• Title/Summary/Keyword: Linear damping

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Enhancement of wave-energy-conversion efficiency of a single power buoy with inner dynamic system by intentional mismatching strategy

  • Cho, I.H.;Kim, M.H.
    • Ocean Systems Engineering
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    • v.3 no.3
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    • pp.203-217
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    • 2013
  • A PTO (power-take-off) mechanism by using relative heave motions between a floating buoy and its inner mass (magnet or amateur) is suggested. The inner power take-off system is characterized by a mass with linear stiffness and damping. A vertical truncated cylinder is selected as a buoy and a special station-keeping system is proposed to minimize pitch motions while not affecting heave motions. By numerical examples, it is seen that the maximum power can actually be obtained at the optimal spring and damper condition, as predicted by the developed WEC(wave energy converter) theory. Then, based on the developed theory, several design strategies are proposed to further enhance the maximum PTO, which includes the intentional mismatching among heave natural frequency of the buoy, natural frequency of the inner dynamic system, and peak frequency of input wave spectrum. By using the intentional mismatching strategy, the generated power is actually increased and the required damping value is significantly reduced, which is a big advantage in designing the proposed WEC with practical inner LEG (linear electric generator) system.

Development of equivalent linear algorithm procedure that accounts for the loading frequency dependent soil behavior (하중의 주파수에 지배받는 흙의 동적거동을 고려하는 등가선형해석방법 개발)

  • Park, Du-Hee;Lee, Hyun-Woo;Lee, Seung-Chan;Kim, Jae-Yeon;Chun, Byung-Sik
    • Proceedings of the Korean Geotechical Society Conference
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    • 2006.03a
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    • pp.617-624
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    • 2006
  • Site response analysis is widely used in estimating local seismic site effects. The soil behavior in the analysis is assumed to be Independent of the rate of the seismic loading laboratory results, however, indicate that cohesive soil behavior is greatly influenced by the rate of loading. A new equivalent linear analysis method is developed that accounts for the rate-dependence of soil behavior and used to perform a series of one dimensional site response analyses. Results indicate that while rate-dependent shear modulus has limited influence on computed site response, rate-dependent soil damping greatly filters out high frequency components of the ground motion and thus results in lower response.

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An Identification of Dynamic Characteristics by Spectral Analysis Technique of Linear Autoregressive Model Using Lattice Filter (Lattice Filter 이용한 선형 AR 모델의 스펙트럼 분석기법에 의한 동특성 해석)

  • Lee, Tae-Yeon;Shin, Jun;Oh, Jae-Eung
    • Journal of the Korean Society of Safety
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    • v.7 no.2
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    • pp.71-79
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    • 1992
  • This paper presents a least-square algorithms of lattice structures and their use for adaptive prediction of time series generated from the dynamic system. As the view point of adaptive prediction, a new method of Identification of dynamic characteristics by means of estimating the parameters of linear auto regressive model is proposed. The fast convergence of adaptive lattice algorithms is seen to be due to the orthogonalization and decoupling properties of the lattice. The superiority of the least-square lattice is verified by computer simulation, then predictor coefficients are computed from the linear sequential time data. For the application to the dynamic characteristic analysis of unknown system, the transfer function of ideal system represented in frquency domain and the estimated one obtained by predicted coefficients are compared. Using the proposed method, the damping ratio and the natural frequency of a dynamic structure subjected to random excitations can be estimated. It is expected that this method will be widely applicable to other technical dynamic problem in which estimation of damping ratio and fundamental vibration modes are required.

