• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.126-130
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• 2000

In this study, flutter characteristics of a composite wing have been studied for the variation of laminate angles in the subsonic, transonic and supersonic flow regime. The laminate angles are selected by the aspect of engineering practice such as 0, $\pm$45 and 90 degrees. To calculate the unsteady aerodynamics for flutter analysis, the Doublet Lattice Method(DLM) in subsonic flow and the Doublet Point Method(DPM) in supersonic flow are applied in the frequency domain. In transonic flow, transonic small disturbance(TSD) code is used to calculate the nonlinear unsteady aerodynamics in the time domain. Aeroelastic governing equation has been solved by v-g method in the frequency domain and also by Coupled Time-Integration Method(CTIM) in the time domain. from the results of present study, characteristics of free vibration responses and aeroelastic instabilities of a composite wing are presented for the set of various lamination angles in the all flow range.

• Wind and Structures
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• v.12 no.5
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• pp.413-424
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• 2009

A series of wind tunnel free-decay sectional model dynamic tests were conducted to examine the effects of torsional-to-vertical natural frequency ratio of 2DOF bridge dynamic systems on the aerodynamic and dynamic properties of bridge decks. The natural frequency ratios tested were around 2.2:1 and 1.2:1 respectively, with the fundamental vertical natural frequency of the system held constant for all the tests. Three 2.9 m long twin-deck bridge sectional models, with a zero, 16% (intermediate gap) and 35% (large gap) gap-to-width ratio, respectively, were tested to determine whether the effects of frequency ratio are dependent on bridge deck cross-section shapes. The results of wind tunnel tests suggest that for the model with a zero gap-width, a model to approximate a thin flat plate, the flutter derivatives, and consequently the aerodynamic forces, are relatively independent of the torsional-to-vertical frequency ratio for a relatively large range of reduced wind velocities, while for the models with an intermediate gap-width (around 16%) and a large gap-width (around 35%), some of the flutter derivatives, and therefore the aerodynamic forces, are evidently dependent on the frequency ratio for most of the tested reduced velocities. A comparison of the modal damping ratios also suggests that the torsional damping ratio is much more sensitive to the frequency ratio, especially for the two models with nonzero gap (16% and 35% gap-width). The test results clearly show that the effects of the frequency ratio on the flutter derivatives and the aerodynamic forces were dependent on the aerodynamic cross-section shape of the bridge deck.

• Journal of KSNVE
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• v.7 no.1
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• pp.91-97
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• 1997

An analysis is presented on the stability of an elastic cantilever column having the elastic restraints at its free end, carrying an added tip mass, and subjected to uniformly distributed follower forces. The elastic restraints are formed by both a translational spring and a rotatory spring. For this purpose, the governing equations and boundary conditions are derived by using Hamilton's principle, and the critical flutter loads and frequencies are obtained from the numerical evaluation of the eigenvalue functions of this elastic system. The added tip mass increases as a whole the critical flutter load of the elastic cantilever column, but the presence of its moment of inertia of mass has a destabilizing effect. The existence of the translational and rotatory springs at the free end increases the critical flutter load of the elastic cantilever column. Nevertheless, their effects on the critical flutter load are not uniform because of their coupling. The translational spring restraining the free end of the cantilever column decreases the critical flutter load by coupling with a large value of tip mass, while by coupling with the moment of inertia of tip pass its effect on the critical flutter load is contrary. The rotatory spring restraining the free end of the cantilever column increases the critical flutter load by coupling with the tip mass, but decreases it by coupling with the moment of inertia of the tip mass.

• Wind and Structures
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• v.14 no.3
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• pp.187-208
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• 2011

Based on the ANSYS, an approach of full-mode aerodynamic flutter analysis for long-span suspension bridges has been presented in this paper, in which the nonlinearities of structure, aerostatic and aerodynamic force due to the deformation under the static wind loading are fully considered. Aerostatic analysis is conducted to predict the equilibrium position of a bridge structure in the beginning, and then flutter analysis of such a deformed bridge structure is performed. A corresponding computer program is developed and used to predict the critical flutter wind velocity and the corresponding flutter frequency of a long-span suspension bridge with double main span. A time-domain analysis of the bridge is also carried out to verify the frequency-domain computational results and the effectiveness of the approach proposed in this paper. Then, the nonlinear effects on aerodynamic behaviors due to aerostatic action are discussed in detail. Finally, the results are compared with those of traditional suspension bridges with single main span. The results show that the aerostatic action has an important influence on the flutter stability of long-span suspension bridges. As for a suspension bridge with double main spans, the flutter mode is the first anti-symmetrical torsional vibration mode, which is also the first torsional vibration mode in natural mode list. Furthermore, a double main-span suspension bridge is better in structural dynamic and aerodynamic performances than a corresponding single main-span structure with the same bridging capacity.

