• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2003년도 춘계학술대회 논문집
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• pp.214-214
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• 2003

During a model test of Hutton TLP, a "ringing" response was first observed about 20 years ago. This phenomenon is a resonant build up over the time of wave period and this burst-like motion can cause the extreme load on the TLP tether. It is often detected in the large and steep irregular waves but the generation mechanism leading to the "ringing" is not yet well understood. According to the research since then, the higher order harmonic components may account for the "ringing" on the floating offshore structures. The main purpose of the present research is, thus, to measure the higher harmonic forces exerted on a vertical truncated circular column and to compare them with available data. A vertical truncated cylinder with a diameter of 3.5inch and a draft of 10.5inch is used as a test structure, which is a scaled model of ISSC TLP column. The cylinder is installed at a distance of 45ft from the wave maker in order to avoid parasitic waves created in the wave flap. Attached to the upper part of the cylinder are two force gages to measure the horizontal (surge) and vertical (heave) forces on the cylinder. The incoming waves are Stokes waves with a slope ranging from 0.06 to 0.24. The forces and waves are measured for 60 seconds with a sampling rate of 50 Hz. Among the recorded data, the first 10 waves are excluded because of transient behavior of the waves and the next The horizontal and vertical forces are analyzed up to 5th order harmonics. The horizontal forces are then compared to the values from the theoretical model called "FNV model". In addition, force transfer functions are also investigated. Major findings in this research are below. 1) The first order forces measured are slightly larger than the theoretical values of "FNV model" 2) The "FNV model" considerably overpredicts the second order forces. 3) The larger the amplitude and more extreme the wave slope, the smaller the predictions are compared to the experimental. 4) The higher harmonic forces are significantly smaller than the first harmonic force for all wave parameters. 5) The normalized forces vs. waves slopes are almost constant in the lower harmonics but vary a lot in the higher harmonics. 6) The trend of forces is more nonlinear in the horizontal forces than in the vertical forces as the wave slope increases. 7) The part of the results above is also observed by other researchers and confirmed again through the present work.

• 대한기계학회논문집B
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• 제20권12호
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• pp.3960-3969
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• 1996

A fluid flow inside a circular cylinder subject to horizontal and circular oscillation is analyzed theoretically. Under the assumption of small-amplitude oscillation, the governing equations take linear forms. The velocity field is obtained in terms of the first kind of Bessel function of order 1. It was found that a particle describes an orbit close to a circle in the central region and an arc near the side wall. We also obtained the Stokes' drift velocity induced by the traveling wave along the circumferential direction. The Eulerian streaming velocities at the edge of the bottom and side boundary layers were also obtained. It was shown that the vertical component of the steady streaming velocity on the side wall is almost proportional to the amplitude of the free surface motion.

• 한국에너지공학회:학술대회논문집
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• 한국에너지공학회 1999년도 추계 학술발표회 논문집
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• pp.231-236
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• 1999

This paper is to propose a concept and performance of the gravity engine which could extract energy from sea or river as a clean and renewable and sustainable power, the tidal or hydro-power. The vertical motion of the buoyancy cylinder of the present gravity engine is converted to the mechanical work directly without any hydraulic loss. The increased gravity potential during high tide is harnessed proportional to the length of the buoyancy cylinder times tidal height which is greater than the conventional tidal power using water mill. This energy amplification results from the net energy gain between the resource energy and the imposed energy to extract water out of the buoyancy cylinder. Its efficiency is higher than the conventional water mill due to its direct mechanical conversion.

• 한국해양공학회지
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• 제19권5호
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• pp.16-25
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• 2005

The effect of vertical riser support system on the dynamic behaviour of a classical spar platform is investigated. Spar platform generally uses buoyancy-can riser support system, but as water depth gets deeper the alternative riser support system is required due to safety and cost issues. The alternative riser support system is to hang risers off the spar platform using pneumatic cylinders rather than the buoyancy-can. The existing numerical model for hull/mooring/riser coupled dynamics analysis treats riser as an elastic rod truncated at the keel (truncated riser model), thus, in this model, the effect of riser support system can not be modeled correctly. Due to this reason, the truncated riser model tends to overestimate the spar pitch and heave motion. To evaluate more realistic global spar motion, mechanical coupling among risers, guide frames and support cylinders inside of spar moon-pool should be modeled. In the newly developed model, the risers are extended through the moon-pool by using nonlinear finite element methods with realistic boundary condition at multiple guide frames. In the simulation, the vertical tension from pneumatic cylinders is modeled by using ideal-gas equation and the vertical tension from buoyancy-cans is modeled as constant top tension. The different dynamic characteristics between buoyancy-can riser support system and pneumatic riser support system are extensively studied. The alternative riser support system tends to increase spar heave motion and needs damper system to reduce the spar heave motion.

