Flapping Amplitude Research Articles

Nonlinear acoustic diametral modes of pressure pulsations during flutter of rotor blades of a turbocompressor

Features of pressure pulsations of flow during flutter on the bending frequency of rotor blades of an axial turbocompressor, which occurs after incomplete resonance at the torsion frequency of blades with a harmonic of rotor speed, are investigated. The frequency spectrum of nonlinear acoustic diametral modes is predicted, based on Navier-Stokes equations ensemble-averaged. This makes it possible to identify characteristic series determined by the bending frequency of flutter and its second harmonic in the frequency spectrum for recording the pressure pulsation of flow during flutter along with harmonics of rotor speed. The occurrence the second harmonic of the bending frequency is the nonlinear reaction of flow, which locally limits the flutter amplitude in time.

The Effect of Aerodynamic Braking on the Inertial Power Requirement of Flapping Flight: Case Study of a Gull

There has been an unresolved question of whether there is any significant degree of aerodynamic braking during wing deceleration in the flapping flight of birds, with direct analogies existing with flapping micro air vehicles. Some authors have assumed a complete conversion of kinetic energy into (useful) aerodynamic work during wing deceleration. Other authors have assumed no aerodynamic braking. The different assumptions have led to predictions of inertial power requirements in birds differing by a factor of 2. Our work is the first to model the aerodynamic braking forces on the wing during wing deceleration. A model has been developed that integrates the aerodynamic forces along the length of the wing and also throughout the wing beat cycle. A ring-billed gull was used in a case study and an adult specimen was used to gather morphometric data including a steady state measurement of the lift coefficient. The model estimates that there is a 50% conversion of kinetic energy into useful aerodynamic work during wing deceleration for minimum power speed. This aerodynamic braking reduces the inertial power requirement from 11.3% to 8.5% of the total power. The analysis shows that energy conversion is sensitive to wing inertia, amplitude of flapping, lift coefficient and wing length. The aerodynamic braking in flapping micro air vehicles can be maximised by maximising flap angle, maximising wing length (for a given inertia), minimising inertia and maximising lift coefficient.

On the quasi-steady aerodynamics of normal hovering flight part I: the induced power factor

An analytical treatment to quantify the losses captured in the induced power factor, k, is provided for flapping wings in normal hover, including the effects of non-uniform downwash, tip losses and finite flapping amplitude. The method is based on a novel combination of actuator disc and lifting line blade theories that also takes into account the effect of advance ratio. The model has been evaluated against experimental results from the literature and qualitative agreement obtained for the effect of advance ratio on the lift coefficient of revolving wings. Comparison with quantitative experimental data for the circulation as a function of span for a fruitfly wing shows that the model is able to correctly predict the circulation shape of variation, including both the magnitude of the peak circulation and the rate of decay in circulation towards zero. An evaluation of the contributions to induced power factor in normal hover for eight insects is provided. It is also shown how Reynolds number can be accounted for in the induced power factor, and good agreement is obtained between predicted span efficiency as a function of Reynolds number and numerical results from the literature. Lastly, it is shown that for a flapping wing in hover k owing to the non-uniform downwash effect can be reduced to 1.02 using an arcsech chord distribution. For morphologically realistic wing shapes based on beta distributions, it is shown that a value of 1.07 can be achieved for a radius of first moment of wing area at 40% of wing length.

A waypoint-tracking controller for a bionic autonomous underwater vehicle with two pectoral fins

This study aims to develop a waypoint-tracking control system for a biomimetic underwater vehicle (BUV). The BUV is propelled by wide paired pectoral foils, and each pectoral foil is driven by three independent fin rays. To simplify the control strategy, the maximum flapping amplitude of the pectoral fin is used to control the forward velocity, and a turning factor is defined for the manoeuvre control. Several swimming experiments are carried out to investigate the influence of the control parameters on the swimming performance of the prototype. Based on the results of the swimming experiments, a waypoint-tracking control system is proposed, which contains two layers: the velocity control layer and the heading angle control layer. A subdivision control method is adopted by the velocity control layer to get the maximum flapping amplitude. The fuzzy control method is employed by the heading angle control layer to obtain the turning factor for steering motion. Several waypoint-tracking experiments are carried out to verify effectiveness of the control system. The results show that the prototype can automatically reach the target area with the designed control system, even though the waypoints are arranged or randomly given.

