Flutter Mechanism Research Articles

Geometrically nonlinear effects in wing aeroelastic dynamics at large deflections

This paper investigates geometrically nonlinear effects due to nonlinear kinematics and follower aerodynamics in the aeroelastic dynamics of a very flexible wing. The test case is the Pazy wing, a benchmark wing model for geometrically nonlinear aeroelastic studies involving large deflections in low-speed flow. The work builds on a low-order model of the wing composed of a geometrically nonlinear beam coupled with potential flow thin airfoil theory. Nonfollower aerodynamics underestimates static deflections by up to 10%, whereas linear kinematics overestimates them by up to 50%. At low static deflections, the wing hump mode flutter mechanism is driven by three-dimensional unsteady aerodynamic effects. As static deflections increase, the flutter onset and offset speeds are driven by curvature effects that can only be captured by nonlinear kinematics, which reduce the natural frequencies associated with torsion and in-plane bending motions. Follower aerodynamics shrinks the hump mode instability region and shifts it to 1–4% lower flow speeds with no change in the flutter frequency, the qualitative trends, and the amplitude of limit-cycle oscillations. Overall, nonlinear kinematics is the primary geometrical nonlinearity influencing the wing aeroelastic behavior: the modal and stability scenarios with and without follower aerodynamics collapse onto the same curves when presented as functions of the tip deflection. However, geometrical nonlinearities alone miss the subcritical nature of the hump mode flutter mechanism, which is attributed to aerodynamic stall and its interaction with large deflections. This work expands the growing literature on the aeroelastic dynamics of very flexible wings and will ease future studies considering aerodynamic nonlinear effects.

New Viewpoint on the Mechanism of Laminar Separation Flutter

At transitional Reynolds numbers, an elastically supported airfoil oscillating in pitch can undergo laminar separation flutter (LSF), which is characterized by self-sustained small-amplitude oscillations. To gain insight into the mechanism of LSF, we conduct wind tunnel tests for to investigate the LSF response of a freely rotating NACA0015 airfoil with various structural natural frequencies and positions of the elastic axis. The experimental results show that the dominant mode of flow around the NACA0015 airfoil abruptly changes at , resulting in a change in the trend of LSF response. Then, an aeroelastic model is constructed to explain how the instability of LSF arises. This model can accurately predict the LSF frequency of airfoils with various structural natural frequencies. Moreover, based on the aeroelastic model, we perform linear stability analysis on the aeroelastic system and propose the instability criterion for LSF as . This instability criterion is identical to that for rotational galloping of square cylinder, indicating that LSF and rotational galloping of square cylinder are essentially the same aeroelastic phenomenon that appears in different aerodynamic profiles. This finding identifies the underlying cause for the remarkable similarities between the two phenomena.

An analytical frequency response function for fluidelastic instability model of a normal triangular array in cross-flow

A new model based on Theodorsen’s classical theory of aerodynamic instability and the mechanism of flutter is proposed to explain the origin of the time delay, an important parameter for fluidelastic instability prediction, between the tube motion and the non-linear fluidelastic forces. Using this model, a novel time delay function has been theoretically derived with no reliance on unsteady fluid force data, as was the case with earlier models. A stability analysis is carried out based on the new time delay function combined with the experimental static fluid forces for a range of mass damping parameters. The predicted stability threshold is found to compare well with experimental stability data.

The design and analysis of flapping wing mechanism based on eccentric crank slider

In order to improve the sharp return characteristics of the flutter mechanism, and then increase the downward power of the flutter aircraft, reduce the drag of the wing upward, and increase the flight efficiency, this paper designs a flutter drive mechanism with sharp return characteristics based on the eccentric crank slider flutter structure. Firstly, the kinematic model of the eccentric crank slider mechanism is established through motion analysis, and then the simulation model of the flutter mechanism is established in SOLIDWORKS to simulate its motion and verify its theoretical analysis. The results show that the designed flutter mechanism has an upper limit angle of 17°, a lower limit angle of -34°, and a travel speed ratio of 1.35, which are consistent with the flight parameters of the selected bionic bird, so it has good aerodynamic performance. And the kinematic parameters obtained from the simulation are consistent with the theoretical calculation, which verifies the correctness of the theoretical calculation.

