Flutter Mechanism Research Articles

Exploration on flutter mechanism of a damaged transonic rotor blade using high-fidelity fluid–solid coupling method

Structural damage in turbomachinery is a primary origin of aeronautic accidents, which is receiving increased attention. This study is thus focused on the aeroelastic analysis of damaged blades, including the onset of flutter and underlying mechanisms. First, a high-fidelity fluid–solid coupling system is established with computational fluid dynamics (CFD) and computational structural dynamics (CSD) technologies, via which the dynamic aeroelastic analysis is conducted based on static aeroelastic deformation. Second, a damaged rotor blade is parametrically modelled with variable damage levels, extents, and positions. Finally, the modal identification method of spectral proper orthogonal decomposition (SPOD) is applied to observe flow details and provide physical insight into the flutter mechanism for damaged blades. Numerical analysis finds that there is a critical damage level below which the aeroelastic stability is positively improved with increasing damage level; otherwise, a significant loss of stability is induced. The damage location and extent further affect this critical damage level and the change rate crossing the threshold. The simulation with CFD/CSD finds that the high pressure near the trailing edge induced from boundary layer separation suppresses vibrations in stable conditions, but motivates vibrations during flutter, which is because of the high-pressure spread to nearing blades. SPOD modes reveal that high-frequency disturbances with large scale are primary factors inducing flutter, which is further stimulated by the high-order disturbances with small scale. This study provides a crucial foundation for the fatigue prediction for rotor blades in service and the optimisation design for high-performance turbomachinery in the near future.

Flow field analysis of self-sustained flutter of a wide-slotted bridge deck

This study delves into the flutter mechanism of a 5,000 m bridge with a wide-slotted deck, finding that the motion is self-sustained and not violently destructive. The system damping ratio is not fixed, and only one stable orbit exists. High-resolution PIV experiments at an experimental wind speed of 10.5 m/s measured the static and dynamic flow fields on the windward and leeward decks. The static results showed leading-edge separation on the windward deck within the reference range, while separation on the leeward deck was difficult to observe. Dynamic testing identified instantaneous vortices around the windward/leeward deck at each phase, with no signs of vortices in the wind speed vectors at all phases after phase-averaging, indicating that the vortex drift hypothesis is not valid. The analysis of the streamline pattern revealed periodic variations in leading-edge separation size and reattachment length on the windward deck during the vibration process, while the leeward deck showed consistently inconspicuous changes. Further examination uncovered a peculiar behavior in the horizontal wind speed profile on the leeward deck during the vibration process, attributed to dynamic changes in the height difference between the windward and leeward decks during flutter. The study suggests that the unusual wind speed profile on the leeward deck is caused by the dynamic changes in height difference between the windward and leeward decks during the flutter process, resulting in additional wind loading. These findings shed light on the complex dynamics of bridge flutter and have implications for the design and maintenance of long-span bridges.

Simulasi Numerik Aeroelastik Model Seksional 2D Jembatan Bentang Panjang untuk Mengetahui Karakteristik Ketidakstabilan Flutter

Flutter is an aerodynamic instability on a long span bridge that can cause damage to the entire bridge deck structure. The interaction between wind and structure in long span bridges can be investigated by numerical simulation. In this study, an aeroelastic simulation was performed on a 2 DoFs sectional model of a long-span bridge deck with free vibration techniques to analyze flutter speed and determine the effect of deck shape on flutter instability characteristics using ANSYS software. The simulation result data was then extracted using the Modified Ibrahim Time-Domain Method (MITD) identification method to obtain the damping ratio and flutter derivatives coefficients. The damping ratio value is used to determine the critical flutter speed, whereas the coefficient flutter derivatives is used to determine the characteristics of flutter instability and the flutter mechanism that occurs in the bridge. The results showed that the rectangular shape (bluff body) is more susceptible to flutter instability than the streamlined shape, and has a lower flutter speed value than the other shapes. The flutter mechanism that occurs is torsional flutter, whereas in the streamline body is coupling flutter.

