• 해양환경안전학회지
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• 제29권7호
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• pp.992-1001
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• 2023

OWEC(Overtopping Wave Energy Converter)는 월파된 파도를 이용한 파력발전시스템이라한다. OWEC의 성능 및 안전성은 파고, 주기 등 파도의 특성에 의해 영향을 받는다. 따라서 해역 특성에 따른 OWEC의 최적 형상과 구조안전성에 관한 연구가 필요하다. 본 연구에서는 울릉읍 연안 해양 환경 데이터를 이용하였으며, SPH(Smoothed Particle Hydrodynamics) 입자법 해석을 통해 기존 케이슨 하부 구조에 변화를 준 모델 4개를 비교하여 월파 효율을 분석하였다. 그 결과, 하부 구조의 변경 및 경량화가 가능함을 확인하였다. 최적화 해석을 통해 설계 하중에 내하력을 가지는 하부 구조인 새로운 트러스형 구조를 제안하였다. 이후 부재 직경 및 두께를 설계변수로 하는 사례 연구를 통해 허용응력조건 하에서 구조 안전성의 확보를 확인하였다. 주기적인 파랑 하중을 받기 때문에 제안하는 구조의 고유 진동수와 해당 해역의 파주기를 비교하였으며, 1년 재현 주기의 파랑을 하중으로 한 조화응답해석을 수행하였다. 제안하는 하부 구조는 동일 가진력에서 기존 설계 대비 응답의 크기가 감소하였으며, 기존 대비 32% 이상의 중량 절감을 수행하였다.

• Advances in materials Research
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• 제8권4호
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• pp.259-274
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• 2019

Flexoelectric effect has a major role on mechanical responses of piezoelectric materials when their dimensions become submicron. Applying differential quadrature (DQ) method, the present article studies dynamic characteristics of a small scale beam made of piezoelectric material considering flexoelectric effect. In order to capture scale-dependency of such piezoelectric beams, nonlocal elasticity theory is utilized and also surface effects are included for better structural modeling. Governing equations have been derived by utilizing Hamilton's rule with the assumption that the scale-dependent beam is subjected to thermal environment leading to uniform temperature variation across the thickness. Obtained results based on DQ method are in good agreement with previous data on pizo-flexoelectric beams. Finally, it would be indicated that dynamic response characteristics and vibration frequencies of the nano-size beam depends on the existence of flexoelectric influence and the magnitude of scale factors.

• 응용통계연구
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• 제26권3호
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• pp.375-387
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• 2013

통계조사에서 단위 무응답은 표본크기를 축소시켜 추정값의 효율을 떨어뜨릴 뿐만 아니라, 주요 관심변수에 대해 응답자와 무응답자간의 차이가 존재하는 경우 무응답 편향(bias)을 발생시키는 요인으로 일반적으로 가중값 조정방법을 이용하여 보정한다. 대부분의 조사에서 무응답 표본에 대한 정보를 얻는 것은 현실적으로 매우 어려우며, 무응답으로 인한 추정값의 편향 정도를 파악하는 것 또한 쉽지 않다. 통계청에서는 2010년, 2011년 가구패널조사인 가계 금융조사를 실시하였다. 2010년 1차 웨이브는 1만 가구에 대해 모두 응답한 정보를 가지고 있으며, 2011년 2차 웨이브에는 부재, 거부 등으로 인한 무응답 표본(약 10%)이 존재한다. 본 연구에서는 2차 웨이브의 무응답 표본들을 무응답 성향이 강한 표본으로 가정하고 1차 웨이브에서 무응답가구로 분류한 후 무응답 가구의 특성과 무응답 편향정도 및 무응답 조정 효과를 분석하였다.

• 한국지진공학회논문집
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• 제21권1호
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• pp.69-76
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• 2017

Seismic analyses of a pile under a large rigid basement foundation embedded in the homogeneous soil layer were performed practically by a response displacement method assuming a sinusoidal wave form. However, it is hard to take into account the characteristics of a large mat foundation and a heterogeneous soil layer with the response displacement method. The response displacement method is relevant to the 2D problems for longitudinal structures such as tunnel, underground cave structure, etc., but might not be relevant with isolated foundations for building structures. In this study, seismic pile analysis by a pseudo 3D finite element method was carried out to compare numerical results with results of the response displacement method considering 3D characteristics of a foundation-soil system which is important for the building foundation analyses. Study results show that seismic analyses results of a response displacement method are similar to those of a pseudo 3D numerical method for stiff and dense soil layers, but they are too conservative for a soft soil layer inducing large soil pressures on the foundation wall and large pile displacements due to ignored foundation rigidity and resistance.

• International Journal of Naval Architecture and Ocean Engineering
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• 제11권2호
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• pp.923-938
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• 2019

Surge motion of top-end platform induced by periodic wave makes marine flexible riser encounter equivalent sheared-oscillatory flow, under which the Vortex-induced Vibration (VIV) response will be more complicated than pure sheared flow or oscillatory flow cases. Based on a time domain force-decomposition model, the VIV response characteristics under sheared-oscillatory flows are investigated numerically in this paper. Firstly, the adopted numerical model is validated well against laboratory experiments under sheared flow and oscillatory flow. Then, 20 sheared-oscillatory flow cases with different oscillation periods and top maximum current velocities are designed and simulated. Under long and short oscillation period cases, the structural response presents several similar features owing to the instantaneous sheared flow profile at each moment, but it also has some different patterns because of the differently varying flow field. Finally, the effects and essential mechanism of oscillation period and top maximum current velocity on VIV response are discussed systematically.

