Phasing of dragonfly wings can improve aerodynamic efficiency by removing swirl

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.     

Wrinkles enhance the diffuse reflection from the dragonfly Rhyothemis resplendens

The dorsal surfaces of the hindwings of the dragonfly Rhyothemis resplendens (Odonata: Libellulidae) reflect a deep blue from the multilayer structure in its wing membrane. The layers within this structure are not flat, but distinctly ‘wrinkled’, with a thickness of several hundred nanometres and interwrinkle crest distances of 5 µm and greater. A comparison between the backscattered light from R. resplendens and a similar, but un-‘wrinkled’ multilayer in the damselfly Matronoides cyaneipennis (Odonata: Calopterygidae) shows that the angle over which incident light is backscattered is increased by the wrinkling in the R. resplendens structure. Whereas the reflection from the flat multilayer of M. cyaneipennis is effectively specular, the reflection from the wrinkled R. resplendens multilayer spans 1.47 steradians (equivalent to ±40° for all azimuthal angles). This property enhances the visibility of the static wing over a broader angle range than is normally associated with a smooth multilayer, thereby markedly increasing its conspicuousness.     

Incompressible flow past oscillatory wings of low aspect ratio by the integral equations method

In the framework of the linearized perturbation theory, the pressure jump over the oscillating wing is the solution of a two‐dimensional integral equation. Performing an asymptotic expansion with respect to the aspect ratio and keeping the leading terms, we reduce the integral equation to a one‐dimensional one, obtaining a simplified method of solving the lifting surface integral equation for a class of thin wings of low aspect ratio with a straight trailing edge. The one‐dimensional integral equation is solved for the delta flat plate and the pressure coefficient field and the lift and moment coefficients are calculated. The range of validity of the new method is discussed in the final part of the paper. Copyright © 1999 John Wiley & Sons, Ltd.     

Predicting power-optimal kinematics of avian wings

A theoretical model of avian flight is developed which simulates wing motion through a class of methods known as predictive simulation. This approach uses numerical optimization to predict power-optimal kinematics of avian wings in hover, cruise, climb and descent. The wing dynamics capture both aerodynamic and inertial loads. The model is used to simulate the flight of the pigeon, Columba livia, and the results are compared with previous experimental measurements. In cruise, the model unearths a vast range of kinematic modes that are capable of generating the required forces for flight. The most efficient mode uses a near-vertical stroke–plane and a flexed-wing upstroke, similar to kinematics recorded experimentally. In hover, the model predicts that the power-optimal mode uses an extended-wing upstroke, similar to hummingbirds. In flexing their wings, pigeons are predicted to consume 20% more power than if they kept their wings full extended, implying that the typical kinematics used by pigeons in hover are suboptimal. Predictions of climbing flight suggest that the most energy-efficient way to reach a given altitude is to climb as steeply as possible, subjected to the availability of power.     

Defects of steel units of the high-lift devices of aircraft wings caused by fretting corrosion

We study the defects of a flap actuator of the wing of an An-124 aircraft. The fractographic and metallographic investigations of the surface defects of a screw–nut friction couple are performed. A device for wear tests under the conditions of fretting corrosion is developed. Recommendations concerning the reduction of possible damage to flap actuators are proposed.     

Nonlinear stochastic stability analysis of wind turbine wings by Monte Carlo simulations