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Effects of Forward Speed on the Linear and Nonlinear Hydrodynamic Forces Acting on Advancing Submerged Cylinders in Oscillation (동요(動搖)하는 2차원몰수체(次元沒水體)에 작용(作用)하는 선형(線形) 및 비선형(非線形) 동유체력(動流體力)에 미치는 전진속도(前進速度)의 영향(影響))

  • J.H.,Hwang;Y.J.,Kim;S.S.,Lee
    • Bulletin of the Society of Naval Architects of Korea
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    • v.24 no.2
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    • pp.47-54
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    • 1987
  • Linear and nonlinear hydrodynamic force, which acts on submerged circular and eilliptic cylinders in oscillations as well as in advancing motion, are investigated as an initial-boundary value problem using a numerical method, which makes use of the source distribution on the body surface and the spectral method for treating the free surface waves. In the numerical code developed here, the boundary condition at the body surface is linearized. Using the numerical code so attained, nonlinear effects for different forward speeds and of the large-amplitude motion are computed. One of the major findings is that, when the forward speed is large, the added mass has its minimum and the damping force change rapidly around the frequency corresponding to the speed-frequency parameter, $\tau$=0.25, Compared to the result of Grue's [10], who used linear theory to get abrupt changes in values of the added mass and the damping force at the frequency corresponding to $\tau$=0.25, the present study, which takes nonlinear effects into account, shows much smoother variations near the frequency.

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A new practical equivalent linear model for estimating seismic hysteretic energy demand of bilinear systems

  • Samimifar, Maryam;Massumi, Ali;Moghadam, Abdolreza S.
    • Structural Engineering and Mechanics
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    • v.70 no.3
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    • pp.289-301
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    • 2019
  • Hysteretic energy is defined as energy dissipated through inelastic deformations during a ground motion by the system. It includes frequency content and duration of ground motion as two remarkable parameters, while these characteristics are not seen in displacement spectrum. Since maximum displacement individually cannot be the appropriate criterion for damage assessment, hysteretic energy has been evaluated in this research as a more comprehensive seismic demand parameter. An innovative methodology has been proposed to establish a new equivalent linear model to estimate hysteretic energy spectrum for bilinear SDOF models under two different sets of earthquake excitations. Error minimization has been defined in the space of equivalent linearization concept, which resulted in equivalent damping and equivalent period as representative parameters of the linear model. Nonlinear regression analysis was carried out for predicting these equivalent parameter as a function of ductility. The results also indicate differences between seismic demand characteristics of far-field and near-field ground motions, which are not identified by most of previous equations presented for predicting seismic energy. The main advantage of the proposed model is its independency on parameters related to earthquake and response characteristics, which has led to more efficiency as well as simplicity. The capability of providing a practical energy based seismic performance evaluation is another outstanding feature of the proposed model.

Non linear vibrations of stepped beam system under different boundary conditions

  • Ozkaya, E.;Tekin, A.
    • Structural Engineering and Mechanics
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    • v.27 no.3
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    • pp.333-345
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    • 2007
  • In this study, the nonlinear vibrations of stepped beams having different boundary conditions were investigated. The equations of motions were obtained using Hamilton's principle and made non dimensional. The stretching effect induced non-linear terms to the equations. Forcing and damping terms were also included in the equations. The dimensionless equations were solved for six different set of boundary conditions. A perturbation method was applied to the equations of motions. The first terms of the perturbation series lead to the linear problem. Natural frequencies for the linear problem were calculated exactly for different boundary conditions. Second order non-linear terms of the perturbation series behave as corrections to the linear problem. Amplitude and phase modulation equations were obtained. Non-linear free and forced vibrations were investigated in detail. The effects of the position and magnitude of the step, as well as effects of different boundary conditions on the vibrations, were determined.

