• 한국소음진동공학회논문집
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• 제17권7호
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• pp.605-610
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• 2007

In this paper a dynamic behavior(natural frequency) of a cracked cantilever beam subjected to follower force is presented. In addition, an analysis of the flutter and buckling instability of a cracked cantilever beam subjected to a follower compressive load is presented. Based on the Euler-Bernoulli beam theory, the equation of motion can be constructed by using the Lagrange's equation. The vibration analysis on such cracked beam is conducted to identify the critical follower force for flutter instability based on the variation of the first two resonant frequencies of the beam. Besides, the effect of the crack's intensity and location on the flutter follower force is studied. The crack section is represented by a local flexibility matrix connecting two undamaged beam segments. The crack is assumed to be in the first mode of fracture and to be always opened during the vibrations.

• Structural Engineering and Mechanics
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• 제76권2호
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• pp.163-173
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• 2020

As a vehicle moves on multiple equal-span beams at constant speed, the running vehicle would be subjected to repetitive excitations from the beam vibrations under it. Once the exciting frequency caused by the vibrating beams coincides with any of the vehicle's frequencies, resonance would take place on the vehicle. A similar resonance phenomenon occurs on a beam subject to sequential moving loads with identical axle-intervals. To reduce both resonant phenomena of a vehicle moving on guideway girders, this study proposed an additional feedback controller based the condensed virtual dynamic absorber (C-VDA) scheme. This condensation scheme has the following advantages: (1) the feedback tuning gains required to adapt the control currents or voltages are directly obtained from the tuning forces of the VDA; (2) the condensed VDA scheme does not need additional DoFs of the absorber to control the vibration of the maglev-vehicle/guideway system. By decomposing the maglev vehicle-guideway coupling system into two sub-systems (the moving vehicle and the supporting girders), an incremental-iterative procedure associated with the Newmark method is presented to solve the two sets of sub-system equations. From the present studies, the proposed C-VDA scheme is a feasible approach to suppress the interaction response for a maglev vehicle in resonance moving on a series of guideway girders.

• Journal of Advanced Marine Engineering and Technology
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• 제10권2호
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• pp.146-155
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• 1986

Until recently, the calculation of torsional vibration for the marine diesel engine shafting has been performed only for vibratory stresses of resonant points and vibratory stresses for other engine speeds are determined by the estimation. With the advent of energy-saving engines which have a long stroke and a small number of cylinders, the first major critical torsional vibration of the propulsion shaft appears ordinarily near the MCR speed of engine and the flank of its vibratory stress exceeds now and then the limit stress defined by the rules of Classification Society. In order to know the above condition in the design stage of propulsion shafting, it is necessary to calculate the forced torsional vibration with the damping of propulsion shafting for all orders and to synthesize its calculated results according to their phase angles. In this study, the forced torsional vibrations with the damping of propulsion shafting are calculated for several orders by mechanical impedance method, and their results are synthesized. A computer program for above calculations are developed and some test-runs of the developed program are performed for propulsion shaftings of actual ships. The results of calculations are compared with measured values and also with those of the modal analysis method. They show fairly good agreements and the developed program is checked up on its reliability.

• Advances in nano research
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• 제3권1호
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• pp.29-37
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• 2015

In the current study, the nonlinear vibration properties of an embedded zigzag single-walled carbon nanotube (SWCNT) are investigated. Winkler-type model is used to simulate the interaction of the zigzag SWCNTs with a surrounding elastic medium. The relation between deflection amplitudes and resonant frequencies of the SWCNT is derived through harmonic balance method. The equivalent Young's modulus and shear modulus for zigzag SWCNT are derived using an energy-equivalent model. The amplitude - frequency curves for large-amplitude vibrations are graphically illustrated. The simulation results show that the chirality of zigzag carbon nanolube as well as surrounding elastic medium play more important roles in the nonlinear vibration of the single-walled carbon nanotubes.

