• Smart Structures and Systems
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• 제9권3호
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• pp.231-251
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• 2012

The stochastic optimal control for a piezoelectric spherically symmetric shell subjected to stochastic boundary perturbations is constructed, analyzed and evaluated. The stochastic optimal control problem on the boundary stress output reduction of the piezoelectric shell subjected to stochastic boundary displacement perturbations is presented. The electric potential integral as a function of displacement is obtained to convert the differential equations for the piezoelectric shell with electrical and mechanical coupling into the equation only for displacement. The displacement transformation is constructed to convert the stochastic boundary conditions into homogeneous ones, and the transformed displacement is expanded in space to convert further the partial differential equation for displacement into ordinary differential equations by using the Galerkin method. Then the stochastic optimal control problem of the piezoelectric shell in partial differential equations is transformed into that of the multi-degree-of-freedom system. The optimal control law for electric potential is determined according to the stochastic dynamical programming principle. The frequency-response function matrix, power spectral density matrix and correlation function matrix of the controlled system response are derived based on the theory of random vibration. The expressions of mean-square stress, displacement and electric potential of the controlled piezoelectric shell are finally obtained to evaluate the control effectiveness. Numerical results are given to illustrate the high relative reduction in the root-mean-square boundary stress of the piezoelectric shell subjected to stochastic boundary displacement perturbations by the optimal electric potential control.

• Earthquakes and Structures
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• 제18권1호
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• pp.27-44
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• 2020

This work presents a comparison of three performance-based seismic design methods (PBSD) as applied to plane steel frames having eccentric braces (EBFs) and buckling restrained braces (BRBFs). The first method uses equivalent modal damping ratios (ξ_k), referring to an equivalent multi-degree-of-freedom (MDOF) linear system, which retains the mass, the elastic stiffness and responds in the same way as the original non-linear MDOF system. The second method employs modal strength reduction factors (${\bar{q}}_k$) resulting from the corresponding modal damping ratios. Contrary to the behavior factors of code based design methods, both ξ_k and ${\bar{q}}_k$ account for the first few modes of significance and incorporate target deformation metrics like inter-storey drift ratio (IDR) and local ductility as well as structural characteristics like structural natural period, and soil types. Explicit empirical expressions of ξ_k and ${\bar{q}}_k$, recently presented by the present authors elsewhere, are also provided here for reasons of completeness and easy reference. The third method, developed here by the authors, is based on a hybrid force/displacement (HFD) seismic design scheme, since it combines the force-base design (FBD) method with the displacement-based design (DBD) method. According to this method, seismic design is accomplished by using a behavior factor (q_h), empirically expressed in terms of the global ductility of the frame, which takes into account both non-structural and structural deformation metrics. These expressions for q_h are obtained through extensive parametric studies involving non-linear dynamic analysis (NLDA) of 98 frames, subjected to 100 far-fault ground motions that correspond to four soil types of Eurocode 8. Furthermore, these factors can be used in conjunction with an elastic acceleration design spectrum for seismic design purposes. Finally, a comparison among the above three seismic design methods and the Eurocode 8 method is conducted with the aid of non-linear dynamic analyses via representative numerical examples, involving plane steel EBFs and BRBFs.

• Structural Engineering and Mechanics
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• 제24권3호
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• pp.375-393
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• 2006

Under high velocity, pulse type near source earthquakes semi-active control systems are very effective in reducing seismic response base isolated structures. Semi-active control systems can be classified as: 1) independently variable stiffness, 2) independently variable damping, and 3) combined variable stiffness and damping systems. Several researchers have studied the effectiveness of independently varying damping systems for seismic response reduction of base isolated structures. In this study effectiveness of a combined system consisting of a semi-active independently variable stiffness (SAIVS) device and a magnetorheological (MR) damper in reducing seismic response of base isolated structures is analytically investigated. The SAIVS device can vary the stiffness, and hence the period, of the isolation system; whereas, the MR damper enhances the energy dissipation characteristics of the isolation system. Two separate control algorithms, i.e., a nonlinear tangential stiffness moving average control algorithm for smooth switching of the SAIVS device and a Lyapunov based control algorithm for damping variation of MR damper, are developed. Single and multi degree of freedom systems consisting of sliding base isolation system and both the SAIVS device and MR damper are considered. Results are presented in the form of nonlinear response spectra, and effectiveness of combined variable stiffness and variable damping system in reducing seismic response of sliding base isolated structures is evaluated. It is shown that the combined variable stiffness and variable damping system leads to significant response reduction over cases with variable stiffness or variable damping systems acting independently, over a broad period range.