Influence mechanism and quantificational evaluation of key factors affecting flutter stability of a transonic fan

The blade flutter stability directly affects the stable operating range of the aero-engine. A few factors related to flutter stability, such as the shock wave, separation flow, inter-blade phase angle (IBPA), and so on, have been found in existing flutter mechanism studies. However, the mechanisms of several factors affecting flutter stability remain to be determined. Taking a transonic fan, NASA rotor 67, as the research object, this paper studies the main factors affecting the blade flutter stability and their influence mechanisms through the unidirectional Fluid-Structure-Interaction (FSI) simulation. The amplitude and phase of unsteady pressure caused by various factors determine the aerodynamic damping distributions of the blade under different operating conditions, which are obtained by the energy method. A quantificational method based on increment proportion is proposed to evaluate the effects of various factors on blade flutter stability by the ratio of the work increment done by each influencing factor to the total work increment, and the flutter stability analysis method is further established. Using this proposed method, it is found that the dominant factors affecting flutter stability before and after design point are inconsistent, which are the shock wave on the suction surface and tip leakage flow, respectively. Furthermore, the mapping relationship of nodal diameter (ND) and flutter stability is also investigated, and the influence mechanism is revealed in this research. The results demonstrate that the nodal diameter affects the amplitude and phase of unsteady pressure by affecting the amplitude and phase of shock wave oscillation, thus affecting the blade flutter stability.

Wind energy harvesting inspired by Palm leaf flutter: Observation, mechanism and experiment

Environmental adaptation of wind energy harvester (WEH) would be improved by mimicking the leaf flutter in Mother Nature. This study presents aeroelastic modelling and laboratory testing of Palm leaves to understand the mechanism of leaf flutter, furtherly designs an artificial leaf WEH and tests it in wind tunnel. Three modes of investigation are fulfilled: (1) Observation: Palm leaf flutter is observed in urban wind. The material properties of Palm leaves are measured by microscopic images or tensile tests. (2) Modelling: The aeroelastic model is established using finite element method and doublet-lattice aerodynamics to determine the flutter boundary of Palm leaves. The mechanism of mode coalescence between bending and twisting is found, and the model is further applied in designing the artificial leaf. (3) Experiments: Palm leaf oscillations under several flow speeds are measured with ultra-high-speed laser displacement sensor in wind tunnel. The critical speed and frequency of Palm leaf flutter are then obtained. Furthermore, the artificial leaf is mounted on the piezoelectric cantilever and evaluated in wind tunnel. At preliminary trial, the leaf harvester starts working at as low wind speed as 2 m/s, with stable frequency of 3.56 Hz. It reaches output power density of 1.238 µW/cm3 in our laboratory. The study highlights the bio-inspired design of wind energy harvester with low cut-in wind speed and stable output frequency.

Supervised Machine Learning Based Noninvasive Prediction of Atrial Flutter Mechanism from P-to-P Interval Variability under Imbalanced Dataset Conditions.

Atrial flutter (AFL) is a common arrhythmia with two significant mechanisms, namely, focal (FAFL) and macroreentry (MAFL). Discrimination of the AFL mechanism through noninvasive techniques can improve radiofrequency ablation efficacy. This study aims to differentiate the AFL mechanism using a 12-lead surface electrocardiogram. P-P interval series variability is hypothesized to be different in FAFL and MAFL and may be useful for discrimination. 12-lead ECG signals were collected from 46 patients with known AFL mechanisms. Features for a proposed classifier are extracted through descriptive statistics of the interval series. On the other hand, the class ratio of MAFL and FAFL was 41 : 5, respectively, which was highly imbalanced. To resolve this, different data augmentation techniques (SMOTE, modified-SMOTE, and smoothed-bootstrap) have been applied on the interval series to generate synthetic interval series and minimize imbalance. Modification is introduced in the classic SMOTE technique (modified-SMOTE) to properly produce data samples from the original distribution. The characteristics of modified-SMOTE are found closer to the original dataset than the other two techniques based on the four validation criteria. The performance of the proposed model has been evaluated by three linear classifiers, namely, linear discriminant analysis (LDA), logistic regression (LOG), and support vector machine (SVM). Filter and wrapper methods have been used for selecting relevant features. The best average performance was achieved at 400% augmentation of the FAFL interval series (90.24% sensitivity, 49.50% specificity, and 76.88% accuracy) in the LOG classifier. The variation of consecutive P-wave intervals has been shown as an effective concept that differentiates FAFL from MAFL through the 12-lead surface ECG.

