Flutter Phenomenon Research Articles

A Study on Post-Flutter Characteristics of a Large-Span Double-Deck Steel Truss Main Girder Suspension Bridge

To investigate the nonlinear flutter characteristics of long-span suspension bridges under different deck ancillary structures and configurations, including those with and without a central wind-permeable zone, as well as to analyze the hysteresis phenomenon of a subcritical flutter and elucidate the mechanisms leading to the occurrence of nonlinear flutter, this paper studies first the post-flutter characteristics of the torsion single-degree-of-freedom (SDOF) test systems and vertical bending–torsion two-degree-of-freedom (2DOF) test systems under different aerodynamic shape conditions are further analyzed, and the role of the vertical vibration in coupled nonlinear flutter is discussed. The results indicate that better flutter performance is achieved in the absence of bridge deck auxiliary structures with a central wind-permeable zone. The participation of vertical vibrations in the post-flutter vibration increases with the increase in wind speed, reducing the flutter performance of the main girder. Furthermore, the hysteresis phenomenon in the subcritical flutter state is observed in the wind tunnel experiment, and its evolution law and mechanism are discussed from the perspective of amplitude-dependent damping. Finally, the vibration-generating mechanism of the limit oscillation ring is elaborated in terms of the evolution law of the post-flutter vibration damping.

Aero-thermodynamic responses of a novel FGM sector disks using mathematical modeling and deep neural networks

Composite sector disks have extensive applications in aerospace industries, particularly when exposed to challenging conditions such as supersonic airflow and thermal environments. These applications leverage the superior properties of composite materials, including high strength-to-weight ratios, enhanced durability, and excellent thermal resistance, to meet the stringent requirements of aerospace operations. Multi-directional functionally graded (MD-FG) materials due to high-temperature resistance and other amazing properties in each direction have gotten plenty of attention recently. So, in this research, a thermoelasticity solution has been presented to study fundamental frequency traits of an MD-FG sector disk in supersonic airflow via both mathematics simulation and deep neural networks technique. For obtaining exact displacement fields, along with defining the changes of transverse shear strains along the system's thickness, the refined zigzag hypothesis is utilized. For obtaining the temperature-dependent equations, heat conduction relation and thermal boundary conditions of the MD-FG structure are presented. A coupled quasi-3D new refined theory (Q3D-NRT) and generalized differential quadrature method (GDQM) are presented for obtaining and solving the partial differential equations in the time-displacement domain. After obtaining the mathematics results, appropriate datasets are made for testing, training, and validation of the deep neural networks technique. Finally, the results have shown that aerodynamic pressure, temperature changes, Mach number, free stream speed, and air yaw angle have a major role in the stability/instability analyses of the thermally affected MD-FG sector disk in supersonic airflow. As an amazing outcome, increasing the sector angle, FG indexes, and temperature change lead to the reduction of the critical Mach number, and aerodynamic pressure associated with the flutter phenomenon.

Optimization of Composite Wing Structure with Static, Buckling, and Flutter Constraints using the Finite Element Method

In airplane structural design, creating a strong yet lightweight structure is crucial. During the preliminary design phase, structural sizing optimization is necessary to achieve an efficient and lightweight structure that meets safety standards. Composite materials offer a high strength-to-weight ratio, enabling a lighter structure without compromising strength. This work focuses on optimizing composite structure wings by considering laminae thicknesses as design variables and minimizing structure weight as the objective, using the gradient-based method. The optimization constraints include the Tsai-Hill criterion, buckling factor, and flutter speed to ensure the structure's safety against static load, buckling phenomena, and flutter phenomena. The optimization results indicate that the buckling factor is the most critical constraint, carrying the highest weight. The weight of the wing structure decreased by 53% and 79% after optimizing with static and flutter constraints, respectively. Despite minimal changes in weight after optimizing with the buckling constraint, the structure now meets the buckling safety criteria and is safe from buckling.

