• 전기화학회지
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• 제4권3호
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• pp.104-108
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• 2001

용융분사법으로 나프탈렌계 메조페이스 피치(mP)를 방사하여 산화안정화 속도를 변화시켜 흑연화 섬유의 모폴러지를 제어하였으며, 흑연화 섬유를 이용하여 Li-ion 이차전지 부극을 제조하여 충$\cdot$방전 거동 및 용량을 측정하였다. 용융분사조건에 따라 제조된 피치섬유의 직경은 $4{\mu}m$로부터 $16{\mu}m$까지 다양하였다 이중에서 직경 $10{\mu}m$인 피치섬유를 선택하여 세가지 승온속도 조건 $2^{\circ}C/min,\;5^{\circ}C/min,\;10^{\circ}/min$에서 산화안정화 후 $1000^{\circ}C$에서 탄소화하여 $2650^{\circ}C$에서 흑연화 한 결과, 섬유 단면이 산화안정화 조건 $2^{\circ}C/min$의 경우는 라디알 구조, $5^{\circ}C/min$의 것은 라디알-랜덤 구조, $10^{\circ}C/min$의 경우는 skin-core 구조를 형성하였고, 승온속도가 큰 경우일수록 이흑연화성이 컷다. 이것은 큰 승온속도에서는 탄소화$\cdot$흑연화 과정에서 섬유표면에서만 산화안정화가 일어나고, 내부에서는 피치분자가 유동성이 커 승온과정에서 고결정성의 흑연구조가 발달한 것으로 추측된다. 따라서 이흑연화성이 큰 $10^{\circ}C/min$에서 산화안정화 한 것이 충전방전 용량이 $2^{\circ}C/min$의 경우에 비해서 1.3배로 약 400mAh/g, 충방전 효율도 $96.8\%$로 가장 우수한 특성을 나타냈다.

• 한국전기전자재료학회논문지
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• 제23권3호
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• pp.194-198
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• 2010

Recently as the electronic devices are getting to be more and more smaller, transformers are needed to be micro fabricated using MEMS technology. In this paper transformers have been fabricated and measured by depositing insulation layer to reduce the loss of eddy current and in the middle core a high permeability permalloy was designed based on the turns ratio between primary coil and secondary coil which are 1:1 transformers. (the number of turns of primary coil and secondary coil: 3/3, 5/5, 7/7). The size of the transformers including ground shield are $1mm{\times}1.5mm$, $1mm{\times}1.95mm$, $1mm{\times}2.35mm$ respectively. The line width, pitch and the height of post are 50um. Based on the measured data from the micro fabricated transformers, the 3/3 turns in the primary coil and secondary coil showed the lowest insertion loss with 1.5 dB at 480 MHz and the 7/7 turns in the primary coil and secondary coil showed the highest insertion loss with 2.5 dB at 280 MHz. Also confirmed that the bandwidth goes up as the number of turns goes down. There was some difference between the actual measured data and the HFSS simulation result. It looks as if it is an error of the difference between oxidation of copper or the permeability of SU-8.

• 한국정밀공학회지
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• 제29권2호
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• pp.156-160
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• 2012

Recently, the demands of the large scale machine tools gradually increase to machine the large parts, such as large scale crankshaft, yaw and pitch bearings for the wind power generator and the vehicle or aircraft components. But the high technology is necessary in order to develop the huge machine tools. Furthermore, the global market of it has been monopolized by a few companies. So, we need to develop the large scale machine tools and study its core technology to rush into the increasing market. In this study, we carried out the researches for the important core technology of a multi-tasking, machine tool; a large scale 5-axis machine tool of gantry type for multi-task machining. This study is focused on the design of large size gantry type multi-axis machine. In the case of large size of machine the cross rail deflection in the X-axis is significant. To reduce the deflection due to the eccentric spindle head, a special hollow type design in the cross rail with outside ram is adapted in this study. Also, the Zig-Zag motion in the Y-axis is inevitable with the gantry geometry, which is by the un-balancing, different motion at the left and the right columns moving. We tried to reduce the influence of Zig-Zag motion using FEM with different loading conditions at the left and the right side column.

• 한국항공우주학회지
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• 제33권7호
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• pp.9-17
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• 2005

비정상 패널법을 이용하여 복엽기 형태 배치의 복식 플랩핑 에어포일들에 대한 후류의 형상 및 추력 특성을 연구하였다. 에어포일들에서 발생하는 후류 형상은 와핵 모델, 와핵 첨가법 그리고 4계 Runge-Kutta 법을 사용하여 계산하였다. 해석 결과는 유동 가시화, 엄밀해 그리고 전산 해석 결과와 비교하여 검증하였다. 복엽기 배치의 에어포일의 경우, 두께 및 캠버는 추력을 감소시키는 효과가 있었다. 플런징과 피칭 운동들 사이의 위상차가 90도 및 120도 일 때 최대 추력이 발생하였다. 플런지 속도 및 피치 크기가 클수록 추력은 증가하였다. 에어포일 사이의 거리가 감소할수록 추력은 증가하나, 0.6c 이하로 가까워질 경우 추력은 감소하였다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제11권2호
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• pp.736-749
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• 2019

This paper employs computational tools to investigate the diffraction effects in regular head waves on the added resistance and wake on the propeller plane. The objective ships are a 66,000 DWT bulk carrier and a 3,600 TEU container ship. Fixed and free to heave and pitch conditions at design speed have been taken into account. Two-phase unsteady Reynolds averaged Navier-Stokes equations have been solved using the finite volume method; and a realizable k-ε model has been applied for the turbulent closure. The free surface is obtained by solving a VOF equation. The computations are carried out at the same scale of the model tests. Grid and numerical wave damping zones are applied to remove unwanted wave reflection at the boundaries. The computational results are analyzed using the Fourier series. The added resistances in waves at the free condition are higher than those at the fixed condition, which are nearly constant for all wavelengths. The wake velocity in waves is higher than that in calm water, and is accelerated where the wave crest locates on the propeller plane. When the vertical motion at the stern goes upward, the wake velocity also accelerated.

