Showing papers on "Flutter published in 2010"

On the energy harvesting potential of piezoaeroelastic systems

Abstract: This paper investigates the concept of piezoaeroelasticity for energy harvesting. The focus is placed on mathematical modeling and experimental validations of the problem of generating electricity at the flutter boundary of a piezoaeroelastic airfoil. An electrical power output of 10.7 mW is delivered to a 100 kΩ load at the linear flutter speed of 9.30 m/s (which is 5.1% larger than the short-circuit flutter speed). The effect of piezoelectric power generation on the linear flutter speed is also discussed and a useful consequence of having nonlinearities in the system is addressed.

Studies on Fluid-Thermal-Structural Coupling for Aerothermoelasticity in Hypersonic Flow

Abstract: The field of aerothermoelasticity plays an important role in the analysis and optimization of airbreathing hypersonic vehicles, impacting the design of the aerodynamic, structural, control, and propulsion systems at both the component and multi-disciplinary levels. This study aims to expand the fundamental understanding of hypersonic aerothermoelasticity by performing systematic investigations into fluid-thermal-structural coupling, and also to develop frameworks, using innovative modeling strategies, for reducing the computational effort associated with aerothermoelastic analysis. Due to the fundamental nature of this work, the analysis is limited to cylindrical bending of a simply-supported, von K arm an panel. Multiple important effects are included in the analysis, namely: 1) arbitrary, nonuniform, in-plane and through-thickness temperature distributions, 2) material property degradation at elevated temperature, and 3) the effect of elastic deformation on aerodynamic heating. It is found that including elastic deformations in the aerodynamic heating computations results in non-uniform heat flux, which produces non-uniform temperature distributions and non-uniform material property degradations. This results in reduced flight time to the onset of flutter and localized regions in which the material temperature limits may be exceeded. Additionally, the trade-off between computational cost and accuracy is evaluated for aerothermoelastic analysis based on either quasi-static or time-averaged dynamic fluid-thermal-structural coupling, as well as computational fluid dynamics based reduced-order modeling of the aerodynamic heat flux. It is determined that these approaches offer the potential for significant improvements in aerothermoelastic modeling in terms of efficiency and/or accuracy.

A T-shaped piezoelectric cantilever for fluid energy harvesting

Abstract: This paper proposes a T-shaped piezoelectric cantilever for generating electric power from fluid flow. The working principle of the device is based on aeroelastic flutter and utilizes a bimorph cantilever with T-shape which hastens occurrence of flutter at a low fluid speed. A prototype device (100×60×30 mm3) was tested in a wind tunnel. The device was found to provide power from a wind speed of 4 m/s and a continuous peak electrical power output of 4.0 mW. The simplicity of the present device consisting of only a bimorph cantilever is considered to be cost effective.

Long-Duration Time-Resolved PIV to Study Unsteady Aerodynamics

Abstract: A time-resolved particle image velocimetry (PIV) system has been developed at the University of Western Ontario, London, ON, Canada, with long-recording-time capabilities This system is uniquely suited to the study of unsteady aerodynamics and hydrodynamics, such as avian aerodynamics or bluff-body oscillations Measurements have been made on an elongated bluff body through the initial build-up phase of flutter The possibilities to study this instability, which was responsible for the collapse of the Tacoma Narrows Bridge, are significantly broadened by the use of this system The long-time recording capability of the system allows for novel results since it yields data that are spatially and temporally resolved over a long record length The buildup of flutter is shown to exhibit complex dynamics that are heavily influenced by the flow-induced motion of the body Features of the wake turbulence as a function of time are presented and shown to substantially vary

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.

Toward Real-Time Computational-Fluid-Dynamics-Based Aeroelastic Computations Using a Database of Reduced-Order Information

Abstract: This paper describes a computational-fluid-dynamics-based computational methodology for fast on-demand aeroelastic predictions of the behavior of a full aircraft configuration at variable flight conditions and demonstrates its feasibility. The methodology relies on the offline precomputation of a database of reduced-order bases and models associated with a discrete set of flight parameters, and its training for an interpolation method suitable for reduced-order information. The potential of this near-real-time computational methodology for assisting flutter flight testing is highlighted with the aeroelastic identification of an F-16 configuration in the subsonic, transonic, and supersonic regimes.