The Impact of the Nonlinear Integral Positive Position Feedback (NIPPF) Controller on the Forced and Self-Excited Nonlinear Beam Flutter Phenomenon

This article presents a novel approach to impact regulation of nonlinear vibrational responses in a beam flutter system subjected to harmonic excitation. This study introduces the use of a Nonlinear Integral Positive Position Feedback (NIPPF) controller for this purpose. This technique models the system as a three-degree-of-freedom nonlinear system representing the beam flutter, coupled with a first-order and a second-order filter representing the NIPPF controller. By applying perturbation analysis to the linearized system model, the authors obtain analytical solutions for the autonomous system with the controller. This study aims to reduce vibration amplitudes in a nonlinear dynamic system, specifically when 1:1 internal resonance occurs. The Routh–Hurwitz criterion is utilized to evaluate the system’s stability. Furthermore, the frequency–response curves (FRCs) exhibit symmetry across a range of parameter values. The findings highlight that the effectiveness of vibration suppression is directly related to the product of the NIPPF control signal after comparing with different controllers. Numerical simulations, conducted using the fourth-order Runge–Kutta method, validate the analytical solutions and demonstrate the system’s amplitude response. The strong correlation between the analytical and numerical results highlights the accuracy and dependability of the proposed method.

Computational fluid–structure analysis of the impact of leaflet thickness and protrusion height on the flutter phenomenon in aortic valve bioprostheses

Computational fluid–structure analysis of the impact of leaflet thickness and protrusion height on the flutter phenomenon in aortic valve bioprostheses

An Effective Arbitrary Lagrangian-Eulerian-Lattice Boltzmann Flux Solver Integrated with the Mode Superposition Method for Flutter Prediction

An Effective Arbitrary Lagrangian-Eulerian-Lattice Boltzmann Flux Solver Integrated with the Mode Superposition Method for Flutter Prediction

High-frequency visualization of flexible structures using an event-triggered camera: multiple flapping membranes

Optical measurements of fluid–structure interaction (FSI) usually require high-speed imaging techniques. Traditional high-speed cameras are expensive and cannot record data continuously for long periods. An inexpensive and high-frequency measurement method that utilizes an event-triggered camera and a strobe laser is proposed in this paper. In this method, the k-means clustering technique was first employed to determine the precise time window in which to reconstruct frames from event data. Then, a Kalman filtering algorithm was used to extract the dynamic deformation process of the flexible structures from the noisy event data. Wind tunnel flutter tests were conducted using various sets of parallel membranes to examine the proposed method. Under the illumination of a 30 mW strobe laser, membrane flapping was recorded at a resolution of 1280 × 720 pixels with a frame rate of 10 000 fps. With no camera memory storage limitations, the proposed method could continuously record the membrane flapping, thereby making it easy to capture the intermittent flutter phenomenon at the flutter boundary. Two flapping modes, i.e. symmetric mode and in-phase mode, were observed in the flutter of two membranes. Similar symmetric mode and out-of-phase mode were also observed in the flutter of three membranes. Membranes collisions and mode transitions were found in a certain wind speed range. For the first time, the collision processes between membranes were recorded during the flutter of multiple membranes. These collisions were found to be associated with the transitions between different flapping modes.

The Vocal Flutter of Multiple System Atrophy: A Parkinsonian-Type Phenomenon?

Early features of multiple system atrophy (MSA) are similar to those in Parkinson's disease (PD), which can challenge differential diagnosis. Identifying clinical markers that help distinguish MSA from forms of parkinsonism is essential to promptly implement the most appropriate management plan. In the context of a thorough neurological evaluation, the presence of a vocal flutter might be considered a potential feature of MSA-parkinsonian type (MSA-P). This case series describes clinical histories of 3 individuals with MSA-P. In each case, vocal flutter was detected during neurological and motor speech evaluations. It seemed to be a concomitant feature with the constellation of other signs and symptoms that led to the clinical diagnosis. The vocal flutter may be described as pitch and loudness fluctuations during phonation. Different from a vocal tremor, the flutter phenomenon has higher oscillation frequencies. The neuropathological underpinnings of vocal flutter may be related to generalized laryngeal dysfunction that is commonly described in MSA-P. Vocal flutter may be a unique speech feature in some individuals who have MSA-P. Future studies using perceptual and acoustic measures of speech are warranted to quantify these observations and directly compare to other MSA variants, PD, and a control group.

