• International Journal of Naval Architecture and Ocean Engineering
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• 제5권4호
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• pp.513-528
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• 2013

A floating Oscillating Water Column (OWC) wave energy converter, a Backward Bent Duct Buoy (BBDB), was simulated using a state-of-the-art, two-dimensional, fully-nonlinear Numerical Wave Tank (NWT) technique. The hydrodynamic performance of the floating OWC device was evaluated in the time domain. The acceleration potential method, with a full-updated kernel matrix calculation associated with a mode decomposition scheme, was implemented to obtain accurate estimates of the hydrodynamic force and displacement of a freely floating BBDB. The developed NWT was based on the potential theory and the boundary element method with constant panels on the boundaries. The mixed Eulerian-Lagrangian (MEL) approach was employed to capture the nonlinear free surfaces inside the chamber that interacted with a pneumatic pressure, induced by the time-varying airflow velocity at the air duct. A special viscous damping was applied to the chamber free surface to represent the viscous energy loss due to the BBDB's shape and motions. The viscous damping coefficient was properly selected using a comparison of the experimental data. The calculated surface elevation, inside and outside the chamber, with a tuned viscous damping correlated reasonably well with the experimental data for various incident wave conditions. The conservation of the total wave energy in the computational domain was confirmed over the entire range of wave frequencies.

• Structural Engineering and Mechanics
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• 제5권2호
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• pp.177-191
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• 1997

In the paper, a fractional derivative Kelvin-Voigt model describing the dynamic behavior of a special class of fluid viscous dampers, is presented. First of all, in order to verify their mechanical properties, two devices were tested the former behaving as a pure damper (PD device), whereas the latter as an elastic-damping device (ED device). For both, quasi-static and dynamic tests were carried out under imposed displacement control. Secondarily, in order to describe their cyclical behavior, a model composed by an elastic and a damping element connected in parallel was defined. The elastic force was assumed as a linear function of the displacement whereas the damping one was expressed by a fractional derivative of the displacement. By setting an appropriate numerical algorithm, the model parameters (fractional derivative order, damping coefficient and elastic stiffness) were identified by experimental results. The estimated values allowed to outline the main parameter properties on which depend both the elastic as well as the damping behavior of the considered devices.

• 소음진동
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• 제7권5호
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• pp.861-869
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• 1997

An approximate analytical method has been developed for estimating hydrodynamic forces acting on oscillating inner cylinder in concentric annulus. When the rigid inner cylinder executes translational oscillation, fluid inertia and damping forces on the oscillating cylinder are generated by unsteady pressure and viscous skin friction. Considering the dynamic-characteristics of unsteady viscous flow and the added mass coefficient of inviscid fluid, these hydrodynamic forces including viscous effect are dramatically simplified and expressed in terms of oscillatory Reynolds number and the geometry of annular configuration. Thus, the viscous effect on the forces can be estimated very easily compared to an existing theory. The forces are calculated by two models developed for relatively high and low oscillatory Reynolds numbers. The model for low oscillatory Reynolds number is suitable for relatively high ratio of the penetration depth to annular space while the model for high oscillatory Reynolds number is applicable to the case of relatively low ratio. It is found that the transient ratio between two models is approximately 0.2~0.25 and the forcea are expressed in terms of oscillatory Reynolds number, explicity. The present results show good agreements with an existing numerical results, especially for high and low penetration ratios to annular gap.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2008년도 춘계학술대회논문집
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• pp.250-254
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• 2008

Eddy currents are induced when a nonmagnetic conductive material is moving subjected to the magnetic field due to a permanent magnet. These currents circulates in the conductive material and are dissipated, causing a repulsive force between the magnet and the conductor. Using this concept, the eddy current damping can be used as a viscous damping. The present study investigates the characteristics of a magnetic damping analytically and experimentally. The theoretical model of a eddy current damping is developed from electromagnetics and is verified from experiments. The drop test of a magnet in the cooper tube shows that the present model can accurately predict the damping force. Additionally, the dynamic test of a eddy current damping is carried to verify the present model.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2000년도 춘계학술대회 논문집
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• pp.7-12
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• 2000

As the offshore oil fields are moved to the deep ocean, the oil production systems of FPSO(Floating production storage and offloading system) are building these days and so it is the most important to estimate the drift motion and damping effects the drift motion importantly. The components of damping consist of viscous, wave radiation effect and wave drift damping. It is need to estimate the wave drift damping exactly among them. The wave drift damping means the change rate of mean wave drift force with respect to the ship and ocean structures speed. In order to calculate this, the 3-Dimensional panel method used to translating and pulsating Green function is adopted. The calculation is carried out for series 60(CB = 0.7) vessel and the results are compared with other theoretical ones.