Approximate Modeling of Unsteady Aerodynamics for Hypersonic Aeroelasticity

Abstract: DOI: 10.2514/1.C000190 Various approximations to unsteady aerodynamics are examined for the aeroelastic analysis of a thin doublewedge airfoil in hypersonic flow. Flutter boundaries are obtained using classical hypersonic unsteady aerodynamic theories: piston theory, Van Dyke’s second-order theory, Newtonian impact theory, and unsteady shock-expansion theory. The theories are evaluated by comparing the flutter boundaries with those predicted using computational fluid dynamics solutions to the unsteady Navier–Stokes equations. Inaddition, several alternative approaches to the classical approximations are also evaluated: two different viscous approximations based on effective shapes and combined approximate computational approaches that use steady-state computational-fluid-dynamics-based surrogatemodelsinconjunction withpistontheory.Theresultsindicatethat,with theexceptionof first-order piston theory and Newtonian impact theory, the approximate theories yield predictions between 3 and 17% of normalized root-mean-square error and between 7 and 40% of normalized maximum error of the unsteady Navier–Stokes predictions. Furthermore, the demonstrated accuracy of the combined steady-state computational fluid dynamics and piston theory approaches suggest that important nonlinearities in hypersonic flow are primarily due to steadystate effects. This implies that steady-state flow analysis may be an alternative to time-accurate Navier–Stokes solutions for capturing complex flow effects.

Three-dimensional dynamics of a cantilevered pipe conveying fluid, additionally supported by an intermediate spring array

Abstract: In this paper, the three-dimensional (3-D) non-linear dynamics of a cantilevered pipe conveying fluid, constrained by arrays of four springs attached at a point along its length is investigated. In the theoretical analysis, the 3-D equations are discretized via Galerkin's technique. The resulting coupled non-linear differential equations are solved numerically using a finite difference method. The dynamic behaviour of the system is presented in the form of bifurcation diagrams, along with phase-plane plots, time-histories, PSD plots, and Poincare maps for five different spring configurations. Interesting dynamical phenomena, such as 2-D or 3-D flutter, divergence, quasiperiodic and chaotic motions, have been observed with increasing flow velocity. Experiments were performed for the cases studied theoretically, and good qualitative and quantitative agreement was observed. The experimental behaviour is illustrated by video clips (electronic annexes). The effect of the number of beam modes in the Galerkin discretization on accuracy of the results and on convergence of the numerical solutions is discussed.

Conceptual Design of a Multi-utility Aeroelastic Demonstrator

Abstract: This paper presents the conceptual design of a flight vehicle to demonstrate active aeroelastic control technologies for suppressing flutter and aeroelastic instabilities. The technologies are directed at a class of high aspect ratio, flying wing ISR vehicles being pursued by the Air Force Research Laboratory as part of their SensorCraft program. This class of vehicles are susceptible to an unstable coupling of 1 st wing bending and short period mode, known as body freedom flutter (BFF). The constraints on the flight demonstrator vehicle were that the vehicle have a size large enough to replicate the structure and flutter characteristics of a full scale SensorCraft but small enough to fit within AFRL program funding and not present a large debris field in the case of an in-flight break-up. Trade study results revealed that a 15% scale met the study constraints. The structural analysis approach for designing a subscale vehicle to have a specific body freedom flutter speed and frequency while meeting structural strength criteria is discussed. The design of a flight control system to control the combined rigid and flexible dynamics of a vehicle that exhibits BFF is discussed. Flight test considerations of this unique demonstrator vehicle are presented.

