Showing papers on "Flutter published in 2003"

Aeroelastic Dynamic Analysis of a Full F-16 Configuration for Various Flight Conditions

Abstract: An overview is given of recent advances in a three-field methodology for modeling and solving nonlinear fluid-structure interaction problems, and its application to the prediction of the aeroelastic frequencies and damping coefficients of a full F-16 configuration in various subsonic, transonic, and supersonic airstreams is reported. In this three-field methodology the flow is described by the arbitrary Lagrangian-Eulerian form of the Euler equations, the structure is represented by a detailed finite element model, and the fluid mesh is unstructured, dynamic, and updated by a robust torsional spring analogy method. Simulation results are presented for stabilized, accelerated, low-g, and high-g flight conditions, and correlated with flight-test data. Consequently, the practical feasibility and potential of the described computational-fluid-dynamics-based computational method for the flutter analysis of high-performance aircraft, particularly in the transonic regime, are discussed.

Performance comparison of two oscillating positive expiratory pressure devices: Acapella versus Flutter.

Abstract: BACKGROUND: Oscillatory positive expiratory pressure (PEP) with the Flutter device facilitates secretion removal. In the Flutter a steel ball vibrates inside a cone, causing air flow vibration. A new device, the Acapella, uses a counterweighted plug and magnet to create air flow oscillation. The Acapella comes in 2 models: one for patients with expiratory flow > 15 L/min and one for < 15 L/min. We hypothesized that the Acapella and Flutter would produce similar mean PEP, oscillatory pressure amplitude, and frequency over a clinically relevant range of flows. METHODS: We measured oscillatory amplitude, PEP, and frequency. Values for frequency, peak, trough, and mean pressure were recorded automatically every 3 seconds at flows of 5, 10, 15, 20, 25, and 30 L/min. The pressure waveform for 1 second was also graphically displayed and recorded. The devices were adjusted to give low, medium, and high mean expiratory pressure (Flutter angle at 0, 20, and 40°; Acapella by dial setting). Data were analyzed by 2-way repeated measures analysis of variance, and differences were considered significant when p was < 0.05. RESULTS: There were statistically significant differences between the devices for mean pressure, pressure amplitude, and frequency, for all experimental conditions. However, the differences were relatively small and may not be clinically important. Both devices produced similar pressure waveforms at the medium flows. At 5 L/min the Acapella produced a more stable waveform, with a lower frequency, higher amplitude, and a slightly wider range of PEP than the Flutter. CONCLUSIONS: Acapella and Flutter have similar performance characteristics. Acapella’s performance is not gravity-dependent (ie, dependent on device orientation) and may be easier to use for some patients, particularly at low expira

Blade Excitation by Aerodynamic Instabilities: A Compressor Blade Study

Abstract: In this paper, we investigate non-synchronous vibrations (NSV) in turbomachinery, an aeromechanic phenomenon in which rotor blades are driven by a fluid dynamic instability. Unlike flutter, a self-excited vibration in which vibrating rotor blades and the resulting unsteady aerodynamic forces are mutually reinforcing, NSV is primarily a fluid dynamic instability that can cause large amplitude vibrations if the natural frequency of the instability is near the natural frequency of the rotor blade. In this paper, we present both experimental and computational data. Experimental data was obtained from a full size compressor rig where the instrumentation consisted of blade-mounted strain gages and case-mounted unsteady pressure transducers. The computational simulation used a three-dimensional Reynolds averaged Navier-Stokes (RANS) time accurate flow solver. The computational results suggest that the primary flow features of NSV are a coupled suction side vortex shedding and a tip flow instability. The simulation predicts a fluid dynamic instability frequency that is in reasonable agreement with the experimentally measured value.Copyright © 2003 by ASME

Status of Unsteady Aerodynamic Prediction for Flutter of High-Performance Aircraft

Abstract: Current production unsteady aerodynamics codes are based on panel methods. The capabilities present in these methods to model complex shapes represents one of the major advancements in this methodology. A second advancement is that the new codes are now formulated based on a unie ed analytical capability throughout the Mach range. Examples of these capabilities will be demonstrated. Finally, based on the discussion of requirements for e utter analyses of high-performance aircraft, a description of the needs for the future is presented.Itishoped thatthese recommendations willprovide guidance to those investigators working in the e eldof unsteady aerodynamics. II. Background A. Unsteady Aerodynamic Methods Strip theory aerodynamics originated in the early 1940s, 1;2 and this method was the primary aerodynamic tool for e utter analyses for many years.In 1966, Yates 3 proposed modifying CL® toaccount for e nite span effects with the result being referred to as modie ed strip theory. This aerodynamic method, coupled with the normal modeapproach,and the V‐g‐! solution technique formedthe basis for production e utter analyses in the late 1960s, and it was to this methodology that the author was e rst exposed to production e utter analyses.