Wind turbines are increasing in magnitude without a proportional increase of stiffness, for which reason geometrical nonlinearities become increasingly important. In this paper the nonlinear equations of motion are analysed of a rotating Bernoulli–Euler beam including nonlinear geometrical and inertial contributions. A reduced two-degrees-of-freedom modal expansion is used specifying the modal coordinate of the fundamental blade and edgewise fixed base eigenmodes of the beam. The rotating beam is subjected to harmonic and narrow-banded support point motion from the nacelle displacement. It is shown that under harmonic excitation at certain combinations of eigenfrequencies, rotational frequency, amplitude and frequency of the support point motion, the nonlinear system may produce almost periodic response or even chaotic response. The strange attractor of this unstable behaviour is analysed under narrow-banded excitation, and it is shown that the qualitative behaviour of the strange attractor is very similar for the periodic and almost periodic responses, whereas the strange attractor for the chaotic case loses structure as the excitation becomes narrow-banded. Furthermore, the characteristic behaviour of the strange attractor is shown to be identifiable by the so-called information dimension. Due to the complexity of the coupled nonlinear structural system all analyses are carried out via Monte Carlo simulations.     

Aerodynamic effects of flexibility in flapping wings

Recent work on the aerodynamics of flapping flight reveals fundamental differences in the mechanisms of aerodynamic force generation between fixed and flapping wings. When fixed wings translate at high angles of attack, they periodically generate and shed leading and trailing edge vortices as reflected in their fluctuating aerodynamic force traces and associated flow visualization. In contrast, wings flapping at high angles of attack generate stable leading edge vorticity, which persists throughout the duration of the stroke and enhances mean aerodynamic forces. Here, we show that aerodynamic forces can be controlled by altering the trailing edge flexibility of a flapping wing. We used a dynamically scaled mechanical model of flapping flight (Re ≈ 2000) to measure the aerodynamic forces on flapping wings of variable flexural stiffness (EI). For low to medium angles of attack, as flexibility of the wing increases, its ability to generate aerodynamic forces decreases monotonically but its lift-to-drag ratios remain approximately constant. The instantaneous force traces reveal no major differences in the underlying modes of force generation for flexible and rigid wings, but the magnitude of force, the angle of net force vector and centre of pressure all vary systematically with wing flexibility. Even a rudimentary framework of wing veins is sufficient to restore the ability of flexible wings to generate forces at near-rigid values. Thus, the magnitude of force generation can be controlled by modulating the trailing edge flexibility and thereby controlling the magnitude of the leading edge vorticity. To characterize this, we have generated a detailed database of aerodynamic forces as a function of several variables including material properties, kinematics, aerodynamic forces and centre of pressure, which can also be used to help validate computational models of aeroelastic flapping wings. These experiments will also be useful for wing design for small robotic insects and, to a limited extent, in understanding the aerodynamics of flapping insect wings.     

High-speed photogrammetry system for measuring the kinematics of insect wings

We describe and characterize an experimental system to perform shape measurements on deformable objects using high-speed close-range photogrammetry. The eventual application is to extract the kinematics of several marked points on an insect wing during tethered and hovering flight. We investigate the performance of the system with a small number of views and determine an empirical relation between the mean pixel error of the optimization routine and the position error. Velocity and acceleration are calculated by numerical differencing, and their relation to the position errors is verified. For a field of view of approximately 40 mm x 40 mm, a rms accuracy of 30 mum in position, 150 mm/s in velocity, and 750 m/s2 in acceleration at 5000 frames/s is achieved. This accuracy is sufficient to measure the kinematics of hoverfly flight.     

Structural colours of nickel bioreplicas of butterfly wings

The two-angle conformally evaporated-film-by-rotation technique (TA-CEFR) was devised to coat the wings of the monarch butterfly with nickel in order to form a 500-nm thick bioreplica thereof. The bioreplica exhibits structural colours that are completely obscured in actual wings by pigmental colours. Thus, the TA-CEFR technique provides a way to replicate, study and exploit hidden morphologies of biological surfaces.     

