Showing papers on "Flutter published in 2009"

Supersonic Flutter Analysis Based on a Local Piston Theory

Abstract: DOI: 10.2514/1.37750 A highly efficient local-piston theory is presented for the prediction of inviscid unsteady pressure loads at supersonic and hypersonic speeds. A steady mean flow solution is first obtained by an Euler method. The classical pistontheoryismodifiedtoapplylocallyateachpointontheairfoilsurfaceontopofthelocalmean flowtoobtainthe unsteadypressureperturbationscausedbythedeviationoftheairfoilsurfacefromitsmeanlocationwithouttheneed of performing unsteady Euler computations. Results of two- and three-dimensional unsteady air loads and flutter predictions are compared with those obtained by the classical piston theory and an unsteady Euler method to assess theaccuracyandvalidityrangeinairfoilthickness, flightMachnumber,andangleofattackandwiththepresenceof blunt leading edges. The local-piston theory is found to offer superior accuracy and much wider validity range compared with the classical piston theory, with the cost of only a fraction of the computational time needed by an unsteady Euler method.

Feasibility Study on a New Energy Harvesting Electromagnetic Device Using Aerodynamic Instability

Abstract: Energy harvesting systems convert ambient energy from environment such as vibration, sunlight, wind, temperature gradient, etc. into electrical energy. Among several ambient energy sources, wind energy can be considered as one of the most promising sources because of its attractive features such as efficiency and economic merit. However, if an ordinary type of wind turbine is used for providing the electricity to low-power equipments (e.g., light poles, wireless sensors for structural health monitoring, etc.), it might be too inefficient and too costly. Recently, on the other hand, alternative (or innovative) approaches for wind power systems have been investigated by focusing on the aerodynamic instability phenomena such as galloping, flutter and vortex shedding. This paper first proposes a new energy harvesting system using wake galloping. To this end, the energy harvesting system based on wake galloping is designed and manufactured. And then, a series of wind tunnel tests are carried out in order to validate the efficiency and effectiveness of the proposed energy harvesting device. From these tests, the applicability of the proposed energy harvesting system using aerodynamic instability (i.e., wake galloping) is experimentally verified. Therefore, it can be an efficient energy harvesting system. Moreover, it can be used as an alternative energy source for low-power equipment, resulting in much simpler structural health monitoring systems without batteries for wireless sensors.

Nonlinear Aeroelasticity of a Very Flexible Blended-Wing-Body Aircraft

Abstract: Blended-wing-body (BWB) aircraft with high-aspect-ratio wings is an important configuration for high-altitude long-endurance unmanned aerial vehicles (HALE UAV). Recently, Northrop Grumann created a wind tunnel model under the Air Force’s High Lift over Drag Active (HiLDA) Wing program to study the aeroelastic characteristics of blended-wing-body for a potential Sensorcraft concept. This paper presents a study on the coupled aeroelastic / flight dynamics stability and response of a BWB aircraft that is modified from the HiLDA experimental model. An effective method is used to model very flexible BWB vehicles based on a low-order aeroelastic formulation that is capable of capturing the important structural nonlinear effects and couplings with the flight dynamics degrees of freedom. A nonlinear strain-based beam finite element formulation is used. Finite-state unsteady subsonic aerodynamic loads are incorporated to be coupled with all lifting surfaces, including the flexible body. Based on the proposed model, body-freedom flutter is studied, and is compared with the flutter results with all or partial rigid-body degrees of freedom constrained. The applicability of wind tunnel aeroelastic results (where the rigid-body motion is limited) is discussed in view of the free flight conditions (with all 6 rigid-body degrees of freedom). Furthermore, effects of structural and aerodynamic nonlinearities as well as wing bending/torsion rigidity coupling on the stability and gust response are also studied in this paper.