Time-averaged characteristics of a reacting fuel jet in vitiated cross-flow

This paper describes an experimental study of reacting jets in a high-temperature (1775 K) vitiated crossflow at 6 atm. We present an extensive data set based on high speed chemiluminescence imaging and exhaust gas sampling showing the characteristics of the time-averaged trajectory, width of the flame, flame standoff (or ignition) location, and NOx emissions over a momentum flux ratio range of 0.75 < J < 240. Key observations are: (1) Depending upon ignition times, reaction can initiate uniformly around the jet, initiate on the leeward side of the jet and spread around to the windward side farther downstream, or initiate further downstream. (2) The time-averaged trajectory generally follows nonreacting trajectories, but penetrates further in the far-field than for what would be expected of a nonreacting jet. (3) The width of heat release zone increases monotonically with downstream location, J, and flame flapping amplitude, but seems to be dominated by the size of the counter-rotating vortex pair. (4) The measured ignition locations were of the same order of magnitude as values based on calculated ignition time scales and mean jet exit velocities, but with some additional variability. (5) The incremental NOx emissions were controlled primarily by the global temperature rise associated with burning the jet fuel (for the fixed crossflow conditions studied here), and the NOx emissions increased roughly linearly with the temperature rise.

20507 羽ばたき機構を有する楕円翼の空力特性

Recently, the flapping mechanisms such as insects and birds are investigated for the development of MAV technology. We experimented two methods in order to analyze flapping mechanism. The one way is fluid force measurement using by six component force load cells in a low speed wind tunnel. Another one way is visualization tests such as surface tuft method and injection streak line method using fog generator for a flapping elliptical wing varying flapping amplitude and frequency. From the results of these experiments, we have confirmed that, flapping motion raises lift coefficients and there is a leading edge vortex and wing tip vortex generated over the wing upper surface, moreover these vortices varies with flapping motion.

Assessment of PIV-based unsteady load determination of an airfoil with actuated flap

For complex experimental setups involving movable structures it is not trivial to directly measure unsteady loads. An alternative is to deduce unsteady loads indirectly from measured velocity fields using Noca's method. The ultimate aim is to use this method in future work to determine unsteady loads for fluid–structure interaction problems. The focus in this paper is first on the application and assessment of Noca's method for an airfoil with an oscillating trailing edge flap. To our best knowledge Noca's method has not been applied yet to airfoils with moving control surfaces or fluid–structure interaction problems. In addition, wind tunnel corrections for this type of unsteady flow problem are considered.The experiment is performed in a closed wind tunnel with a wing with a chord of 0.5m and a 0.2c trailing edge flap at a Reynolds number of Re=700000. The reduced frequencies of the flap are k=0.1 and k=0.2, whereas the mean flap deflections and amplitudes are 1° or 3°. Velocity fields are obtained with planar particle image velocimetry (PIV) and Noca's method is evaluated at multiple contours along the airfoil. The resulting unsteady loads are compared with loads obtained with Kutta–Joukowski's theorem applied to the experimental data and 2-D panel simulations with mimicked wind tunnel walls.Conclusion is that Noca's approach is relatively sensitive to the contour location and shows small offsets in the force coefficients. Using the experimental data, Noca's momentum flux equation applied to a set of contours gives a mean solution of the unsteady loads with an error bandwidth on average 6.39% of its mean value. The mean aerodynamic forces are slightly underpredicted, on average of about 5%. Among others, a higher resolution of the experimental data and more accurate approximations of velocity gradients will improve the force prediction. Phase and amplitude of the lift confirm 2-D panel computations including modeled wind tunnel walls and a gap correction.

Active Control by Piezoelectric Material Applied to Dynamic Analysis for Suppression of Panel with Uncertain Parameters

This paper studied the dynamic suppression problems of active control of composite panel with uncertain parameters. Considering uncertain factors of panel, finite element vibration dynamic model is established for active vibration control, through interval analysis method to estimate panel in active control of vibration critical air speed and flutter amplitude, to this determined active suppression effects of panel, numerical simulation indicate that through with Piezoelectric patches as active vibration control, the panel with uncertain parameters of whole interval of vibration critical wind speed were changed from vibration to convergence, showing that the control effect was significant.

