• International Journal of Aeronautical and Space Sciences
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• 제8권1호
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• pp.140-147
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• 2007

Whenever the hinge axis of aircraft wing rotates, its stiffness varies. Also, there are nonlinearities in the connection of the actuator and the hinge axis, and it is necessary to inspect the coupled effects between the actuator dynamics and the hinge nonlinearity. Nonlinear aeroelastic characteristics are investigated by using the iterative V-g method. Time domain analyses are also performed by using Karpel's minimum state approximation technique. The doublet hybrid method(DHM) is used to calculate the unsteady aerodynamic forces in subsonic regions. Structural nonlinearity located in the load links of the actuator is assumed to be friction. The friction nonlinearity of an actuator is identified by using the describing function technique. The nonlinear flutter analyses have shown that the flutter characteristics significantly depends on the structural nonlinearity as well as the dynamic stiffness of an actuator. Therefore, the dynamic stiffness of an actuator as well as the nonlinear effect of hinge axis are important factors to determine the flutter stability.

• Wind and Structures
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• 제14권3호
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• pp.221-238
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• 2011

The estimated response of large-scale engineering structures to severe wind loads is prone to modelling uncertainties that can only ultimately be assessed by full-scale testing. To this end ambient vibration data from full-scale monitoring of the historic Clifton Suspension Bridge has been analysed using a combination of a frequency domain system identification method and a more elaborate stochastic identification technique. There is evidence of incipient coupling action between the first vertical and torsional modes in strong winds, providing unique full-scale data and making this an interesting case study. Flutter derivative estimation, which has rarely previously been attempted on full-scale data, was performed to provide deeper insight into the bridge aerodynamic behaviour, identifying trends towards flutter at higher wind speeds. It is shown that, as for other early suspension bridges with bluff cross-sections, single-degree-of-freedom flutter could potentially occur at wind speeds somewhat below requirements for modern designs. The analysis also demonstrates the viability of system identification techniques for extracting valuable results from full-scale data.

• Wind and Structures
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• 제24권4호
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• pp.367-384
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• 2017

The Maanshan Bridge over Yangtze River in China is a new long-span suspension bridge with double main spans of $2{\times}1080m$ and a closed streamline cross-section of single box deck. The flutter and buffeting performances were investigated via wind tunnel tests of a full bridge aeroelastic model at a geometric scale of 1:211. The tests were conducted in both smooth wind and simulated boundary layer wind fields. Emphasis is placed on studying the interference effect of adjacent span via installing a wind deflector and a wind separating board to shelter one span of the bridge model from incoming flow. Issues related to effects of mid-tower stiffness and deck supporting conditions are also discussed. The testing results show that flutter critical wind velocities in smooth flow, with a wind deflector, are remarkably lower than those without. In turbulent wind, torsional and vertical standard deviations for the deck responses at midspan in testing cases without wind deflector are generally less than those at the midspan exposed to wind in testing cases with wind deflector, respectively. When double main spans are exposed to turbulent wind, the existence of either span is a mass damper to the other. Furthermore, both effects of mid-tower stiffness and deck supporting conditions at the middle tower on the flutter and buffeting performances of the Maanshan Bridge are unremarkable.

• 한국항공우주학회지
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• 제34권6호
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• pp.75-82
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• 2006

틸트로터 항공기는 기존의 헬리콥터에 비해 두 배 정도 빠르고 비행 가능 영역은 수배에 이른다. 로터시스템을 틸팅(tilting)하여 전진비행하기 때문에 기존 헬기 블레이드에서 발생되는 전진면의 압축성 효과와 후퇴면의 실속을 방지하는 효과를 얻을 수 있다. 그러나 틸트로터 항공기에서는 훨플러터(whirl flutter)로 알려진 공탄성 불안정성으로 인해 최대 전진속도에 제한을 가지게 된다. 본 논문에서는 우선 로터시스템 자체의 공탄성 안정성에 대한 파라메트릭 연구를 수행하였고, 피치링크 강성, 짐발 스프링 상수, 원추각이 스마트무인기의 훨플러터 안정성에 미치는 영향을 CAMRAD II를 이용한 해석을 통해 고찰하였다.

• 한국항공운항학회지
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• 제21권4호
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• pp.47-52
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• 2013

In this paper, limit cycle flutter induced by Hopf bifurcation is studied with nonlinear system analysis approach and observed for the critical slowing down phenomenon. Considering an attractor of the dynamics of a system, when a small perturbation is applied to the system, the dynamics converge toward the attractor at some rate. The critical slowing down means that this recovery rate approaches zero as a parameter of the system varies and the size of the basin of attraction shrinks to nil. Consequently, in the pre-bifurcation regime, the recovery rates decrease as the system approaches the bifurcation. This phenomenon is one of the features used to forecast bifurcation before they actually occur. Therefore, studying the critical slowing down for limit cycle flutter behavior would have potential applicability for forecasting those types of flutter. Herein, modeling and nonlinear system analysis of the 2D airfoil with torsional nonlinearity have been discussed, followed by observation of the critical slowing down phenomenon.

