's hovering motion. ...
Dragonflies have remarkable flight skills and their excellent flight performance has attracted persistent attention. The multi-degree-of-freedom flapping kinematics and interaction of the tandem wings might account for their extraordinary flight skills. In this paper, the effects of pitching motion on the aerodynamic performance of a dragonfly-like flapping wing have been numerically studied. A transient numerical method based on the overset mesh technique is used to simulate the flapping and pitching movements. Different pitching amplitudes have been evaluated as the forewing and hindwing flap in counter-stroking during the hovering process. It is found that the pitching motion has an obvious influence on the tandem wings' aerodynamic performance, and there is a reasonable pitching amplitude to make the hovering vertical force optimal. Additionally, the interaction of the tandem configuration will lead to an obvious fluctuation in the aerodynamic force. The research in this paper is helpful to understand the flight mechanism of dragonflies flight.
... To study the unsteady aerodynamics of flying animals in forward flight, most researchers have simplified the lateral flapping wing in a flow with fixed freestream velocity. In this way, several researchers have extensively studied the effects of different kinematic [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and structural parameters [21][22][23][24][25][26] on the lift, energy consumption, and efficiency of flapping wings in propulsive motion. It has been found that when these parameters are given suitable values, flapping wings can provide superior flight performance, like high lift and power efficiency. ...
In the natural world, numerous flying creatures generate both thrust and lift by flapping their wings. Aerodynamic mechanisms of forward flight with flapping wings have received much attention from researchers. However, the majority of previous studies have simplified the forward-flight motion of flapping wings to be uniform, and there has been no detailed evaluation of the validity of this simplification. Motivated by this, aerodynamic characteristics of a self-propelled flapping wing with a non-zero angle of attack were investigated. The results showed that the asymmetric leading-edge vortex produced in the wing's upstroke and downstroke leads to transient thrust, driving the self-propelled wing to move with variable forward velocities. Compared to the uniform forward-flight cases, significant losses in lift and severe changes in the flow field were observed in self-propelled flapping wings. In addition, the changes in the aerodynamic performance—including the forward propulsion speed, lift, and power efficiency—of the self-propelled flapping wing with changes in various dimensionless parameters were also investigated. The heaving amplitude was shown to have significant effects on lift and propulsion speed of the self-propelled flapping wing, while the effects of ratio between the airfoil density and fluid density as well as the Reynolds number, were relatively small. In most conditions, when the Strouhal number was in the range 0.2–0.4, the self-propelled flapping wing performed well in terms of both lift generation and propulsive efficiency. These research results suggest that it is necessary to consider the fluctuating forward speed in aerodynamic modeling of propulsive flapping wings.
... Dragonfly flight is aerodynamically advanced, and great maneuverability and different flight modes can be achieved by the precise angling and independent motion of the fore-and hindwings (Salami et al., 2019;Liu et al., 2021). When migrating, globe skimmers have been observed to travel at velocities of 3.1-7 m/s depending on tailwind (Srygley, 2003;Feng et al., 2006). ...
Insect migration redistributes enormous quantities of biomass, nutrients and species globally. A subset of insect migrants perform extreme long-distance journeys, requiring specialized morphological, physiological and behavioral adaptations. The migratory globe skimmer dragonfly (Pantala flavescens) is hypothesized to migrate from India across the Indian Ocean to East Africa in the autumn, with a subsequent generation thought to return to India from East Africa the following spring. Using an energetic flight model and wind trajectory analysis, we evaluate the dynamics of this proposed transoceanic migration, which is considered to be the longest regular non-stop migratory flight when accounting for body size. The energetic flight model suggests that a mixed strategy of gliding and active flapping would allow a globe skimmer to stay airborne for up to 230–286 h, assuming that the metabolic rate of gliding flight is close to that of resting. If engaged in continuous active flapping flight only, the flight time is severely reduced to ∼4 h. Relying only on self-powered flight (combining active flapping and gliding), a globe skimmer could cross the Indian Ocean, but the migration would have to occur where the ocean crossing is shortest, at an exceptionally fast gliding speed and with little headwind. Consequently, we deem this scenario unlikely and suggest that wind assistance is essential for the crossing. The wind trajectory analysis reveals intra- and inter-seasonal differences in availability of favorable tailwinds, with only 15.2% of simulated migration trajectories successfully reaching land in autumn but 40.9% in spring, taking on average 127 and 55 h respectively. Thus, there is a pronounced requirement on dragonflies to be able to select favorable winds, especially in autumn. In conclusion, a multi-generational, migratory circuit of the Indian Ocean by the globe skimmer is shown to be achievable, provided that advanced adaptations in physiological endurance, behavior and wind selection ability are present. Given that migration over the Indian Ocean would be heavily dependent on the assistance of favorable winds, occurring during a relatively narrow time window, the proposed flyway is potentially susceptible to disruption, if wind system patterns were to be affected by climatic change.
... In 1951, Waloff & Rainey [6,7] presented a quantitative study on the East African desert locust's swarming behaviour concluding that their endured gliding is directly associated with their tendency to fly in swarms. A few years later, Weis-Fogh [8] commenced a detailed study on the locust's aerodynamic footprint that is continuously pursued by other researchers presented in the contemporary literature [9][10][11][12][13][14]. Recent studies suggest that the wing mechanical properties and morphology in tandem configuration are the other complementary factors contributing to locust's ultra-high aerodynamic performance. ...
In aviation, gliding is the most economical mode of flight explicitly appreciated by natural fliers. They achieve it by high-performance wing structures evolved over millions of years in nature. Among other prehistoric beings, locust is a perfect example of such natural glider capable of endured transatlantic flights that could inspire a practical solution to achieve similar capabilities on micro aerial vehicles. Although an investigation in this study demonstrates the effects of haemolymph on the flexibility of several flying insect wings proving that many species exist with further simplistic yet well designed wing structures.
However, biomimicry of such aerodynamic and structural properties is hindered by the limitations of modern as well as conventional fabrication technologies in terms of availability and precision, respectively. Therefore, here we adopt finite element analysis to investigate the manufacturing-worthiness of a 3D digitally-reconstructed locust wing, and propose novel combinations of economical and readily-available manufacturing methods to develop the model into prototypes that are structurally similar to their counterparts in nature while maintaining the optimum gliding ratio previously obtained in the aerodynamic simulations. The former is assessed here via an experimental analysis of the flexural stiffness and maximum deformation rate as EI_s=1.34e-4 Nm2, EI_c=5.67e-6 Nm2, and >148.2%, respectively.