NDOF Simulation Model for Flight Control Development with Flight Test Correlation

Abstract: traditional design constraints. One such issue is the coupling of an aircraft’s rigid body dynamics and structural modes that can result in issues ranging from poor flying qualities to a phenomenon known as Body Freedom Flutter (BFF). This phenomenon was first seen over five decades ago and is significant in vehicles with low tail volume and high aspect ratio wings or with a high fineness ratio fuselage. When encountered in flight, BFF can lead to divergent oscillations and catastrophic destruction of the aircraft. The solution to BFF often results in too much of a compromise in some areas of the aircraft design. In previous cases where BFF existed within the flight envelope, additional structure was added at the expense of performance. In other cases, BFF has been avoided by limiting the vehicle’s flight envelope. A possible alternative solution of active suppression has been forwarded that holds the promise of a full flight envelope and reduced structural weight. To assist the development of active flutter suppression the Flight Controls and Flutter communities must work together and develop a common language and methodology. This is important in the modeling of the vehicle so that control systems can be properly developed. This paper discusses the development of an Integrated Flight and Aeroelastic Control (IFAC) Integrated Product Team (IPT). The paper describes the modeling used by the IFAC IPT to develop the vehicle dynamic model and how the model was validated and correlated through ground and flight test.

Recent extensions and applications of the ‘CST’ universal parametric geometry representation method

Abstract: For aerodynamic design optimisation as \Veil as for multidisciplinary design optimisation stlldies , it is very desirable to limit the number of the geometric design variables. In Ref I , a ' fundamental' parametric :Jcrofoil geometry represcntation method was presented. The method included the introduction of a geometric 'class function/shape function' transformation technique, CST, s ueh that roLlnd nose/sharp aft end geometries as well as o ther c lasses of geometries could be represented exactly by analytic well behaved and simple mathematical function s having easily observed physical features, The CST method was s hown to dcscrihe a n cssentially limillcss dcsign spacc composed entirel y of analytically smooth geometri es. In Ref. 2, th e CST methodo'iogy was extended to more gene ral three dimensional applications such as wing, body, ducts and nace ll cs. It was shown that any general 3D geometry can be repre­ sented by a distribution of fund amental shapes, and that th e 'shape function/c la ss funct ion ' methodology can be used to describe th e fundame ntal shapes as wcll as thc distributi ons of the fundamental shapes, A number of applications of the 'CST' method to nacelles, ducts, wings and bodies were presented to illustrate th e versatility of this new methodology, In this paper, the CST tllethou is extended to include geometric warping such as variable camber, simple flap, aeroelastic and flutter deflections. The usc of the CST method for geometric morphing of one geometric shape into another is a lso shown. The use of CST analy ti c wings in design optimisation will also be discussed.

Time-Linearized and Time-Accurate 3D RANS Methods for Aeroelastic Analysis in Turbomachinery

Abstract: Graham Ashcroft Institute of Propulsion Technology, German Aerospace Center (DLR), Linder HA¶he, 51147 Cologne, Germany A computational method for performing aeroelastic analysis using either a time-linearized or an unsteady time-accurate solver for the compressible Reynolds averaged Navier--Stokes (RANS) equations is described. The time-linearized solver employs the assumption of small time-harmonic perturbations and is implemented via finite differences of the nonlinear flux routines of the time-accurate solver. The resulting linear system is solved using a parallelized generalized minimal residual (GMRES) method with block-local preconditioning. The time accurate solver uses a dual time stepping algorithm for the solution of the unsteady RANS equations on a periodically moving computational grid. For either solver, and both flutter and forced response problems, a mapping algorithm has been developed to map structural eigenmodes, obtained from finite element structural analysis, from the surface mesh of the finite element structural solver to the surface mesh of the finite volume flow solver. Using the surface displacement data an elliptic mesh deformation algorithm, based on linear elasticity theory, is then used to compute the grid deformation vector field. The developed methods are validated first using standard configuration 10. Finally, for an ultra-high bypass ratio fan, the results of the time-linearized and the unsteady module are compared. The gain in prediction time using the linearized methods is highlighted.