Coupled flutter of parallel plates

Abstract: Experimental visualizations of the coupled flutter of an assembly of two, three, and four flexible parallel cantilevered plates immersed in an axial uniform flow are presented. Depending on the flow velocity, on the interplate distance, and on the plate length, different coupled modes are observed. Selected modes and the associated thresholds and frequencies are compared with the results of a linear stability analysis.

Design of Composite Wings Including Uncertainties : A Probabilistic Approach

Abstract: A probabilistic method is developed to optimize the design of an idealized composite wing through consideration of the uncertainties in the material properties, fiber-direction angle, and ply thickness. The polynomial chaos expansion method is used to predict the mean, variance, and probability density function of the flutter speed, making use of an efficient Latin hypercube sampling technique. One-dimensional, two-dimensional, and three-dimensional polynomial chaos expansions are introduced into the probabilistic flutter model for different combinations of material, fiber-direction-angle, and ply-thickness uncertainties. The results are compared with Monte Carlo simulation and it is found that the probability density functions obtained using second- and third-order polynomial chaos expansion models compare well but require much less computation. A reliability criterion is defined, indicating the probability of failure due to flutter, and is used to determine successfully the optimal robust design of the composite wing.

Toward an Improvement in the Identification of Bridge Deck Flutter Derivatives

Abstract: This paper presents a short review of the state-of-the-art methods to identify bridge deck flutter derivatives and proposes a new algorithm to simultaneously extract the aeroelastic coefficients from free-vibration section-model tests, which is based on the improvement of the unifying least-squares (ULS) method and is therefore called modified unifying least-squares method. The advantages with respect to ULS are the faster and better convergence and the improvement in accuracy due to the introduction of weighting factors in the unifying error function. The method has been validated through numerically simulated noisy signals and experimental heaving and pitching time histories for two different bridge deck cross sections: a single-box and a multiple-box girder section model. The analysis of the artificial signals shows that a few system parameters are very difficult to be identified due to the fact that the problem is strongly ill-conditioned. Nevertheless, all the diagonal and off-diagonal components of the stiffness and damping matrices which significantly contribute to the output of the system are correctly estimated. The improvement with respect to other methods is extensively discussed. For the wind-tunnel test cases the accuracy of the identification procedure is evaluated through the comparison between measured signals and those simulated through the estimated mechanical and aerodynamic system parameters with very satisfactory results. With respect to many previous attempts of validation, this approach clearly shows the degree of accuracy that can be expected from the identification algorithm. Finally, for the considered test cases the linear model which stands behind the method seems to be an acceptable approximation of the physics of the phenomenon.

Effects of Turbulent Boundary Layer on Panel Flutter

Abstract: Numerical studies were carried out to investigate the effects of turbulent boundary layers on panel flutter at supersonic speeds. In this study, Reynolds-averaged Navier-Stokes equations were solved to take into account the turbulent boundary layer and its viscous effects. First, the fluid-structure coupling code was validated. The computed flutter boundaries agreed well with experimental data. Moreover, the results showed that the viscous effects were important and should be taken into account for flutter computation. Second, the boundary-layer effects were investigated in the Mach number range of 1.0-2.4. We compared the Reynolds-averaged Navier-Stokes computation with the inviscid computation and discussed the differences between them. We found that the boundary layer has not only a stabilizing effect but also a destabilizing effect, depending on the Mach number. The most important finding is that the flutter dynamic pressure slowly increases due to the boundary layer as the Mach number increases. In addition, the design boundary methodology was reviewed in terms of the turbulent boundary-layer effect, which will be helpful for the development of a new boundary-layer correction for the design boundary.

Adjoint-Based Sensitivity Formulation for Fully Coupled Unsteady Aeroelasticity Problems

Abstract: A method to compute sensitivities for use in gradient-based shape optimization applied to unsteady aeroelasticity problems is presented. The method takes into account the coupling between all three fundamental aspects of computational aeroelasticity: namely, unsteady flow equations, time-varying structural response to aerodynamic loads, and dynamic meshes that accommodate geometric deformations. The devised method provides discretely exact sensitivities of time-integrated and non-time-integrated objective functions with respect to design variables that control the shape of geometry. The algorithm is formulated in a general manner and can be readily extended to coupled multidisciplinary problems involving any number of disciplines. The algorithm is applied to a simple two-dimensional flutter model to demonstrate the proof of concept.