Ultimately, a comparative study of the mechanical properties reveals the feasibility of each prototype for gliding micro aerial vehicle applications.
... To study the unsteady aerodynamics of flying animals in forward flight, most researchers have simplified the lateral flapping wing in a flow with fixed freestream velocity. In this way, several researchers have extensively studied the effects of different kinematic [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and structural parameters [21][22][23][24][25][26] on the lift, energy consumption, and efficiency of flapping wings in propulsive motion. It has been found that when these parameters are given suitable values, flapping wings can provide superior flight performance, like high lift and power efficiency. ...
Flapping rotary wing (FRW) is a promising wing layout applicable to micro air vehicles design due to its capability in high lift production. Previous aerodynamic investigations of FRW have mainly focused on hovering flight, while forward flight has been relatively ignored. Therefore, this numerical study aims to address the unsteady aerodynamics of FRW in forward flight based on a FRW configuration with a forward-tilted rotational plane. Results show that compared with hovering flight, forward flying FRW shows a rotational-cycle-averaged thrust and lift reduction and an enhanced rotating moment, all of which can be mainly ascribed to the changes in the aerodynamics on the retreating side. For FRW rotating at a specified rotational speed, the larger advance ratio and severe forward tilt of its rotational plane are both disadvantageous to thrust production and can lead to significant lift reduction. However, they are beneficial for rotational moment enhancement and conducive to FRW rotating at higher speeds, thereby further compensating for the losses in thrust and lift. By further taking a real FRW at near-zero rotating moment into consideration, high thrust and lift can be achieved at the same time in cases of high advance ratio and large rotational plane tilt angle due to fast passive rotating, which differs from flapping wing and rotary wing. These findings can expand our understanding of the unsteady aerodynamics of flapping wing with complex kinematics in forward flight and provide guidance to the design of an FRW micro air vehicle that is capable of forward flight.
The four-winged form of dragonfly and damselfly allows them to fly with great agility and endurance, which is accomplished by independently controlling the kinematics of each wing. In this study, we have performed numerical simulations of two tandem airfoils oscillating along an inclined stroke plane at Re=157. We investigated the effect of stroke plane angle (β) of forefoil and hindfoil on the aerodynamic performance of dragonflies (or damselflies) hovering flight. The simulations are carried out for parallel and non-parallel stroke planes of forefoil and hindfoil oscillating with three phase differences i.e., ϕ=0^o, 90^o and 180^o. For parallel stroke planes, the results show that total lift increases with β whereas total thrust decreases. Also, the total lift as well as thrust reduces with an increase in ϕ. The forefoil performance is affected by the flow induced by hindfoil leading-edge vortices (LEVs), and the hindfoil interaction with forefoil wake vortices significantly affects the hindfoil performance. The results show that non-parallel stroke planes have detrimental effects on the total lift for ϕ=0^o and 90^o. However, for ϕ=180^o, lift augmentation of 46% is obtained in the case where the forefoil stroke plane angle is smaller than the hindfoil. The results obtained during this investigation can help in optimizing the wing kinematics during the Micro-Air-Vehicles (MAVs) development.
This research is taking the first steps toward applying a 2D dragonfly wing skeleton in the design of an airplane wing using artificial intelligence. The work relates the 2D morphology of the structural network of dragonfly veins to a secondary graph that is topologically dual and geometrically perpendicular to the initial network. This secondary network is referred as the reciprocal diagram proposed by Maxwell that can represent the static equilibrium of forces in the initial graph. Surprisingly, the secondary graph shows a direct relationship between the thickness of the structural members of a dragonfly wing and their in-plane static equilibrium of forces that gives the location of the primary and secondary veins in the network. The initial and the reciprocal graph of the wing are used to train an integrated and comprehensive machine-learning model that can generate similar graphs with both primary and secondary veins for a given boundary geometry. The result shows that the proposed algorithm can generate similar vein networks for an arbitrary boundary geometry with no prior topological information or the primary veins' location. The structural performance of the dragonfly wing in nature also motivated the authors to test this research's real-world application for designing the cellular structures for the core of airplane wings as cantilever porous beams. The boundary geometry of various airplane wings is used as an input for the design proccedure. The internal structure is generated using the training model of the dragonfly veins and their reciprocal graphs. One application of this method is experimentally and numerically examined for designing the cellular core, 3D printed by fused deposition modeling, of the airfoil wing; the results suggest up to 25% improvements in the out-of-plane stiffness. The findings demonstrate that the proposed machine-learning-assisted approach can facilitate the generation of multiscale architectural patterns inspired by nature to form lightweight load-bearable elements with superior structural properties.
Odonata flight performance capabilities and behaviour and their body and wing form diversity are explored, and their interrelationships discussed theoretically and from observational evidence. Overall size and particularly wing loading appear predictably to be related to speed range. In Anisoptera at least, relatively short bodies and long wings should favour high speed manoeuvrability, though further information is needed. Medium and low aspect ratio wings are associated with gliding and soaring, but the significance of aspect ratio in flapping flight is less straightforward, and much depends on kinematics. Narrow wing bases, petiolation, basal vein fusion, distal concentration of area and a proximally positioned nodus – described by a newly defined variable, the “nodal index” – all allow high torsion between half-strokes and favour habitually slow flight, while broad wing bases are useful at higher speeds. The “basal complex” in all families seems to be a mechanism for automatic lowering of the trailing edge and maintenance of an effective angle of attack, but the relative merits of different configurations are not yet clear. There is serious need for more quantitative information on a wider range of species and families.