Overview of Computational Methods to Predict Flutter in Aircraft

Abstract Aeroelastic flutter is a dynamically complex phenomenon that has adverse and unstable effects on elastic structures. It is crucial to better predict the phenomenon of flutter within the scope of aircraft structures to improve the design of their wings. This review aims to establish fundamental guidelines for flutter analysis across subsonic, transonic, supersonic, and hypersonic flow regimes, providing a thorough overview of established analytical, numerical, and reduced-order models as applicable to each flow regime. The review will shed light on the limitations and missing components within the previous literature on these flow regimes by highlighting the challenges involved in simulating flutter. In addition, popular methods that employ the aforementioned analyses for optimizing wing structures under the effects of flutter—a subject currently garnering significant research attention—are also discussed. Our discussion offers new perspectives that encourage collaborative effort in the area of computational methods for flutter prediction and optimization.

The Effect of Cubic Damping on Geometric Nonlinear Flutter in Long‐Span Suspension Bridges

This study investigates the wind‐induced flutter phenomenon of long‐span suspension bridges. A mathematical model is established using the energy method, considering the nonlinear aeroelastic behavior of a bridge deck constrained by vertical suspenders under cubic nonlinear damping. The aerodynamic self‐excited force is obtained using the Scanlan flutter theory. The critical state of flutter and the relationship between the amplitude and parameters of the limit cycle oscillation (LCO) are obtained by solving the nonlinear flutter equation using the average perturbation method. The Hopf bifurcation is confirmed to occur in the critical state of the nonlinear motion model using the Jacobian matrix. The results are verified using the Runge–Kutta numerical method, and it is found that the nonlinear response of the flutter LCO is in general agreement with the numerical solution. The study also shows that the degenerate Hopf bifurcation appears at the critical state under the condition of lower cubic damping coefficient, and the nonlinear flutter bifurcation changes from a degenerate Hopf bifurcation to a supercritical Hopf bifurcation as the cubic damping coefficient grows.

Flutter Phenomenon and Safety Implications in Transonic Flow

As advancements in aircraft manufacturing technology persistently elevate the boundaries of flight speed, challenges emerge when aircraft velocities verge on sonic speeds. Notably, empirical observations have highlighted instances of pronounced vibrations and, in extreme cases, structural failures, casting shadows over flight stability and safety. This research endeavors to dissect the ramifications of shock waves and flutter phenomena, precipitated by fluid discontinuities during supersonic flight, on the overarching safety of aircraft. Methodologically, the study delineates a series of simulation protocols, commencing with the derivation of simplified functions representing the aircraft's 2D contour, followed by error function computations, and finally, using the finite difference method in conjunction with the successive over-relaxation (SOR) iterative technique to model the aircraft's velocity distribution. Preliminary findings indicate that for β values spanning 0.1, 0.15, and 0.175, the surrounding airflow retains its continuity and stability, with the Mach number's rate of change exhibiting a decelerating trend. Conversely, at a β threshold of 0.2, the airflow descends into turbulence, manifesting in erratic Mach number fluctuations. Such fluidic discontinuities underscore potential threats to the aircraft's flight safety, necessitating further exploration and mitigation strategies.

Composite Fins Subsonic Flutter Prediction Based on Machine Learning

In the present work, the potential application of machine learning techniques in the flutter prediction of composite materials missile fins is investigated. The flutter velocity data set required for different fin aerodynamic geometries and materials is generated using a hybrid data collection method: from the wind tunnel experiments at flows ranging from 5 to 30 m/s at Re = 300,000 to 500,000, whereas synthetic data is collected using modified NACA flutter boundary model. Once the flutter data are collected, different regression algorithms were investigated, and the results were compared in terms of accuracy (when compared to the experimentally obtained results and the numerical flutter models), training time (minimization), and R-squared values (maximization). The algorithms investigated and their performance analyzed are fast forest regression, SDCA regression (stochastic dual coordinate ascent), and the light GBM (light gradient boosting machine) regression algorithm that belongs to the gradient boosting regression algorithm family. It was found that the light GBM algorithm renders the most accurate results. Based on this research, it can be concluded that artificial intelligence (machine learning) techniques can be successfully deployed in the analysis of complex flutter phenomena.