Vibration phase difference analysis of long-span suspension bridge during flutter

The research on vibration characteristics of long-span bridges is of great importance to flutter analysis, yet the literature is limited which concerns the phase difference characteristic of the full bridge. This study thus aims to investigate the vibration phase difference characteristics and mechanism along the main span during flutter. The time-domain flutter analysis was conducted in a long-span suspension bridge. The results manifest that the vibration in different directions at the same position and the vibration in the same direction at different positions of the main girder and the main cable all exist phase difference, which varies along the main span. The deduced phase difference equation reveals that the variable of the distance from mid-span is in cosine term of the denominator, making the increasing of the distance resulting in the growth of phase difference. Due to the variation of phase difference, the vibration of the main girder and main cable along the span is asynchronous. The motion trajectories of individual points are elliptic and vary along the main span. These findings can help understand the flutter performance and mechanism of long-span bridges.

Use of nicorandil is associated with increased risk of incident atrial fibrillation.

Background: Nicorandil will activate ATP-sensitive potassium channel (KATP). However, activation of potassium channels plays an important role in the mechanism of atrial fibrillation (AF) or atrial flutter (AFL). Whether use of nicorandil might contribute to initiation and/or perpetuation of AF/AFL remained unknown. We determined the relationship between use of nicorandil and risk of atrial fibrillation and determined its molecular mechanism.Methods: We performed a nested case-control study using a cohort from the National Health Insurance Research Database (NHIRD) of Taiwan. The association between nicorandil use and risk of atrial fibrillation/flutter was estimated by logistic regression model. We also performed molecular, cellular and animal studies to explain the association.Results: A total of 715 individuals who experienced AF/atrial flutter were matched to 72,215 controls. New use of nicorandil was found to be associated with increased risk for AF/AFL (odds ratio [OR], 2.34; 95% CI 1.07–5.13) compared to nitrate use. We found the expression of KATP subunits Kir6.2 and SUR2A in human and rat left atrial tissues. Furthermore, nicorandil directly shortened action potential duration (APD) in rat left atrium and shortened the QT interval of cultured human induced pluripotent stem cell (iPSC) derived cardiomyocytes (iPSC-CMs).Conclusions: Use of nicorandil was found to be associated with increased risk of AF/AFL. We also showed the expression of KATP subunits in human atria, and a possible mechanism that use of nicorandil increases the risk of AF through activation of KATP and shortening of atrial APD.

Energy extraction in the dynamic modes of flow for airfoil's laminar separation flutter

This paper aims to gain new insight into the physical mechanism of laminar separation flutter (LSF) from the perspective of energy transfer and dynamic mode decomposition (DMD) modes of flow. An online DMD method accounting for the airfoil's pitch motion is developed, and the relationship between the topology of energy map and DMD modes is established. Simulation results indicate that there are two limit cycle branches in energy map, but only one branch is stable. The LSF time response state can be predicted accurately by the stable limit cycle branch. The topology of an energy map is dominated by the DMD mode corresponding to the airfoil's pitch frequency. The developed DMD method can extract the variation of flow structures effectively. The pressure distribution of DMD mode corresponding to the pitch frequency is dominated by the leading-edge suction and bubble's suction. The bubble's suction is induced by the trailing-edge laminar separation bubble or laminar separation bubble (LSB). When the pitch amplitude is larger than 4°, the trailing-edge laminar separation bubble transforms to LSB. The inherent mechanism is that increasing the trailing-edge separation bubble's intensity promotes the energy extraction while the occurrence of LSB mitigates it.