Modeling of nonlinear self-excited forces: A study on flutter characteristics and mechanisms of post-flutter behaviors

The intrinsic physical relevance of higher-order self-excited force (SEF) components has received limited attention, and there is a dearth of formulas that adequately analyze the influence of SEF components on the post-flutter characteristics. Based on Taylor formulas and the principle of independence, semi-empirical polynomial SEF models are developed and validated. The energy input efficiency and role of each order SEF component are examined using the proposed models. By introducing the principle of energy equivalence and approximate average power, theoretical formulas designed to calculate the post-flutter characteristics are established. Finally, the applicability and robustness of the SEF models and theoretical formulas are discussed. Results show that the proposed models can obtain independent higher-order SEF components, which is conducive to the correct analysis of the SEF driving mechanisms. The theoretical formulas can accurately reconstruct the time-varying curves of the flutter characteristics, and the terms in the formulas can explicitly calculate and analyze the mechanism of each SEF model element. It is observed that the higher-order SEF components have a significant impact on the accurate reconstruction of SEFs while barely affecting the system energy. Moreover, the limit cycle oscillation generation mechanisms of the investigated two rectangular cylinders are different, but the variation of the flutter characteristics with time remain the same.

Data-Driven Approach to the Development of an Aeroelastic Flutter Precursor

This paper presents a novel algorithm for aeroelastic flutter early detection. Two new features for flutter onset detection are presented. Flutter early warning is accomplished using only measured signals, with essentially no prior knowledge needed about the aircraft or the flutter mechanism involved. The algorithm consists of three stages: 1) extraction of regularity features, 2) calibration by addition of white noise to nominal measurements, and 3) thresholding. Four types of datasets were used: a) synthetic data, b) simulated data generated using aeroelastic response simulations to stochastic gusts, c) measured data from a wind tunnel experiment, and d) flight test data including actual flutter onsets. The algorithm was shown to be able to flag an impending flutter event before critical onset occurs. (The Python code for paper is available at https://github.com/bmeivar/flutter.)

Flutter instability characteristics and mechanisms of Ziegler double pendulum with arbitrary masses, stiffness and damping

Flutter instability characteristics and mechanisms of Ziegler double pendulum with arbitrary masses, stiffness and damping

Coupled Flutter Behavior and Mechanism of Flexible Suspended Footbridge

Coupled Flutter Behavior and Mechanism of Flexible Suspended Footbridge

A Detailed Physical Explanation of an Aircraft Flutter Mechanism

A Detailed Physical Explanation of an Aircraft Flutter Mechanism

Flutter analysis of a rigid-flexible coupled composite space structure with momentum wheels under thermal load

The communication and observation spacecraft generally consists of the spacecraft body, the composite connecting beams, the working payload and the CMG/momentum wheel for integrated attitude-vibration control. When such complex space structure operates in near-Earth orbit, the sudden change in thermal load may cause a solar flux shock to the structure, which is likely to excite the flutter motion of the structure. The flutter mechanism of a rigid-flexible coupled composite space structure configured with the momentum wheel is investigated. In this study, the heat conduction equation of the composite beam appendage is derived. Then, the dynamic equations of the system influenced by the momentum wheel and composite material parameters are obtained based on the Kane's equations. Using the Routh–Hurwitz stability criterion, the stability boundaries of the flexible appendage are obtained for different composite material parameters, different operating conditions of the momentum wheel, and different attitude angular velocities. Finally, the flutter behavior is analyzed based on the dynamic response of the flexible structure and the flutter stability boundaries are verified.

Mechanism of single-mode panel flutter in low supersonic flow

Supersonic panel flutter can be of two possible types: single-mode flutter and coupled-mode flutter. Compared to coupled-mode flutter, the physical mechanism of single-mode flutter is less studied at present. In order to reveal the inducing mechanism of single-mode flutter, the present paper constructed a reduced-order fluid model by using the system identification and the Auto Regressive with eXogenous input (ARX) model. Coupling the reduced-order model (ROM) for unsteady aerodynamics in low supersonic flow with the structural equation, a highly efficient ROM-based aeroelastic model in state space is formulated, then the flutter boundaries are obtained and the stability of aeroelastic modes are investigated by the complex eigenvalue analysis. According to the physical meaning of relevant parameters in unsteady aerodynamic reduced order model ARX, it is found that the unsteady characteristic of the flow, specifically the history effects of the unsteady aerodynamic forces, plays a dominant role in causing the single-mode panel flutter at low supersonic speeds. It is also shown that the higher modes are indeed weakly unstable in the absence of any structural damping or a viscous boundary layer.