Nonlinear control of earthquake excited high raised buildings by approximate disturbance decoupling

Summary This paper deals with a class of hybrid control systems for earthquake excited high raised buildings which consist of a passive base isolation and an additional active damper. The mechanical model of the building is a shear wall structure with nonlinear, hysteretic restoring forces. The differential geometric approach based on exterior differential systems is used to solve the disturbance decoupling problem for the mechanical model. Since the ideal controller is difficult to implement, a new control law is proposed. It solves the decoupling problem only approximately but its implementation is simple because only signals easy to measure are used. The stability is proved by means of the theory of Liapunov in combination with dissipative systems, and it turns out that the closed loop is robust with respect to all the variations of practical interest, too. Finally, computer simulations demonstrate the feasibility and efficiency of the discussed approach.Dedicated to Prof. Dr. Dr. h. c. Franz Ziegler on the occasion of his 60th birthday     

考虑发动机推力影响的机翼颤振分析

发动机安装在机翼上时,其推力具有典型的随动特征,并对机翼颤振产生重要影响.基于有限元分析软件MSC/Nastran的DMAP开发,提出了一种考虑发动机推力和几何非线性影响的机翼颤振分析方法.作为验证,分析了推力对某高空长航时飞行器机翼颤振速度的影响,与已有结果吻合良好.对一带有两个发动机的复杂机翼结构进行了结构建模和颤振分析,重点分析了推力大小及作用位置对颤振速度的影响.结果表明,发动机的推力效应在颤振分析中是不应忽略的.     

Low-speed aerodynamic characteristics of wings with sweep discontinuities

It has been shown that, using the concept of a discrete vortex filament shedding, Weissinger’s method can be modified to determine the span-wise loading for wings with a sweep discontinuity. The sweep discontinuity is modelled by the introduction of a discontinuity in the equivalent vortex pattern chosen to replace the wing. The total lift, induced drag, pitching moment coefficients and span-wise location of the centre of pressure have been found for a few wing geometries using De Young and Harper’s procedure. An increase in the outboard sweep shifts the maximum load position outboard besides decreasing the overall lift and induced drag coefficients. High suction peaks appear at the pivot.     

On active control of laminar-turbulent transition on two-dimensional wings

This paper gives an overview of drag reduction on aerofoils by means of active control of Tollmien-Schlichting (TS) waves. Wind-tunnel experiments at Mach numbers of up to M(x)=0.42 and model Reynolds numbers of up to Re(c)=2 × 10(6), as well as in-flight experiments on a wing glove at Mach numbers of M<0.1 and at a Reynolds number of Re(c)=2.4 × 10(6), are presented. Surface hot wires were used to detect the linearly growing TS waves in the transitional boundary layer. Different types of voice-coil- and piezo-driven membrane actuators, as well as active-wall actuators, located between the reference and error sensors, were demonstrated to be effective in introducing counter-waves into the boundary layer to cancel the travelling TS waves. A control algorithm based on the filtered-x least mean square (FxLMS) approach was employed for in-flight and high-speed wind-tunnel experiments. A model-predictive control algorithm was tested in low-speed experiments on an active-wall actuator system. For the in-flight experiments, a reduction of up to 12 dB (75% TS amplitude) was accomplished in the TS frequency range between 200 and 600 Hz. A significant reduction of up to 20 dB (90% TS amplitude) in the flow disturbance amplitude was achieved in high-speed wind-tunnel experiments in the fundamental TS frequency range between 3 and 8 kHz. A downstream shift of the laminar-turbulent transition of up to seven TS wavelengths is presented. The cascaded sensor-actuator arrangement given by Sturzebecher & Nitsche in 2003 for low-speed wind-tunnel experiments was able to shift the transition Δx=240 mm (18% x/c) downstream by a TS amplitude reduction of 96 per cent (30 dB). By using an active-wall actuator, which is much shorter than the cascaded system, a transition delay of seven TS wavelengths (16 dB TS amplitude reduction) was reached.     