Effects of Nonlinear Soil Characteristics on the Dynamic Stiffnesses of a Foundation-Soil System Excited with the Horizontal Motion (비선형 지반특성이 수평 방향운동을 받는 기초지반체계의 동적강성에 미치는 영향)

  • 김용석
    • Proceedings of the Earthquake Engineering Society of Korea Conference
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    • 2000.04a
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    • pp.120-129
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    • 2000
  • As structure-soil interaction analysis for the seismic analysis of structures requires a nonlinear analysis of a structure-soil system considering the inelastic characteristics of soil layers nonlinear analyses of the foundation-soil system with the horizontal excitation were performed considering the nonlinear soil conditions for the nonlinear seismic analysis of structures. Stiff soil profile of SD and soft soil profile of SE specified in UBC were considered for the soil layers of a foundation and Ramberg-Osgood model was assumed for the nonlinear characteristics of soil layers. Studies on the changes of dynamci stiffnesses and damping rations of surface and embedded foundations depending on foundation size soil layer depth and piles were performed to investigate the effects of the nonlinear soil layer on the horizontal and rotational dynamic stiffnesses and damping ratios of the foundation-soil system According to the study results nonlinear prperties of a soil laryer decreeased horizontal and rotational linear stiffnesses and increased damping ratios largely Effects of foundation size soil layer depth and piles were also significant suggesting the necessity of nonlinear seismic analyses of structures.

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A Study on Optimal Multi-dynamic Absorber of Damped Linear Vibration System under the Harmonic Motion of the Base (기반의 조화운동을 받는 감쇠선형진동계의 최적 복합동흡진기에 관한 연구)

  • 안찬우;김동영;홍도관
    • Journal of KSNVE
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    • v.10 no.2
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    • pp.325-330
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    • 2000
  • A dynamic absorber is used to protect the primary vibration system under the steady-state harmonic disturbance. In a number of cases it appears expedient to install several absorbers of smaller masses instead of one. This may be due to the need of distribute the absorber's response along the construction, restrictions on the absorber's installation. So, we studied characteristics of the primary vibration system for the optimal natural frequency ratio and the optimal damping ratio of serial multi-dynamic absorber. Also we obtained the optimum values of the serial multi-dynamic absorber parameters using computer simulation for the damped primary vibration system. In designing multi-dynamic absorber, we presented for the optimal natural frequency and the optimal damping ratio of multi-dynamic absorbers.

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Vibration Control of MR Suspension System Considering Damping Force Hysteresis (댐핑력 히스테리시스를 고려한 MR 서스펜션의 진동제어)

  • Seong, Min-Sang;Sung, Kum-Gil;Han, Young-Min;Choi, Seung-Bok;Lee, Ho-Guen
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • 2007.11a
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    • pp.381-386
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    • 2007
  • This paper presents vibration control performances of a commercial magnetorheological (MR) suspension via new control strategy considering hysteresis of the field-dependent damping force of MR damper. A commercial MR damper which is applicable to high class passenger vehicle is adopted and its field-dependent damping force is experimentally evaluated. Preisach hysteresis model for the MR damper is identified using experimental first order descending (FOD) curves. Then, a feed-forward compensation strategy for the MR damper is formulated and integrated with a linear quadratic regulation (LQR) feedback controller for the suspension system. Control performances of the proposed control strategy for the MR suspension is experimentally evaluated with quarter vehicle test facility.

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Vibration Control of MR Suspension System Considering Damping Force Hysteresis (댐핑력 히스테리시스를 고려한 MR 서스펜션의 진동제어)

  • Seong, Min-Sang;Sung, Kum-Gil;Han, Young-Min;Choi, Seung-Bok;Lee, Ho-Guen
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.18 no.3
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    • pp.315-322
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    • 2008
  • This paper presents vibration control performances of a commercial magnetorheological(MR) suspension via new control strategy considering hysteresis of the field-dependent damping force of MR damper. A commercial MR damper which is applicable to high class passenger vehicle is adopted and its field-dependent damping force is experimentally evaluated. Preisach hysteresis model for the MR damper is identified using experimental first order descending(FOD) curves. Then, a feed-forward compensation strategy for the MR damper is formulated and integrated with a linear quadratic regulation(LQR) feedback controller for the suspension system. Control performances of the proposed control strategy for the MR suspension is experimentally evaluated with quarter vehicle test facility.