• Wind and Structures
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• v.7 no.4
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• pp.281-291
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• 2004

The flutter characteristics of long span bridges are discussed from the point of the unsteady pressure distribution on bridge deck surface during heaving/torsional vibration related to the aerodynamic derivatives. In particular, it is explained that the coupling terms, which consist of $A_1^*$ and $H_3^*$, play a substantial role on the coupled flutter, in comparison with the flutter characteristics of various structural sections. Also the effect of the torsional/heaving frequency ratio of bridge structures on the flutter instability is discussed from the point of the coupling effect between heaving and torsional vibrations.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.14 no.2
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• pp.17-26
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• 2006

The flutter characteristics of all movable tail wing with honeycomb sandwich structure have been studied in this study. The present wing model has a airfoil cross section and the linear variation of spanwise thickness. Structural vibration analysis is performed based on the finite element method using sandwich and beam elements. Unsteady aerodynamic technique used on the doublet lattice method has been effectively used to conduct the frequency-domain flutter analyses. The parametric flutter studies have been performed for various structural design parameters. Computational results on flutter stability due to the variation of structural parameters are presented and its related characteristics are investigated through the comparison of results.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.11b
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• pp.1197-1201
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• 2001

Flutter suppression of a wing/store model is investigated. An aircraft wing with a store is modeled as a 2-D typical section. Unsteady aerodynamics of the wing/store model are computed by using Doublet Hybrid Method(DHM) in the frequency-domain, and are approximated by Minimum-state(MS) approximation. LQG controller is used to suppress the flutter of the wing/store model and the aeroelastic characteristics of the closed-loop system are investigated. The flutter characteristics of the wing/store model are improved and the flutter speed is increased up to about 16 ％.

• Wind and Structures
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• v.35 no.5
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• pp.323-336
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• 2022

This study investigates the applicability of the direct identification of flutter derivatives in the time domain using Rational Function Approximation (RFA), where the extraction procedure requires either a combination of at least two wind speeds or one wind speed. In the frequency domain, flutter derivatives are identified at every wind speed. The ease of identifying flutter derivatives in the time domain creates a paradox because flutter derivative patterns sometimes change in higher-order polynomials. The first step involves a numerical study of RFA extractions for different deck shapes from existing bridges to verify the accurate wind speed combination for the extraction. The second step involves validating numerical simulation results through a wind tunnel experiment using the forced vibration method in one degree of freedom. The findings of the RFA extraction are compared to those obtained using the analytical solution. The numerical study and the wind tunnel experiment results are in good agreement. The results show that the evolution pattern of flutter derivatives determines the accuracy of the direct identification of RFA.

• International Journal of Aeronautical and Space Sciences
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• v.15 no.1
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• pp.20-31
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• 2014

This paper deals with the development of a realistic shape optimization of damaged columns that are subjected to conservative and non-conservative forces, using the Genetic Algorithm (GA). The analysis is based on the design of the most optimized shape of the column under the constraint of constant weight, considering the Static, Vibrational, and Flutter characteristics. Under the action of conservative and non-conservative longitudinal forces, an elastic column loses its stability. A numerical analysis based on FEM has been performed on a uniform damaged column, to compute the fundamental buckling load, vibration frequency, and flutter load, under various end restraints. An optimization search based on the Genetic Algorithm is then executed, to find the optimal shape design of the column. The optimized column references the one having the highest buckling load, highest vibration frequency, and highest flutter load, among all the possible shapes of the column, for a given volume. A comparison is then made between the values obtained for the optimized damaged column, and those obtained for the optimized undamaged column. The comparison reveals that the incorporation of damage in the column alters its optimal shape to only a certain extent. Also, the critical load and frequency values for the optimized damaged column are comparatively low, compared with those obtained for the optimized undamaged column. However, these results hold true only for moderate-intensity damage cases. For high intensity damage, the optimal shape may not remain the same, and may vary, according to the severity of damage.

• International Journal of Aeronautical and Space Sciences
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• v.14 no.2
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• pp.133-139
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• 2013

Morphing aircraft capable of varying their wing form can operate efficiently at various flight conditions. However, radical morphing of the aircraft leads to increased structural complexities, resulting in occurrence of dynamic instabilities such as flutter, which can lead to catastrophic events. Therefore, it is of utmost importance to investigate and understand the changes in flutter characteristics of morphing wings, to ensure uncompromised safety and maximum reliability. In this paper, a study on the flutter characteristics of the folding wing type morphing concept is conducted, to examine the effect of changes in folding angles on the flutter speed and flutter frequency. The subsonic aerodynamic theory Doublet Lattice Method (DLM) and p-k method are used, to perform the flutter analysis in MSC.NASTRAN. The present baseline flutter characteristics correspond well with the results from previous study. Furthermore, enhancement of the flutter characteristics of an aluminum folding wing is proposed, by varying the outboard wing folding angle independently of the inboard wing folding angle. It is clearly found that the flutter characteristics are strongly influenced by changes in the inboard/outboard folding angles, and significant improvement in the flutter characteristics of a folding wing can be achieved, by varying its outboard wing folding angle.