Numerical Investigation of Classic Flutter Mechanism Under Circumferentially Non-Uniform Inlet Condition

Abstract Over recent years, the aeroelastic problem of fan blades has become significantly serious due to the demand for high performance of aero-engines. To explore the aeroelastic issue of flutter, fan instability is represented by the aerodynamic damping. When the inlet condition is uniform, aerodynamic damping is mainly caused by the temporal factor, and the classic flutter usually does not occur due to the usage of mistuning. However, with the existence of circumferentially non-uniform components in the distorted inflow, aerodynamic damping is also affected by the spatial factor, which may alter the occurrence mechanism of classic flutter. To investigate this issue, the full-annulus 3D (three-dimensional) unsteady calculation is conducted on a transonic fan NASA Rotor 67 with a typical circumferential distortion inflow. As demonstrated by the energy method for the rotor blades at each nodal diameter, the coefficients of aerodynamic damping contain an interval value rather than a fixed datum. Some of the blades have positive damping, and others are negative. The positive and negative aerodynamic damping is determined by the relative position between the rotor blade and the distorted region in the circumferential direction. Furthermore, a fast calculation method based on the fundamental principle regarding flow periodicity is developed to obtain the aerodynamic damping at different nodal diameters under non-uniform inlet conditions.

About the Wing and Whirl Flutter of a Slender Wing–Propeller System

Next-generation aircraft designs often incorporate multiple large propellers attached along the wingspan (distributed electric propulsion), leading to highly flexible dynamic systems that can exhibit aeroelastic instabilities. This paper introduces a validated methodology to investigate the aeroelastic instabilities of wing–propeller systems and to understand the dynamic mechanism leading to wing and whirl flutter and transition from one to the other. Factors such as nacelle positions along the wing span and chord and its propulsion system mounting stiffness are considered. Additionally, preliminary design guidelines are proposed for flutter-free wing–propeller systems applicable to novel aircraft designs. The study demonstrates how the critical speed of the wing–propeller systems is influenced by the mounting stiffness and propeller position. Weak mounting stiffnesses result in whirl flutter, while hard mounting stiffnesses lead to wing flutter. For the latter, the position of the propeller along the wing span may change the wing mode shapes and thus the flutter mechanism. Propeller positions closer to the wing tip enhance stability, but pusher configurations are more critical due to the mass distribution behind the elastic axis.

A novel multi-modal analytical method focusing on dynamic mechanism of bridge flutter

A novel three-dimensional (3D) multi-modal analytical method is proposed in the present study for investigating the dynamic mechanism of long-span bridge flutter. The multi-modal coupled flutter differential equations are derived in a practical manner utilizing the principle of virtual work in this method, and the explicit expressions of system damping and stiffness are established through the excitation-feedback mechanism. Consequently, this method can not only conveniently identify the critical point of 3D bridge flutter, but also provide a comprehensive understanding of the underlying mechanism involved in 3D bridge flutter. The well-known coupled aerodynamic damping related to flutter derivatives A1∗ and H3∗, which is principally responsible for the coupled flutter, is further subdivided in this study to reveal the intrinsic coupled mechanism of flutter participating modes: The aerodynamic coupled degree between a certain vertical bending mode and the fundamental torsional mode can be ascertained by evaluating the similarity factor between their mode shapes; the phase angle between a certain vertical bending mode and the fundamental torsional mode, which is largely dominated by the numerical relationship between natural and flutter frequency, determines whether the aerodynamic coupled effect associated with the two modes drives or restrains flutter.

High efficacy and safety of pulsed electrical field ablation via contact force-controlled radiofrequency catheters for atrial fibrillation ablation with linear lesions in the left atrium