Optimal design of stochastic aeroviscoelastic systems in subsonic regime using the doublet lattice

In a concerted effort to mitigate the global environmental impact, various governments and international organizations have put forth strategies aimed at enhancing the efficiency of products and solutions. The aeronautical and aerospace industries have set forth an ambitious target of eliminating their CO2 emissions by the year 2050. To realize this objective, manufacturers are adopting innovative approaches, including the incorporation of new blade engines, utilization of Sustainable Aviation Fuel (SAF), integration of lighter materials, and the adoption of novel geometric wing shapes designed for enhanced efficiency. However, the implementation of structural and aerodynamic modifications, such as those required for aeroelastic systems, can lead to instability effects, notably the flutter phenomenon. Addressing this challenge, one viable strategy involves the application of vibration control techniques. In this context, the use of passive control with viscoelastic materials emerges as an intriguing option due to its cost-effectiveness and ease of application. Additionally, considering the inherent variabilities in structural and aerodynamic parameters within these systems, there is a need for an efficient stochastic modeling methodology to cater to realistic applications of industrial significance. The present study endeavors to model a stochastic aeroviscoelastic system, specifically focusing on a plate-like wing in a subsonic regime. This is achieved through the integration of stochastic finite element modeling and the Doublet Lattice Method (DLM). The study aims to quantify the impact of increased mass and stiffness, particularly related to the addition of layers and the inclusion of damping from the viscoelastic material. The findings of the research demonstrate a noteworthy 31% increase in flutter speed with a corresponding 38% increase in mass. Although the ratio of speed gain to mass gain is below one in this instance, with a partial treatment approach, it can potentially reach 2. This underscores the advantageous nature of treating aeronautical panels with layers of viscoelastic material. Importantly, the engineer possesses the capability to strategically select deterministic parameters to align with specific objectives between the two defined functions.

Evaluation of numerical techniques for modeling flutter phenomenon into two geometries: the 1:4.9 rectangle and the Great Belt East Bridge in scale 1:7

Evaluation of numerical techniques for modeling flutter phenomenon into two geometries: the 1:4.9 rectangle and the Great Belt East Bridge in scale 1:7

Modeling the flutter phenomenon by CFD of rectangular profiles

Modeling the flutter phenomenon by CFD of rectangular profiles

Flutter analysis of multi-directional functionally graded sector poroelastic disks

For the first time, in this article, stable/unstable zones of multi-directional functionally graded (MD-FG) sector disks under aerodynamic pressures are sought. Also, because of porous media, the system is modeled as a poroelastic system. The damping matrix, together with the aerodynamic stiffness matrix, is modeled upon Krumhaar's modified supersonic piston hypothesis. For more accuracy of the results, the Series-Fourier expansion of displacement fields in two-dimension is considered. To solve the partial differential equations, the singular convolution integration method with a singular convolution, as a numerical solver, is used. The precision of the outcomes is verified with those available in the other published research. It is demonstrated that the 3D model has great accuracy in predicting the flutter phenomenon and supersonic vibrations of the MD-FG sector disks. Finally, it has been shown that MD-FG power indices, radius ratio, boundary condition, poroelastic media, geometry, Mach number, aerodynamic pressure, and air yaw angle, in addition to Biot's coefficient, have a crucial role in the stable/unstable zones of the MD-FG sector disks under various conditions. One of the most important results of the current report is that the alteration in material properties of the MD-FG sector disk does not alter the yaw angle in which maximum frequency occurs.

An Explanation for the Flutter Paradox in the Supercritical Region of a Simply-Supported Fluid-Conveying Pipe

Abstract Employing traditional Galerkin method, a coupled-mode flutter is predicted in the supercritical region of simply-supported pipes which constitutes a paradox since the internal flow effect is conservative and there is no energy to sustain the oscillation. Although there is a consensus that the flutter does not exist, the intrinsic mechanism remains to be clarified. This study has found that the internal flow induced Coriolis force term cannot be decoupled in traditional Galerkin method which leads to the dissatisfaction of the convergence conditions required in weighted residual approach (WRA). Moreover, the disparities in the predicted complex frequencies have been witnessed at different base function numbers when the internal flow velocity is sufficiently large. A modified Galerkin method adopting a new set of weighting functions is proposed based on WRA, and the Coriolis force term disappears by use of the orthogonality relations (it is stated that the Coriolis force is not directly omitted). Thus, a convergent solution for the set of residual functions which are identically equal to zeros can be guaranteed. Employing the modified method, the convergence in simulations is confirmed and the flutter phenomenon does not occur. This study can be a workbench for the study on the unsolved or partly solved issues in simulations of fluid-conveying pipes. Moreover, it has demonstrated that the predictions in traditional Galerkin method overestimate the natural frequencies, and it becomes more profound in higher-order natural modes at larger internal flow velocities which are of practice significance for dynamic analysis of flexible pipeline systems.