In the recent decades, biomimetic robots have attracted scientific communities’ attention increasingly, as people try to learn from nature in which exist astonishing and uniquely evolved mechanisms shown by very species. Dragonfly, as such one example, demonstrates unique and superior flight performance than most of the other insect species and birds. Researchers are obsessed with the aerodynamic characteristics of an in-flight dragonfly as two pairs of independently controlled wings provide them with an unmatchable flying performance and robustness. In this paper, an extensive review of recent studies related to the flight aerodynamics of dragonflies has been conducted. The main research findings about effect of the motion parameters and body attitude on the resulting aerodynamic forces and power requirements in different flight modes of a dragonfly are summarized. Particular attention is given to functional characteristics of dragonfly wings and the importance of mutual interaction between forewing and hindwing for its flyability. This article aims to bring together current understandings of dragonfly aerodynamics and thus has certain reference value to design and control of dragonfly-inspired biomimetic devices.
This work is a synthesis of our current understanding of the mechanics, aerodynamics and visually mediated control of dragonfly and damselfly flight, with the addition of new experimental and computational data in several key areas. These are: the diversity of dragonfly wing morphologies, the aerodynamics of gliding flight, force generation in flapping flight, aerodynamic efficiency, comparative flight performance and pursuit strategies during predatory and territorial flights. New data are set in context by brief reviews covering anatomy at several scales, insect aerodynamics, neuromechanics and behaviour. We achieve a new perspective by means of a diverse range of techniques, including laser-line mapping of wing topographies, computational fluid dynamics simulations of finely detailed wing geometries, quantitative imaging using particle image velocimetry of on-wing and wake flow patterns, classical aerodynamic theory, photography in the field, infrared motion capture and multi-camera optical tracking of free flight trajectories in laboratory environments. Our comprehensive approach enables a novel synthesis of datasets and subfields that integrates many aspects of flight from the neurobiology of the compound eye, through the aeromechanical interface with the surrounding fluid, to flight performance under cruising and higher-energy behavioural modes.
This article is part of the themed issue ?Moving in a moving medium: new perspectives on flight?.
The phase change between the forewing and hindwing is a distinct feature that sets dragonfly apart from other insects. In this paper, we investigated the aerodynamic effects of varying forewing-hindwing phase difference with a 60° inclined stroke plane during hovering flight. Force measurements on a pair of mechanical wing models showed that in-phase flight enhanced the forewing lift by 17% and the hindwing lift was reduced at most phase differences. The total lift of both wings was also reduced at most phase differences and only increased at a phase range around in-phase. The results may explain the commonly observed behavior of the dragonfly where 0° is employed in acceleration. We further investigated the wing-wing interaction mechanism using the digital particle image velocimetry (PIV) system, and found that the forewing generated a downwash flow which is responsible for the lift reduction on the hindwing. On the other hand, an upwash flow resulted from the leading edge vortex of the hindwing helps to enhance lift on the forewing. The results suggest that the dragonflies alter the phase differences to control timing of the occurrence of flow interactions to achieve certain aerodynamic effects.
We investigate how the blood flow in the veins in the flapping wings of a dragonfly affects their dynamic response. An idealized model of an elastic tube conveying fluid and rotating around a fixed axis is adopted in this study, based on which governing partial differential equations of motion are obtained by invoking the extended Hamilton’s principle. Separation of variables techniques and assumed modes method are employed to solve the resulting equations, and the stabilization analysis is performed to assess the stability of the system. In particular, the coupling effects of tube rotation, deformation, and the movement of the fluid inside are evaluated under different flow rates and rotation speeds. This demonstrates that if the blood in the dragonfly wings flows from humeral angle distally to the wing apex, a stabilization effect can be obtained, and the higher the blood flow rate is, the faster the system will be stabilized. Contrary cases are also studied for further validation of the model.
In this study, we experimentally studied the relationship between wingbeat frequency and resonant frequency of 30 individuals of eight insect species from five orders: Odonata (Sympetrum flaveolum), Lepidoptera (Pieris rapae, Plusia gamma and Ochlodes), Hymenoptera (Xylocopa pubescens and Bombus rupestric), Hemiptera (Tibicen linnei) and Coleoptera (Allomyrina dichotoma). The wingbeat frequency of free-flying insects was measured using a high-speed camera while the natural frequency was determined using a laser displacement sensor along with a Bruel and Kjaer fast Fourier transform analyzer based on the base excitation method. The results showed that the wingbeat frequency was related to body mass (m) and forewing area (Af), following the proportionality f ∼ m(1/2)/Af, while the natural frequency was significantly correlated with area density (f0 ∼ mw/Af, mw is the wing mass). In addition, from the comparison of wingbeat frequency to natural frequency, the ratio between wingbeat frequency and natural frequency was found to be, in general, between 0.13 and 0.67 for the insects flapping at a lower wingbeat frequency (less than 100 Hz) and higher than 1.22 for the insects flapping at a higher wingbeat frequency (higher than 100 Hz). These results suggest that wingbeat frequency does not have a strong relation with resonance frequency: in other words, insects have not been evolved sufficiently to flap at their wings' structural resonant frequency. This contradicts the general conclusion of other reports--that insects flap at their wings' resonant frequency to take advantage of passive deformation to save energy.
The role of leading-edge vortex (LEV) in enhancing aerodynamic lift during flapping flight is discussed. The LEV is generated from the balance between the pressure gradient, the centrifugal force, and the Coriolis force in the momentum equation. The LEV generates a lower pressure area, which results in a large suction on the upper surface. The LEV's main characteristics and the implications on lift generation change as the Reynolds number varies. The LEV can enhance lift by attaching a bounded vortex core to the upper leading edge during wing translation. The LEV needs to maintain a high axial flow velocity in the core and remains stable along the spanwise direction to be effective in enhancing lift. The higher Reynolds number yields much more pronounced axial flow at the core of the LEV, which together with the LEV forms a helical flow structure near the leading edge.
Aerial predation is a highly complex, three-dimensional flight behavior that affects the individual fitness and population dynamics of both predator and prey. Most studies of predation adopt either an ecological approach in which capture or survival rates are quantified, or a biomechanical approach in which the physical interaction is studied in detail. In the present study, we show that combining these two approaches provides insight into the interaction between hunting dragonflies (Libellula cyanea) and their prey (Drosophila melanogaster) that neither type of study can provide on its own. We performed >2500 predation trials on nine dragonflies housed in an outdoor artificial habitat to identify sources of variability in capture success, and analyzed simultaneous predator-prey flight kinematics from 50 high-speed videos. The ecological approach revealed that capture success is affected by light intensity in some individuals but that prey density explains most of the variability in success rate. The biomechanical approach revealed that fruit flies rarely respond to approaching dragonflies with evasive maneuvers, and are rarely successful when they do. However, flies perform random turns during flight, whose characteristics differ between individuals, and these routine, erratic turns are responsible for more failed predation attempts than evasive maneuvers. By combining the two approaches, we were able to determine that the flies pursued by dragonflies when prey density is low fly more erratically, and that dragonflies are less successful at capturing them. This highlights the importance of considering the behavior of both participants, as well as their biomechanics and ecology, in developing a more integrative understanding of organismal interactions.