Abstract Background Pulsed Field Ablation have been shown to be efficient and safe for Pulmonary vein isolation (PVI) in catheter ablation of atrial fibrillation (AF). The bipolar application of pulsed electrical energy for electroporation of myocardium has been established with specially designed catheters, which are not used for linear ablation. Purpose We evaluated a novel monophasic PEF ablation technique applied by commercially available CF-controlled catheters for linear ablation lines beyond PVI for treatment of complex AF patients. Methods Patients (n=31, age 64±12 years, 29% females, BMI 29±4) referred for re-ablation of AF and presented with recurrence of persistent AF (n=13) or atypical atrial flutter (n=14) after a PVI procedure, or patients with longstanding persistent AF and severe enlarged atrium (n=4) were included in our patient cohort for additional AF substrate ablation with PEF applications. After high density voltage mapping of the left atrium with multipolar microelectrode catheters, and PVI, or re-PVI was completed (8 patients), additional left atrial linear lesions were performed according to the atrial flutter mechanism or as fibrosis-guided substrate ablation. All but one patient showed low voltage areas (bipolar electrograms <0.5mV) in 35±31% of the left atrial surface. Using contact force sensing catheters, maximal PEF energy pulses were applied with a minimum of 5g contact force and an interlesion distance ≤6mm. Performed linear lesions were left atrial roof line, lateral mitral isthmus line, anterior mitral line, left posterior wall isolation, and cavotricuspid isthmus (CTI) line. Before PEF application close-by a coronary artery (lateral mitral line, CTI) nitroglycerie 0.2mg i.v. was injected. Results Complete lines with exclusively PEF applications and proofed bidirectional conduction block were 100% for roof lines (n=20), posterior wall isolation (n=8), anterior mitral lines (n=10). Exclusively endocardial PEF applications blocked the lateral mitral line in 14 out of 16 cases (88%). Only in two cases additional epicardial RF applications via coronary sinus and endocardial additional RF applications were necessary to complete the mitral isthmus block. In 7 out of 8 cases CTI were blocked by linear PEF applications (88%), however, in one case additional RF applications became necessary. No ECG ST segment elevation was recorded. No serious complications occurred throughout all procedures. Follow up will be available at presentation time. Conclusion Catheter ablation of complex AF patients with linear lesions is performd very efficient and safe with PEF applications by CF-sensing RF ablation catheters. Especially the high efficacy of exclusively endocardial PEF applications for creation of a lateral mitral isthmus block is very promising. However, the long term permanence of the linear lines have to be awaited.

A Conceptual Model for the Flutter Mechanism in Interlocked Turbine Blades

Abstract Many turbine rotor blades adopt the interlock configuration; among its advantages, there is a large increase in the natural frequencies with respect to the simpler cantilever configuration. Most widespread assessment methods estimate that this frequency increase should lead to a large improvement to the flutter stability of the interlocked rotor rows; some well-known approaches, such as the Panovsky–Kielb stability maps, suggest that flutter should be very unlikely for interlock configurations. Nevertheless, both the experience and detailed simulations indicate that it is possible to find flutter in interlocked blades, even with moderate aspect ratio. In this paper, the authors study the underlying physics behind flutter for interlocked blades, and present a very simplified model that is able to reproduce the main modal characteristics of interlocked blades, including the frequency increase and change in mode-shape as a function of the nodal diameter. This model also identifies a flutter mechanism, which is dependent on the aerodynamic interaction between the bending and torsion motion of the blade; even if both basic motions are individually stable, their interaction may lead to a coupled aeroelastic instability. The results from this simplified model are fully consistent with detailed three-dimensional linearized unsteady computational fluid dynamics simulations of representative turbine rotor blades, and should be of application in many realistic cases of interlock flutter. Finally, the authors present a study on the influence of the main parameters of the simplified model on the flutter stability, and derive some conclusions about the implications on flutter-free interlock design.

Aeroelastic Analysis of Highly Flexible Wings with Linearized Frequency-Domain Aerodynamics

Aeroelastic flutter analysis of configurations with geometric structural nonlinearities typically is done with time-domain analysis. The results from this process are computationally expensive and can yield cumbersome results that may be difficult to manage and interpret. Compared to time-domain methods, frequency-domain flutter analysis can provide additional insight into the characteristics of a flutter instability. By linearizing the aeroelastic problem about the nonlinear equilibrium state, this work applies frequency-domain aeroelastic analysis to the Pazy wing, the subject of the Large Deformation Working Group in the Aeroelastic Prediction Workshop. Generalized aerodynamic forces (GAFs) are computed with both a doublet-lattice method and a computational fluid dynamics solver at a range of reduced frequencies as well as a range of dynamic pressures to account for the dependence of the mode shapes on the nonlinear equilibrium state. These GAFs are used in a flutter solver, which is modified to handle the nonlinear dependence of the stiffness matrix and GAFs on the dynamic pressure. The hump mode flutter mechanism predicted by the linear doublet-lattice method is found to underpredict the severity of the instability, relative to the computational fluid dynamics-based tool, though the overall static and dynamic aeroelastic mechanisms predicted by the two fidelities are similar.