Effects of aspect-ratio on the flapping behaviour of energy-harvesting membrane

Experimental studies were carried out on aspect-ratios of 2 and 3 rectangular polyethylene terephthalate membranes to clarify the effects of aspect-ratio on membrane flapping dynamics and strain energy distribution for Reynolds numbers from 3228 to 9732. Flapping modes similar to our previous study are identified and an increase in aspect-ratio is observed to promote transition of membrane flapping from limited cantilever-like mode to quasi-periodical traveling wave mode. This leads to corresponding increases in the flapping amplitude and its growth rate. On the other hand, both membranes achieve “lock-in” states at the same Reynolds number with nearly identical flapping frequency growth rates. Increasing the membrane aspect-ratio leads to strong variations in the strain distributions and reduces its increments with the Reynolds number Lastly, estimated membrane strain energy increases more with the Reynolds number for the higher aspect-ratio membrane. Compared to the lower aspect-ratio membrane however, it reaches an asymptotic level once it attains “lock-in” condition due to the more three-dimensional flapping motions.

Downstream flow structures of a fluttering piezoelectric energy harvester

The flow structures downstream of a fluttering piezoelectric energy harvester are examined, since 40% increases in power output have been noted from a downstream harvester. Smoke wire flow visualisation with a novel bi-lighting technique, and simultaneous voltage measurements are used to understand this phenomenon. The bi-lighting technique helped to confirm the out-of-plane velocity fields caused by the tilted cone vortices shed from the upstream harvester. It was speculated that the time-varying local pressure gradients induced by the propagating vortices coupled beneficially with the free-stream dynamic pressure and flutter instabilities, acting to elevate the flutter amplitude of the downstream harvester. Also, the phase lag between the harvesters showed a nonlinear relationship with the free-stream wind speed.

Stability of underwater periodic locomotion

Most aquatic vertebrates swim by lateral flapping of their bodies and caudal fins. While much effort has been devoted to understanding the flapping kinematics and its influence on the swimming efficiency, little is known about the stability (or lack of) of periodic swimming. It is believed that stability limits maneuverability and body designs/flapping motions that are adapted for stable swimming are not suitable for high maneuverability and vice versa. In this paper, we consider a simplified model of a planar elliptic body undergoing prescribed periodic heaving and pitching in potential flow. We show that periodic locomotion can be achieved due to the resulting hydrodynamic forces, and its value depends on several parameters including the aspect ratio of the body, the amplitudes and phases of the prescribed flapping.We obtain closedform solutions for the locomotion and efficiency for small flapping amplitudes, and numerical results for finite flapping amplitudes. This efficiency analysis results in optimal parameter values that are in agreement with values reported for some carangiform fish. We then study the stability of the (finite amplitude flapping) periodic locomotion using Floquet theory. We find that stability depends nonlinearly on all parameters. Interesting trends of switching between stable and unstable motions emerge and evolve as we continuously vary the parameter values. This suggests that, for live organisms that control their flapping motion, maneuverability and stability need not be thought of as disjoint properties, rather the organism may manipulate its motion in favor of one or the other depending on the task at hand.

Study on Fault Diagnosis Technology for Band Sawing Machine

Aiming at the low automation level and the non-completeness of the fault diagnosis technology for the band sawing products in China and in our province, this paper studied the digital control system based on extracting the flutter amplitude characteristics of the band saw blades in real time, developed the adjustable device which clears the sticky chips between the saw teeth based on the brush friction change according to the inclination angle Φ between the brush disk and the saw blade, and designed the state recognition system for the hydraulic oil circuits of the sawing machine based on detecting real-timely the oil temperature, pressure, flow rate and hydraulic cylinders displacement and transmitting wirelessly. So it can improve the grade, efficiency, reliability and service life for the band sawing machine.

