Showing papers on "Flutter published in 2004"

Aeroelastic stability predictions for a MW-sized blade

Abstract: Classical aeroelastic flutter instability historically has not been a driving issue in wind turbine design. In fact, rarely has this issue even been addressed in the past. Commensurately, among the wind turbines that have been built, rarely has classical flutter ever been observed. However, with the advent of larger turbines fitted with relatively softer blades, classical flutter may become a more important design consideration. In addition, innovative blade designs involving the use of aeroelastic tailoring, wherein the blade twists as it bends under the action of aerodynamic loads to shed load resulting from wind turbulence, may increase the blade's proclivity for flutter. With these considerations in mind it is prudent to revisit aeroelastic stability issues for a MW-sized blade with and without aeroelastic tailoring. Focusing on aeroelastic stability associated with the shed wake from an individual blade turning in still air, the frequency domain technique developed by Theodorsen for predicting classical flutter in fixed wing aircraft has been adapted for use with a rotor blade. Results indicate that the predicted flutter speed of a MW-sized blade is slightly greater than twice the operational speed of the rotor. When a moderate amount of aeroelastic tailoring is added to the blade, a modest decrease (12%) in the flutter speed is predicted. By comparison, for a smaller rotor with relatively stiff blades the predicted flutter speed is approximately six times the operating speed. When frequently used approximations to Theodorsen's method are implemented, drastic underpredictions result, which, while conservative, may adversely impact blade design. These underpredictions are also evident when this MW-sized blade is analysed using time domain methods. Published in 2004 by John Wiley & Sons, Ltd.

Numerical Analysis of Store-Induced Limit-Cycle Oscillation

Abstract: Store-induced limit-cycle oscillation of a rectangular wing with tip store in transonic flow is simulated using a variety of mathematical models for the flowfield: transonic small-disturbance theory (with and without inclusion of store aerodynamics) and transonic small-disturbance theory with interactive boundary layer (without inclusion of store aerodynamics). For the conditions investigated, assuming inviscid flow, limit-cycle oscillations are observed to occur as a result of a weakly subcritical Hopf bifurcation and are obtained at speeds lower than those predicted 1) nonlinearly for clean-wing flutter and 2) linearly for wing/store flutter. The ability of transonic small-disturbance theory to predict the occurrence and strength of this type of limit-cycle oscillation is compared for the different models. Differences in unmatched and matched aeroelastic analysis are described. Solutions computed for the clean rectangular wing are compared to those computed with the Euler equations for a case of static aeroelastic behavior and for a case of forced, rigid-wing oscillation at Mach 0.92.

The Secondary Bifurcation of an Aeroelastic Airfoil Motion: Effect of High Harmonics

Abstract: The nonlinear dynamical response of a two-degree-of-freedom aeroelastic airfoil motion with cubic restoring forces is investigated. A secondary bifurcation after the primary Hopf (flutter) bifurcation is detected for a cubic hard spring in the pitch degree-of-freedom. Furthermore, there is a hysteresis in the secondary bifurcation: starting from different initial conditions the motion may jump from one limit cycle to another at different fluid flow velocities. A high-order harmonic balance method is employed to investigate the possible bifurcation branches. Furthermore, a numerical time simulation procedure is used to confirm the stable and unstable bifurcation branches.

Flutter, Tumble and Vortex Induced Autorotation

Abstract: The phenomenon of flutter and tumble of objects in free fall has been studied using two-dimensional numerical simulations of uniform flow past a plate which is free to rotate about a fixed axis through its centroid. Particular focus is on the effect of Reynolds number and plate thickness-to-length ratio on the flutter-to-tumble transition and on the observed frequency of the angular motions. Simulations indicate that the tendency to tumble increases with increasing Reynolds number and decreasing thickness ratio. A case is also made that the tumbling frequency for two-dimensional plates is governed by a Karman type vortex shedding process. These results for this pinned plate have also been verified by simulating a limited number of free-fall cases.