New Approach for the Identification and Validation of a Nonlinear F/A-18 Model by Use of Neural Networks

Abstract: This paper presents a new approach for identifying and validating the F/A-18 aeroservoelastic model, based on flight flutter tests. The neural network (NN), trained with five different flight flutter cases, is validated using 11 other flight flutter test (FFT) data. A total of 16 FFT cases were obtained for all three flight regimes (subsonic, transonic, and supersonic) at Mach numbers ranging between 0.85 and 1.30 and at altitudes of between 5000 and 25 000 ft. The results obtained highlight the efficiency of the multilayer perceptron NN in model identification. Optimization of the NN requires mixing of two proprieties: the hidden layer size reduction and four-layered NN performances. This paper shows that a four-layer NN with only 16 neurons is enough to create an accurate model. The fit coefficients were higher than 92% for both the identification and the validation test data, thus demonstrating accuracy of the NN.

Transonic Aeroelastic Stability Predictions Under the Influence of Structural Variability

Abstract: DOI: 10.2514/1.46971 Flutter prediction as currently practiced is almost always deterministic in nature, based on a single structural model that is assumed to represent a fleet of aircraft. However, it is also recognized that there can be significant structural variability, even for different flights of the same aircraft. The safety factor used for flutter clearance is in part meant to account for this variability. Simulation tools can, however, represent the consequences of structural variability in the flutter predictions, providing extra information that could be useful in planning physical tests and assessing risk. The main problem arising for this type of calculation when using high-fidelity tools based on computational fluid dynamics is the computational cost. The current paper uses an eigenvalue-based stability method together with Euler-level aerodynamics and different methods for propagating structural variability to stability predictions.The propagation methodsare Monte Carlo,perturbation, andinterval analysis. Thefeasibility of this type of analysis is demonstrated. Results are presented for the Goland wing and a generic fighter configuration.

Some Recent Advances in Nonlinear Aeroelasticity: Fluid- Structure Interaction in the 21 st Century

Abstract: Aeroelasticity is the field that examines, models and seeks to understand the interaction of the forces from an aerodynamic flow and the deformation of an elastic structure. The forces produce deformation, but the structural deformation in turn changes the aerodynamic forces. This feedback between force and deformation leads to a variety of dynamic responses of the fluid and the structure including flutter (a Hopf bifurcation), limit cycle oscillations and sometimes chaos. Selected recent advances in nonlinear aeroelasticity and fluidstructure interaction are reviewed to identify and model the fundamental elements that they share. Topics discussed include the following.

Reliability-Based Design Optimization of Nonlinear Aeroelasticity Problems

Abstract: This paper introduces a methodology for the reliability-based design optimization (RBDO) of nonlinear aeroelastic problems. It is based on the construction of explicit flutter and subcritical limit cycle oscillations (LCO) boundaries in terms of the design variables. The boundaries, generated using a Support Vector Machine (SVM), can then be used to efficiently evaluate probabilities of failure and solve an RBDO problem. Test results are presented demonstrating the construction of flutter boundaries as well as LCO boundaries for problems with structural nonlinearities. The solution of an example of RBDO problem is also provided.

Aeroelastic Airfoil with Free Play at Angle of Attack with Gust Excitation

Abstract: A theoretical and experimental aeroelastic study of a typical airfoil section with control surface free play for nonzero angle of attack in low subsonic flow is presented. The study includes the flutter and limit cycle oscillation behavior and also the linear and nonlinear aeroelastic responses excited by periodic gust loads. The theoretical approach uses Peters's finite state airloads model. The experimental investigation has been carried out in the Duke University wind tunnel using a rotating slotted cylinder gust generator. The theoretical and experimental results show that the self-excited aeroelastic limit cycle oscillation is sensitive to the effect of initial pitch angle. For the gust response, the effect of initial pitch angle is smaller for the plunge and pitch responses and is larger for the flap response. The fair to good quantitative agreement between theory and experiment verifies that the present analytical approach has reasonable accuracy and good computational efficiency for nonlinear aeroelastic response analysis of such systems.