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(Abstract) Hummingbirds fly with their wings almost fully extended during their entire wingbeat. This pattern, associated with having proportionally short humeral bones, long distal wing elements, and assumed to be an adaptation for extended hovering flight, has lead to predictions that the aerodynamic mechanisms exploited by hummingbirds during hovering should be similar to those observed in insects. To test these predictions, we flew rufous hummingbirds (Selasphorus rufus, 3.3 g, n = 6) in a variable-speed wind tunnel (0-12 ms-1) and measured wake structure and dynamics using digital particle image velocimetry (DPIV). Unlike hovering insects, hummingbirds produced 75% of their weight support during downstroke and only 25% during upstroke, an asymmetry due to the inversion of their cambered wings during upstroke. Further, we have found no evidence of sustained, attached leading edge vorticity (LEV) during up or downstroke, as has been seen in similarly-sized insects - although a transient LEV is produced during the rapid change in angle of attack at the end of the downstroke. Finally, although an extended-wing upstroke during forward flight has long been thought to produce lift and negative thrust, we found circulation during downstroke alone to be sufficient to support body weight, and that some positive thrust was produced during upstroke, as evidenced by a vortex pair shed into the wake of all upstrokes at speeds of 4 - 12 m s-1.

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... 7(b) that the attached vortex of the turbine under fluctuating wind is closer to the blade surface, which indicates that the turbine under fluctuating wind has better delay stall mechanism. According to the study on the bird flight by LIANG and DONG[18]and also WARRICK[19], the delay stall is the reason why the birds produce larger aerodynamic force(the Reynolds number Re of the bird flight is similar to the VAWT considered in this work), which indicates that the unsteady working mechanic of VAWT is similar to the birds flight. This conclusion is consist with the opinion proposed by WORASINCHAI et al[20]. ...

The present work deals with an investigation of the self-starting aerodynamic characteristics of VAWT under fluctuating wind. In contrast to the previous studies, the rotational speed of the turbine is not fixed, the rotation of the turbine is determined by the dynamic interaction between the fluctuating wind and turbine. A weak coupling method is developed to simulate the dynamic interaction between the fluctuating wind and passive rotation turbine, and the results show that if the fluctuating wind with appropriate fluctuation amplitude and frequency, the self-starting aerodynamic characteristics of VAWT will be enhanced. It is also found that compared with the fluctuation amplitude, the fluctuation frequency of the variation in wind velocity is shown to have a minor effect on the performance of the turbine. The analysis will provide straightforward physical insight into the self-starting aerodynamic characteristics of VAWT under fluctuating wind. © 2016, Central South University Press and Springer-Verlag Berlin Heidelberg.

... It is also found in Fig. 7(b) that the attached vortex of the turbine under fluctuating wind is closer to the blade surface, which indicates that the turbine under fluctuating wind has better delay stall mechanism. According to the study on the bird flight by LIANG and DONG [18] and also WARRICK [19], the delay stall is the reason why the birds produce larger aerodynamic force(the Reynolds number Re of the bird flight is similar to the VAWT considered in this work), which indicates that the unsteady working mechanic of VAWT is similar to the birds flight. This conclusion is consist with the opinion proposed by WORASINCHAI et al [20]. ...

The solid state transformer (SST) can be viewed as an energy router in energy internet. This work presents sliding mode control (SMC) to improve dynamic state and steady state performance of a three-stage (rectifier stage, isolated stage and inverter stage) SST for energy internet. SMC with three-level hysteresis sliding functions is presented to control the input current of rectifier stage and output voltage of inverter stage to improve the robustness under external disturbance and parametric uncertainties and reduce the switching frequency. A modified feedback linearization technique using isolated stage simplified model is presented to achieve satisfactory regulation of output voltage of the isolated stage. The system is tested for steady state operation, reactive power control, dynamic load change and voltage sag simulations, respectively. The switching model of SST is implemented in Matlab/ Simulink to verify the SST control algorithms.

... The only available information about these turns comes from other species like gliding or flapping birds, and hovering insects [5,13,3,2,10,6]. These animals are very different since hummingbird is entailed with simple wings that operate over a larger range of speeds [11]. ...

We propose a new method to analyse and charac-terise the hummingbird wing dynamics using statistical measures from an optical flow field. The method starts by computing a dense optical flow, using a multi-scale spatial representation. First the pixels with larger velocities are retained as the support of the flow field, formed by the wings motion. Then, the global angular velocity is calculated as a function of the time during the hummingbird stroke. Additionally, each wing is segmented and the spatial variance of the velocity orientation is computed, providing the instants of principal stroke deformation. Finally a local measure of orientation deviation allows to localise the region of maximal torsion. Preliminary results show a compact and coherent description of the hovering flight dynamics of the hummingbird.