Energy harvesting: a key to wireless sensor nodes

Abstract: Energy harvesting has enabled new operational concepts in the growing field of wireless sensing. A novel energy harvesting device driven by aeroelastic flutter vibrations has been developed and could be used to complement existing environmental energy harvesters such as solar cells in wireless sensing applications. An analytical model of the mechanical, electromechanical, and aerodynamic systems suitable for designing aeroelastic energy harvesters for various flow applications are derived and presented. Wind tunnel testing was performed with a prototype energy harvester to characterize the power output and flutter frequency response of the device over its entire range of operating wind speeds. Finally, two wing geometries, a flat plate and a NACA 0012 airfoil were tested and compared.

On-Demand CFD-Based Aeroelastic Predictions Using a Database of Reduced-Order Bases and Models

Abstract: This paper demonstrates the feasibility of a CFD-based computational strategy aimed at on-demand predictions of aeroelastic responses of full aircraft configurations for variable flight conditions. The strategy relies on the pre-computation of a database of reduced-order bases and models for discrete flight parameters, and an interpolation method suitable for adapting in real-time the stored reduced-order information to parameter values not populated in the database. It also features a database training and reduction scheme based on concepts from machine learning to maximize both the robustness and performance of local interpolations. The application of this computational strategy to the broad aeroelastic identification of a complete F-16 fighter configuration highlights its near-real-time processing capability and demonstrates its potential for assisting flutter flight testing.

Vibration and Flutter Characteristics of a Folding Wing

Abstract: Studies are presented that characterize the dynamic aeroelastic aspects of a morphing aircraft design concept. The notion of interest is a folding wing design resulting in large-scale wing area changes. A finite element approach is used to investigate the sensitivity of natural frequencies and flutter instabilities to the wing position (e.g., fold angle), actuator stiffness, and vehicle weight. Sensitivities in these areas drive design requirements and raise flight envelope awareness issues. The study is presented in two parts as a comparison between two models of varying complexity. A simple folding wing model, based on the Goland wing, is analyzed and results are compared with a built-up structural model of the proposed full scale morphing vehicle.

Development of an aeroelastic vibration power harvester

Abstract: Aeroelastic vibration of structures represents a novel energy harvesting opportunity that may offer significant advantages over traditional wind power devices in many applications. Such a system could complement existing alternative energy sources by allowing for distributed power generation and placement in urban areas. The device configuration of a simple two degree aeroelastic system suitable for piezoelectric power harvesting is presented. The mechanical, electromechanical, and aerodynamic equations of motion governing the dynamics and electrical output of the system as a function of incident wind speed are derived. The response and current output of one design for a bench top scale harvester are simulated and presented. Finally, a strategy for expanding the operating envelope of the power harvester is proposed and discussed.

Mechanisms for Wide-Chord Fan Blade Flutter

Abstract: This paper describes a detailed wide-chord fan blade flutter analysis with emphasis on flutter bite. The same fan was used with three different intakes of increasing complexity to explain flutter mechanisms. Two types of flutter, namely stall flutter and acoustic flutter, were identified. The first intake is a uniform cylinder for which there are no acoustic reflections. Only stall flutter, driven by flow separation, can exist in this case. The second intake, based on the first one, has a ‘bump’ feature to reflect the fan’s forward pressure wave at a known location so that detailed parametric studies can be undertaken. The analysis revealed a mechanism for acoustic flutter, which is driven by the phase of the reflected wave. The third intake has the typical geometric features of a flight intake. The results indicate that flutter bite occurs when both stall and acoustic flutter happen at the same speed. It is also found that blade stiffening has no effect on aero-acoustic flutter.Copyright © 2009 by Rolls-Royce plc