• 한국해양공학회지
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• 제14권4호
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• pp.17-22
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• 2000

As the offshore oil fields are moved to the deep ocean, the oil production system of FPSO(Floating Production Storage and Offloading System) Type are constructed frequently these days. So, it is very important to estimate the drift motion and damping effects due to the drift motion simultaneously. The components of slow drift motion damping consist of viscous, wave radiation effect and wave drift damping. It is needed to estimate the wave drift damping more accurately than others. The wave drift damping signifies the time-rate of mean wave drift force on oscillating ship or ocean structure which constant speed. In order to calculate this, the 3-Dimensional panel method is employed with the translating and pulsating Green function in the frequency domain. The calculation is carried out for a Series 60 ($C_B$/=0.7) and the results are compared with other numerical ones.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 춘계학술대회논문집
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• pp.947-951
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• 2007

This paper is concerned with a new concept for the damper without neither a coil spring nor fluid. The new damper concept consists of the permanent magnets and the cylinder of the conducting material. The opposite pole magnets produces the repulsive forces and this is substituted for the coil spring. The relative motion between the magnets and conducting cylinder produces eddy currents thus resulting in the electromagnetic force, which turns out to be the damping force thus and is substituted for a damping fluid. This damper is called the eddy current damper(ECD). The important advantage of the proposed ECD is that it does not require the damping fluid and any external power and is non-contacting and relatively insensitive to temperature. In the present study, the proposed ECD was constructed and experiments were performed to investigate its dynamic characteristics. The experiments shows that the proposed ECD has the excellent damping ability.

• Structural Engineering and Mechanics
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• 제33권6호
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• pp.709-724
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• 2009

In this paper a free-free uniform beam with damping effects subjected to follower and transversal forces at its end is considered as a model for a space structure. The effect of damping on the stability of the system is first investigated and the effects of the follower and transversal forces on the vibration of the beam are shown next. Proportional damping model is used in this work, hence, the effects of both internal (material) and external (viscous fluid) damping on the system are noted. In order to derive the frequency of the system, the Ritz method has been used. The mode shapes of the system must therefore be extracted. The Newmark method is utilized in the study of the system vibration. The results show that an increase in the follower and transversal forces leads to an increase of the vibrational motion of the beam which is not desirable.

• 한국가스학회지
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• 제12권4호
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• pp.46-51
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• 2008

본 연구에서는 비선형 디스크 스프링과 고무링으로 구성된 완충장치의 감쇠에 관하여 연구하였다. 고무링의 마찰력과 디스크 스프링의 이력현상을 측정하여, 디스크 스프링의 적층 배열에 따른 이력곡선을 근사하고, 소산되는 에너지의 양을 추산하였다. 마찰력과 소산 에너지에 근거하여 네 종류의 감쇠 해석모델을 제시하였으며, 각각의 모델에 대한 충격응답을 고찰하였다. 고무링의 마찰력보다는 디스크 스프링의 이력현상이 완충장치의 감쇠거동에서 더 큰 영향을 미치었다. 가장 실용적인 감쇠 모델로는 소산되는 총 에너지의 양에 근거한 등가 점성 감쇠 모델이 다른 감쇠 모델보다 적합하다는 결론을 얻었다.

• 한국기계가공학회지
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• 제13권6호
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• pp.82-87
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• 2014

Small MR dampers with a simple structure were designed and manufactured. The Bingham model was used to represent the dynamic characteristics of the damper, and the parameters of the model were estimated from experimental data which were obtained by harmonic tests. The value of the estimated yield shear force remains positive when no electric current is applied, and it increases slowly with the current. The estimated viscous damping coefficient has a value close to zero when no electric current is applied, and it increases almost linearly with the current.