Prolonging the plasma circulation of proteins by nano-encapsulation with phosphorylcholine-based polymer

Short in vivo circulation is a major hindrance to the widespread adoption of protein therapeutics.Protein nanocapsules generated by encapsulating proteins with a thin layer of phosphorylcholine-based polymer via a two-step encapsulation process exhibited significantly prolonged plasma half-life.Furthermore,by constructing nanocapsules with similar sizes but different surface charges and chemistry,we demonstrated a generic strategy for prolonging the plasma half-life of therapeutic proteins.In an in vitro experiment,four types of bovine serum albumin (BSA) nanocapsules were incubated with fetal bovine serum (FBS) in phosphate buffer saline (PBS);the cell uptake by HeLa cells was monitored to systematically evaluate the characteristics of the surface chemistry during drculation.Single positron emission tomography-computed tomography (SPECT)was employed to allow real-time observation of the BSA nanoparticle distribution in vivo,as well as quantification of the plasma concentration after intravenous administration.This study offers a practical method for translating a broad range of proteins for clinical use.     

变体机翼大变形梯形蒙皮结构研究

首先介绍了变体机翼及蒙皮结构的发展研究现状,提出了大变形梯形蒙皮结构的构想.随后,建立了梯形蒙皮结构的力学分析数学模型,与有限元仿真分析结果进行了比较和分析,验证了所建立数学分析模型的正确性,为进一步开展制备和实验研究奠定了理论基础.     

Development of computerized methods for estimation of peripheral blood circulation in children under screening investigations

Experience gained in estimation of the conditions of peripheral hydrodynamics in children with various forms of vegetative dysfunctions was used to develop algorithms for diagnosis of hemodynamic disturbances, a set of recording devices, and software using statistical probability diagnostic methods. Belarusian State University of Computer Science and Radioelectronics, Minsk; Belarusian State Institute for Advanced Medical Training, Ministry of Public Health of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 3, pp. 413–418. May–June, 1996.     

Numerical investigation of lake circulation around islands by the finite element method

Lake circulations around island are calculated based on the shallow lake model using the finite element method. Wind stress, bottom friction, Coriolis force and shore line geometry are incorporated in the mathematical model and Lake Erie is used in the computations. A closure condition is derived for the multiply connected region of interest. The discussions are made in the light of accuracy and applicability of the numerical scheme used, and the influence of the circulation pattern due to the presence of islands.     

Three-dimensional vortex wake structure of flapping wings in hovering flight

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.     

Three-dimensional nonlinear flow over finite symmetrical wings of arbitrary planform

Summary An integral equation method is applied to the solution of the transonic small disturbance equation for finite wings in subsonic flows. The spanwise influence of the nonlinear compressibility sources is expressed in terms of Bessel and related functions. Additional nonlinear compressibility effects are considered two dimensional. Numerical results for high aspect ratio wings approach correctly the corresponding planar values. Results obtained for slender pointed small aspect ratio wings are transformed to the solution of the equivalent body of revolution and are then favorable compared with the solution given by the method of local linearization.

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Dreidimensionale nichtlineare Strömung um endliche symmetrische Flügel beliebigen Grundrisses

Zusammenfassung Eine Integralgleichungsmethode ist auf die Lösung der schallnahen Gleichung für kleine Störungen und für endliche Flügel im Unterschall angewandt. Die Einflüsse der nichtlinearen Kompressibilitätsquellen in Spannweitenrichtung sind durch Besselund verwandte Funktionen ausgedrückt. Die zusätzlichen nichtlinearen Kompressibilitätseffekte sind als zweidimensional angenommen. Numerische Ergebnisse für Flügel großer Spannweite zeigen die richtige Annäherung zu entsprechenden ebenen Lösung. Erhaltene Lösungen für spitze Flügel kleiner Spannweite sind auf Lösungen für den äquivalenten Rotationskörper transformiert und sind in zufriedenstellender Übereinstimmung mit nach der parabolischen Methode gewonnenen Lösungen.

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With 10 Figures

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This paper was presented at the Sixth U.S. National Congress of Applied Mechanics, Harvard University, Cambridge, Mass., June 15–19, 1970 and is dedicated to Prof. Dr.K. Oswatitsch on the occasion of his 60^th birthday.