... In many cases however, the only available information about these turns comes from other species like the gliding or flight patterns observed in insects [5], [6], [7], [8], [10], [11]. However these animals are actually very different since they are entailed with simple wings that operate over a larger range of speeds [9] and are supported by an outer skeleton. In addition, while birds roll their body to orient the wing forces, insects can easily turn by asymmetric left-right wingbeats [4], [45], [34], [35] . ...

A new method for automatic analysis and characterization of recorded hummingbird wing motion is proposed. The method starts by computing a multiscale dense optical flow field, which is used to segment the wings, i.e., pixels with larger velocities. Then, the kinematic and deformation of the wings were characterized as a temporal set of global and local measures: a global angular acceleration as a time function of each wing and a local acceleration profile that approximates the dynamics of the different wing segments. Additionally, the variance of the apparent velocity orientation estimates those wing foci with larger deformation. Finally a local measure of the orientation highlights those regions with maximal deformation. The approach was evaluated in a total of 91 flight cycles, captured using three different setups. The proposed measures follow the yaw turn hummingbird flight dynamics, with a strong correlation of all computed paths, reporting a standard deviation of [Formula: see text] and [Formula: see text] for the global angular acceleration and the global wing deformation respectively.

In this paper, the interpretation of hovering flight for hummingbirds is firstly presented and discussed from a hummingbird morphology perspective (muscle and skeleton) including weight distribution, followed by a discussion of hovering aerodynamics. Next, by studying the scale laws, geometry similarity, and statistical analysis on wing parameters, the parametric relation between wing performances and weight is achieved before applying in wing design of flapping wing micro autonomous drones (FWMADs). The efficiency of designed wing based on the scaling law is verified by flying test. Designed wings based on different materials and methods are summarized. Last, the morphology of bird’s tails is presented, and then the designed tails inspired by hummingbirds are introduced before tail performances are also discussed simply. The results show that the tail could be predicted to apply to the stability of hovering twin-wing FWMADs. The current studies provide a simple but powerful guideline for biologists and engineers who study the morphology of hummingbirds and design FWMADs.

Variable camber deformation is observed during the flight of some insects and bird species; however, the effect of this special airfoil shape motion on the aerodynamic characteristics of the airfoil is not well understood, especially for the airfoil under gust wind. We did a numerical study to investigate the aerodynamic characteristics of a variable camber plunge airfoil under wind gusts. A weak coupling program was developed to simulate the interaction between the variable camber plunge airfoil and fluid, and the flow field, aerodynamic force and energy efficiency of different camber airfoils under different wind gust conditions are investigated. It was found that camber deformation influences the aerodynamic characteristics of the airfoil greatly. If the airfoil has an appropriate camber deformation, the deformation can increase the mean thrust and the propulsive efficiency of the airfoil. Moreover, the aerodynamic characteristics of the appropriate camber airfoils are not significantly affected by the gust frequency, and there exists a range of gust amplitude where the aerodynamic characteristics of the airfoils are also not significantly affected by the gust amplitude, which may be beneficial for aerodynamic stability of the airfoil. The results also show that appropriate camber deformation can suppress leading edge vortex separation, which improves the aerodynamic characteristics of the airfoil. © 2015, The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

The state of the art of flapping wing ornithopter MAV is reviewed to provide a comprehensive insight into the geometrical, ki nematic and aerodynamic characteristics of flapping biosystems. Then a generic approach is carried out to model the kinematics and aerodynamics of ornithopter to mimic flapping wing to produce lift and thrust for hovering and forward flight, by considering the motion of a three-dimensional rigid thin wing in flapping and pitching motion, using simple approach, applied to a two-and quad-wing flapping ornithopter, which are modeled and analyzed to mimic flapping wing biosystem to produce lift and thrust for forward flight. Considering bird's scale ornithopter, basic unsteady aerodynamic approach incorporating salient features of viscous effect and leading-edge suction are utilized. Parametric study is carried out to reveal the aerodynamic characteristics of flapping quad-wing ornithopter flight characteristics and for comparative analysis with various selected simple models in the literature, in an effort to de velop a flapping wing ornithopter model. Further, numerical and flow visualization studies are carried out to simulate the aerodynamics of generic rigid and flexible flapping ornithopter wings. Two different solvers are utilized; FLUENT for fluid flow analysis and ABAQUS for structural analysis. The resulting coupled procedure retains second order temporal accuracy. The simulation of phenomena of aeroelasticity is performed with a FSI method.

The dynamics of a flapping-wing vehicle are inherently aeroservoelastic since the interaction of aerodynamics and structural dynamics are critical to performance and will be altered by any control effectors. These dynamics have been shown to exhibit nonlinear behaviors in the time-frequency domain for a variety of wings. This paper generates a model of the aeroservoelastic dynamics for a wing as a function of control effectors. These models are polynomial functions of each control effector and capture the nonlinear behavior in the system. Most importantly, the resulting models are a basis from which to compute the deflection in response to any control command.