A local activation time histogram-An invaluable tool to diagnose a rare and complex atrial flutter mechanism.

A local activation time histogram-An invaluable tool to diagnose a rare and complex atrial flutter mechanism.

Accurate closed-form eigensolutions of three-dimensional panel flutter with arbitrary homogeneous boundary conditions

Highly accurate closed-form eigensolutions for flutter of three-dimensional (3D) panel with arbitrary combinations of simply supported (S), glide (G), clamped (C) and free (F) boundary conditions (BCs), such as cantilever panels, are achieved according to the linear thin plate theory and the first-order piston theory as well as the complex modal analysis, and all solutions are in a simple and explicit form. The iterative Separation-of-Variable (iSOV) method proposed by the present authors is employed to obtain the highly accurate eigensolutions. The flutter mechanism is studied with the benefit of eigenvalue properties from mathematical senses. The effects of boundary conditions, chord-thickness ratios, aerodynamic damping, aspect ratios and in-plane loads on flutter properties are examined. The results are compared with those of Kantorovich method and Galerkin method, and also coincide well with analytical solutions in literature, verifying the accuracy of the present closed-form results. It is revealed that, (A) the flutter characteristics are dominated by the cross section properties of panels in the direction of stream flow; (B) two types of flutter, called coupled-mode flutter and zero-frequency flutter which includes zero-frequency single-mode flutter and buckling, are observed; (C) boundary conditions and in-plane loads can affect both flutter boundary and flutter type; (D) the flutter behavior of 3D panel is similar to that of the two-dimensional (2D) panel if the aspect ratio is up to a certain value; (E) four to six modes should be used in the Galerkin method for accurate eigensolutions, and the results converge to that of Kantorovich method which uses the same mode functions in the direction perpendicular to the stream flow. The present analysis method can be used as a reference for other stability issues characterized by complex eigenvalues, and the highly closed-form solutions are useful in parameter designs and can also be taken as benchmarks for the validation of numerical methods.

Influence of Fluid Viscosity and Compressibility on Nonlinearities in Generalized Aerodynamic Forces for T-Tail Flutter

The numerical assessment of T-tail flutter requires a nonlinear description of the structural deformations when the unsteady aerodynamic forces comprise terms from lifting surface roll motion. For linear flutter, a linear deformation description of the vertical tail plane (VTP) out-of-plane bending results in a spurious stiffening proportional to the steady lift forces, which is corrected by incorporating second-order deformation terms in the equations of motion. While the effect of these nonlinear deformation components on the stiffness of the VTP out-of-plane bending mode shape is known from the literature, their impact on the aerodynamic coupling terms involved in T-tail flutter has not been studied so far, especially regarding amplitude-dependent characteristics. This term affects numerical results targeting common flutter analysis, as well as the study of amplitude-dependent dynamic aeroelastic stability phenomena, e.g., Limit Cycle Oscillations (LCOs). As LCOs might occur below the linear flutter boundary, fundamental knowledge about the structural and aerodynamic nonlinearities occurring in the dynamical system is essential. This paper gives an insight into the aerodynamic nonlinearities for representative structural deformations usually encountered in T-tail flutter mechanisms using a CFD approach in the time domain. It further outlines the impact of geometrically nonlinear deformations on the aerodynamic nonlinearities. For this, the horizontal tail plane (HTP) is considered in isolated form to exclude aerodynamic interference effects from the studies and subjected to rigid body roll and yaw motion as an approximation to the structural mode shapes. The complexity of the aerodynamics is increased successively from subsonic inviscid flow to transonic viscous flow. At a subsonic Mach number, a distinct aerodynamic nonlinearity in stiffness and damping in the aerodynamic coupling term HTP roll on yaw is shown. Geometric nonlinearities result in an almost entire cancellation of the stiffness nonlinearity and an increase in damping nonlinearity. The viscous forces result in a stiffness offset with respect to the inviscid results, but do not alter the observed nonlinearities, as well as the impact of geometric nonlinearities. At a transonic Mach number, the aerodynamic stiffness nonlinearity is amplified further and the damping nonlinearity is reduced considerably. Here, the geometrically nonlinear motion description reduces the aerodynamic stiffness nonlinearity as well, but does not cancel it.