Aeroelastic analysis of bridges: effects of turbulence and aerodynamic nonlinearities

Abstract: Current linear aeroelastic analysis approaches are not suited for capturing the emerging concerns in bridge aerodynamics introduced by aerodynamic nonlinearities and turbulence effects. These issues may become critical for bridges with increasing spans and/or with aerodynamic characteristics sensitive to the effective angle of incidence. This paper presents a nonlinear aerodynamic force model and associated time domain analysis framework for predicting the aeroelastic response of bridges under turbulent winds. The nonlinear force model separates the aerodynamic force into low- and high-frequency components according to the effective angle of incidence. The low-frequency force component is modeled utilizing quasi-steady theory. The high-frequency force component is based on the frequency dependent unsteady aerodynamic characteristics, which are similar to the traditional force model but vary in space and time following the low-frequency effective angle of incidence. The proposed framework provides an effective analysis tool to study the influence of structural and aerodynamic nonlinearities and turbulence on the bridge aeroelastic response. The effectiveness of this approach is demonstrated by utilizing an example of a long span suspension bridge with aerodynamic characteristics sensitive to the angle of incidence. The influence of mean wind angle of incidence on the aeroelastic modal properties and the associated aeroelastic response and the sensitivity of bridge response to nonlinear aerodynamics and low-frequency turbulence are examined.

Efficacy of Tuned Mass Dampers for Bridge Flutter Control

Abstract: This paper examines efficacy of tuned mass dampers (TMD) in controlling self-excited motion resulting from negative damping. New optimal TMD parameters are suggested which provide better performance than those suggested in the literature. The dependence of TMD performance on structural damping is highlighted. The equations of motion of a combined system comprised of multiple TMDs attached to a bridge deck are presented where the bridge motion is described in terms of reduced-order modal coordinates. Details concerning the multimode coupled flutter of long-span bridges with auxiliary TMDs are provided. The effectiveness and limitations of TMDs for controlling multimode bridge flutter are examined, emphasizing the dependence of TMD performance on the bridge dynamic and aerodynamic characteristics. This study shows that the effectiveness of TMDs is rather limited in controlling a hard-type flutter characterized by negative aerodynamic damping that grows rapidly with increasing wind speed beyond the onset of flutter. However, it is relatively effective in controlling a soft-type flutter in which the negative damping builds up slowly with increasing wind speed. Robust TMD design issues are also discussed in light of their sensitivity to design parameters in the vicinity of optimal values.

Implications of cubic physical/aerodynamic non-linearities on the character of the flutter instability boundary

Abstract: The present paper deals with a study of the benign and catastrophic characters of the flutter instability boundary of 2-D lifting surfaces in a supersonic flow field. The objectives of this work are: (i) to contribute to a better understanding of the implications of aerodynamic and physical non-linearities on the character of the flutter boundary and (ii), to outline the effects exerted in the same respect by some important parameters of the aeroelastic system. With the aim of addressing this problem, the method based on the First Liapunov Quantity is used to study the bifurcational behavior of the aeroelastic system in the vicinity of the flutter boundary. The expected outcomes of this study are: (a) to greatly enhance the scope and reliability of the aeroelastic analysis and design criteria of advanced aircraft and, (b) to provide a theoretical basis for the analysis of more complex non-linear aeroelastic systems.