The influence of the bending rigidity of a flexible heaving wing on its propulsive performance in a two-dimensional imposed parallel flow is investigated in the inviscid limit. Potential flow theory is used to describe the flow over the flapping wing. The vortical wake of the wing is accounted for by the shedding of point vortices with unsteady intensity from the wing's trailing edge. The trailing-edge flapping amplitude is shown to be maximal for a discrete set of values of the rigidity, at which a resonance occurs between the forcing frequency and a natural frequency of the system. A quantitative comparison of the position of these resonances with linear stability analysis results is presented. Such resonances induce maximum values of the mean developed thrust and power input. The flapping efficiency is also shown to be greatly enhanced by flexibility. Comment: 21 pages, 13 figures, to appear in Physics of Fluids
The free flapping flight of the dragonfly Sympetrum sanguineum and the damselfly Calopteryx splendens was filmed in a large flight enclosure at 3000 frames s-1. The wingtip kinematics are described for these flights. Despite the two species being similar in size, the damselfly flew with wingbeat frequencies half those of the dragonfly. The damselfly could perform a clap and fling, and the proximity to which the wings approached each other during this manoeuvre correlated with the total force produced during the wingstroke. The dragonfly beat its wings with a set inclination of the stroke planes with respect to the londitudinal body axis; the damselfly, in contrast, showed a greater variation in this angle. Both species aligned their stroke planes to be nearly normal to the direction of the resultant force, the thrust. In order to achieve this, the dragonfly body alignment correlated with the direction of thrust. However, the damselfly body alignment was independent of the thrust direction. Velocities and accelerations were greater for the dragonfly than for the damselfly. However, non-dimensional velocities and accelerations normalised by the wingbeat periods were greater for the damselfly.
Here we show, by qualitative free- and tethered-flight flow visualization, that dragonflies fly by using unsteady aerodynamic mechanisms to generate high-lift, leading-edge vortices. In normal free flight, dragonflies use counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing downstroke, attached flow on the forewing upstroke, and attached flow on the hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats with highly separated downstroke flows, with a single LEV attached across both the fore- and hindwings. We use smoke visualizations to distinguish between the three simplest local analytical solutions of the Navier-Stokes equations yielding flow separation resulting in a LEV. The LEV is an open U-shaped separation, continuous across the thorax, running parallel to the wing leading edge and inflecting at the tips to form wingtip vortices. Air spirals in to a free-slip critical point over the centreline as the LEV grows. Spanwise flow is not a dominant feature of the flow field--spanwise flows sometimes run from wingtip to centreline, or vice versa--depending on the degree of sideslip. LEV formation always coincides with rapid increases in angle of attack, and the smoke visualizations clearly show the formation of LEVs whenever a rapid increase in angle of attack occurs. There is no discrete starting vortex. Instead, a shear layer forms behind the trailing edge whenever the wing is at a non-zero angle of attack, and rolls up, under Kelvin-Helmholtz instability, into a series of transverse vortices with circulation of opposite sign to the circulation around the wing and LEV. The flow fields produced by dragonflies differ qualitatively from those published for mechanical models of dragonflies, fruitflies and hawkmoths, which preclude natural wing interactions. However, controlled parametric experiments show that, provided the Strouhal number is appropriate and the natural interaction between left and right wings can occur, even a simple plunging plate can reproduce the detailed features of the flow seen in dragonflies. In our models, and in dragonflies, it appears that stability of the LEV is achieved by a general mechanism whereby flapping kinematics are configured so that a LEV would be expected to form naturally over the wing and remain attached for the duration of the stroke. However, the actual formation and shedding of the LEV is controlled by wing angle of attack, which dragonflies can vary through both extremes, from zero up to a range that leads to immediate flow separation at any time during a wing stroke.
Dragonflies are dramatic, successful aerial predators, notable for their flight agility and endurance. Further, they are highly capable of low-speed, hovering and even backwards flight. While insects have repeatedly modified or reduced one pair of wings, or mechanically coupled their fore and hind wings, dragonflies and damselflies have maintained their distinctive, independently controllable, four-winged form for over 300Myr. Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation. Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving aerodynamic efficiency. This is achieved by recovering energy from the wake wasted as swirl in a manner analogous to coaxial contra-rotating helicopter rotors. With the appropriate fore-hind wing phasing, aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.
Insect-scale flapping wing flight vehicles can conduct environmental monitoring, disaster assessment, mapping, positioning and security in complex and challenging surroundings. To develop bio-inspired flight vehicles, systematic probing based on the particular category of flight vehicles is needed. This Element addresses the aerodynamics, aeroelasticity, geometry, stability and dynamics of flexible flapping wings in the insect flight regime. The authors highlight distinct features and issues, contrast aerodynamic stability between rigid and flexible wings, present the implications of the wing-aspect
ratio, and use canonical models and dragonflies to elucidate scientific insight as well as technical capabilities of bio-inspired design.
The forewing and the hindwing of a dragonfly have different geometry that could be an evolutionary specialization for better aerodynamic performance via sophisticated wing pitch control. Under different extent of wing pitching by the wing root musculature, the fore- and hindwings could exhibit different shape deformation and aerodynamic characteristics as a result of passive shape deformation. We measured the flow around the flapping wings using time-resolved particle image velocimetry (TR-PIV) to investigate the consequences of shape and the pitching mechanisms of the wings on the aerodynamics of dragonflies. The flow fields and pitching angle variations of the naturally actuated wing of the dragonfly were compared with that of the same wing artificially actuated only by flapping motion. We found that the trailing edge vortex dynamics and the wake were affected by the wing shape only for the in-vivo experiment with muscle induced pitching. Under the in-vivo with muscle induced pitching, the hindwing took more part in generating horizontal momentum with larger pitching magnitude, due to the larger chord length compared with forewing. Meanwhile, when there was only pitching due to the wing membrane deformation of artificially actuated flapping, a slight difference in the surrounding flow structures was found between the hindwing and the forewing, and the net flow in one period was reduced nearly to zero. These results provided quantified evidence to the extent and importance of the pitching motion of the wings in dragonfly flight. The results of this work can be useful for the design of wings, their actuation mechanism, and the in-flight kinematics control of flapping wing micro air vehicles (MAVs).
