• Journal of Ship and Ocean Technology
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• 제8권4호
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• pp.1-9
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• 2004

High lifting devices used for control purposes have received much attention in the marine field. Hydrofoils for supporting the hull, roll stabilizer fins for developing the motion damping performance, rudders for maneuverability are the well-known devices. In the present study, the ability of the rudder with flap to produce high lift was analyzed. The boundary layer control, one of the flow control techniques, was adopted. Especially, to build the blown flap, a typical and representative type of a boundary layer control, a flapped rudder was designed and manufactured so that it could eject the water jet from the gap between the main foil and the flap to the flap surface tangentially. And it was tested in the towing tank. Simultaneously, to know the information about the 2-dimensional flow field, a fin model with similar characteristics as the rudder model applicable for the motion control was made and tested in the cavitation tunnel. In addition, local flow measurements were carried out to obtain physical information, for example, a surface pressure measurement and flow visualization around the flap. And CFD simulation was used to obtain information difficult to collect from the experiment about the 2-dimensional flow.

• 한국음향학회지
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• 제31권6호
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• pp.384-390
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• 2012

수동 소나는 데몬 처리를 통해 수중 표적의 방사 소음으로부터 프로펠러 정보 추출한다. 그러나 기존 데몬 처리 기법은 프로펠러 신호 뿐 아니라 토널 신호 성분도 추출하므로 토널 신호의 간섭으로 인한 성능 저하가 있다. 다시 말해 데몬 처리 주파수 영역 내에 토널 신호가 존재하면 기존 데몬 처리 기법은 토널 신호의 간섭에 의한 신호 성분을 추가적으로 추출한다. 따라서 본 논문에서는 토널 신호 간섭을 제거할 수 있는 데몬 처리 기법을 제안한다. 제안된 기법은 데몬 처리를 위한 방사 소음 신호의 복조 과정 이전에 토널 신호를 추출 및 제거한다. 그러므로 제안된 데몬 처리 기법은 토널 신호의 간섭에 강인하다. 그리고 시뮬레이션을 통해서 제안된 기법이 기존의 데몬 처리 기법보다 성능이 우수함을 검증하였다.

• 대한조선학회논문집
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• 제52권3호
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• pp.206-221
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• 2015

Supercavitation is a modern technique which can be used to surround an underwater vehicle with a bubble in order to reduce the resistance of the vehicle. When the vehicle is at low speed in the deep sea, the cavitation number is relatively big and it is difficult to generate a cavity large enough to envelope the vehicle. In this condition, the artificial cavity, called ventilated cavity, can be used to solve this problem by supplying gas into the cavity and can maintain supercavitating condition. In this paper, a relationship between the ventilation gas supply rate and the cavity shape is determined. Based on the relationship a ventilation rate control is developed to maintain the supercavitating state. The performance of the ventilation control is verified with a depth change control. In addition, dynamics modeling for the supercavitating vehicle is performed by defining forces and moments acting on the vehicle body in contact with water. Simulation results show that the ventilation control can maintain the supercavity of an underwater vehicle at low speed in the deep sea.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2606-2611
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• 2008

The high energy fluid leakage from the high temperature and high differential pressure drop system of NPPs (Nuclear Power Plants) decreases efficiency and consequently leads to considerable economic loss due to less power production. Also, the leakage possibly damages critical parts of components such as valve and trim with the effect of cavitation, flashing, and erosion, etc. and deteriorates its performance. Thus, in this study, we diagnosed the as-is leakage for four (4) main steam drain valves and two (2) steam traps of Yonggwang 1,2 units during normal operation by using multi-measuring technique and observed the occurrence of fine leakage. In the course of measuring fluid leakage, the sign of fine leakage is estimated to be the leakage from orifice. By converting the leakage to energy loss, it is equivalent to the amount of several hundred thousand won per each unit, which supports the basis for the justification of fine leakage.

• 대한조선학회논문집
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• 제56권4호
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• pp.327-334
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• 2019

Recently, there has been a growing interest in artificial supercavitation as a way to reduce friction drag of submerged vehicles. A cavitator plays an important role to generate the supercavity, so many studies have focused on the case of cavitator only. However, the body shape behind the cavitator affects the growth of the supercavity and this effect must be considered for evaluating the overall performance of the system. In this work, we conducted experimental investigation on artificial supercavitation generated by different combinations of the cavitator and body. We observed the supercavity pattern by using a high-speed camera and measured the pressure inside the cavity by using an absolute pressure transducer. We estimated the relation between the amount of injected air and the supercavity shape for different combinations. In summary, the disk type cavitator generates larger supercavity than that of the cone and ellipsoidal cavitators, but cavity development speed is relatively slower rather than the others. Furthermore, fore body angle plays an important role to generate the supercavity enveloping the entire body.