The flight dynamics of flapping-wing micro air vehicles result from a complicated relationship between aerodynamics and structural dynamics. This relationship has both frequency-domain aspects and time-domain aspects that are each critical. As such, analyzing data from a flapping wing requires techniques that can process information related to both of these domains. This paper introduces wavelet analysis as a tool that determine the frequency content of time-varying deflections. The resulting wavelet map presents a time-frequency domain representation that relates both the time-domain aspects and frequency-domain aspects. Data is analyzed using wavelet processing from a set of wings with different structural dynamics and different flapping parameters whose responses in the vertical and span-wise directions are recorded using visual image correlation. The resulting wavelet maps demonstrate the variations in energy content and temporal distribution associated with the deflections. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

Recent flow visualization experiments with the hawkmoth, Manduca sexta, revealed a small but clear leading-edge vortex and a pronounced three-dimensional flow. Details of this flow pattern were studied with a scaled-up, robotic insect ('the flapper') that accurately mimicked the wing movements of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing confirmed the existence of a small, strong and stable leading-edge vortex, increasing in size from wingbase to wingtip. Between 25 and 75% of the wing length, its diameter increased approximately from 10 to 50% of the wing chord. The leading-edge vortex had a strong axial flow velocity, which stabilized it and reduced its diameter. The vortex separated from the wing at approximately 75% of the wing length and thus fed vorticity into a large, tangled tip vortex. If the circulation of the leading-edge vortex were fully used for lift generation, it could support up to two-thirds of the hawkmoth's weight during the downstroke. The growth of this circulation with time and spanwise position clearly identify dynamic stall as the unsteady aerodynamic mechanism responsible for high lift production by hovering hawkmoths and possibly also by many other insect species.

Visualization experiments with Manduca sexta have revealed the presence
of a leading-edge vortex and a highly three-dimensional flow pattern. To
further investigate this important discovery, a scaled-up robotic insect
was built (the 'flapper') which could mimic the complex movements of the
wings of a hovering hawkmoth. Smoke released from the leading edge of
the flapper wing revealed a small but strong leading-edge vortex on the
downstroke. This vortex had a high axial flow velocity and was stable,
separating from the wing at approximately 75% of the wing length. It
connected to a large, tangled tip vortex, extending back to a combined
stopping and starting vortex from pronation. At the end of the
downstroke, the wake could be approximated as one vortex ring per wing.
Based on the size and velocity of the vortex rings, the mean lift force
during the downstroke was estimated to be about 1.5 times the body
weight of a hawkmoth, confirming that the downstroke is the main
provider of lift force.

The structure of the wake behind a kestrel in medium-speed flight down a 36 m length of corridor was analysed qualitatively and quantitatively by stereophoto-grammetry of multiple flash photographs of the motion of small soap-covered helium bubbles. The wake consists of a pair of continuous, undulating trailing vortices. The upstroke is therefore aerodynamically active and the circulation appears to remain constant along the wing whose geometry is altered during the course of the wing-stroke. It is argued that the flight kinematics, and so the wake structure, of the kestrel may be typical of flapping flight at medium speeds and a flight model based on this wake geometry is presented. Rough estimates of the rate of momentum generated in the wake balance the weight almost exactly and a direct estimate of the induced power requirement from the wake measurements is obtained. The significance of these results for the various alternative aerodynamic descriptions and energetic predictions of models of flapping animal flight is briefly assessed.

To examine the performance compromises necessitated by adaptations for high efficiency in flight, such as high aspect ratio wings, the flight morphology and acceleration performance of a guild of coursing aerial insectivores (swifts and swallows) were compared with those of a guild of avian generalists. Though phylogenetic non-independence made inference of adaptation difficult, biologically significant differences in aspect ratio and acceleration performance probably exist between the two groups of birds. A model of aerial insectivory is presented to illustrate the performance demands of this foraging method and the impacts of the compromises between high efficiency in sustained flight and turning- and linear-maneuvering performance.

Theoretical considerations and available experimental studies are combined for a discussion on the aerodynamic mechanisms of lift generation in hovering animal flight. A comparison of steady-state thin-aerofoil theory with measured lift coefficients reveals that leading edge separation bubbles are likely to be a prominent feature in insect flight. Insect wings show a gradual stall that is characteristic for thin profiles at Reynolds numbers (Re) less than about 105. In this type of stall, flow separates at the sharp leading edge and then re-attaches downstream to the upper wing surface, producing a region of limited separation enclosing a recirculating flow. The resulting leading edge bubble enhances the camber and thickness of the thin profile, improving lift at low Re. Some of the results for bird wing profiles indicate that the complications of leading edge bubbles might even be found in the fast forward flight of birds.

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.