A multi-fidelity modelling approach for evaluation and optimization of wing stroke aerodynamics in flapping flight

AbstractThe aerodynamics of hovering flight in a hawkmoth (Manduca sexta) are examined using a computational modelling approach which combines a low-fidelity blade-element model with a high-fidelity Navier–Stokes-based flow solver. The focus of the study is on understanding the optimality of the hawkmoth-inpired wingstrokes with respect to lift generation and power consumption. The approach employs a tight coupling between the computational models and experiments; the Navier–Stokes model is validated against experiments, and the blade-element model is calibrated with the data from the Navier–Stokes modelling. In the first part of the study, blade-element and Navier–Stokes modelling are used concurrently to assess the predictive capabilities of the blade-element model. Comparisons between the two modelling approaches also shed insights into specific flow features and mechanisms that are lacking in the lower-fidelity model. Subsequently, we use blade-element modelling to explore a large kinematic parameter space of the flapping wing, and Navier–Stokes modelling is used to assess the performance of the wing-stroke identified as optimal by the blade-element parameter survey. This multi-fidelity optimization study indicates that even within a parameter space constrained by the animal’s natural flapping amplitude and frequency, it is relatively easy to synthesize a wing stroke that exceeds the aerodynamic performance of the hawkmoth wing stroke. Within the prescribed constraints, the optimal wing stroke closely approximates the condition of normal hover, and the implications of these findings on hawkmoth flight capabilities as well as on the issue of biomimetic versus bioinspired design of flapping wing micro-aerial vehicles, are discussed.

Limit vibration and pressure-pulsation cycles of the flow in the case of turbocharger rotor blade flutter

In this paper, the feasibility of limiting the flutter amplitudes of the axial turbocharger rotor blades by stabilizing the rotor speed is based on a study of experimentally confirmed required conditions. Two composite tasks are analyzed, the role of the contributions of structural damping and pressure pulsations in the air flow to the stability of the turbocharger blades’ vibrations and the parametric conditions for limiting the blade’s flutter amplitude at a stabilized rotor speed. It is shown that the blade’s flutter amplitude can be stabilized by increasing the quadratic damping of the acoustic diametral mode of the pressure pulsations in the air flow.

A control and automation system for wave basins

This paper presents the new active absorption wave basin, named Hydrodynamic Calibrator (HC), constructed at the University of São Paulo (USP), in the Laboratory facilities of the Numerical Offshore Tank (TPN). The square (14m×14m) tank is able to generate and absorb waves from 0.5Hz to 2.0Hz, by means of 148 active hinged flap wave makers. An independent mechanical system drives each flap by means of a 1HP servo-motor and a ball-screw based transmission system. A customized ultrasonic wave probe is installed in each flap, and is responsible for measuring wave elevation in the flap. A complex automation architecture was implemented, with three Programmable Logic Computers (PLCs), and a low-level software is responsible for all the interlocks and maintenance functions of the tank. Furthermore, all the control algorithms for the generation and absorption are implemented using higher level software (MATLAB®/Simulink block diagrams). These algorithms calculate the motions of the wave makers both to generate and absorb the required wave field by taking into account the layout of the flaps and the limits of wave generation. The experimental transfer function that relates the flap amplitude to the wave elevation amplitude is used for the calculation of the motion of each flap. This paper describes the main features of the tank, followed by a detailed presentation of the whole automation system. It includes the measuring devices, signal conditioning, PLC and network architecture, real-time and synchronizing software and motor control loop. Finally, a validation of the whole automation system is presented, by means of the experimental analysis of the transfer function of the waves generated and the calculation of all the delays introduced by the automation system.

Transient Loads Control of a Variable Speed Rotor During Lagwise Resonance Crossing

Varying rotor speed during operation is a potential way to improve rotorcraft performance. The transient aeroelastic response of a stiff in-plane rotor system undergoing variable speed operation in forward flight is considered. During crossing of the fundamental lag mode near 2/rev, high transient lag bending moments are observed. The flapping amplitude and duration of the resonance crossing event have a strong influence on the peak lagwise root bending moment. Embedded chordwise fluidlastic dampers are explored as a way to reduce the peak bending moments. Determination of the fluidlastic damper properties is based on the analysis of a two degree-of-freedom blade-damper system. Parametric studies show that tuning port area ratios, loss factors, and device mass can be modified to enhance damper performance, and control the stroke. Results indicate that more than 6% critical lag damping can be provided around the resonance rotor speed, and that approximately 65% peak-to-peak moment reduction can be achieved with those devices. The stroke of the damper is limited to less than 2.5% blade chord length in the worst-case scenarios. Embedded chordwise fluidlastic dampers can successfully control the lagwise transient loads during the lagwise resonance crossing for the variable speed stiff in-plane rotors studied.