Laminar flow over a rectangular cylinder experiencing torsional flutter: Dynamic response, forces and coherence modes

This study investigates the flutter response of a rectangular cylinder model with an aspect ratio of 5 at the Reynolds number Re = 100 via direct numerical simulation. The effects of two key parameters, i.e., the moment of inertia and reduced flow velocity, on the aerodynamic performance and dynamic responses of the cylinder in the state of torsional flutter are investigated. To reveal the flutter mechanism, the high-order dynamic mode decomposition (HODMD) analysis is conducted to decompose the flow field. The results show that both an increase in the moment of inertia and a higher reduced flow velocity lead to a larger torsional amplitude and a corresponding decrease in torque. At the same time, the primary frequency decreases and the size of the shedding vortex gradually enlarges. The vortices shed from the leading edge and the trailing edge of the model form a 2P wake pattern. The leading-edge vortex is significantly larger than the trailing-edge vortex in terms of strength and size. The leading edge plays a dominant role and only contributes to the odd-order HODMD modes while the even-order modes are deemed inconsequential. As the moment of inertia increases, the total energy of the higher-order modes increases, which has the same results as the power spectral density of torque, reflecting increased nonlinearity and complexity of the system. Similarly, increasing the reduced flow velocity at the same moment of inertia has similar excitation effects.

Mechanism of characteristic change of panel flutter caused by oblique shock impingement

Compared to the shock-free condition, the weak shock impingement stabilizes the flexible panel, while the strong shock impingement leads to the early onset of panel flutter with a significant increase in flutter amplitude and frequency. However, the reason for this change by shock impingement remains unclear. The current research examines the mechanism of this change by an in-house code where the von Kármán’s large deflection plate theory is coupled with two-dimensional Euler equations. Compared to the shock-free condition, the oblique shock impingement leads to the change of local dynamic pressure on the panel as well as the static pressure differential across the panel. The analysis on the influence of these changes indicates that, on the one hand, the average dynamic pressure on the panel becomes larger than the shock-free condition, accelerating the onset of panel flutter. On the other hand, the change of the static pressure differential across the panel alters the coupling characteristic between different natural frequencies (modes) of the panel structure. The dynamic response of panel flutter under shock impingements is dominated by the coupling between the second and third modes instead of the first two modes for panel flutter under the shock-free condition. The combined effect of these two changes leads to the change of flutter characteristics of the panel under shock impingement. These findings provide valuable insights into the mechanism of shock-induced panel flutter.

Fan Flutter Mechanisms Related to Blade Mode Shape and Acoustic Properties

Abstract The blade mode shape and acoustic properties are important factors that affect the aeroelasticity of fan blades. The goal of this paper is to investigate the stall flutter mechanism associated with the first mode and its components. The paper numerically studies the impact of blade modes on fan flutter under different acoustic propagation conditions. The research focus includes unsteady characteristics inside blade passages, variations in pressure waves under different acoustic modes, as well as the effects of blade modes on the least stable phase angle. The results show that the twist-induced pressure leads to destabilization. Compared with the work of other authors, this study discovered that when the twist-induced pressure on the pressure side act on the plunge rather than twist, the instability effect will be greater, while the effect of twist-induced pressure on the suction side is weak. The phase of unsteady pressure in two-dimensional flow regions is linear with frequency, while the amplitude is highly sensitive to acoustic properties. The plunge-induced pressure inside the passage undergoes significant changes when downstream from acoustic cut-off to cut-on. The twist-induced pressure is more sensitive to changes in the acoustic propagation state, with the peak of the aerodynamic damping curve near upstream acoustic resonance being solely related to the twist-induced pressure acting on the suction side. The study also finds that the position of the blade torsion axis, represented by the twist-to-plunge ratio, does not affect the most unstable nodal diameter.

Free wake panel method simulations of a highly flexible wing in flutter and gusts

This paper presents low speed fluid structure interaction simulations of a highly flexible wing at various flow conditions, including flutter and excitation from sinusoidal gusts. Such wings are becoming more relevant in recent years, due to their potential for improving aerodynamics and reducing weight, while their flutter characteristics are particularly challenging to address, as the modal properties of the wings change as deflections increase. Calculations are based on time domain coupling of a geometrically exact beam structural model and a 3D free wake panel method, modeling the outer surface of the wing, which allow for nonlinear effects in terms of geometrical deformations and the flow at low computational cost. Static and aeroelastic wing deflections are in line with experimental data of the Pazy wing, which is a benchmark for highly flexible wings from Technion. Two flutter mechanisms are predicted within 1 to 3 m/s of the experimental range. An analysis of the flutter modes is performed, showing that the second torsion mode plays a role in flutter, something that had not been published before. Limit cycle oscillations are achieved and are shown to compare well with reference data, with the frequency being within 1% of the experimental value. Finally, results of gust simulations of the Pazy wing are compared to data from experiments and corrections for the wind tunnel measurements are proposed, which should facilitate future validation efforts. This work serves as a contribution to the Pazy wing dataset and is a step towards mid-fidelity simulations for more complex configurations.