• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1999년도 춘계학술대회논문집
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• pp.14-17
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• 1999

The wrinkling behavior of a thin sheet with perfect geometry is a kind of compressive instability. The compressive instability is influenced by many factors such as stress state mechanical properties of the sheet material geometry of the body contact conditions and plastic anisotropy. The analysis of compressive instability in plastically deforming body is difficult considering all the factors because the effects of the factors are very complex and the instability behavior may show wide variation for small deviation of the factors. In this study the bifurcation theory is introduced for the finite element analysis of puckering initiation and growth of a thin sheet with perfect geometry. All the above mentioned analysis and the post-bifurcation behavior is analyzed by introducing the branching scheme proposed by Riks. The finite element formulation is based on the incremental deformation theory and elastic-plastic material modeling. in order to investigate the effect of plastic anisotropy on the compressive instability a square plate that is subjected to compression in one direction and tension in the other direction is analyzed by the above-mentionedfinite element analysis. The critical stress ratios above which the buckling does not take place are found for various plastic anisotropic modeling method and discussed. Finally the effect of plastic anisotropy on the puckering behavior in the spherical cup deep drawing process is investigated.

• 한국소음진동공학회논문집
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• 제24권1호
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• pp.14-20
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• 2014

Recent developments for high altitude, long endurance conventional UAVs(HALE UAVs) have revealed new issues regarding aircraft structure design and analysis. First of all, due to intensive mission requirements, the structures of HALE UAVs have lightweight and very flexible main wing with high aspect ratio, and slender fuselage. For this kind of structures, aeroelastic characteristics are different from conventional aircrafts. Hence, currently developed analysis methods are not suitable to fully understand strucutral dynamics of the very flexible aircraft, and to guarantee structural reliability. Therefore, various structural studies considering nonlinear behaviors which are generally ignored for the conventional aircraft strucutral analyis have been attracting researchers interests. Nonlinear flutter of the very flexible wing is one of the subject to be studied in combination with strong coupling between aeroelastic characteristics and flight dynamics. Herein, as preliminary study, modeling and nonlinear system analysis of the 2D airfoild with torsional nonlinearity have been discussed.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2013년도 추계학술대회 논문집
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• pp.226-231
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• 2013

Recent developments for high altitude, long endurance conventional UAVs (HALE UAVs) have revealed new issues regarding aircraft structure design and analysis. First of all, due to intensive mission requirements, the structures of HALE UAVs have lightweight and very flexible main wing with high aspect ratio, and slender fuselage. For this kind of structures, aeroelastic characteristics are different from conventional aircrafts. Hence, currently developed analysis methods are not suitable to fully understand strucutral dynamics of the very flexible aircraft, and to guarantee structural reliability. Therefore, various structural studies considering nonlinear behaviors which are generally ignored for the conventional aircraft strucutral analyis have been attracting researchers interests. Nonlinear flutter of the very flexible wing is one of the subject to be studied in combination with strong coupling between aeroelastic characteristics and flight dynamics. Herein, as preliminary study, modeling and nonlinear system analysis of the 2D airfoild with torsional nonlinearity have been discussed.

• 한국산학기술학회논문지
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• 제19권12호
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• pp.14-24
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• 2018

몇 가지 전형적인 기존 및 진보된 초탄성 구성모델들의 고무패치 이축인장 및 구형 또는 원통형 풍선 팽창에서의 불안정성에 대해서 밝힌다. 적용할 구성모델은 neo-Hookean 모델, Mooney-Rivlin 모델, Gent 모델, Arruda-Boyce 모델, Fung 모델, Pucci-Saccomandi 모델 등이다. 팽창 및 분기 해석은 이들 변형에너지 함수들의 막 방정식을 이용하여 수행할 수 있다. 해석에는 사각패치에 대한 Kearsley의 분기현상, 고무풍선의 일반화 한 팽창현상, 고무풍선의 분기현상을 다룬다. 이들 변형에너지 함수들 중에서도 오직 Mooney-Rivlin 모델에서만 Kearsley의 분기현상이 일어남을 확인하였다. 팽창 방정식은 구형풍선과 원통형 풍선을 함께 다룰 수 있도록 일반화 시켰다. 팽창해석에 의하여 극한점과 임계 물성치들을 무차원 압력 및 팽창 부피의 항들로 구하였다. 그렇게 구해진 결과들로부터 분기현상을 구할 수 있었다. 또한 유한요소법을 사용하여 고무류의 구조적 불안정 문제들을 다룰 때 필요한 특별한 조처에 대해서 제안하였다. 결론적으로 고무류의 불안정성을 포함하는 문제를 다룰 때는 해석기법은 물론 구성모델의 선택에 따라 결과가 달라질 수 있으므로 신중한 처리가 요구된다.