• 한국공간구조학회논문집
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• 제6권3호
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• pp.59-67
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• 2006

수동동조질량감쇠기의 진동제어 효과는 질량감쇠기와 구조물과의 동조로 인하여 나타나고 있다. 그러나 바닥판 구조물의 질량과 강성의 변화로 인하여 실제 구조물에 있어서 비동조 현상이 일어나기도 한다. 이러한 상황에서는 수동동조질량감쇠기의 성능이 비효율적이며 경우에 따라서는 구조물의 진동을 증가시키기도 한다. 본 논문에서는 기계나 사람에 의한 바닥판 구조물의 진동을 줄이기 위하여 자기유체감쇠기의 적용성을 알아보고자 한다. 준 능동제어와 groundhook 제어 알고리즘을 적용하여 준능동감쇠기의 성능과 수동질량감쇠기의 성능을 비교 분석하였다. 또한 비동조 상황에서의 준능동감쇠가와 수동질량감쇠기의 견인성을 비교 분석하여 다음과 같은 결론을 얻었다. 자기유체감쇠기는 동조상황에서 우수한 제어 성능을 가지고 있으며 비동조 상황에서도 우수한 견인성을 보여주고 있다.

• 한국공간구조학회논문집
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• 제7권2호
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• pp.53-61
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• 2007

경간이 길어지고 강성이 유연한 바닥판 구조물은 처짐과 진동에 같은 사용성에 있어서 많은 문제점을 가지고 있다. 따라서 다양한 진동제어 시스템이 제공되고 있으며 TMD와 같은 수동제어 시스템은 적용에 있어서 한계가 있다. 본 논문에서는 MR감쇠기와 TMD를 조합한 준능동 TMD의 제어기법에 따른 제어성능을 알아보았다. 준능동 TMD의 감쇠기를 Groundhook 모델로 모형화한 경우에 주구조물인 바닥판 구조물의 진동제어에 있어서 보다 효과적인 것을 볼 수 있으나 TMD의 변위를 제어해야 하는 경우에 준능동 TMD의 감쇠기를 Skyhook 모델로 모형화하여 진동을 제어할 필요가 있다. 그리고 바닥판 구조물과 TMD를 동시에 제어해야하는 경우에는 Hybrid 제어기법이 우수한 제어성능을 보이고 있다.

• Investigative Magnetic Resonance Imaging
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• 제3권2호
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• pp.188-195
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• 1999

fMRI의 신호는 매우 다양한 종류의 선호들이 혼합된 상태이며 , 비록 몇 가지의 요소에 대해 모델링하여 그 선호 형태를 추측할 수 있으나 모든 신호를 정확하게 분리하여 뇌신경의 활성화를 반영하는 신호만을 선택적으로 알아 내기는 어려운 일이다. 또한 뇌와 신체의 생리적 현상으로 발생하는 잡음뿐아니라 움직임이나 계기의 잡음은 fMRl의 데이터 분석을더욱 어렵게 한다. 따라서 실제 뇌신경의 활성화를 정확히 나타내는 참고데이터(reference data)를 선택하는 것은 힘든 일이며, 뇌신경의 활성화를 반영하는 의미 있는 여러 신호 형태에 대한 분석은 현재 fMRl의 후처리 (post-processing) 분석 방법에서 하나의 연구 과제라 할 수 있다. 본 연구에서는 prioriknow­-ledge 혹은 참고 데이터가 필요 없는 분석 방법인 Independent Component Analysis (lCA) 를 이용하여 fMRI선호를 분석하였다. ICA는 현재 많이 사용되고 있는 상관 분석 방법에 비해 신호의 형태를 분석하는 데에 보다 효과적일 수 있으며, 지연된 반응 형태를 갖는 신호나 움직임에 의한 신호의 패턴을 분리하여 분석할 수 있다. 한편, ICA만으후 fMRl의 신호에 따라 분석이 효과적이지 못한 경우 Principal Component Analysis(PCA) threshold, wavelet spatial f filtering, 부분적 영상 분석 방법들을 ICA전에 수행 함으로써 보다 효과적인 분석을 수행할 수 있다. ICA는 fMRl 신호의 형태 분석에 효과적인 방법이라고 생각하며, 데이터의 자유도를 감소 하기 위해서는 선 필터링 (pre-filtering) 방법들이 적용될 수 있다.