• 한국생산제조학회지
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• 제20권3호
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• pp.268-273
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• 2011

In light guide panels, the PMMA sheet that is used in liquid crystal displays plays an important role in scattering the incident light and requires very fine machining as the sheet is directly related to the optical characteristics of the panels. High speed end milling(HSE) and high speed shaping(HSS) processes that are widely adopted and applied to the precise machining of light incident plane through vibration-assisted HSS for increasing the optical quality by minimizing the above-mentioned problems. The cutting tool and the tool post presented in this paper are designed by the authors to increase the magnitude of the cutting stroke by adopting the resonant frequency without weakening the stiffness and to reduce vibrations during even high speed feeding. The dynamic haracteristics of thecutting tool and the tool post are evaluated through simulation and experiment as well. The results reveal very appropriate dynamic characteristics for vibration-assisted HSS.

• Steel and Composite Structures
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• 제49권6호
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• pp.667-684
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• 2023

In the context of finite element method, a computational simulation is presented to study and analyze the dynamic behavior of regularly perforated functionally graded rotating beam for the first time. To investigate the effect of perforation configurations, both regular circular and squared perforation patterns are studied. To explore impacts of graded material distributions, both axial and transverse gradation profiles are considered. The material characteristics of graded materials are assumed to be smoothly and continuously varied through the axial or the thickness direction according the nonlinear power gradation law. A computational finite elements procedure is presented. The accuracy of the numerical procedure is verified and compared. Resonant frequencies, axial displacements as well as internal stress distributions throughout the perforated graded rotating cantilever beam are studied. Effects of material distributions, perforation patterns, as well as the rotating beam speed are investigated. Obtained results proved that the graded material distribution has remarkable effects on the dynamic performance. Additionally, circular perforation pattern produces more softening effect compared with squared perforation configuration thus larger values of axial displacements and maximum principal stresses are detected. Moreover, squared perforation provides smaller values of nondimensional frequency parameters at most of vibration modes compared with circular pattern.

• 한국안광학회지
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• 제6권2호
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• pp.85-97
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• 2001

눈물 층을 사이에 두고 각막 위에 부착되어 있는 렌즈(하드렌즈)에는 모세관작용에 따른 장력이 렌즈가장자리에 균일하게 방사형으로 향하여 작용한다. 순목등에 의한 충격으로 평형상태의 렌즈가 평형 위치에서 벗어나게 되면 눈물층의 간격에 변화가 발생하고 이 변화에 의해 불균일 모세관작용에 기인하는 장력에 따라 렌즈에는 복원력이 발생하고 이 힘에 의해 렌즈는 감쇄운동(진동)을 하게 된다. 이러한 복원력을 계산하고 렌즈의 운동을 예측할 수 있는 미분방정식과 컴퓨터프로그램을 수립하였으며 이 컴퓨터 모델을 사용하여 렌즈의 구경, 베이스 커브, 눈물 층의 두께 등의 변수가 렌즈의 운동에 미치는 영향을 모사(模寫)하였다. 눈물층의 점성에 의한 마찰력이 관성력에 비해 크기 때문에 렌즈는 진동을 하지 않고 시간의 경과에 따라 일률적으로 변위가 감소하는 운동양상을 나타내고 있으며 렌즈의 구경이 증가할수록, 눈물층의 두께가 얇아질수록 복원력이 증가하며 따라서 렌즈가 원위치로 되돌아오는데 걸리는 시간이 짧아지고 있다. 그러나 렌즈의 베이스커브는 그 값이 특정 값을 가질 때 원위치 도달 시간이 최소가 된다. 렌즈의 공진진동수는 눈물층의 두께가 증가할수록 렌즈구경이 감소할수록 낮아지고 있으며 베이스커브가 특정 값을 가질 때 공진진동수 역시 최대가 된다. 실제로 콘택트렌즈를 착용한 상태에서 렌즈의 공진진동수와 동일한 진동수의 외부 충격이 렌즈에 가해지는 경우 급격한 렌즈의 상하 또는 좌우 진동이 예상되며 따라서 렌즈가 탈착 된다든지 또는 렌즈의 형상변형으로 인해 각막에 통증이 발생할 수도 있을 것이다. 고함수(高含水) 소프트렌즈와 강은 diaphragm 그 자체는 탄성이 거의 없다. 그러나 함수 소프트렌즈가 각막 상에 눈물 층을 사이에 두고 부착되어 있는 경우에는 눈물의 표면 장력에 의해 탄성이 유기(誘起)될 수 있으므로 진동의 영향이 있을 것으로 본다.