We present a quantitative characterization of the unsteady aerodynamic features of a live, free-flying dragonfly under a well-established flight condition. In particular, our investigations cover the span-wise features of vortex interactions between the fore- and hind-pairs of wings that could be a distinctive feature of a high aspect ratio tandem flapping wing pair. Flapping kinematics and dynamic wing-shape deformation of a dragonfly were measured by tracking painted landmarks on the wings. Using it as the input, computational fluid dynamics analyses were conducted, complemented with time-resolved particle image velocimetry flow measurements to better understand the aerodynamics associated with a dragonfly. The results show that the flow structures around hindwing’s inner region are influenced by forewing’s leading edge vortex, while those around hindwing’s outer region are more influenced by forewing’s shed trailing edge vortex. Using a span-resolved approach, we found that the forewing–hindwing interactions affect the horizontal force (thrust) generation of the hindwing most prominently and the modulation of the force generation is distributed evenly around the midspan. Compared to operating in isolation, the thrust of the hindwing is largely increased during upstroke, albeit the drag is also slightly increased during the downstroke. The vertical force generation is moderately affected by the forewing–hindwing interactions and the modulation takes place in the outer 40% of the hindwing span during the downstroke and in the inner 60% of the span during the upstroke.
Remarkable flight performance is key to the survival of adult Odonata. They integrate varied three-dimensional architectures and kinematics of the wings, unsteady aerodynamics, and sensory feedback control in order to achieve agile flight. Therefore, a diverse range of approaches are necessary to understand their flight strategy comprehensively. Recently, new data have been presented in several key areas in Odonata such as measurement of surface topographies, computational fluid dynamic analyses, quantitative flow visualisation using particle image velocimetry, and optical tracking of free flight trajectories in laboratory environments. In this paper, we briefly review those findings alongside more recent studies that have advanced our understanding of the flight mechanics of Odonata still further.
Insect wings have no flight muscles, except those situated in the thorax. However, they continuously respond to forces acting on them during flight. This ability is achieved by the specialised design of the wings and plays a key role in their aerodynamic performance. Dragonfly (Anisoptera) wings represent an extreme example of this automatic shape control among flying insects. The functionality of the wings results from complex interactions between several structural components of which they are composed. Here we put together the results of our recent works, to review the functional roles of some of the key wing components including vein, membrane, vein microjoint, nodus, basal complex and corrugation. Our results help to understand the relationship between the structure, material and function of each of these wing components in complex dragonfly wings. We further use our data to explain how the interactions between the wing components provide dragonflies with fully functional wings.
The lift force was reported not to be high enough to support the dragonfly’s weight during flight in some conventional investigations, and higher lift force is required for its takeoff. In this study, by employing a thin plate model, impact effect is investigated for the wing deformation in dragonfly flapping during takeoff. The static displacement is formulated to compare with the dynamical displacement caused by impact. The governing equation of motion for the impact dynamics of a dragonfly wing is derived based on Newton’s second law. Separation of variables technique and assumed modes method are introduced to solve the resulting equations. Further, lift force is presented for the cases of considering and without considering the impact on the wing flapping which indicates that the impact has prominent effects for the dragonfly’s aerodynamic performance. Numerical simulations demonstrate that considering the impact effect on the wing flapping can increase the wing deformation, which results in the rise of the lift force. The enhanced lift force is of critical importance for the dragonfly’s takeoff.
In this work we studied the differences in flight kinematics and aerodynamics that could relate to differences in wing morphologies of a dragonfly and a damselfly. The damselflies and dragonflies normally fly with the fore wing or hind wing in the lead, respectively. The wing of the damselfly is petiolate, which means that the wing root is narrower than that of the dragonfly. The influence of the biological morphology between the damselfly and the dragonfly on their hovering strategies is worthy of clarification. The flight motions of damselflies and dragonflies in hovering were recorded with two high-speed cameras; we analyzed the differences between their hovering motions using computational fluid dynamics. The distinct mechanisms of the hovering flight of damselflies (Matrona cyanoptera) and dragonflies (Neurothemis ramburii) with different phase lags between fore and hind wings were deduced. The results of a comparison of the differences of wing phases in hovering showed that the rotational effect has an important role in the aerodynamics; the interactions between fore and hind wings greatly affect their vortex structure and flight performance. The wake of a damselfly sheds smoothly because of slender petiolation; a vertical force is generated steadily during the stage of wing translation. Damselflies hover with a longer translational phase and a larger flapping amplitude. In contrast, the root vortex of a dragonfly impedes the shedding of wake vortices in the upstroke, which results in the loss of a vertical force; the dragonfly hence hovers with a large amplitude of wing rotation. These species of Odonata insects developed varied hovering strategies to fit their distinct biological morphologies.
The effects of the phase difference between fore- and hindwings are experimentally investigated on the aerodynamic force, aerodynamic efficiency, and longitudinal control forces for tandem flapping wings in hovering and forward flight. The aerodynamic force and power are measured using a dynamically scaled mechanical model in a water tunnel at a Reynolds number of 7660–11,400. The experimental results indicate that a tandem configuration, except in-phase motion, is detrimental to the generation of vertical and horizontal aerodynamic forces in hovering and slow forward flight; whereas the vertical force is enhanced in out-of-phase motion when the hindwing leads the forewing in faster forward flight. In-phase motion has the largest aerodynamic efficiency of all the phase differences in hovering and forward flight, which does not correspond to the phase difference that dragonflies use in hovering and forward flight. In contrast, a shift in the phase difference in the range of 45–180 deg (hindwing leads forewing) does not change the total vertical or horizontal force but largely and monotonically changes the pitching moment about the body caused by the force difference between the fore- and hindwings in hovering and forward flight, which corresponds to the fact that dragonflies use antiphase motion in hovering and out-of-phase motion of 50–130 deg in forward flight.