Numerical Investigation of the Fan Flutter Mechanism Related to Acoustic Propagation Characteristics

AbstractIn this study, the flutter mechanism related to acoustic propagation characteristics of a wide-chord fan rotor was investigated numerically. The first bending mode and its two and three nodal diameters traveling wave patterns were considered. The unsteady disturbance induced by the blade vibration in the duct gradually changed from cut-off to cut-on mode by increasing the blade frequency. Flutter occurred in some specific frequency range. The upstream cut-on and downstream cut-off condition with the risk of flutter previously identified in other studies was also observed in this paper. A new flutter risk frequency in which the blade frequency was less than the upstream cut-on frequency was found. The results showed that the effects of flow features and acoustic propagation characteristics on aeroelasticity were independent. To explain why flutter was more likely to occur near stall, two sets of frequencies representing different acoustic propagation characteristics were selected to carry out aeroelastic simulations for different working conditions along the same speed line. When flutter occurred, the suction side always provided positive damping, and the pressure side always provided negative damping. This study analyzed the effect of different flow features, such as shock and radial migration, in detail. The phase difference between the pressure fluctuations on the pressure side and the blade velocity played a vital role in fan flutter.

Design and Motion Simulation Analysis of a Novel 2D Bionic Flutter Aircraft

In order to improve the symmetry of the flutter mechanism in the bionic flutter aircraft, and then improve the overall motion stability of the flutter aircraft. In this paper, an innovative flutter wing configuration is designed based on a planar crank-slider mechanism and a sparrow model. The geometric relationship of the flutter mechanism is established, the motion of the flutter aircraft is modeled, and the dimensions of the model are optimized. The geometric relationship of the flutter structure was simulated and analyzed by CATIA. The results show that the upper limit angle of the designed flutter structure is βmax=25.82° and the lower limit angle is βmin=6.34°. The CATIA simulation data are consistent with the theoretical values of the established structural geometry model. Meeting the needs of small and medium-sized flutter aircraft.The design provides a theoretical basis for the design and development of the bionic flutter vehicle and the fabrication of a solid prototype.

Paradoxical delayed capture proved the dual-loop tachycardia mechanism of a cavotricuspid isthmus-dependent atrial flutter

A 37-year-old man underwent catheter ablation for a cavotricuspid isthmus-dependent atrial flutter. Two 20-pole deflectable electrode catheters were placed in a parallel position on the tricuspid annulus and right atrial lateral wall. The dual-loop tachycardia mechanism of the atrial flutter was suggested by paradoxical delayed capture of the lateral wall of the right atrium during entrainment pacing from the lateral tricuspid annulus.

Flutter Predictions for Very Flexible Wing Wind Tunnel Test

The stability boundaries of a very flexible wing are sought to inform a wind-tunnel flutter test campaign. The objective is twofold: to identify via simulation the relevant physical processes to be explored while ensuring safe and nondestructive experiments, and to provide a benchmark case for which computational models and test data are freely available. Analyses have been independently carried out using two geometrically nonlinear structural models coupled with potential flow aerodynamics. The models are based on a prototype of the wing for which static load and aeroelastic tests are available, and the experimental results have been successfully reproduced numerically. The wing displays strong geometrically nonlinear effects with static deformations as high as 50% of its span. This results in substantial changes to its structural dynamics, which display several mode crossings that cause the flutter mechanisms to change as a function of deformation. Stability characteristics depend on both the freestream velocity and the angle of attack. A fast drop of the flutter speed is observed as the wing deforms as the angle of attack is increased, whereas a large stable region is observed for wing displacements over 25%. The corresponding wind tunnel dynamic tests have validated these predictions.