Dynamic Response of Suspension Bridge to High Wind and Running Train

Abstract: A framework is presented for predicting the dynamic response of long suspension bridges to high winds and running trains. A three-dimensional finite-element model is used to represent a suspension bridge. Wind forces acting on the bridge, including both buffeting and self-excited forces, are generated in the time domain using a fast spectral representation method and measured aerodynamic coefficients and flutter derivatives. Each 4-axle vehicle in a train is modeled by a 27-degrees-of-freedom dynamic system. The dynamic interaction between the bridge and train is realized through the contact forces between the wheels and track. By applying a mode superposition technique to the bridge only and taking the measured track irregularities as known quantities, the number of degrees of freedom of the bridge-train system is significantly reduced and the coupled equations of motion are efficiently solved. The proposed formulation is then applied to a real wind-excited long suspension bridge carrying a railway inside the bridge deck of a closed cross section. The results show that the formulation presented in this paper can predict the dynamic response of the coupled bridge-train systems under fluctuating winds. The extent of interaction between the bridge and train depends on wind speed and train speed.

Effects of Flow Instabilities on the Linear Analysis of Turbomachinery Aeroelasticity

Abstract: The linear analysis of turbomachinery aeroelasticity is based on the linearization of the unsteady flow equations around the mean flow field, which can be determined by a nonlinear steady solver. The unsteady periodic flow can be decomposed into a sum of harmonics, each of which can be computed independently by solving a set of linearized equations. The analysis considers just one particular frequency of unsteadiness at a time, and the objective is to compute a complex flow solution that represents the amplitude and phase of the unsteady flow. The solution procedure of both the nonlinear steady and the linear harmonic Euler/Navier-Stokes solvers of the HYDRA suite of codes consists of a preconditioned fixed-point iteration. The numerical instabilities encountered while solving the linear harmonic equations for some turbomachinery test cases are documented, their physical origin highlighted, and the implementation of a GMRES algorithm aiming at the stabilization of the linear code summarized. Presented results include the flutter analysis of a two-dimensional turbine section and a civil engine fan.

Chaotic Analysis of Nonlinear Viscoelastic Panel Flutter in Supersonic Flow

Abstract: In this paper chaotic behavior of nonlinear viscoelastic panels in asupersonic flow is investigated. The governing equations, based on vonKaarman's large deflection theory of isotropic flat plates, areconsidered with viscoelastic structural damping of Kelvin's modelincluded. Quasi-steady aerodynamic panel loadings are determined usingpiston theory. The effect of constant axial loading in the panel middlesurface and static pressure differential have also been included in thegoverning equation. The panel nonlinear partial differential equation istransformed into a set of nonlinear ordinary differential equationsthrough a Galerkin approach. The resulting system of equations is solvedthrough the fourth and fifth-order Runge–Kutta–Fehlberg (RKF-45)integration method. Static (divergence) and Hopf (flutter) bifurcationboundaries are presented for various levels of viscoelastic structuraldamping. Despite the deterministic nature of the system of equations,the dynamic panel response can become random-like. Chaotic analysis isperformed using several conventional criteria. Results are indicative ofthe important influence of structural damping on the domain of chaoticregion.

Flutter Suppression Using Micro-Trailing Edge Effectors

Abstract: Recent developments in actuator technology have resulted in small, simple flow control devices capable of affecting the flow field over flight vehicles sufficiently to generate control forces. One of the devices which has been under investigation is the Micro-Trailing Edge Effector (MiTE), which consists of a small, 1-5% chord, vertically sliding flap mounted at the trailing edge. The high bandwidth and good control authority with little required power makes the device an ideal candidate for active control of flutter in high aspect ratio wings. Unfortunately traditional control techniques do not address the non-linear nature of the device or the competing performance goals arising from large numbers of distributed devices. Novel approaches to control design, such as reinforcement learning, are therefore required. To demonstrate the aeroelastic control capability of the MiTEs and to explore reinforcement learning techniques, an experimental model has been designed, fabricated, and tested. This paper details the experimental model and the accompanying analytical model. Design, manufacturing and open loop testing of the experimental model and comparisons with the analytical predictions are presented. This paper also covers the controller design for flutter suppression using reinforcement learning policy search techniques. The results of closed loop testing, resulting in successful flutter suppression with the MiTEs, is presented.