This practical guide provides comprehensive information on PIV. The third edition extends many aspects of Particle image Velocimetry, in particular the tomographic PIV method, high-velocity PIV, Micro-PIV, and accuracy assessment.
In this book, relevant theoretical background information directly support the practical aspects associated with the planning, performance and understanding of experiments employing the PIV technique. It is primarily intended for engineers, scientists and students, who already have some basic knowledge of fluid mechanics and non-intrusive optical measurement techniques. It shall guide researchers and engineers to design and perform their experiment successfully without requiring them to first become specialists in the field. Nonetheless many of the basic properties of PIV are provided as they must be well understood before a correct interpretation of the results is possible
We investigate the characteristics of inter-wing aerodynamic interactions across the span of the high-aspect-ratio, flexible wings of dragonflies under tethered and free-flying conditions. The effects of the interactions on the hindwings vary across four spanwise regions. (I) Close to the wing root, a trailing-edge vortex (TEV) is formed by each stroke, while the formation of a leading-edge vortex (LEV) is limited by the short translational distance of the hindwing and suppressed by the forewing-induced flow. (II) In the region away from the wing root but not quite up to midspan, the formation of the hindwing LEV is influenced by that of the forewing LEV. This vortex synergy can increase the circulation of the hindwing LEV in the corresponding cross-section by 22% versus that the hindwing in isolation. (III) The region about half way between the wing root and wing tip is there is a transition dominated by downwash from the forewing resulting in flow attached to the hindwing. (IV) An LEV is developed in the remaining, outer region of the wing at the end of a stroke when the hindwing captures the vortex shed by the forewing. The interaction effects depend not only on the wing phasing, but also the flapping offset and flight direction. The aerodynamics of the hindwings vary substantially from the wing root to the wing tip. For a given phasing, this spanwise variation in the aerodynamics can be exploited in the design of artificial wings to achieve greater agility and higher efficiency.
Nowadays, there is a growing need for flying drones with diverse capabilities for both civilian and military applications. There is also a significant interest in the development of novel drones which can autonomously fly in different environments and locations and can perform various missions. In the past decade, the broad spectrum of applications of these drones has received most attention which led to the invention of various types of drones with different sizes and weights. In this review paper, we identify a novel classification of flying drones that ranges from unmanned air vehicles to smart dusts at both ends of this spectrum, with their new defined applications. Design and fabrication challenges of micro drones, existing methods for increasing their endurance, and various navigation and control approaches are discussed in details. Limitations of the existing drones, proposed solutions for the next generation of drones, and recommendations are also presented and discussed.
The effects of changing the trailing edge shape on the wake and propulsive performance of a pitching rigid panel are examined experimentally. The panel aspect ratio is AR=1, and the trailing edges are symmetric chevron shapes with convex and concave orientations of varying degree. Concave trailing edges delay the natural vortex bending and compression of the wake, and the mean streamwise velocity field contains a single jet. Conversely, convex trailing edges promote wake compression and produce a quadfurcated wake with four jets. As the trailing edge shape changes from the most concave to the most convex, the thrust and efficiency increase significantly.
More than a million insects and approximately 11,000 vertebrates utilize flapping wings to fly. However, flapping flight has only been studied in a few of these species, so many challenges remain in understanding this form of locomotion. Five key aerodynamic mechanisms have been identified for insect flight. Among these is the leading edge vortex, which is a convergent solution to avoid stall for insects, bats and birds. The roles of the other mechanisms - added mass, clap and fling, rotational circulation and wing-wake interactions - have not yet been thoroughly studied in the context of vertebrate flight. Further challenges to understanding bat and bird flight are posed by the complex, dynamic wing morphologies of these species and the more turbulent airflow generated by their wings compared with that observed during insect flight. Nevertheless, three dimensionless numbers that combine key flow, morphological and kinematic parameters - the Reynolds number, Rossby number and advance ratio - govern flapping wing aerodynamics for both insects and vertebrates. These numbers can thus be used to organize an integrative framework for studying and comparing animal flapping flight. Here, we provide a roadmap for developing such a framework, highlighting the aerodynamic mechanisms that remain to be quantified and compared across species. Ultimately, incorporating complex flight maneuvers, environmental effects and developmental stages into this framework will also be essential to advancing our understandingofthe biomechanics, movement ecologyand evolution of animal flight.
The kinematics of free unimpeded hovering flight of Aeschna juncea L. was analysed from films taken in the field with 80 frames sec−1, and from still pictures taken with a motorized camera.
The body is held almost horizontal, and the wing stroke plane is tilted 60° relative to the horizontal. In these respects the dragonfly differs strongly from most other hovering animals. The wing beats essentially in the same plane on the downstroke and upstroke. All wings are strongly supinated (pitched-up) during the upstroke. The stroke angle is ca. 60° and the wing beat frequency ca. 36 Hz.
Average, minimum force coefficients were calculated with use of steady-state aerodynamic theory. Calculations were made under several alternative assumptions and gave lift coefficients of 3. 5 to 6. 1, which are all far too large to be explainable with steady-state aerodynamics. At least 60% of the force generated in hovering flight are due to non-steady-state aerodynamics. The pitching rotations of the wings at top and bottom of the stroke are believed to contribute much force, although the exact mechanism is not clear.
At the leading edge of the wing of dragonflies there is a unique morphological arrangement, the node. It permits elastic tension of the leading edge and seems to be an adaptation permitting strong wing twistings. The node may also function as a shock absorber.
By analysis of slow-motion films of dragonflies and damselflies in free flight, released in front of a backdrop or startled during flight, the following flight parameters have been quantified for symmetrical manoeuvres: wingbeat frequency, relative durations of up-and downstroke, phase relationships of the beats of fore-and hindwings, stroke amplitude, mean stroke velocity, flight velocity, nondimensional flight velocity, advance ratio, acceleration, angle of attack and stroke plane.