• International Journal of Naval Architecture and Ocean Engineering
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• 제8권3호
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• pp.252-261
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• 2016

A spar-type floating substructure that is being widely used for offshore wind power generation is vulnerable to resonance in the heave direction because of its small water plane area. For this reason, the stable dynamic response of this floating structure should be ensured by accurately identifying the resonance characteristics. The purpose of this study is to analyze the characteristics of the combination resonance between the excitation frequency of a regular wave and natural frequencies of the floating substructure. First, the nonlinear equations of motion with two degrees of freedom are derived by assuming that the floating substructure is a rigid body, where the heaving motion and pitching motions are coupled. Moreover, to identify the characteristics of the combination resonance, the nonlinear term in the nonlinear equations is approximated up to the second order using the Taylor series expansion. Furthermore, the validity of the approximate model is confirmed through a comparison with the results of a numerical analysis which is made by applying the commercial software ANSYS AQWA to the full model. The result indicates that the combination resonance occurs at the frequencies of ${\omega}{\pm}{\omega}_5$ and $2{\omega}_{n5}$ between the excitation frequency (${\omega}$) of a regular wave and the natural frequency of the pitching motion (${\omega}_{n5}$) of the floating substructure.

• 한국소음진동공학회논문집
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• 제23권5호
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• pp.407-413
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• 2013

This paper deals with the analysis of dynamic characteristics of mooring system of floating-type offshore wind turbine. A spar-type floating structure which consists of a nacelle, a tower and the platform excepting blades, is used to model the floating wind turbine and connect three catenary cables to substructure. The motion of floating structure is simulated when the mooring system is attached using irregular wave Pierson-Moskowitz model. The mooring system is analyzed by changing cable position of floating structure. The dynamic behavior characteristics of mooring system are investigated comparing with cable tension and 6-dof motion of floating structure. These characteristics are much useful to initial design of floating-type structure. From the simulation results, the optimized design parameter that is cable position of connect point of mooring cable can be obtained.

• Journal of Mechanical Science and Technology
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• 제17권6호
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• pp.860-869
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• 2003

Dynamic characteristics of a catenary that supplies electrical power to high-speed railway is investigated. The catenary is a slender structure composed of repeating spans. Each span is in turn composed of the contact and messenger wires connected by the hangers in regular intervals. A finite element based dynamic model is developed, and numerical simulations are performed to determine the dynamic characteristics of the catenary The influence of the structural parameters on the response characteristics is investigated. The structural parameters considered include tension on the contact and messenger wires, stiffness of the hangers, and the hanger and span spacing. The hanger characteristics are found to be the dominant factors that influence the overall dynamic characteristics of the catenary.

• 한국해안·해양공학회논문집
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• 제34권6호
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• pp.233-246
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• 2022

기존 케이슨방파제의 전면 또는 후면에 추가로 케이슨을 설치하여 구조물의 안정성을 향상시키기 위한 설계 및 시공사례가 보고되고 있다. 본 연구에서는 상용프로그램인 ANSYS AQWA를 이용하여 신규 케이슨이 추가로 설치되는 경우 입사하는 파랑과 케이슨들간의 상호작용 영향에 의해 개별 케이슨에 작용하는 파력 특성을 분석하였다. 앞뒤 케이슨간의 간격변화에 따른 개별 케이슨에 작용하는 파력 특성을 주파수영역에서 수치해석을 수행하였다. 그리고 특이한 파력분포 특성이 발생되는 주파수에 대해서는 불규칙파 조건에서의 시간영역 수치해석을 통해 개별 케이슨에 작용하는 동적파력 특성을 면밀하게 검토하였다.

• International Journal of Ocean System Engineering
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• 제4권1호
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• pp.1-18
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• 2014

Heave plates have been widely used to enhance viscous damping and thus reduces the heave response of Spar platforms. Single heave plate attached to the keel of the Spar has been reported in literature (Tao and Cai 2004). The effect of double heave plates on hydrodynamic response in random waves has been investigated in this study. The influence of relative spacing $L_d/D_d$ ($D_d$-the diameter of the heave plate) on the hydrodynamic response in random waves has been simulated in wave basin experiments and numerical model. The experimental investigation has been carried out using 1:100 scale model of Spar with double heave plates in random waves for different relative spacing and varying wave period. The influence of relative spacing between the heave plates on the motion responses of Spar are evaluated and presented. Numerical investigation has been carried out to investigate effect of relative spacing on hydrodynamic characteristics such as heave added mass and hydrodynamic responses. The measured results were compared with those obtained from numerical simulation and found to be in good agreement. Experimental and numerical simulation shows that the damping coefficient and added mass does not increase for relative spacing of 0.4 and the effect greater than relative spacing on significant heave response is insignificant.