Parametric aerodynamic and aeroelastic study of a deformable flag-based energy harvester for powering low energy devices

Extensive wind tunnel experiments were performed to find the optimal and most influential parameters for energy harvesting from piezoelectric membranes (thin elastic cantilevered plates), herein called flags. Four different piezoelectric flags were tested in conjunction with the seven different bluff bodies at various wind speeds and streamwise gaps between the bluff body and flag. Stiffness of the flags was also varied along with its length to assess the impact on harvested power. Various flapping modes are observed for different flags which correspond to the amount of harnessed power. The results revealed that altering the stiffness of flag can produce an increase of 29 and 31 times in power generation for long and short flags, respectively. It was found that the 120-degree bluff body produced the largest power with the capability to generate a strong wake region. Further analysis of the best performing flag (short and stiff) for various bluff bodies shows a remarkable increase of 661.2 % in generated power. This membrane shows rhythmic periodic flapping with higher modes of deformation in comparison to its other flag counterparts. These alternate flags show out-of-plane and twisting motion which wastes strain energy and causes a cancelation effect, reducing total energy output. The results show that in designing an efficient and reliable energy harvester, the flag’s stiffness and length as well as the shape of the bluff body play an important role. Furthermore, in comparison with the flutter phenomenon (i.e. a dynamic instability of the flag in the absence of a bluff body), the results with a bluff body demonstrate that higher energy generation can be achieved at a lower wind speed, thus paving the way for small and practical energy harvesting devices specifically in remote areas.

Flutter and divergence instability regions of a column subjected to the load realized by means of a jet engine with an additional guiding structure

In this paper, a new slender system is considered — a column that is subjected to a compressive follower load. The studied structure uses a system of rigid transoms guiding the loaded end of the column with the limitation of their mutual rotation by means of rotational springs. This additional element does not transfer the compressive load but affects guidance of the loaded end of the column. In the presented form, the solution used to guide the end of the column was not previously taken into account. The boundary problem was formulated on the basis of Hamilton’s principle, taking into account (due to the non-linearity) the small parameter method. The theoretical, numerical, and experimental studies were conducted by determining an influence of the system parameters (in particular the element guiding the loaded end) on the instability type. On the basis of the kinetic stability criterion, critical divergence and flutter loads as well as limit values of parameters in the system were determined, at which instability occurs. The research was limited to the determination of the first linear component of vibration frequency. As a part of numerical research, detailed considerations were made on characteristic curves. It has been shown that with an appropriate selection of the parameters of the guiding system, a new shape of the characteristic curves can be obtained. This new feature shows that in the boundary case (instability change from flutter to divergence) the critical divergence load is greater than the critical flutter. In order to confirm the correctness of the mathematical model and thus the results of numerical tests concerning the occurrence of flutter and divergence areas the experimental tests were carried out consisting of recording the behavior of the system with a quick camera when it was loaded with a model jet engine. The correctness of the shape of characteristic curves was confirmed by the experimental frequency analysis of considered columns at different jet engine thrust magnitudes. The conducted research may be of great importance in the control of the flutter phenomenon of slender systems exposed to follower forces, including those resulting from the action of the wind.

Efficient parametric study of a stochastic airfoil system based on hybrid surrogate modeling with advanced automatic kriging construction

Flutter is one of the most important aeroelastic instability phenomena that arises from the interaction between the structural dynamics of the mechanical airfoil system and the surrounding airflow. This instability phenomenon can lead not only to a reduction in aircraft performance but also to catastrophic structural failure.Therefore, one of the major challenges is to perform parametric and sensitivity studies on the stability behavior of a wing system subject to many random uncertainties in order to achieve a thorough understanding and reliable estimation of the role played by each parameter in the flutter phenomenon. To carry out such a study, an advanced surrogate modeling technique based on kriging and polynomial chaos expansion (PCE) is proposed for the prediction of flutter instability. In addition, a methodology based on hybrid surrogate modeling with advanced automatic kriging construction is discussed to promote an efficient parametric study of the airfoil system with uncertainties subjected to flutter. The Sobol indices highlight that the role played by each random parameter depends strongly on the flow speed and airfoil geometry with complex behaviors, giving valuable insights into the physics and the complexity of flutter.