The wingbeat frequencies are higher in the smaller species and in those with relatively large wing loading. As a rule, Zygoptera have a wingbeat frequency only half that of Anisoptera. The stroke amplitude is almost always much larger in Zygoptera than in Anisoptera, which have a greater range of variation in this respect. Stroke velocity is higher in Anisoptera than in Zygoptera; it is also higher in the more elaborate flight manoeuvres than in others. The calculated stroke velocities resemble those actually measured.
Anisoptera fly more rapidly than Zygoptera. With respect to the nondimensional flight velocities, it is notable that although the values for Anisoptera are higher than those for Zygoptera, they are exceeded by the Calopterygidae; the latter can fold their wings back during rapid forward flight and shoot away, as in the ‘ballistic’ flight of small songbirds. However, the advance ratio is higher in Anisoptera than in Calopterygidae.
Anisoptera also perform better than Zygoptera with respect to acceleration. Three categories of phase relationships between the beats of the fore-and hindwings are established: counterstroking, phase-shifted stroking and parallel stroking. Counterstroking produces uniform flight, whereas the flight produced by phase-shifted and, in particular, parallel stroking is irregular. The angles of attack of the wings are shown to be associated with particular flight manoeuvres, as are the stroke planes. Flight manoeuvres are discussed without drawing detailed aerodynamic conclusions. The flight of Anisoptera is compared with that of Zygoptera.
Aerodynamic forces, power consumptions and efficiencies of flexible and rigid tandem wings undergoing combined plunging/pitching motion were measured in a hovering flight and two forward flights with Strouhal numbers of 0.6 and 0.3. Three flexible dragonfly-like tandem wing models termed Wing I, Wing II, and Wing III which are progressively less flexible, as well as a pair of rigid wings as the reference were operated at three phase differences of 0°, 90° and 180°. The results showed that both the flexibility and phase difference have significant effects on the aerodynamic performances. In both hovering and forward flights at a higher oscillation frequency of 1 Hz (St = 0.6), the Wing III model outperformed the other wing models with larger total horizontal force coefficient and efficiency. In forward flight at the lower frequency of 0.5 Hz (St = 0.3), Wing III, rigid wings and Wing II models performed best at 0°, 90° and 180° phase difference, respectively. From the time histories of force coefficients of fore- and hind-wings, different peak values, phase lags, and secondary peaks were found to be the important reasons to cause the differences in the average horizontal force coefficients. Particle image velocimetry and deformation measurements were performed to provide the insights into how the flexibility affects the aerodynamic performance of the tandem wings. The spanwise bending deformation was found to contribute to the horizontal force, by offering a more beneficial position to make LEV more attached to the wing model in both hovering and forward flights, and inducing a higher-velocity region in forward flight.
The dragonfly, Anaxparthenope Julius (Brauer) was observed in free flight, and a theoretical analysis of flight performance at various speeds was carried out. The variation with time of forces and moments acting on wings and body in steady trimmed flight was calculated by the local circulation method. Measures of flight performance, such as top speed, cruising speed and maximum endurance speed, were estimated from a necessary power curve required in steady flight and from the estimated available power. The results show that without using any novel unsteady aerodynamic force generated by a separated flow over the wings, the dragonfly can make steady trimmed flight at various flight speeds, from hovering to top speed.
This practical guide intends to provide comprehensive information on the PIV technique that in the past decade has gained significant popularity
throughout engineering and scientific fields involving fluid mechanics. Relevant theoretical background information directly support the practical
aspects associated with the planning, performance and understanding of experiments employing the PIV technique. The second edition includes
extensive revisions taking into account significant progress on the technique as well as the continuously broadening range of possible
applications which are illustrated by a multitude of examples. Among the new topics covered are high-speed imaging, three-component methods,
advanced evaluation and post-processing techniques as well as microscopic PIV, the latter made possible by extending the group of authors by
an internationally recognized expert. This book is primarily intended for engineers, scientists and students, who already have some basic
knowledge of fluid mechanics and non-intrusive optical measurement techniques. It shall guide researchers and engineers to design and perform
their experiment successfully without requiring them to first become specialists in the field. Nonetheless many of the basic properties of PIV are
provided as they must be well understood before a correct interpretation of the results is possible.
The aerodynamics of dragonflies, discussed by C. Somps and M. Luttges (14 June, p. 1326), present an intriguing problem. I share the author's conviction that an understanding of unsteady aerodynamics is essential for an understanding of insect flight. I also agree that dragonflies may use nonconventional mechanisms for producing lift.
A full derivation is presented for the vortex theory of hovering flight outlined in preliminary reports. The theory relates the lift produced by flapping wings to the induced velocity and power of the wake. Suitable forms of the momentum theory are combined with the vortex approach to reduce the mathematical complexity as much as possible. Vorticity is continuously shed from the wings in sympathy with changes in wing circulation. The vortex sheet shed during a half-stroke convects downwards with the induced velocity field, and should be approximately planar at the end of a half-stroke. Vorticity within the sheet will roll up into complicated vortex rings, but the rate of this process is unknown. The exact state of the sheet is not crucial to the theory, however, since the impulse and energy of the vortex sheet do not change as it rolls up, and the theory is derived on the assumption that the extent of roll-up is negligible. The force impulse required to generate the sheet is derived from the vorticity of the sheet, and the mean wing lift is equal to that impulse divided by the period of generation. This method of calculating the mean lift is suitable for unsteady aerodynamic lift mechanisms as well as the quasi-steady mechanism. The relation between the mean lift and the impulse of the resulting vortex sheet is used to develop a conceptual artifice - a pulsed actuator disc - that approximates closely the net effect of the complicated lift forces produced in hovering. T he disc periodically applies a pressure impulse over some defined area, and is a generalized form of the Froude actuator disc from propeller theory. The pulsed disc provides a convenient link between circulatory lift and the powerful momentum and vortex analyses of the wake. The induced velocity and power of the wake are derived in stages, starting with the simple Rankine-Froude theory for the wake produced by a Froude disc applying a uniform, continuous pressure to the air. The wake model is then improved by considering a ‘modified’ Froude disc exerting a continuous, but non-uniform pressure. This step provides a spatial correction factor for the Rankine-Froude theory, by taking into account variations in pressure and circulation over the disc area. Finally, the wake produced by a pulsed Froude disc is analysed, and a temporal correction factor is derived for the periodic application of spatially uniform pressures. Both correction factors are generally small, and can be treated as independent perturbations of the Rankine-Froude model. Thus the corrections can be added linearly to obtain the total correction for the general case of a pulsed actuator disc with spatial and temporal pressure variations. The theory is compared with Rayner’s vortex theory for hovering flight. Under identical test conditions, numerical results from the two theories agree to within 3%. Rayner presented approximations from his results to be used when applying his theory to hovering animals. These approximations are not consistent with my theory or with classical propeller theory, and reasons for the discrepancy are suggested.