• 한국지반공학회논문집
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• 제15권1호
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• pp.175-183
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• 1999

최근 우리나라도 내진설계의 필요성에 따라 지진시 등의 고변형률로부터 정밀기계기초에 미치는 미소변형률 진동까지 기초지반의 동적거동에 관한 연구가 점증하고 있는 실정이다. 한편, 화강풍화토는 국내에 가장 널리 분포되어 있는 대표적인 흙으로써 지반진동문제 해결에 필요한 지반계수 등의 동적거동 파악이 시급한 형편이나 이에 관한 연구는 극히 한정되어 있다. 본 연구는 수원지방의 화강풍화토에 대하여 간극비, 포화도가 다른 공시체를 제작하여 구속압력 및 반복응력비를 변화시켜 공진주시험과 반복삼축시험을 실시하여 불포화 화강풍화토의 동적거동을 고찰하였고 지반보강을 위하여 흔히 사용되는 흙-시멘트 혼합토에 대하여도 시멘트 혼합비와 간극비를 달리한 공시체에 대하여 같은 조건하에 동적거동을 구명하기 위한 두가지 같은 실험을 수행하였다. 연구결과 불포화 화강풍화토의 전단탄성계수는 표면장력에 지배되며 최적포화도는 간극비에 따라 공진주 시험인 경우 약 17%~l8%, 반복삼축시험인 경우 약 10%~l5% 로 나타났다. 또한, 수원지역의 화강풍화토에 대한 한계 변형률 범위와 평균전단탄성계수도 추정할 수 있었다. 한편, 일련의 공진주시험과 반복삼축시험의 결과를 토대로 하여 화강풍화토의 시멘트 혼합량에 따른 최대전단탄성계수 추정식을 비교적 조밀한 경우와 비교적 느슨한 경우로 나누어 제안하였다.

• 한국강구조학회 논문집
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• 제13권3호
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• pp.301-311
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• 2001

내풍설계에 있어서, 아스펙트비가 6정도인 초고층 건물의 각주형 단면의 바람에 의한 공력 불안정 진동 또는 갤로핑 진동이 발생할 수 있다. 본 연구에서는 등류에서 아스펙트비 6정도이고 변장비가 1/4간격으로 1에서 2까지의 초고층 건물의 각주형 단면에대한 공력불안정 진동과 갤로핑 진동에 대해 실험적으로 고찰하였다. 실험 결과, 등류시 코너 컷이 없는 경우가 코너 컷을 가진 초고층 건물의 각주형 단면에 비해 공력 불안정 진동이 커지는 경향이 있으며, 또한 갤로핑 진동도 나타났다. 따라서, 코너 컷에 의해 각주형 초고층 건물에서의 공력 불안정 진동은 효과적으로 저감되었다.

• Bulletin of the Korean Chemical Society
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• 제34권12호
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• pp.3641-3648
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• 2013

Energy transfer and bond dissociation of $C-H_{methyl}$ and $C-H_{ring}$ in excited toluene in the collision with $H_2$ and $D_2$ have been studied by use of classical trajectory procedures at 300 K. Energy lost by the vibrationally excited toluene to the ground-state $H_2/D_2$ is not large, but the amount increases with increasing vibrational excitation from 5000 and $40,000cm^{-1}$. The principal energy transfer pathway is vibration to translation (V-T) in both systems. The vibration to vibration (V-V) step is important in toluene + $D_2$, but plays a minor role in toluene + $H_2$. When the incident molecule is also vibrationally excited, toluene loses energy to $D_2$, whereas it gains energy from $H_2$ instead. The overall extent of energy loss is greater in toluene + $D_2$ than that in toluene + $H_2$. The different efficiency of the energy transfer pathways in two collisions is mainly due to the near-resonant condition between $D_2$ and C-H vibrations. Collision-induced dissociation of $C-H_{methyl}$ and $C-H_{ring}$ bonds occurs when highly excited toluene ($55,000-70,400cm^{-1}$) interacts with the ground-state $H_2/D_2$. Dissociation probabilities are low ($10^{-5}{\sim}10^{-2}$) but increase exponentially with rising vibrational excitation. Intramolecular energy flow between the excited C-H bonds occurring on a subpicosecond timescale is responsible for the bond dissociation.