• Structural Engineering and Mechanics
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• 제57권6호
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• pp.1085-1105
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• 2016

The drainage problem in bifurcations causes pecuniary losses when hydropower stations are undergoing periodic overhaul. A new design philosophy for Y-typed bifurcations that are flat-bottomed is proposed. The bottoms of all pipe sections are located at the same level, making drainage due to gravity possible and shortening the draining time. All fundamental curves were determined, and contrastive analysis with a crescent-rib reinforced bifurcation in an actual project was conducted. Feasibility demonstrations were researched including structural characteristics based on finite element modeling and hydraulic characteristics based on computational fluid dynamics. The new bifurcation provided a well-balanced shape and reasonable stress state. It did not worsen the flow characteristics, and the head loss was considered acceptable. The proposed Y-typed bifurcation was shown to be suitable for pumped storage power stations.

• Kyungpook Mathematical Journal
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• 제62권1호
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• pp.119-132
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• 2022

Virotherapy is an effective method for the treatment of cancer. The oncolytic virus specifically infects the lyse cancer cell without harming normal cells. There is a time delay between the time of interaction of the virus with the tumor cells and the time when the tumor cells become infectious and produce new virus particles. Several types of viruses are used in virotherapy and the delay varies with the type of virus. This delay can play an important role in the success of virotherapy. Our present study is to explore the impact of this delay in cancer virotherapy through a mathematical model based on delay differential equations. The partial success of virotherapy is guarenteed when one gets a stable non-trivial equilibrium with a low level of tumor cells. There exits Hopf-bifurcation by considering the delay as bifurcation parameter. We have estimated the length of delay which preserves the stability of the non-trivial equilibrium point. So when the delay is less than a threshold value, we can predict partial success of virotherapy for suitable sets of parameters. Here numerical simulations are also performed to support the analytical findings.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1996년도 추계학술대회 논문집
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• pp.799-805
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• 1996

In many cases, the magnetic farce model is linearized at the origin in designing the controller of a magnetic bearing system. However. this linear assumption is violated by the unmodeled nonlinear effect such as sensor dislocation and backup bearing dislocation. Therefore, a direct probe into the nonlinear magnetic force model in an active magnetic bearing system is necessary. To analyze the nonlinear magnetic force model of a magnetic bearing system, phase plot analysis which is to plot the numerical solution of the nonlinear equation in several initial points in the interested region is applied. Phase plot analysis is used to observe a nonlinear dynamic system qualitatively (not quantitatively). With this method, we can get much useful information of the nonlinear system. Among this information, a bifurcation graph that represents stability and locations of fixed points is essential. From the bifurcation graph, a stability criterion of magnetic bearing system is derived.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2000년도 추계 학술대회논문집
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• pp.115-122
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• 2000

본 연구에서는 2 차원 벽구동 캐비티 유동에 의하여 나타나는 이력효과에 의한 분기(Bifurcation)현상을 전산유체기법을 사용하여 연구하였다. 캐비티는 북쪽과 동쪽벽이 움직일 수 있고, 다른 두 벽은 고정되어있는 구조이다. 실험은 Reynolds 수 100 에서 1000까지 증가시켜가면서 북쪽벽과 동쪽벽을 동시에 가속 시켜 정상상태에 이르게 한 경우와 북쪽벽이 먼저 가속되어 정상해에 이른 후 동쪽벽을 나중에 가속하여 재차 정상상태에 이르게 한 경우를 비교하였다. 그 결과 Reynolds수가 약 200이상부터 벽에 작용하는 항력, 유량함수의 값, 재부착점등이 분기현상을 나타냄을 확인하였다.

• 대한수학회지
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• 제46권4호
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• pp.713-731
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• 2009

In this paper, a three-dimensional eco-epidemiological model with delay is considered. The stability of the two equilibria, the existence of Hopf bifurcation and the permanence are investigated. It is found that Hopf bifurcation occurs when the delay ${\tau}$ passes though a sequence of critical values. The estimation of the length of delay to preserve stability has also been calculated. Numerical simulation with a hypothetical set of data has been done to support the analytical findings.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2001년도 춘계학술대회논문집
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• pp.289-294
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• 2001

Three kinds of viscoelastic damper model, which has a non-linear spring as an element is studied analytically and numerically. The behavior of the damper model shows non-linear hysteresis curves which is qualitatively similar to those of real viscoelastic materials. The motion is governed by a non-linear constitutive equation and an additional equation of motion. Harmonic balance method is applied to get analytic solutions of the system. The frequency-response curves show that multiple solutions co-exist and that the jump phenomena can occur. In addition, it is shown that separate solution branch exists and that it can merge with the primary response curve. Saddle-node bifurcation sets explain the occurrences of such non-linear phenomena.