The distribution of vorticity in the wake of a hovering bird or insect is considered. The wake is modelled by a chain of coaxial small-cored circular vortex rings stacked one upon another; each member of the chain is generated by a single wing-stroke. Circulation is determined by the animal's weight and the time for which a single ring must provide lift; ring size is calculated from the circulation distribution on the animal's wing. The theory is equally applicable to birds and insects, although the mechanism of ring formation differs. This approach avoids the use of lift and drag coefficients and is not bound by the constraints of steady-state aerodynamics; it gives a wake configuration in agreement with experimental observations. The classical momentum jet approach has steady momentum flux in the wake, and is difficult to relate to the wing motions of a hovering bird or insect; the vortex wake can be related to the momentum jet, but adjacent vortex elements are disjoint and momentum flux is periodic.
The evolution of the wake starting from rest is considered by releasing vortex rings at appropriate time intervals and allowing them to interact in their own velocity fields. The resulting configuration depends on the feathering parameter f (which depends on the animal's morphology); f increases with body size. At the lower end of the wake rings coalesce to form a single large vortex, which breaks away from the rest of the wake at intervals. Wake contraction depends on f ; the minimum areal contraction of one-half (as in momentum-jet theory) occurs only in the limit f → 0, but values calculated for smaller insects of just over one-half suggest that the momentum jet may be a good approximation to the wake when f is small.
Induced power in hovering is calculated as the limit of the mean rate of increase of wake kinetic energy as time progresses. It can be related to the classical momentum-jet induced power by a simple conversion factor. For an insect or hummingbird the usual momentum-jet estimate may be between 10 and 15% too low, but for a bird it may be as much as 50% too low. This suggests that few, if any, birds are able to sustain aerobic hovering, and that as small a value of f as possible would be necessary if the bird were to hover.
Tip losses (energy cost of the vortex-ring wake compared with the equivalent momentum jet) are negligible for insects, but can be in the range 15–20% for birds.
Birds, bats, and insects flap their wings to produce lift as well as thrust. The aerodynamics associated with flapping wings can be enhanced by large-scale vortical flow structures by wing kinematics, wing shapes, and flexible wing structures. Consequently, with appropriate kinematics, frequency, and amplitude, flapping wings can generate substantially higher lift and perform well beyond stall angle of attack for fixed wings. In this chapter, we highlight the fluid physics associated with low Reynolds number flapping wings, including leading edge vortices, pitch-up rotation, wake capture, clap-and-fling mechanisms, and tip vortices, as well as the impact of wing flexibility on flapping wing aerodynamics.
In the present study, we conduct an experiment using a one-paired dynamically scaled model of an insect wing, to investigate how asymmetric strokes with different wing kinematic parameters are used to control the aerodynamics of a dragonfly-like inclined flapping wing in still fluid. The kinematic parameters considered are the angles of attack during the mid-downstroke (α(md)) and mid-upstroke (α(mu)), and the duration (Δτ) and time of initiation (τ(p)) of the pitching rotation. The present dragonfly-like inclined flapping wing has the aerodynamic mechanism of unsteady force generation similar to those of other insect wings in a horizontal stroke plane, but the detailed effect of the wing kinematics on the force control is different due to the asymmetric use of the angle of attack during the up- and downstrokes. For example, high α(md) and low α(mu) produces larger vertical force with less aerodynamic power, and low α(md) and high α(mu) is recommended for horizontal force (thrust) production. The pitching rotation also affects the aerodynamics of a flapping wing, but its dynamic rotational effect is much weaker than the effect from the kinematic change in the angle of attack caused by the pitching rotation. Thus, the influences of the duration and timing of pitching rotation for the present inclined flapping wing are found to be very different from those for a horizontal flapping wing. That is, for the inclined flapping motion, the advanced and delayed rotations produce smaller vertical forces than the symmetric one and the effect of pitching duration is very small. On the other hand, for a specific range of pitching rotation timing, delayed rotation requires less aerodynamic power than the symmetric rotation. As for the horizontal force, delayed rotation with low α(md) and high α(mu) is recommended for long-duration flight owing to its high efficiency, and advanced rotation should be employed for hovering flight for nearly zero horizontal force. The present study suggests that manipulating the angle of attack during a flapping cycle is the most effective way to control the aerodynamic forces and corresponding power expenditure for a dragonfly-like inclined flapping wing.
Aerodynamic force generation and mechanical power requirements of a dragonfly (Aeschna juncea) in hovering flight are studied. The method of numerically solving the Navier–Stokes equations in moving overset grids is used.
When the midstroke angles of attack in the downstroke and the upstroke are set to 52° and 8°, respectively (these values are close to those observed), the mean vertical force equals the insect weight, and the mean thrust is approximately zero. There are two large vertical force peaks in one flapping cycle. One is in the first half of the cycle, which is mainly due to the hindwings in their downstroke; the other is in the second half of the cycle, which is mainly due to the forewings in their downstroke. Hovering with a large stroke plane angle (52°), the dragonfly uses drag as a major source for its weight-supporting force (approximately 65% of the total vertical force is contributed by the drag and 35% by the lift of the wings).
The vertical force coefficient of a wing is twice as large as the quasi-steady value. The interaction between the fore- and hindwings is not very strong and is detrimental to the vertical force generation. Compared with the case of a single wing in the same motion, the interaction effect reduces the vertical forces on the fore- and hindwings by 14% and 16%, respectively,of that of the corresponding single wing. The large vertical force is due to the unsteady flow effects. The mechanism of the unsteady force is that in each downstroke of the hindwing or the forewing, a new vortex ring containing downward momentum is generated, giving an upward force.
The body-mass-specific power is 37 W kg-1, which is mainly contributed by the aerodynamic power.
- W Shyy
- H Aono
- C Kang
- H Liu