• Title/Summary/Keyword: Damping depth

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Depth Scaling Strategy Using a Flexible Damping Factor forFrequency-Domain Elastic Full Waveform Inversion

  • Oh, Ju-Won;Kim, Shin-Woong;Min, Dong-Joo;Moon, Seok-Joon;Hwang, Jong-Ha
    • Journal of the Korean earth science society
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    • v.37 no.5
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    • pp.277-285
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    • 2016
  • We introduce a depth scaling strategy to improve the accuracy of frequency-domain elastic full waveform inversion (FWI) using the new pseudo-Hessian matrix for seismic data without low-frequency components. The depth scaling strategy is based on the fact that the damping factor in the Levenberg-Marquardt method controls the energy concentration in the gradient. In other words, a large damping factor makes the Levenberg-Marquardt method similar to the steepest-descent method, by which shallow structures are mainly recovered. With a small damping factor, the Levenberg-Marquardt method becomes similar to the Gauss-Newton methods by which we can resolve deep structures as well as shallow structures. In our depth scaling strategy, a large damping factor is used in the early stage and then decreases automatically with the trend of error as the iteration goes on. With the depth scaling strategy, we can gradually move the parameter-searching region from shallow to deep parts. This flexible damping factor plays a role in retarding the model parameter update for shallow parts and mainly inverting deeper parts in the later stage of inversion. By doing so, we can improve deep parts in inversion results. The depth scaling strategy is applied to synthetic data without lowfrequency components for a modified version of the SEG/EAGE overthrust model. Numerical examples show that the flexible damping factor yields better results than the constant damping factor when reliable low-frequency components are missing.

Hydrodynamic Forces Characteristics of a Circular Cylinder with a Damping Plate (감쇠판이 부착된 원기둥의 동유체력 특성)

  • Cho, Il-Hyoung
    • Journal of Ocean Engineering and Technology
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    • v.25 no.1
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    • pp.1-7
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    • 2011
  • The radiation of water waves by a heaving truncated circular cylinder with damping plate is solved in the frame of the three-dimensional linear potential theory. The damping plate has a distinct advantage in reducing the motion response of a floating circular cylinder by increasing the added mass and the damping coefficient. Using the matched eigenfunction expansion method, the characteristics of hydrodynamic added mass and the damping coefficient are investigated with various system parameters, such as the radius and submergence depth of the damping plate. It is found that both added mass and the damping coefficient are significantly increased due to the arranged features of the larger damping plate with shallow submergence, which are positive factors as a motion reduction device of the floating offshore platform. Also the numerical results for an oscillating submerged disk show that the added mass is negative and that the damping coefficient has a peak value at resonant frequency when submergence depth is sufficiently small.

Wave Damping Rate Over Multi-layer Permeable Bed of Finite Depth (깊이가 유한한 다중 투수층 위에서의 파의 감쇠율)

  • Suh, Kyung-Duck;Do, Ki-Deok
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.21 no.2
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    • pp.127-135
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    • 2009
  • Reid and Kajiura(1957) has studied on the wave damping rate over a permeable bed of infinite depth. In this study, wave damping rate over a permeable bed of finite depth is derived by linear wave theory. It is then extended to derive wave damping rates over a double or triple layer, each of which consist of different material. Applying the wave damping rate to the mild slope equation, the wave transmission coefficient over a permeable bed has been calculated. The model has been certificated by comparing with the result of Flaten and Rygg(1991)'s integral equation method in the case of a single-layer bed.

A study on performance assessment of WEC rotor in the Jeju western waters

  • Poguluri, Sunny Kumar;Bae, Yoon Hyeok
    • Ocean Systems Engineering
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    • v.8 no.4
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    • pp.361-380
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    • 2018
  • The dynamic performance of the wave energy converter (WEC) rotor with different geometric parameters such as depth of submergence and beak angle has been assessed by considering the linear potential flow theory using WAMIT solver and along with the computational fluid dynamics (CFD). The effect of viscous damping is incorporated by conducting numerical free decay test using CFD. The hydrodynamic coefficients obtained from the WAMIT, viscous damping from the CFD and estimated PTO damping are used to solve the equation of motion to obtain the final pitch response, mean optimal power and capture width. The viscous damping is almost 0.9 to 4.6 times when compared to the actual damping. It is observed that by neglecting the viscous damping the pitch response and power are overestimated when compared to the without viscous damping. The performance of the pitch WEC rotor in the Jeju western coast at the Chagwido is analyzed using Joint North Sea Wave Project (JONSWAP) spectrum and square-root of average extracted power is obtained. The performance of WEC rotor with depth of submergence 2.8 m and beak angle $60^{\circ}$ found to be good compared to the other rotors.