Identification of computational-fluid-dynamics based unsteady aerodynamic models for aeroelastic analysis

Abstract: Three approaches for reduced-order modeling of computational-fluid-dynamics-(CFD) based unsteady aerodynamics, employing system-identification methods, are presented, and used for generation of three models: A frequency-domain model, a time-domain autoregressive-moving-average model, and a discrete-time state-space model. All models are identified based on the same identification data, which consists of the time histories of the generalized aerodynamic forces developed in response to filtered white-Gaussian-noise modal excitation, computed in a CFD analysis. The models are used for rapid flutter analysis via traditional frequency-domain methods, linear stability analysis, and time simulation. The method is applied for flutter analysis of the AGARD 445.6 wing. The filtered white-Gaussian-noise input is found to be applicable within the framework of CFD, yielding informative identification data sets. The identification process is simple, and the resulting reduced-order models closely reproduce the CFD system response to various excitations. Reduced-order model-based flutter analysis is rapid and yields accurate results compared with wind-tunnel test, CFD, and linear aerodynamics results.

The mu-k Method for Robust Flutter Solutions

Abstract: A straightforward frequency-domain method for robust flutter analysis is presented. First, a versatile uncertainty description for the unsteady aerodynamic forces is derived by assigning uncertainty to the frequency-domain pressure coefficients. The uncertainty description applies to any frequency-domain aerodynamic method, benefits from the same level of geometric detail as the underlying aerodynamic model, exploits the modal formulation of the flutter equation, and is computed by simple postprocessing of standard aerodynamic data. Next, structured singular value analysis is applied to derive an explicit criterion for robust flutter stability based on the flutter equation and a parametric uncertainty description. The resulting procedure for computation of a worst-case flutter boundary resembles a p-k or g-method flutter analysis, produces match-point flutter solutions and allows for detailed aerodynamic uncertainty descriptions. Finally, the proposed method is successfully applied to a wind-tunnel model in low-speed airflow.

Impact of actuation concepts on morphing aircraft structures

Abstract: *†‡§¶ Design optimization studies are presented that characterize specifications and design implication for integrated actuation systems of morphing aircraft structures. In a morphing aircraft design concept accommodated by articulating wing folds, the actuated system stiffness, load capacity, and integral volumetric requirements drive flutter, strength, and aerodynamic performance. Design studies concerning aircraft flight speed, maneuver load factor, and actuator response provide sensitivities in structural weight, aeroelastic performance, and actuator flight load distributions. Overall design requirements are developed for new actuation systems. The studies provide insight to the aircraft design process and flight criteria specification for morphing aircraft structures.

CFD‐Based Nonlinear Computational Aeroelasticity

Abstract: This chapter overviews a three-field formulation of nonlinear aeroelastic problems, where the fluid is modeled by the arbitrary Lagrangian–Eulerian (ALE) form of either the Euler or Navier–Stokes equations; the structure is represented by a detailed finite element model, and the fluid-mesh is unstructured, dynamic, and constructed by a robust structure-analogy–based method. It also discusses recent advances in the computational algorithms associated with this approach for modeling nonlinear fluid/structure interaction problems. These include CFD schemes that are formally second-order time-accurate on moving grids, energy-transfer–conserving methods for discretizing transmission conditions across nonmatching fluid and structure discrete interfaces, and state-of-the-art loosely coupled algorithms for solving efficiently coupled systems of fluid and structural equations arising from the discretization of real aircraft configurations. A taste of the capabilities of such algorithms is provided by reporting on their validation for an F-16 fighter aircraft configuration in various subsonic, transonic, and supersonic airstreams, and at various load factors. The chapter concludes with a discussion of the feasibility and merit of the overviewed computational technology for accurately extracting the flutter envelopes of civilian and military aircraft, and a perspective on future research in CFD-based nonlinear computational aeroelasticity. Keywords: computational aeroelasticity; CFD; fluid/structure interaction; flutter; loosely coupled algorithms; validation

Finite Element Modal Formulation for Hypersonic Panel Flutter Analysis with Thermal Effects

Abstract: A finite element time-domain modal formulation for analyzing nonlinear flutter of panels subjected to hypersonic airflow has been developed. Von Karman large deflection plate theory is used for description of the structural nonlinearity, and third-order piston theory is employed to consider the aerodynamic nonlinearity. The thermal loadings of uniformly distributed surface temperatures and temperature gradients through the panel thickness are considered. By the application of the modal truncation technique, the number of governing equations of motion is reduced dramatically so that the computational costs are reduced significantly. All possible types of panel behavior, including flat and stable, buckled but dynamically stable, limit cycle oscillation (LCO), periodic motion, and chaotic motion were observed. Examples of the applications of the proposed methodology were flutter responses of isotropic and composite panels. Special emphasis was placed on the boundary between LCO and chaos and on the route to chaos. Time history, phase plane plot, Poincare map, bifurcation diagram, and Lyapunov exponent are employed in the chaos study. It is found that at low or moderately high dynamic pressures, the fluttering panel typically takes a period-doubling route to evolve into chaos, whereas, at high dynamic pressures, the route generally involves bursts of chaos and rejuvenations of periodic motions.

