• 한국광학회지
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• 제10권4호
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• pp.328-334
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• 1999

Acrylamide를 홑몸체로 하는 포토폴리머에서 두광파혼합(two-wave energy coupling)실험을 하였다. 외부의 전기장이나 비선형 매질의 이동, 또는 두 입사파 중 광파의 위상이동 없이 자체회절 진동(self-diffraction oscillations)로 측정되었다. 이러한 현상을 설명하기 위하여 위상격자와 흡수격자가 혼합된 격자(mixed grating)와 두 광파의 에너지 결합동안에 형성되는 강도 간섭무늬에 대해 공간 위상 이동(nonlocal response)이 있는 경우를 가정하여, 수정된 Kogelnik의 결합파동 방정식을 유도하였다. 측정된 자체회절 진동을 통하여 포토폴리머의 위상격자(phase grating)는 강도격자(intensity grating)에 대해$\pm$50。의 위상 이동이 있음을 알았다.

• International Journal of Aerospace System Engineering
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• 제2권2호
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• pp.58-61
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• 2015

Combustion instability in solid rocket motors is a long-term open problem since the first rockets were used. Based on the numerous previous studies, it is known that the limit cycle amplitude is one of the key characteristics of the nonlinear combustion instability in solid rocket motors. Flandro's extended energy balance corollary, aims to predict the limit cycle amplitude of complex, nonlinear pressure oscillations for rockets or air-breathing engines, and leads to a precise assessment of nonlinear combustion instability in solid rocket motors. However, based on the comparison with experimental data, it is revealed that the Flandro's method cannot accurately describe such a complex oscillatory pressure. Thus in this work we make modifications of the nonlinear term in the nonlinear wave equations which represents the interaction of different modes. Through this modified method, a numerical simulation of the cylindrical solid rocket has been carried out, and the simulated result consists well with the experimental data. It means that the added coefficient makes the nonlinear wave growth equations describe the experimental data better.

• 한국항공우주학회지
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• 제33권5호
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• pp.28-33
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• 2005

이선형 플런지 스프링을 가지는 2차원 단면 익형 모델에 대하여 초음속 비선형 공탄성 해석을 수행하였다. 초음속 비정상 공기력 계산을 위해 DPM을 사용하였고 최소상태접근법을 사용하여 근사하였다. 비선형 플러터 해석을 위해 구조 비선형성을 비대칭 이선형 스프링으로 모델링하고 기술 함수 법을 사용하여 선형화하였다. 선형 및 비선형 플러터 해석 결과들은 공력탄성학적 특성들이 주파수 비에 중요한 영향을 받는다는 것을 보여준다. 비선형 플러터 해석으로부터 다양한 제한 주기 운동이 선형플러터 속도 이하 또는 이상에서 관측되었다. 또한 플러터 특성과 응답을 시간영역에서도 조사하였다.

• International Journal of Aeronautical and Space Sciences
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• 제4권1호
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• pp.53-62
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• 2003

In this study, nonlinear aeroelastic characteristics of an supersonic missile wing with strong shock interferences are investigated. The missile wing model has a freeplay structural nonlinearity at its pitch axis. To practically consider the effects of freeplay structural nonlinearity, the fictitious mass method is applied to structural vibration analysis based on finite element method. Nonlinear aerodynamic flows with unsteady shock waves are also considered in supersonic flow regions. To solve the nonlinear aeroelastic governing equations including the freeplay effect, a modal-based coupled time-marching technique based on the fictitious mass method is used in the time-domain. Various aeroelastic computations have been performed for the nonlinear wing structure model. Linear and nonlinear aeroelastic analyses have been conducted and compared with each other in supersonic flow regions. Typical nonlinear limit cycle oscillations and phase plots are presented to show the complex vibration phenomena with simultaneous fluid-structure nonlinearities.

• Advances in aircraft and spacecraft science
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• 제3권4호
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• pp.447-470
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• 2016

Limit cycle oscillations (LCO) as well as nonlinear aeroelastic analysis of a swept aircraft wing with cubic restoring moments in the pitch degree of freedom is investigated. The unsteady aerodynamic loading applied on the wing is modeled by using the strip theory. The harmonic balance method is used to calculate the LCO frequency and amplitude for the swept wing. Finally the super and subcritical Hopf bifurcation diagrams are plotted. It is concluded that the type of bifurcation and turning point location is sensitive to the system parameters such as wing geometry and sweep angle.