The Effect of Forebody Forms on the Ship Motion in Water of Finite Depth (유한(有限)깊이의 물에서의 선체운동(船體運動) -선수선형(船首船型)의 영향(影響)-)

  • J.H.,Hwang;K.P.,Rhee
    • Bulletin of the Society of Naval Architects of Korea
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    • v.13 no.1
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    • pp.11-16
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    • 1976
  • The effect of the bow shape on the ship motion response in longitudinal regular waves of water of finite depth is investigated by employing the strip theory. The two-dimensional hydrodynamic forces(added mass and damping) were calculated by close-fit method for water of finite depth. The models for investigation are U and V bow ship forms of block coefficient 0.8 with constant after body which were used by Yourkov [2] and recently by Kim [3] for their deep water investigations. The following results are obtained by the present numerical experiments. (1) It is confirmed that the damping coefficient of the V-bow ship is greater than that of U-bow ship and in consquence the amplitude of heave and pitch of V-bow ship is smaller than that of U-bow ship among longitudinal regular head waves in water of finite depth (2) The merit of the V-bow ship on the motion damping is more significant in heave than in pitch, and is decreasing with the shallowness of water depth. (3) The change of bow form gives little effect on the wave exciting force and moment compared with the motion responce.

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Damping of Water Waves over Permeable Bed of Finite Depth (유한한 깊이의 투수층에 의한 파랑의 감쇠)

  • Kim, Gun-Woo;Lee, Myung-Eun
    • Journal of the Korean Society of Marine Environment & Safety
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    • v.18 no.3
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    • pp.199-205
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    • 2012
  • In this study, wave transformation by damping due to the permeable bed of finite depth is investigated. The relationship between wave damping rate and relative water depth are presented. The damping rate is used in the eigenfunction expansion method to calculate the wave dissipation over the permeable bed. For a permeable shoal, the eigenfunction expansion model result is compared with that of the integral equation method to show good agreement. The model is also used to examine the wave reflection over the permeable planar slope of various frequency. It has been found that in general relatively short waves are more influenced by the permeability of the permeable seabed than relatively long waves unless the water depth is so large that the influence of permeable bed on surface water waves disappears.

Lubrication Performance Analyses of Spiral Groove Dry Gas Seals - Part II: Detailed Performance Evaluation of Groove Design Parameters (스파이럴 그루브 드라이 가스 시일의 윤활 성능해석 - Part II: 그루브 설계 파라미터의 상세 성능평가)

  • Lee An Sung;Yang Jae-Hun;Choi Dong-Hoon
    • Tribology and Lubricants
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    • v.20 no.2
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    • pp.68-76
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    • 2004
  • Applying a general Galerkin FE lubrication analysis method to spiral groove dry gas seals, this study intends to analyze in detail the effects of groove design parameters, such as a spiral angle, groove width ratio, groove radius ratio, groove depth ratio, and groove taper ratio, on the lubrication performances of an opening force, leakage, axial stiffness and damping, and angular stiffness and damping at low and high rotating speeds: 3,600 and 15,000 nm. Results show that, for the primary design consideration performances such as the opening force and axial and angular stiffnesses, a spiral angle of $25^{\circ}$, a groove width ratio of 0.46, a groove radius ratio of 1.1, a groove depth ratio of 1.0, and a groove taper ratio of 0.0 are preferred. Where the recommended relatively low values of groove depth and taper ratios are to keep the axial and angular dampings positive or higher than 0 particularly at the high rotating speed.

COMPUTATION OF ADDED MASS AND DAMPING COEFFICIENTS DUE TO A HEAVING CYLINDER

  • Bhatta Dambaru D.
    • Journal of applied mathematics & informatics
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    • v.23 no.1_2
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    • pp.127-140
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    • 2007
  • We present the boundary value problem (BVP) for the heave motion due to a vertical circular cylinder in water of finite depth. The BVP is presented in terms of velocity potential function. The velocity potential is obtained by considering two regions, namely, interior region and exterior region. The solutions for these two regions are obtained by the method of separation of variables. The analytical expressions for the hydrodynamic coefficients are derived. Computational results are presented for various depth to radius and draft to radius ratios.

Sloshing Damping in a Swaying Rectangular Tank Using a Porous Bulkhead (투과성 격벽을 이용한 수평 운동하는 사각형 탱크내의 슬로싱 감쇠)

  • Cho, Il-Hyoung
    • Journal of Ocean Engineering and Technology
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    • v.32 no.4
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    • pp.228-236
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    • 2018
  • The performance of a porous swash bulkhead for the reduction of the resonant liquid motion in a swaying rectangular tank was investigated based on the assumption of linear potential theory. The Galerkin method (Porter and Evans, 1995) was used to solve the potential flow model by adding a viscous frictional damping term to the free-surface condition. By comparing the experimental results and the analytical solutions, we verified that the frictional damping coefficient was 0.4. Darcy's law was used to consider the energy dissipation at a porous bulkhead. The tool that was developed with a built-in frictional damping coefficient of 0.4 was confirmed by small-scale experiments. Using this tool, the free-surface elevation, hydrodynamic force (added mass, damping coefficient) on a wall, and the horizontal load on a bulkhead were assessed for various combinations of porosity and submergence depth. It was found that the vertical porous bulkhead can suppress sloshing motions significantly when properly designed and by selecting the appropriate porosity(${\approx}0.1$) and submergence depth.

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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