Nonlinear Open-/Closed-Loop Aeroelastic Analysis of Airfoils via Volterra Series

Abstract: Determination of the subcritical aeroelastic response to arbitrary time-dependent external excitation and determination of the flutter instability of open/closed-loop two-dimensional nonlinear airfoils constitute the main topics. To address these problems, Volterra series and indicial aerodynamic functions are used, and, in the same context, the pertinent aeroelastic nonlinear kernels are determined. Flutter instability predictions obtained within this approach compared with their counterparts generated via the frequency eigenvalue analysis and via experiments reveal excellent agreements. Implications of a number of important parameters characterizing the lifting surface and control law on the aeroelastic response/flutter are discussed, and pertinent conclusions are outlined.

Coherent structures and their influence on the dynamics of aeroelastic panels

Abstract: A panel forced by a supersonic unsteady flow is numerically investigated using a finite difference method, a Galerkin approach, and proper orthogonal decomposition (POD). The aeroelastic model investigated is based on piston theory for modeling the flow-induced forces, and von Karman plate theory for modeling the panel. Structural non-linearity is considered, and it is due to the non-linear coupling between bending and stretching. Several novel facets of behavior are explored, and key aspects of using a Galerkin method for modeling the dynamics of the panel exhibiting limit cycle oscillations and chaos are investigated. It is shown that multiple limit cycles may co-exist, and they are both symmetric and asymmetric. Furthermore, the level of spatial coherence in the dynamics is estimated by means of POD. Reduced order models for the dynamics are constructed. The sensitivity to initial conditions of the non-linear aeroelastic system in the chaotic regime limits the capability of the reduced order models to identically model the time histories of the system. However, various global characteristics of the dynamics, such as the main attractor governing the dynamics, are accurately predicted by the reduced order models. For the case of limit cycle oscillations and stable buckling, the reduced order models are shown to be accurate and robust to parameter variations.

Parameter Adaptation of Reduced Order Models for Three-Dimensional Flutter Analysis

Abstract: The accurate prediction of flutter for large 3-D structures in the transonic flight regime requires the construction of high-fidelity models of the fluid system in order to represent the complex flow characteristics present in this nonlinear regime. The computational expense can be quite considerable when the model is large (many degrees of freedom) and/or long integration times are required to determine the stability properties of the flow. The computational expense of this type of flutter analysis on large systems makes it impractical for use as a tool in applications such as aircraft design, flight testing, or controller design. Recently, the proper orthogonal decomposition (POD) method has been used to construct reduced order models (ROMs) of the fluid system in order to reduce the computational expense of the flutter analysis. The POD method constructs a reduced basis by using either experimental or simulation data at a particular flight condition. The full fluid system is then projected onto this reduced basis in order to form the ROM which is used for the flutter analysis. Here, we investigate the robustness of the POD-based ROM constructed in the time domain in terms of its accuracy when used for flight conditions that are dierent from those at which the ROM was created. We construct a ROM of the fluid system and use it along with a modal representation of the three dimensional structure to perform time domain aeroelastic analyses at dierent freestream Mach numbers, pressures, and densities to define the flutter boundary. We compare these results to experimental data as well as full order nonlinear simulation data. The POD-based ROM is applicable to a very wide range of freestream pressures and densities mostly owing to the fact that the pressure and density only appear as a constant factor in computing the aerodynamic lift force. Thus, a single ROM constructed at a given Mach number can be used to perform aeroelastic analyses over a large range of pressures and densities which increases its application potential in a design process. However, as expected, the POD-based ROM does exhibit sensitivity to the Mach number. We present two interpolation methods for adapting the POD basis vectors to varying Mach numbers. The first approach uses a simple and conventional Lagrange interpolation. The second approach is based on interpolating the angles between the ROM subspaces. We demonstrate these approaches on the AGARD Wing 445.6

Modern developments in computational aeroelasticity

Abstract: A survey of the main challenges in computational aeroelasticity is presented and recent ideas and developments are discussed. Advances over the past 25 years have to a large extent been paced by the required developments in computational fluid dynamics (CFD). The fluid-structure coupling problem remains of central importance and must be addressed in a rational manner in order to obtain accurate and reliable flutter solutions. In the direct Eulerian-Lagrangian computational scheme, a consistent and efficient fluid-structure coupling is obtained by modelling and integrating the fluid-structure system as a single dynamical system, without introducing normal or assumed modes, or an artificial 'virtual surface' at the boundary. This computational approach effectively eliminates the phase integration errors associated with classical methods, where the fluid and the structure are integrated sequentially using different schemes. Numerical results are presented to contrast the efficacy of the various schemes in non-linear aeroelastic and aeroservoelastic calculations.