• 한국산학기술학회논문지
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• 제20권8호
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• pp.23-28
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• 2019

본 연구에서는 선박 및 해양플랜트에 사용되어지는 유체기기에 관하여 공동에 의한 손상을 줄이기 위해 표면 금속코팅을 적용할 경우 구조건전성을 평가하고 효율개선을 위한 수치해석적 결과에 대해 기술하였다. 소재 경량화 및 표면 코팅을 통해 날개 두께 등이 얇아지더라도 구조 강도와 성능이 확보됨을 간접적으로 입증하기 위해 코팅 하지 않은 경우, 코팅 후 동일 두께, 코팅 후 두께 3% 경량화 그리고, 코팅 후 두께 5% 경량화 것의 4 가지 모델에 대해 코팅의 유효성을 해석적 방법으로 평가하였다. 코팅 전 후 및 두께를 3% 줄여 무게를 경량화한 경우에 대해 구조해석을 수행한 결과, 응력은 7% 증가하였고 안전율은 2.7%로 나타났으며 기준 안전율인 1보다 큰 값이 도출되었다. 코팅 전 후및 두께를 5% 줄여 무게를 경량화한 경우에 대해 구조해석을 수행한 결과, 응력은 12%까지 증가하였고 안전율은 0.99%로 나타났으며 0.1% 정도 차이로 안전율이 확보되지 않음에 따라 구조건전성이 부족한 것으로 나타났다. 하지만 부족한 안전율은 좀 더 좋은 소재를 선정하거나 코팅두께를 현재보다 두껍게 한다면 충분한 안전율을 확보할 수 있다고 판단된다. 최종적으로 코팅을 함으로 인해 구조건전성이 개선되는 것을 확인하였고, 5% 경량화한 경우에도 코팅의 효과로 인해 충분히 구조건전성이 확보될 수 있음을 확인하였다.

• Tribology and Lubricants
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• 제32권1호
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• pp.24-31
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• 2016

In this paper hydrodynamic lubrication analysis is carried out to investigate the effects of groove location on the lubrication performance of a piston and cylinder system in a linear compressor. The rectangle shaped grooves having a constant groove depth and width are applied on the lubrication area of the piston. The Universal Reynolds equation is used to calculate the oil film pressure, and the Elrod algorithm with the finite different method is used to solve the governing equation. The JFO boundary condition is applied to predict cavitation regions. Transient analysis for different locations of the grooves on the piston is carried out using the typical operating condition of the linear compressor in order to estimate the variations of frictional power losses and minimum film thicknesses. When the grooves are applied on the lubrication area, both the frictional power loss and the minimum film thickness decrease. The frictional power loss can be reduced effectively, while maintaining a minimum film thickness to enable the piston operation without direct contact with the cylinder surface, by means of choosing a proper location of the grooves. The optimum location of the grooves to improve a lubrication performance depends on the operation condition or the system requirements specification.

• Tribology and Lubricants
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• 제34권5호
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• pp.169-174
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• 2018

A multi-stage compressor (MSC) is comprised of several impellers installed in the pinion gear shaft driven by a main bull gear. In the pinion shaft, a thrust collar (TC) is installed to support the thrust load. The TC makes the lubrication system simpler in the MSC; therefore, it is widely used in similar kinds of machinery. Typically, TCs are installed on both sides of the bull gear and pressure is developed in the lubricated area by creating a taper angle on the TC and bull gear surface. In the current study, we developed a numerical analysis model to evaluate the performance of the TC considering its design parameters. We sloved the Reynolds equation using the finite element method and applied the half Sommerfeld condition to consider cavitation. Based on the pressure calculated in the lubricated area, we calculated the power loss and minimum film thickness. In addition, we calculated stiffness and damping using perturbation method. We performed parametric studies using the developed model. The results of the analysis show that the maximum pressure presents in the center area of the TC and it increases with the taper angle. The area over which pressure is developed decreases with the taper angle. The results also show that there is an optimum taper angle providing minimum power loss and maximum film thickness. Additionally, the stiffness and damping decrease with the taper angle. As the applied load increases, the power loss increases and the minimum film thickness decreases. However, the stiffness and damping increase with the applied load.

• 대한기계학회논문집
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• 제12권3호
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• pp.520-527
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• 1988

본 연구에서는 액체용 이젝터의 성능을 결정하는 여러가지 제약인자중 특히 레이놀즈수 변화에 따른 구동노즐의 면적비 및 목부길이가 액체용 이제터성능에 미치 는 영향을 체계적인 실험을 통하여 연구함으로써 기 개발된 CAD용 전산프로그램의 타 당성을 보다 세밀히 검토하고 이에 보완을 가하는데 연구의 목적이 있다.

• 한국유체기계학회 논문집
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• 제18권6호
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• pp.45-56
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• 2015

This paper aims to develop a submerged propeller turbine for micro hydropower plant which allows to sustain high values of efficiency in a broad range of hydrological conditions (H=2~6 m, $Q=0.15{\sim}0.39m^3/s$). The two aspects to be considered in this development are mechanical simplicity and high-efficiency operation. Unlike conventional turbines that have spiral casing and gear box, this is directing driving and no spiral casing. A 10 kW class turbine which has the most high potential of the power generation has been developed. The most important element in the design of turbine is the runner blade. The initial blade is designed using inverse design method and then the runner geometry is modified by classical hydraulic method. The design process is carried out in two steps. First, the blade shape is fix and then other components of submerged propeller turbine are designed. Computational fluid dynamics analyses based on the Navier-Stokes equations have been used to obtain overall performance data for the blade and the full turbine, respectively. The results generated by performance parameters(head, guide vane opening angle and rotational speed) variations are theoretically analysed. The evaluation criteria for the blade and the turbine performances are the pressure distribution and flow's behavior on the runner blades and turbine. The results of simulation reveals an efficiency of 91.5% and power generation of 10.5kW at the best efficiency point at the head of 4m and a discharge of $0.3m^3/s$.