• Journal of Ship and Ocean Technology
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• 제9권4호
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• pp.1-10
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• 2005

The effect of buoyancy orientation on turbulent channel flow has been investigated using DNS (direct numerical simulation). Grashof number is kept at 9.6 $\times 10^{5}$ while changing the orientation of the buoyancy vector to be parallel or perpendicular to the channel walls. Four study cases can be distinguished during this research namely; streamwise, wall-normal unstable stratification, wall-normal stable stratification and spanwise oriented buoyancy. The driving mean pressure gradient used in all cases is adjusted to keep mass flow rate constant while friction Reynolds number is around 150. At this Grashof number, the skin friction shows decrement in the unstable and stable stratification and increment in the other two cases. Analyses of the changes of flow structure for the four cases are presented highlighting on the mean quantities and second order statistics.

• 해양환경안전학회지
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• 제23권7호
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• pp.820-828
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• 2017

익수자의 부력을 지원할 수 있는 도구나 장비는 익수자의 생명 연장에 중요하다. 본 연구의 목적은, 최소 1분의 시간 동안 사람의 머리가 수면위로 부상하여 호흡할 수 있는 것으로 가정한 최소 부력의 형성에 필요한 가스의 양과 뉴턴(newton) 단위의 부력을 추정함에 있다. 부력 실험 장치는 이산화탄소 가스를 주입한 긴 고무풍선을 이용하였고, 6명의 대학생을 대상으로 부력측정 실험을 하였다. 부력의 정도는 5점 척도를 이용하여 측정하였고, 측정한 데이터의 통계 값을 분석하여 최소 부력에 필요한 이산화탄소의 양을 추정하였다. 실험 결과, 이산화탄소 8 그램은 72 %의 신뢰수준에서 최소부력 조건을 만족하고, 부력은 44.66 뉴턴으로 계산되었다. 이산화탄소 12 그램은 100 %의 신뢰수준에서 최소부력 조건을 만족하고, 부력은 66.99 뉴턴으로 계산되었다. 본 연구를 통해서 저가이면서 휴대가 간편한 최소부력지원 장치의 개발이 가능할 것으로 기대된다.

• 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.

• 한국화재소방학회:학술대회논문집
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• 한국화재소방학회 1997년도 International Symposium on Fire Science and Technology
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• pp.625-632
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• 1997

A two-dimensional zone-like model is developed to predict the interaction between hot gas layer and water droplets after sprinkler activation. The model combines the motion equations for each droplet with heat and mass transfer between the gas and water. The results indicate that negative buoyancy in the hot layer can only be obtained if the initial temperature profile is uniform. If an experimental profile Is used instead, positive buoyancy results. This conclusion has been confirmed with experimental data.

• 한국해양공학회지
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• 제28권5호
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• pp.466-473
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• 2014

Underwater gliders do not have any external propulsion systems that can generate and control their motion. Generally, underwater gliders would obtain a propulsive force through the lift force generated on the body by a fluid. Underwater gliders should be equipped with mechanisms that can induce heave and pitch motions. In this study, an inner movable and rotatable mass mechanism was proposed to generate the pitch and roll motions of an underwater glider. In addition, a buoyancy control unit was presented to adjust the displacement of the underwater glider. The buoyancy control unit could generate the heave motion of the underwater glider. In order to analyze the underwater dynamic behavior of this system, nonlinear 6-DOF dynamic equations that included mathematical models of the inner movable mass and buoyancy control unit were derived. Only kinematic characteristics such as the location of the inner movable mass and the piston position of the buoyancy control unit were considered because the velocities of these systems are very slow. The effectiveness of the proposed dynamic modeling was verified through sawtooth and spiraling motion simulations.