Influence of common zones of low-amplitude activity on the mechanism and treatment of atrial arrhythmias

Background: The treatment of left atrial flutter is a problem that requires a deep understanding of the underlying complex mechanism of arrhythmia. Although a considerable experience exists already in understanding the mechanisms underlying atrial flutter after ablation or surgery, little is known about atypical forms of atrial flutter in patients who have not previously undergone ablation or other cardiac surgery.
 Clinical case description: We present a clinical case of interventional treatment of a patient with atypical atrial flutter who had not previously undergone surgical or interventional heart surgery. This clinical observation demonstrates the role of common zones of low-amplitude activity on the mechanism and treatment of atrial arrhythmias. Widespread areas of low-amplitude activity in the left atrium can create barriers to the propagation of excitation, which can cause atypical atrial flutter.
 Conclusion: When performing a surgical intervention, high-density mapping will help to visualize the mechanism of this arrhythmia. Understanding the mechanism of atypical atrial flutter will help to minimize the RF exposure during the treatment.

Atrial Flutter Mechanism Detection Using Directed Network Mapping.

Atrial flutter (AFL) is a common atrial arrhythmia typically characterized by electrical activity propagating around specific anatomical regions. It is usually treated with catheter ablation. However, the identification of rotational activities is not straightforward, and requires an intense effort during the first phase of the electrophysiological (EP) study, i.e., the mapping phase, in which an anatomical 3D model is built and electrograms (EGMs) are recorded. In this study, we modeled the electrical propagation pattern of AFL (measured during mapping) using network theory (NT), a well-known field of research from the computer science domain. The main advantage of NT is the large number of available algorithms that can efficiently analyze the network. Using directed network mapping, we employed a cycle-finding algorithm to detect all cycles in the network, resembling the main propagation pattern of AFL. The method was tested on two subjects in sinus rhythm, six in an experimental model of in-silico simulations, and 10 subjects diagnosed with AFL who underwent a catheter ablation. The algorithm correctly detected the electrical propagation of both sinus rhythm cases and in-silico simulations. Regarding the AFL cases, arrhythmia mechanisms were either totally or partially identified in most of the cases (8 out of 10), i.e., cycles around the mitral valve, tricuspid valve and figure-of-eight reentries. The other two cases presented a poor mapping quality or a major complexity related to previous ablations, large areas of fibrotic tissue, etc. Directed network mapping represents an innovative tool that showed promising results in identifying AFL mechanisms in an automatic fashion. Further investigations are needed to assess the reliability of the method in different clinical scenarios.

Low-speed flutter of artificial stalk-leaf and its application in wind energy harvesting

Developing wind energy harvester (WEH) by mimicking the leaf flutter may provide an innovative way for increasing the power efficiency and decreasing the cut-in speed. The low-speed flutter mechanism of the stalk-leaf system is investigated through introducing a frictionless hinge into the stalk-leaf finite element model. The aeroelastic system is established by the usage of doublet-lattice aerodynamics and the spline interpolation between structural motion and flow downwash. The critical flutter speed and frequency are analyzed via V-g method. The evolution of damping and frequency with wind speed which various from static air to Beaufort level 5 are simulated. The influence of inclined angle of the stalk on the flutter characteristics is studied. The stainless-steel artificial stalk-leaf systems with inclined angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90° are fabricated and tested in wind tunnel. The wind energy harvesting performance is also measured by attaching macro-fiber composite patches on root of the stalk. It is found that the 30° stalk-leaf WEH has the lowest critical flutter speed, while the energy harvesting output (voltage and power) increases slowly with the increase of the wind speed. On the contrary, although the 90° stalk-leaf (vertical stalk) WEH has the steepest velocity-voltage and velocity-power curve, it also has the highest cut-in speed. In the preliminary tests, the 30° stalk-leaf WEH outputs steady power density of 47.46 μW cm−3 with stable oscillating frequency of 6.6 Hz at 11 m s−1 wind, while the 90° stalk-leaf WEH outputs power density of 92.88 μW cm−3 with oscillating frequency of 7.2 Hz at the same wind speed. The stalk-leaf design presents a possible way to balance the performance between the high efficiency and the low cut-in speed for the WEHs.