• International Journal of Aeronautical and Space Sciences
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• 제6권1호
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• pp.17-26
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• 2005

An optical phase tracking technique for an extrinsic Fabry-Perot interferometer (EFPI) is proposed in order to overcome interferometric non-linearity. Basic idea is utilizing strain-rate information, which cannot be easily obtained from an EFPI sensor itself. The proposed phase tracking system consists of a patch-type EFPI sensor and a simple on-line phase tracking logic. The patch-type EFPI sensor comprises an EFPI and a piezoelectric patch. An EFPI sensor itself has non-linear behavior due to the interferometric characteristics, and a piezoelectric material has hysteresis. However, the composed patch-type EFPI sensor system overcomes the problems that can arise when they are used individually. The dynamic characteristics of the proposed phase tracking system were investigated, and then the patch-type EFPI sensor system was applied to the active suppression of flutter, dynamic aeroelastic instability, of a swept-back composite plate structure. The proposed system has effectively reduced the amplitude of the flutter mode, and increased flutter speed.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.6-15
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• 2008

This paper deals with the aeroelastic instability of vibrating multiple blade rows under aerodynamic coupling with each other. A model composed of three blade rows, e.g., rotor-stator-rotor, where blades of the two rotor cascades are simultaneously vibrating, is considered. The displacement of a blade vibrating under aerodynamic force is expanded in a modal series with the natural mode shape functions, and the modal amplitudes are treated as the generalized coordinates. The generalized mass matrix and the generalized stiffness matrix are formulated on the basis of the finite element concept. The generalized aerodynamic force on a vibrating blade consists of the component induced by the motion of the blade itself and those induced not only by vibrations of other blades of the same cascade but also vibrations of blades in another cascade. To evaluate the aerodynamic forces, the unsteady lifting surface theory for the model of three blade rows is applied. The so-called k method is applied to determine the critical flutter conditions. A numerical study has been conducted. The flutter boundaries are compared with those for a single blade row. It is shown that the effect of the aerodynamic blade row coupling substantially modifies the critical flutter conditions.

• Wind and Structures
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• 제35권5호
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• pp.337-351
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• 2022

As central-slotted box decks usually have excellent flutter performance, studies on this type of deck mostly focus on the vortex-induced vibration (VIV) control. Yet with the increasing span lengths, cable-supported bridges may have critical wind speeds of wind-induced static instability lower than that of the flutter. This is especially likely for bridges with a central-slotted box deck. As a result, the overall aerodynamic performance of such a bridge will depend on its wind-induced static stability. Taking a 1400 m-main-span cable-stayed bridge as an example, this study investigates the influence of a series of deck shape parameters on both static and flutter instabilities. Some crucial shape parameters, like the height ratio of wind fairing and the angle of the inner-lower web, show opposite influences on the two kinds of instabilities. The aerodynamic shape optimization conducted for both static and flutter instabilities on the deck based on parameter-sensitivity studies raises the static critical wind speed by about 10%, and the overall critical wind speed by about 8%. Effective VIV countermeasures for this type of bridge deck have also been proposed.

• Composites Research
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• 제20권1호
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• pp.8-14
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• 2007

플랩을 갖는 복합재 평판날개에 대해서 유격 비선형성과 구동장치의 동적 강성을 고려하여 비선형 공탄성 해석을 수행하였다. DHM 방법을 사용하여 아음속 비정상 공기력을 계산하였으며 유격은 기술함수를 적용하여 이선형 스프링으로 가정하였다. 동적 강성을 기어시스템의 지배방정식으로부터 계산하고, 적층각과 재질에 따른 공탄성 특성을 살펴보았다. 선형 및 비선형 공탄성 해석 결과들은 플러터 특성이 유격과 동강성에 따라 크게 달라지는 것을 보여주었다. 다양한 형태의 제한주기거동이 선형플러터 속도 이전과 이후에서 관찰되었다.

• 항공우주시스템공학회지
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• 제17권6호
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• pp.82-93
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• 2023

본 논문에서는 층간분리 현상을 고려한 복합재 미사일 핀의 비선형 천음속 플러터 해석을 수행하였다. 층간분리 효과를 고려한 유한요소 진동해석 기법은 시험 결과와 비교 및 검증하였다. 비선형 천음속 플러터 해석은 자체 개발한 천음속 미소교란 방정식 기반의 시간영역 플러터 해석 프로그램을 개선하여 복합재 날개의 층간분리 효과까지 고려할 수 있도록 확장하여 활용하였다. 복합재 미사일 핀 모델에 대해 층간분리 영역에 따른 아음속, 천음속 및 초음속 플러터 해석을 수행하고 층간분리 영향에 따른 공력탄성학적 특성을 고찰하였다.