• 대한기계학회논문집B
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• 제24권4호
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• pp.495-503
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• 2000

The present numerical study investigates flow characteristics and heat transfer enhancement of the viscoelastic non-Newtonian fluid in a 2:1 rectangular duct. The combined effect of temperature-dependent viscosity, buoyancy and secondary flow caused by second normal stress difference are all considered. The Reiner-Rivlin model is used as a viscoelastic fluid model to simulate the secondary flow and temperature-dependent viscosity model is adopted. Three types of thermal boundary conditions involving different combinations of heated walls and adiabatic walls are considered in this study. Calculated Nusselt numbers are in good agreement with experimental results in both the thermal developing and thermally developed regions. The heat transfer enhancement can be explained by the combined viscoelasticity-driven secondary flow, buoyancy-induced secondary flow and temperature-dependent viscosity.

• 설비공학논문집
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• 제15권4호
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• pp.274-286
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• 2003

Numerical simulations are performed to investigate the turbulent convective heat transfer of the supercritical carbon dioxide flows in vertical and horizontal square ducts. The gas cooling process at the supercritical state experiences a sudden change in thermodynamic and transport properties. This results in the extraordinary variations of the heat transfer coefficients in the supercritical state, which are much different from those of single or two phase flows. Algebraic second moment closure which can include the effects of large thermophysical property variations of carbon dioxide and of buoyancy is employed to model the Reynolds stresses and turbulent heat fluxes in the governing equations. The previous correlations for the turbulent heat transfer coefficient for the supercritical carbon dioxide flows couldn't reflect the buoyancy effect. The present results are used to establish a new heat transfer coefficient correlation including the effects of large thermophysical property variation and buoyancy on in-duct cooling process of supercritical carbon dioxide.

• 한국자동차공학회논문집
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• 제12권4호
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• pp.42-49
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• 2004

When a truck cab is conveyed at a constant speed by a hanger and immersed into the painting reservoir, it may fall off from the hanger by buoyancy. In order to reduce the buoyancy, on the bottom of a cab panel are holes placed, which allow paint to flow into the inside of a cab. In this study, a differential equation is derived which can be solved numerically by using 4th-Order Runge-Kutta method to calculate transient behavior of the buoyant force with sizes and locations of the holes given. The solution is utilized to optimally determine sizes and locations of the holes.

• 해양환경안전학회지
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• 제23권3호
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• pp.301-312
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• 2017

CE RCD (Recreational Craft Directive) is a certification for the design and construction of small vessels, including pleasure yachts, which are widely used not only in the countries within the European Union, but also in Japan and Southeast Asia. Recently, South Korean leisure craft shipyards have developed interest in exporting to foreign leisure craft markets such as Europe; however, they have encountered difficulties because of the CE RCD regulations, which are relatively complex and difficult to understand. The requirements for buoyancy and stability, which are essential properties that must be understood within the early stage of ship design, are defined based on ISO 12217. However, preparing this assessment according to ship classification regulations is an exceedingly complex task, even with knowledge of naval architecture. In this research, we have developed design support tools to systematically support assessments and preemptively define design information so that buoyancy and stability assessments based on ISO 12217 can be systematically prepared. Our research results were applied to actual examples of yacht design to confirm validity. We believe that the improved yacht design process presented in this research can act as a foundational reference for enhancing the effectiveness and systematic buoyancy and stability assessments.

• 한국산업융합학회 논문집
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• 제27권3호
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• pp.629-634
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• 2024

The Korea Research Institute of Ships and Ocean Engineering (KRISO) has developed a test module that can vertically ascend and descend with a buoyancy engine to verify the performance of the developed buoyancy engine. The independent test module has been tested in the Ocean Engineering Basin(C.M.Lee et al., 2023). After that, more tests were performed in the Deep Ocean Engineering Basin and at sea. In the 50-meter depth pit test of the Deep Ocean Engineering Basin, there were no problems with the ascent and descent operations, but the buoyancy engine was not properly maintained due to various problems in the independent test module, resulting in a difference between the calculated results using the solution of the equations of motion and the actual measurement results. The East Sea test was conducted at a depth of approximately 110 meters north-east of Pohang, with a dive to 100 meters. The difference between the pressure sensor value and the calculated value was observed, but after checking the results of the underwater position tracking device(USBL, Ultra Short Base Line system), it was estimated that the difference was caused by the influence of the current.