• Korean Membrane Journal
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• 제5권1호
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• pp.43-53
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• 2003

The selectivity of a gas in the carbon molecular sieve membrane (CMSM) can be expressed as the ratio of the product of the diffusivity and the solubility of two different gases. The diffusivity is also expressed as the product of the entropy and the total energy (kinetic and potential energy) in the nano-sized pore of the membrane. The present study calculates the entropic-energy and selectivity of penetrant gases such as H$_2$, O$_2$, N$_2$, and CO$_2$ from the gas-in-a box theory to physically analyze the diffusivity of penetrant gas in slit-shaped pore of CMSM focusing on the restriction of gas motion based on the size difference between penetrant gas pairs. The contribution of each energy term is converted to entropic term separately. By the conjugated calculation for each entropic-energy, the entropic effects on diffusivity-selectivity for gas pairs such as H$_2$/N$_2$, CO$_2$/N$_2$, and O$_2$/N$_2$ were analyzed within active pore of CMSM. In the activated diffusion domain, the calculated value of entropic-selectivity lies between 9.25 and 111.6 for H$_2$/N$_2$, between 3.36 and 6.0 for CO$_2$/N$_2$, and between 1.25 and 16.94 for O$_2$/N$_2$, respectively. The size decrement of active pore in CMSM had the direct effect on the reduction of translational entropic-energy and the contribution of vibrational entropic-energy for N$_2$, O$_2$, and H$_2$ was almost negligible. However, the vibrational entropic term of CO$_2$ might extravagantly affect on the entropic-selectivity.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2000년도 추계학술대회 논문집
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• pp.189-195
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• 2000

New concept of LNG-FPSO ship with moonpool and bilge step in bottom is considered and investigated in the point of motion reduction and sloshing phenomena of the cargo and operation tanks. The cargo capacity of the ship of which principle dimensions is L x B x D x t(design) =270.0 x 51.0 x 32.32 x 13.7(m) 16K at 98% loading condition. The two moonpools and rectangular step at bilge part are setted up specially for getting the effect of motion decrease. For the motion analysis, linearized three dimensional diffraction theory with the simplified boundary conditions is used. The six-degree of freedom coupled motion responses are calculated for the LNG-FPSO ship. Viscous effects on the roll motion responses of a vessel are taken into account in this calculation program using an empirical formula suggested by Ikeda, Himeno and Tanaka is used. The case study for the moonpool size had been carried out by theoretical estimation and experimental method. For the optimization of the moonpool size and effect of the step, 9 cases of its size and with and without step are considered. From the results of calculation and experiment, it can be concluded that this designed LNG-FPSO ship have possibility to carry out her missions in the rough sea as for the owner's demand waves condition. The motion responses, especially roll motion, for the designed LNG-FPSO ship are much lower than those of another drillship and shuttle tanker and limit criterions are satisfied. For the check of the cargo tank and operation tank sizes we have performed sloshing analysis in the irregular waves which focuses on the pressure distribution on the tank wall and the time history of pressure and free surface for No.2 and No5. tanks of LNG-FPSO with chamfers. Finally we got the tank size which has no resonance and no impact pressure in all filling in the bow quartering and beam sea.

• 한국소음진동공학회논문집
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• 제13권5호
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• pp.375-383
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• 2003

For a performance analysis of vibration isolation systems, the concept of vibration power flow can be employed preferably when noise radiated from the supporting structure with finite impedances is of interest. The idea is basically simple to understand and formulas for precise estimation of the vibration power are easy to derive. However, It is often required to simplify the process of experimentation under several assumptions due to instrumental limitations. For an example, rotational degree of freedom has not been well treated in bending vibrations of beam or plate-like structures. Yet, several recent studies showed that the moments and rotations play an important role in power transmission and should be taken into consideration carefully as the frequency range of interest goes to audibly high. Therefore, it is readily agreed that reduction of the noise radiation over the high frequency range can be effectively accomplished by adjusting the rotational stiffness of the isolator without changing the vibration isolator efficiency in low frequency range relevant to the translational stiffness of the isolator In this paper, the vibration power flow approach is applied to an AC motor installed on a finite plate in order to illustrate the contribution of the rotational vibration power to the total vibration power transmission. The effects of rotational stiffness of the isolator on the vibration power transmission are investigated by inserting various shapes of Isolators with different rotational stiffness but with $ame translational stiffness between the motor and the plate. The resultant noise radiation from the plate is presented to verify the proposed approach.

• 한국해안해양공학회지
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• 제5권1호
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• pp.1-10
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• 1993

1차 자유도를 갖는 해양구조물을 선정하여 long crested, stationary, Gaussian random waves가 구조물의 동적응답 스펙트럼에 미치는 영향에 대해 규명하였다. 해수면 부근에서 주기적으로 수면위로 부상하는 연직부재를 고려하기 위해 hybrid Morison equation을 제안했고 이 방정식을 기초로 파력을 계산했다. 주기적으로 수면위로 부상하는 효과를 고려한 결과 동적응답 스펙트럼은 감소하였으며 고차 응당성분이 발생했다. 또한 동적응답 스펙트럼은 주파수가 증가할수록 감소하는 경향을 보였으며 이는 해수면 위로 주기적으로 부상하는 현상을 고려할 때 발생하는 고차응답성분 때문인 것으로 사료된다.