Experimental and Numerical Study of Stall Flutter in a Transonic Low-Aspect Ratio Fan Blisk

Abstract: Experiments are performed on a modern design transonic shroudless low-aspect ratio fan blisk that experienced both subsonic/transonic and supersonic stall-side flutter. High-response flush mounted miniature pressure transducers are utilized to measure the unsteady aerodynamic loading distribution in the tip region of the fan for both flutter regimes, with strain gages utilized to measure the vibratory response at incipient and deep flutter operating conditions. Numerical simulations are performed and compared with the benchmark data using an unsteady three-dimensional nonlinear viscous computational fluid dynamic (CFD) analysis, with the effects of tip clearance, vibration amplitude, and the number of time steps-per-cycle investigated. The benchmark data are used to guide the validation of the code and establish best practices that ensure accurate flutter predictions.Copyright © 2003 by ASME

Parallel Computation of Wing Flutter with a Coupled Navier-Stokes/CSD Method

Abstract: A code is developed for the computation of three-dimensional aeroelastic problems such as wing flutter. The unsteady Navier-Stokes flow solver is based on a finite-volume approach with centered flux discretization and artificial diffusion. For the structural displacements a modal approach is applied. The temporal discretization is implicit for both the flow equations and the structural equations. An explicit dual-time method is used to integrate the coupled governing equations. A multigrid method is applied to advance the flow solution, and the computation is performed in parallel with a multiblock approach. A supercritical 2-D wing and the AGARD 445.6 wing serve as test cases for flutter investigations. Results for inviscid flow are compared with results obtained by solving the Navier-Stokes equations with the Baldwin-Lomax and k-ω turbulence models, respectively. Inclusion of viscous effects is critical for the 2-D wing. LCO of the 2-D wing is predicted, but with larger amplitude compared to experimental measurements. Predicted flutter boundary for the AGARD wing agrees well with experimental data in subsonic and transonic range but deviates significantly from experimental data in the supersonic range. Inclusion of viscous effects only slightly improves the result for this case.

Using Aeroelastic Modes for Nonlinear Panel Flutter at Arbitrary Supersonic Yawed Angle

Abstract: It is commonly accepted that six in vacuo natural modes are needed for converged, limit-cycle oscillations of isotropic rectangular plates exposed to supersonic flow at zero yaw angle to the principle panel length. For isotropic or orthotropic rectangular plates under an arbitrary nonzero yawed supersonic flow, then 36 or 6 x 6 natural modes are needed; for laminated anisotropic rectangular plates even at zero yaw angle, 36 or fewer natural modes are needed. To deal with such a large number of modes is computationally costly for flutter analysis, causing complexity and difficulty in designing controllers for flutter suppression. A thorough examination and understanding of the panel limit-cycle behavior leads to the use of aeroelastic modes for supersonic nonlinear panel flutter analysis. The system equations of motion are formulated first in structural node degrees of freedom. Aeroelastic modes are selected and determined, and the system equations is expressed in the aeroelastic modal coordinates. Limit-cycle amplitudes are then determined using numerical integration. Examples show that the number of modes could be greatly reduced by using aeroelastic modes. For determining limit-cycle oscillations of isotropic or anisotropic composite rectangular plates at zero or an arbitrary yawed flow angle, only two aeroelastic modes are needed; but six to seven aeroelastic modes are needed for designing controllers for flutter suppression.

The effect of laminate lay-up on the flutter speed of composite wings:

Abstract: This paper presents an analytical study on optimization of a laminated composite wing structure for achieving a maximum flutter speed and a minimum weight without strength penalty. The investigation is carried out within the range of incompressible airflow and subsonic speed. In the first stage of the optimization, attention has been paid mainly to the effect on flutter speed of the bending, torsion and, more importantly, the bending-torsional coupling rigidity, which is usually associated with asymmetric laminate lay-up. The study has shown that the torsional rigidity plays a dominant role, while the coupling rigidity has also quite a significant effect on the flutter speed. In the second stage of the optimization, attention has been paid to the weight and laminate strength of the wing structure, which is affected by the variation in laminate lay-up in the first stage. Results from a thin-walled wing box made of laminated composite material show that up to 18 per cent increase in flutter speed an...

Feedforward control for reducing disk-flutter-induced track misregistration

Abstract: We show in this paper that the disk vertical vibration signal picked from the slider in a hard disk drive has a fairly good correlation to the track misregistration at the disk flutter frequencies. We propose to introduce a feedforward controller into the usual servo loop to reduce disk-flutter-induced track misregistration (TMR). The feedforward controller is an approximated differential element with the disk vertical direction velocity as input. In our experimental setup, we show that such a method can reduce the TMR induced by the first four disk flutter modes by an average of about 56%. The method can potentially be used in high-track-per-inch servo track writers to reduce the written-in repeatable runouts.