Linear/Nonlinear Supersonic Panel Flutter in a High-Temperature Field

Abstract: An analysis of the flutter and postflutter behavior of infinitely long flat panels in a supersonic/hypersonic flowfield exposed to a high-temperature field is presented. In the approach to the problem, the thermal degradation of thermoelastic characteristics of the material is considered. A third-order piston theory aerodynamic model in conjunction with the von Karman nonlinear plate theory is used to obtain the pertinent aerothermoelastic governing equations. The implications of temperature, thermal degradation, and of structural and aerodynamic nonlinearities on the character of the flutter instability boundary are analyzed. As a byproduct, the implications of the temperature on the linearized flutter instability of the system are discussed. The behavior of the structural system in the vicinity of the flutter boundary is studied via the use of an encompassing methodology based on the Lyapunov First Quantity. Numerical illustrations, supplying pertinent information on the implications of the temperature field and of the thermal degradation are presented, and pertinent conclusions are outlined.

Flutter of Mistuned Bladed Disks and Blisks With Aerodynamic and FMM Structural Coupling

Abstract: This paper presents the initial results from a research effort on blisk and bladed disk mistuning including both structural and aerodynamic coupling. The structural coupling model is based on the Fundamental Mistuning Model (FMM, developed by Feiner & Griffin). This effort extends the FMM technique to accept the aerodynamic coupling coefficients from computational fluid dynamic (CFD) codes. The extended model is applied to a representative engine case. The model and initial studies to identify flutter sensitivities to random and near alternate mistuning are presented. Comparisons are given of tuned and mistuned flutter for only aerodynamic coupling, and with both aerodynamic and structural coupling. For the case studied herein the beneficial effect of mistuning on flutter predicted by aerodynamic coupling models is shown to be greatly inhibited by the inclusion of the structural coupling effects.Copyright © 2004 by ASME

Identification of eighteen flutter derivatives of an airfoil and a bridge deck

Abstract: Wind tunnel experiments are often performed for the identification of aeroelastic parameters known as flutter derivatives that are necessary for the prediction of flutter instability for flexible structures. Experimental determination of all the eighteen flutter derivatives for a section model facilitates complete understanding of the physical mechanism of flutter. However, work in the field of identifying allrnthe eighteen flutter derivatives using section models with all three degree-of-freedom (DOF) has been limited. In the current paper, all eighteen flutter derivatives for a streamlined bridge deck and an airfoil section model were identified by using a new system identification technique, namely, Iterative Least Squares (ILS) approach. Flutter derivatives of the current bridge and the Tsurumi bridge are compared. Flutter derivatives related to the lateral DOF have been emphasized. Pseudo-steady theory for predicting some of the flutter derivatives is verified by comparing with experimental data. The three-DOF suspension system and the electromagnetic system for providing the initial conditions for free-vibration of the section model are also discussed.

Lower Loop Reentry as a Mechanism of Clockwise Right Atrial Flutter

Abstract: Background— Right atrial reentrant tachycardia resulting from lower loop reentry (LLR) around the inferior vena cava (IVC) has been described recently. However, all reported cases of LLR in the literature have negative flutter waves on the inferior surface ECG leads similar to that of counterclockwise typical atrial flutter around the tricuspid annulus (TA). Right atrial flutter with positive flutter waves in the inferior ECG leads has been assumed to rotate as a single reentrant activation wave front around the TA, and the role of LLR in those patients is not known. Methods and Results— Twelve consecutive patients with flutter wave morphology on surface ECG consistent with clockwise atrial flutter were studied. The endocardial activation pattern recorded from conventional multipolar electrode catheters was characteristic of clockwise atrial flutter around the TA. Entrainment pacing in all 12 patients and 3D activation sequence mapping in 7 patients, however, revealed clockwise LLR involving the lower rig...