• 대한설비공학회지:설비저널
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• 제14권2호
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• pp.118-124
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• 1985

The motion of a fluid in the closed annular cavity formed by two concentric vertical cylinders with externally finned tube has been analysed by a numerical solutions of the equation of momentum and energy. For the calculation procedure, the fluid is assumed to have constant thermo-dynamic and transporties except for the density, which is temperature-dependent in the buoyancy term of the vertical momentum equation (Boussinesq approximation). The govern ins equations for velocity and temperature are solved by a finite difference technique which incoorporates a scheme for treating the coupled variables. Results are presented for a range of the Rayleigh number and for various values of the fin height and the number of fins.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.680-685
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• 2000

Investigation on ignition and flame propagation of pyrolyzing fuel in a cylindrical enclosure is accomplished. The pyrolyzing fuel of cylindrical shape is located in an outer cylinder sustained at high-temperature. Due to gravity, the buoyancy motion is inevitably incurred in the enclosure and this affects the flame initiation and propagation behavior. The radiative heat transfer plays an important role since a high temperature difference is involved in the problem. Numerical studies have been performed over overheat ratio, and vertical fuel eccentricity. The location of flame onset is affected by the vertical eccentricity of inner pyrolyzing fuel as well as thermal conditions applied.

• 한국해안·해양공학회논문집
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• 제30권1호
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• pp.29-38
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• 2018

축대칭 진동수주형 부유식 파력발전의 성능을 평가하기 위하여 PTO를 고려한 선형화된 자유수면경계조건을 유도하여 유한요소 수치모델을 수립하였다. 발전효율에 영향을 미칠 것으로 판단되는 동력인출장치(PTO)가 설치되는 실린더내 수주의 공진현상과 부유체의 heave 공진현상에 직접적으로 영양을 줄 수 있는 설계변수인 실린더 및 스커트 길이, 그리고, 수심을 변화시키며 수치실험을 실시하여 발전시스템의 최적설계에 필요한 기초 자료를 확보하였다. 연구결과, 발전시스템의 효율은 실린더내 진동수주의 공진보다는 부유체의 heave 운동 공진에 지배되며, 부이 외측에 스커트를 부착함으로써 효율적으로 공진조건을 변화시킬 수 있음을 확인하였다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제7권5호
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• pp.906-919
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• 2015

The Deep Draft Semi-Submersible (DDS) concepts are known for their favourable vertical motion performance. However, the DDS may experience critical Vortex-Induced Motion (VIM) stemming from the fluctuating forces on the columns. In order to investigate the current-induced excitation forces of VIM, an experimental study of flow characteristics around four square-section cylinders in a square configuration is presented. A number of column spacing ratios and array attack angles were considered to investigate the parametric influences. The results comprise flow patterns, drag and lift forces, as well as Strouhal numbers. It is shown that both the drag and lift forces acting on the cylinders are slightly different between the various L/D values, and the fluctuating forces peak at L/D = 4.14. The lift force of downstream cylinders reaches its maximum at around ${\alpha}=15^{\circ}$. Furthermore, the flow around circular-section-cylinder arrays is also discussed in comparison with that of square cylinders.

• 에너지공학
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• 제9권1호
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• pp.47-53
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• 2000

This paper is to propose the concept and performance of a gravity engine which could extract energy from sea or river as a clean. renewable and sustainable power. the vertical motion of the buoyancy cylinder of the present gravity engine is converted to the mechanical work directly without any hydraulic loss. The positive net energy between the imposed and harnessed one is achieved by the specific operating procedure. The detailed derivation of the energy balance is made based on the first principle of thermodynamics. The calculation demonstrates that the present gravity engine could harness more energy than the conventional turbine system in the same basin area because of the relatively high efficiency in the energy conversion system and added mass from the buoyancy cylinder.

• 한국항공우주학회지
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• 제33권6호
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• pp.8-14
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• 2005

지면 근처에 존재하는 뭉뚝한 물체의 유동장 이해는 자동차 및 항공 업계에 매우 중요한 분야이다. 이를 위해 비압축성 평균 Navier-Stokes 방정식에 $\varepsilon{-SST}$ 난류 모델을 적용하여 정사각주와 이동 지면의 간극 유동을 해석하였다. 비정상 진동을 억제하기 위하여 사각주 하부에 수직/수평의 펜스 설치 효과를 연구하였다. 지면이 운동할 경우에는 지면의 박리 전단층의 강도가 약화되어 사각주 상/하부의 박리 전단층 상호 작용을 촉진시키므로 고정 지면에 비하여 더 낮은 간극에서도 와류 배출이 발생한다.