• Smart Structures and Systems
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• 제21권1호
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• pp.99-111
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• 2018

Wind induced large bending oscillation amplitudes in tall and slender telecommunication steel towers may lead to precocious fatigue cracks and consequent risk of collapse of these structures, many of them installed in rural areas alongside highways and in highly populated urban areas. Varying stress amplitudes at hot spots may be attenuated by means of passive control mechanical devices installed in the tower. This paper gives an account of both mathematical-numerical model and the technique applied to design and evaluate the performance of a double controller installed in existing towers which is composed by a nonlinear pendulum and a novel type of passive controller described herein as a planar motion disk mounted on shear springs. Results of experimental measurements carried out on two slender tubular steel towers under wind action demonstrate the efficiency of the double controllers in attenuating the towers bending oscillation amplitudes and consequent stress amplitudes extending the towers fatigue life.

• 한국가시화정보학회지
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• 제9권1호
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• pp.17-19
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• 2011

The bubble oscillations play an important role in ultrasonic cleaning processes. In the ultrasonic cleaning of semiconductor wafers, the cleaning process often damages micro/nano scale patterns while removing contaminant particles. However, the understanding of how patterns in semiconductor wafers are damaged during ultrasonic cleaning is far from complete yet. Here, we report the observations of the motion of bubbles that induce solid wall damage under 26 kHz continuous ultrasonic waves. We classified the motions into the four types, i.e. volume motion, shape motion, splitting or jetting motion and chaotic motion. Our experimental results show that bubble oscillations get unstable and nonlinear as the ultrasonic amplitude increases, which may exert a large stress on a solid surface raising the possibility of damaging microstructures.

• Journal of applied mathematics & informatics
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• 제41권1호
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• pp.11-31
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• 2023

The paper presents a critical analysis of selected topics related to the modeling of interacting species in which prey has nonlinear reproduction, which is in competition with predator. The mathematical model's stochastic stability is investigated. The method of designing appropriate Lyapunov functions is used to identify permanence conditions among the parameters of the model and conditions for the structure to no longer be extinct. The system's two-dimensional diffusive stability is regarded and studied. The system experiences the process of saddle-node bifurcation by varying the death rate of predator parameter. Further effects of parameters that undergo inherent oscillations are numerically investigated, revealing that as the intensity of predation parameter b is increased, the device encounters non-periodic and damped oscillations.

• Advances in aircraft and spacecraft science
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• 제2권2호
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• pp.125-168
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• 2015

Dynamic aeroelastic behavior of structurally nonlinear Joined Wings is presented. Three configurations, two characterized by a different location of the joint and one presenting a direct connection between the two wings (SensorCraft-like layout) are investigated. The snap-divergence is studied from a dynamic perspective in order to assess the real response of the configuration. The investigations also focus on the flutter occurrence (critical state) and postcritical phenomena. Limit Cycle Oscillations (LCOs) are observed, possibly followed by a loss of periodicity of the solution as speed is further increased. In some cases, it is also possible to ascertain the presence of period doubling (flip-) bifurcations. Differences between flutter (Hopf's bifurcation) speed evaluated with linear and nonlinear analyses are discussed in depth in order to understand if a linear (and thus computationally less intense) representation provides an acceptable estimate of the instability properties. Both frequency- and time-domain approaches are compared. Moreover, aerodynamic solvers based on the potential flow are critically examined. In particular, it is assessed in what measure more sophisticated aerodynamic and interface models impact the aeroelastic predictions. When the use of the tools gives different results, a physical interpretation of the leading mechanism generating the mismatch is provided. In particular, for PrandtlPlane-like configurations the aeroelastic response is very sensitive to the wake's shape. As a consequence, it is suggested that a more sophisticate modeling of the wake positively impacts the reliability of aerodynamic and aeroelastic analysis. For SensorCraft-like configurations some LCOs are characterized by a non-synchronous motion of the inner and outer portion of the lower wing: the wing's tip exhibits a small oscillation during the descending or ascending phase, whereas the mid-span station describes a sinusoidal-like trajectory in the time-domain.

• 대한기계학회논문집B
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• 제27권8호
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• pp.1158-1164
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• 2003

Steady-state structure and acoustic-pressure responses of $H_2$/Air counterflow diffusion flames are studied numerically with a detailed chemistry in view of acoustic instability. The Rayleigh criterion is adopted to judge acoustic amplification or attenuation from flame responses. Steady-state flame structures are first investigated and flame responses to various acoustic-pressure oscillations are numerically calculated in near-equilibrium and near-extinction regimes. The acoustic responses of $H_2$/Air flame show that the responses in near-extinction regime always contribute to acoustic amplification regardless of acoustic-oscillation frequency Flames near extinction condition are sensitive to pressure perturbation and thereby peculiar nonlinear responses occur, which could be a possible mechanism in generating the threshold phenomena observed in combustion chamber of propulsion systems.