• 한국전산유체공학회지
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• 제9권2호
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• pp.18-22
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• 2004

Numerical investigation has been conducted to figure out flow behavior and pressure drop characteristics of spirally corrugated steel pipe which is widely used in civil, industrial and agricultural field owing to many advantages such as good corrosion resistance and durability, strength, easy and quick installation. Also the poly-ethylene coating spirally corrugated steel pipe has the long life under condition of sea water immerged. In the present study, flow behavior in the spirally corrugated pipe and influence of P/d/sub h/(ratio of wave pitch to hydraulic diameter) to pressure drop are investigated by CFD with various Reynolds number. And also friction factor is estimated by pressure drop obtained by flow analysis. According to computation results, the flow runs spirally up and down along the spiral corrugation in the vicinity of wall, but the effect of spiral corrugation disappears in core region of pipe. As P/d/sub h/ becomes small, more pressure drop occurs in spirally corrugated Pipe. Besides, friction factor augmentation becomes much larger as Re increases. In case of p/d/sub h/=0.38, Pressure drop and friction factor of spirally corrugated pipe are about four times larger than smooth pipe at Re: 1.46×10/sup 6/.

• 대한기계학회논문집B
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• 제28권8호
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• pp.921-928
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• 2004

Measurements of local heat/mass transfer coefficients in rotating two-pass ducts are presented. Ducts of three different aspect ratios (W/H), 0.5, 1.0 and 2.0, are employed with a fixed hydraulic diameter ($D_h$) of 26.7 nm. $90^{\circ}$-rib turbulators are attached on the leading and trailing walls symmetrically. The rib height-to-hydraulic diameter ratio ($e/D_h$) is 0.056, and the rib pitch-to-rib height ratio (p/e) is 10. The experimental conditions are the same as those of the previous part of the study. As the rib height-to-duct height ratio (e/H) increases, the core flow is more disturbed and accelerated in the midsections of ribs. Therefore, the obtained data show higher heat/mass transfer in the higher aspect ratio duct. Dean vortices also augment heat/mass transfer in the turn and in the upstream region of the second pass. However, the effect becomes less significant for the higher aspect ratio because the surface area increases in the present geometric condition. The effect of rotation produces heat/mass transfer discrepancy.

• Journal of Mechanical Science and Technology
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• 제14권8호
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• pp.816-822
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• 2000

In the core of the nuclear power plant of PWR, several cases of fuel failure by unknown causes have been experienced for various fuel types. From the common features of the failure pattern, failure lead time, flow conditions, and flow induced vibration characteristics in nuclear fuel bundles, it is deduced that the fretting wear failure of the fuel rod at the spacer grid position is due to the fluidelastic vibration. In the past, fluidelastic vibration was simulated by quasi -static semi-analytical model, so called the static model, which could not account for the interaction between the rods within a bundle. To overcome this defect and to provide for more flexibilities applicable to the fuel bundle, Tanaka's unsteady model was modified to accomodate the geometrical differences and governing parameter changes during the operations such as the number of rods, pitch to diameter ratio (P/D), spring force, damping coefficient, etc. The critical velocity was calculated by solving the governing equations with the MATLAB code. A comparison between the estimated critical velocity and the test result shows a good agreement. Finally, the level of decrease of the critical velocity due to the reduction in the spring force and reduced damping coefficient due to the radiation exposure is also estimated.

• 한국정밀공학회지
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• 제28권7호
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• pp.761-768
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• 2011

The large surface micro machining system includes the equipments and processes for manufacturing the ultra precision micro patterned products with large surface through the mechanical machining. Recent major issue on the micro machining technology may be the development of optical parts for the back light unit of display which has the largest market. This special issue makes up with three parts; the large surface micro machining system and machine, machining process and forming process. In this paper, the state-of-the-art and core technology of large surface micro machining system is introduced with focus on the manufacturing technology for the back light unit of LCD TV. Then, some research results on the development of a roll die lathe is introduced which involves the concept of machine design, improvement of thermal characteristics in the spindle system, improvement of relative parallelism and straightness between spindle system and long stroke feed table, machining of micro pitch patterns. Finally, the direct forming process is introduced as the future work in the large surface micro machining field.

• Structural Engineering and Mechanics
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• 제61권6호
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• pp.785-794
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• 2017

This paper is concerned with the comparative numerical and experimental study on the natural behavior and the motion responses of a 1/75 moored scale model of a 2.5 MW spar-type floating offshore wind turbine subject to 1-D regular wave. Heave, pitch and surge motions and the mooring tensions are investigated and compared by numerical and experimental methods. The upper part of wind turbine which is composed of three rotor blades, hub and nacelle is modeled as a lumped mass and three mooring lines are pre-tensioned by means of linear springs. The numerical simulations are carried out by a coupled FEM-cable dynamics code, while the experiments are performed in a wave tank equipped with the specially-designed vision and data acquisition system. Using the both methods, the natural behavior and the motion responses in RAOs are compared and parametrically investigated to the fairlead position, the spring constant and the location of mass center of platform. It is confirmed, from the comparison, that both methods show a good agreement for all the test cases. And, it is observed that the mooring tension is influenced by all three parameters but the platform motion is dominated by the location of mass center. In addition, from the sensitivity analysis of RAOs, the coupling characteristic of platform motions and the sensitivities to the mooring parameters are investigated.