A Study of a Miniature Modularized Electro-Hydraulic High-Frequency Flutter

In order to solve the problem that traditional flutters can’t reach the requirements of vibration-assisted machining (VAM), a miniature modularized electro-hydraulic high-frequency flutter was proposed, which can output large force in high frequency. The structure and working principle of the flutter were elaborated in this paper. The equations of the valve in this flutter can be obtained based on equations of tradition valves, which established the mathematical model of the flutter. The flutter output waveforms were analysed by MATLAB in different input frequencies. The simulation results show that the flutter amplitude reach the maximum value in resonant frequency.

Flexible ring flapping in a uniform flow

AbstractAn improved version of the immersed boundary (IB) method for simulating an initially circular or elliptic flexible ring pinned at one point in a uniform flow has been developed. The boundary of the ring consists of a flexible filament with tension and bending stiffness. A penalty method derived from fluid compressibility was used to ensure the conservation of the internal volume of the flexible ring. At$\mathit{Re}= 100$, two different flapping modes were identified by varying the tension coefficient for a fixed bending stiffness, or by changing the bending coefficient for a fixed tension coefficient. The optimal tension and bending coefficients that minimize the drag force of the flexible ring were found. Visualization of the vorticity field showed that the two flapping modes correspond to different vortex shedding patterns. We observed the hysteresis property of the flexible ring, which exhibits bistable states over a range of flow velocities depending on the initial inclination angle, i.e. one is a stationary stable state and the other a self-sustained periodically flapping state. The Reynolds number range of the bistability region and the flapping amplitude were determined for various aspect ratios$a/ b$. For$a/ b= 0. 5$, the hysteresis region arises at the highest Reynolds number and the flapping amplitude in the self-sustained flapping state is minimized. A new bistability phenomenon was observed: for certain aspect ratios, two periodically flapping states coexist with different amplitudes in a particular Reynolds number range, instead of the presence of a stationary stable state and a periodically flapping state.

Numerical simulations of the flapping of a three-dimensional flexible plate in uniform flow

The self-sustained flapping of a three-dimensional flexible plate in a uniform viscous flow is numerically simulated with a fictitious domain method. The effects of the various non-dimensional control parameters including the Reynolds number, the density ratio, the dimensionless shear modulus, the length–thickness ratio, and the width–length ratio on the flapping of the plate are investigated. The results show that there exist two flapping modes: symmetrical and asymmetrical flapping about the centerline in the spanwise direction. Near the critical point a decrease in the plate width–length ratio, or the increase in the Reynolds number or the reduced velocity (a combination of the density ratio, the dimensionless shear modulus, and the length–thickness ratio) can make symmetric (or nearly symmetric) flapping become asymmetric. It is found that the flapping amplitude is mainly controlled by the density ratio and the dimensionless elastic modulus, while the frequency by the density ratio and the length–thickness ratio. In addition, the flapping amplitude and frequency are affected significantly by the confinement effect of the computational domain, and normally enhanced as the confinement effect becomes stronger. The effects of the plate width and the mass ratio (i.e., the ratio of the length–thickness and density ratios) on the critical reduced velocities are examined. The results indicate that when the fluid–plate mass ratio (or the plate length–thickness ratio) is relatively small there exist two significantly different critical velocities for the flapping instability, depending on the strength of initial plate deformation, a hysteresis phenomenon. No obvious hysteresis can be observed when the fluid–plate mass ratio (or the plate length–thickness ratio) is large.

Flapping dynamics of a flexible filament

This paper investigates the dynamics of the flapping regime of a filament placed in a two-dimensional soap-film flow for different filament lengths and flow speeds. It was found that the onset of flapping is quasiperiodic, with the main flapping amplitude and frequency modulated by low-amplitude, low-frequency oscillation. At higher flow velocities, the oscillation becomes chaotic. The transition to chaos occurs via the quasiperiodic route to chaos. A new bistability phenomenon was discovered in which the system alternates between the stretched-straight and oscillatory states, which is here referred to as "switching oscillation." Unlike some previously reported forms of bistability, in this case the system alternates between the two states continuously, without any external perturbation.