Static and dynamic stabilities of a microbeam actuated by a piezoelectric voltage

Abstract: In this paper, flutter and divergence instabilities of a cantilever, a clamped–clamped, and a cantilever with intermediate simply-support microbeam sandwiched by piezoelectric layers have been studied. By presenting a mathematical formulation and numerical solution, critical piezoelectric force for avoiding of the instability in a cantilever microbeam has been calculated and validated by known buckling capacity of Beck column. By applying a similar mathematical analysis it has been introduced a critical piezoelectric voltage for a clamped–clamped microbeam. It has been shown that for cantilever microbeams, increasing of the follower piezoelectric force leads to: first flutter and then divergence instabilities whereas in the clamped–clamped microbeams only divergence instability can be occurred. Also effects of the intermediate simply support position on the critical piezoelectric voltage of a cantilever microbeam have been investigated. It has been shown that for case when the intermediate simply support is near to the fixed end of the cantilever increasing of the follower piezoelectric force leads to flutter instability but for case when the intermediate simply support is near to the free end of the cantilever it leads to divergence instability.

Variable area fan nozzle fan flutter management system

Abstract: A system and method of controlling a fan blade flutter characteristic of a gas turbine engine includes adjusting a variable area fan nozzle (42) in response to a neural network (68).

Camber effects in the dynamic aeroelasticity of compliant airfoils

Abstract: This paper numerically investigates the effect of chordwise flexibility on the dynamic stability of compliant airfoils. A classical two-dimensional aeroelastic model is expanded with an additional degree of freedom to capture time-varying camber deformations, defined by a parabolic bending profile of the mean aerodynamic chord. Aerodynamic forces are obtained from unsteady thin airfoil theory and the corresponding compliant-airfoil inertia and stiffness from finite-element analysis. V–g and state-space stability methods have been implemented in order to compute flutter speeds. The study looks at physical realizations with an increasing number of degrees of freedom, starting with a camber-alone system. It is shown that single camber leads to flutter, which occurs at a constant reduced frequency and is due to the lock in between the shed wake and the camber motion. The different combinations of camber deformations with pitch and plunge motions are also studied, including parametric analyses of their aeroelastic stability characteristics. A number of situations are identified in which the flutter boundary of the compliant airfoil exhibits a significant dip with respect to the rigid airfoil models. These results can be used as a first estimation of the aeroelastic stability boundaries of membrane-wing micro air vehicles.

Blade Tip Clearance Flow and Compressor Nonsynchronous Vibrations: The Jet Core Feedback Theory as the Coupling Mechanism

Abstract: This paper investigates the role of tip clearance flow in the occurrence of nonsynchronous vibrations (NSVs) observed in the first axial rotor of a high-speed high-pressure compressor in an aeroengine. NSV is an aeroelastic phenomenon where the rotor blades vibrate at nonintegral multiples of the shaft rotational frequencies in operating regimes where classical flutter is not known to occur. A physical mechanism to explain the NSV phenomenon is proposed based on the blade tip trailing edge impinging jetlike flow, and a novel theory based on the acoustic feedback in the jet potential core. The theory suggests that the critical jet velocity, which brings a jet impinging on a rigid structure to resonance, is reduced to the velocities observed in the blade tip secondary flow when the jet impinges on a flexible structure. The feedback mechanism is then an acoustic wave traveling backward in the jet potential core, and this is experimentally demonstrated. A model is proposed to predict the critical tip speed at which NSV can occur. The model also addresses several unexplained phenomena, or missing links, which are essential to connect tip clearance flow unsteadiness to NSV. These are the pressure level, the pitch-based reduced frequency, and the observed step changes in blade vibration and mode shape. The model is verified using two different rotors that exhibited NSV.

Supersonic Flutter of Functionally Graded Panels Subject to Acoustic and Thermal Loads

Abstract: of the volume fractions of the constituents. The governing equations are derived using the classical plate theory with von Karman geometric nonlinearity and the principle of virtual work. The first-order piston theory is adopted to model aerodynamic pressures induced by supersonic airflows. The thermal load is assumed to be steady-state constant temperature distribution, and the acoustic excitation is considered to be a stationary white-Gaussian random pressure with zero mean and uniform magnitude over the plate surface. The governing equations are transformed to modal coordinates to reduce the computational efforts. The Newton–Raphson iteration method is employed to obtain the dynamic response at each time step of the Newmark scheme for numerical integration. Finally, numerical results are provided to study the effects of the volume fraction exponent, aerodynamic pressure, temperature rise, and the random acoustic load on the panel response.

Theoretical Predictions of F-16 Fighter Limit Cycle Oscillations for Flight Flutter Testing

Abstract: A computational investigation of the flutter onset and limit cycle oscillation behavior of various F-16 fighter weapons and stores configurations is presented. A nonlinear harmonic balance compressible Reynolds-averaged Navier–Stokescomputational fluiddynamic flowsolverisusedtomodeltheunsteadyaerodynamicsoftheF-16wing. Slender body/wing theory is used as an approximate method for accounting for the unsteady aerodynamic effects of wing-tip launchers and missiles. Details of the computational model are presented along with an examination of the sensitivity of computed aeroelastic behavior to characteristics and parameters of the structural and fluid dynamic model. Comparisons with flight-test data are also shown. I. Introduction T HE SEEK EAGLE Office at Eglin Air Force Base performs an essential task in clearing new aircraft/stores configurations through flight tests for safe and effective operation. Many of these flighttestsarefortheF-16aircraftwhichcontinuestobeaworkhorse for the U.S. Air Forcewith continually new stores (missiles, bombs, and fuel tanks) being considered for aircraft operations. Similar aeroelastic flight tests are expected for future fighter aircraft as they go into service in the coming years. The number of needed flight tests is projected to be well beyond the financial and staff resources available. Hence there is a pressing need to identify the most critical aircraft/store configurations for the limited flight-test resources available and also insofar as possibly reduce the number of flight tests needed. Virtual flight testing may be the answer. Using new improved computational capability that provides much more rapid solutions, computational simulation can help identify the most critical aircraft/ store configuration and also hasthe potential of reducingthe number ofneeded flighttestsifconfidencecanbeestablishedinthecapability of simulations to correlate with flight-test data. A new methodology has been developed to produce these computer simulations based upon the notion that because the response is periodic in time, the solution need only be obtained over a single period of oscillation in time. By avoiding the traditional time marching solution which computes the long transient before a steady-state periodic oscillation is reached, computational times are reduced by a factor of 10–100. This enables a sufficiently rapid solutiontomakesuchsimulationsapracticalrealityforthe flight-test engineer and support team. Future developments of this methodology hold the promise of further substantial reductions in computational cost and are being vigorously pursued. Also further refinements in the physical fidelity of the simulation models are being considered.

The Flutter Response of a Piezoelectrically Damped Cantilever Pipe

Abstract: The investigation of passively damped piezoelectric structures within fluid flows is important for two reasons: (a) to increase the critical flutter speed and (b) conversely to generate electrical energy to power small scale electronic systems. In this article, a cantilever pipe with resistive piezoelectric damping is chosen as the model structure to demonstrate behavior over a range of fluid/structure mass ratios. The effects of piezoelectric coupling on the critical flutter velocity, capacitance, load resistance, and piezoelectric location are investigated. The modeling shows that depending on the piezoelectric parameters chosen and the attached electrical load resistance, the addition of a passive piezoelectric element can either increase or decrease stability of the system (i.e., the critical flutter speed of the cantilever pipe can be altered and hence controlled).