유전율 및 투자율에 따른 인공자계도체 특성 및 다이폴 안테나 간 상관관계 분석

이동현,민태홍,이종무,Lee, Donghyun,Min, Taehong,Lee, Jongmoo 한국시뮬레이션학회 2020 한국시뮬레이션학회 논문지 Vol.29 No.1

In this paper, we theoretically examine the characteristics of an Artificial Magnetic Conductor (AMC) constructed of a perfect electric conductor and a normal material having permittivity ε_r, permeability μ_r, and thickness L. First, we derived rigorous equations to describe the infinite AMC structure. Then, we studied how the AMC's characteristics are affected by changes in ε_r, μ_r and L. The operating center frequency exhibiting a 0° reflection coefficient phase occurs when L is one quarter of a guide wavelength. Therefore, the AMC thickness can be reduced by using a material having a high product of ε_r and μ_r. As the ratio μ_r/ε_r increases, the bandwidth of the AMC increases (maximum value: 200 %), and its operating frequency decreases. We also find out he bandwidth of the AMC is improved by introducing a loss in the material. To validate the AMC, we design a dipole antenna on the AMC and demonstrate a relationship between AMC phase and dipole antenna's operating frequency by investigating the dipole on the AMC with different pairs of ε_r and μ_r. 이 논문에서는 도체 위 물질의 유전율 투자율에 따른 인공자계도체 특성을 이론적으로 고찰하고 이 인공자계도체 위에 다이폴 안테나화의 상관관계를 규명했다. 먼저 도체 위 물질이 인공자계도체로 동작하는 주파수 및 대역폭을 수식으로 유도하고, 이를 통해 유전율(ε_r), 투자율(μ_r), 그리고 물질 두께(L)가 인공자계도체의 특성에 어떤 영향이 있는지 분석했다. 물질 두께가 λ/4가 되는 주파수에서 반사계수 위상이 0°이 되므로 높은 유전율과 투자율을 가지는 물질로 인공자계도체 설계 시 두께를 줄일 수 있고, 'μ_r/ε_r' 값이 커질수록 인공자계도체 동작 대역폭은 증가(최대 200%)하며, 동작 주파수는 낮아지는 것을 밝혀냈다. 또한 물질의 손실이 존재하면 인공자계도체의 대역폭이 증가하는 것을 확인했다. 인공자계도체 위에 다이폴 안테나를 설계하고 유전율과 투자율을 변경하면서 인공자계도체 표면 반사 위상과 다이폴 안테나의 동작 주파수를 관계를 시뮬레이션을 통해 규명하였다.

이종입자 강화 SiC/Al7075 금속복합재료의 압축특성 및 마모특성 연구

이동현,조승찬,김양도,이상관,이상복,조일국,Lee, Donghyun,Cho, Seungchan,Kim, Yangdo,Lee, Sang-Kwan,Lee, Sang-Bok,Jo, Ilguk 한국복합재료학회 2017 Composites research Vol.30 No.5

본 연구에서는 액상가압공정을 통해 고체적율의 SiC 입자가 균일 분산된 알루미늄 금속복합재료를 제조하고, 미세조직, 기계적 특성 및 내마모 특성에 대해 분석하였다. 입자크기가 다른 이종 SiC 입자가 약 60 vol.% 이상의 체적율로 균일하게 분산된 SiC/Al7075 복합재료는 단일 SiC 입자로 강화된 복합재료에 비해 체적율이 약 12% 이상 높았으며 압축강도가 200 MPa 이상 증가하였다. 내마모시험 결과 이종 SiC 입자 금속복합재료의 경우 마모너비와 깊이가 각각 $285.1{\mu}m$, $0.45{\mu}m$이며, 마찰계수는 0.16으로 내마모 특성이 가장 우수하였다. In this study, we have investigated microstructure, mechanical properties and wear characteristic of aluminum metal matrix composites with a high volume fraction and uniformly dispersed SiC particles which produced by a liquid pressing process. The volume fraction of bimodal SiC/Al7075 composite was 12% higher than that of the monomodal SiC/Al7075 composite and a compressive strength is increased about 200 MPa. As a result of the abrasion test, the wear width and depth of the bimodal SiC/Al7075 composite were $285.1{\mu}m$ and $0.45{\mu}m$, respectively. The coefficient of friction of bimodal SiC/Al7075 was 0.16.

초임계 CO₂ 발전용 파워 터빈의 회전체 동역학 해석 및 구동 시험

이동현,김병옥,선경호,임형수,Lee, Donghyun,Kim, Byungok,Sun, Kyungho,Lim, Hyungsoo 한국트라이볼로지학회 2017 한국윤활학회지(윤활학회지) Vol.33 No.1

This paper presents a rotordynamic analysis and the operation of a power turbine applied to a 250 kW super-critical $CO_2$ cycle. The power turbine consists of a turbine wheel and a shaft supported by two fluid film bearings. We use a tilting pad bearing for the power turbine owing to the high speed operation, and employ copper backing pads to improve the thermal management of the bearing. We conduct a rotordynamic analysis based on the design parameters of the power turbine. The dynamic coefficients of the tilting pad bearings were calculated based on the iso-thermal lubrication theory and turbine wheel was modeled as equivalent inertia. The predicted Cambell diagram showed that there are two critical speeds, namely the conical and bending critical speeds under the rated speed. However, the unbalance response prediction showed that vibration levels are controlled within 10 mm for all speed ranges owing to the high damping ratio of the modes. Additionally, the predicted logarithmic decrement indicates that there is no unstable mode. The power turbine uses compressed air at a temperature of $250^{\circ}C$ in its operation, and we monitor the shaft vibration and temperature of the lubricant during the test. In the steady state, we record a temperature rise of $40^{\circ}C$ between the inlet and outlet lubricant and the measured shaft vibration shows good agreement with the prediction.

초임계 CO₂ 발전용 파워터빈을 지지하는 틸팅패드 베어링의 열윤활 해석 및 패드 온도 측정

이동현,김병옥,임형수,Lee, Donghyun,Kim, Byungok,Lim, Hyungsoo 한국트라이볼로지학회 2018 한국윤활학회지(윤활학회지) Vol.34 No.2

This paper presents the thermohydrodynamic analysis of tilting journal pad bearings supporting a power turbine rotor applied to a 250 kW super-critical $CO_2$ cycle. In the analysis, the generalized Reynolds equation and 3D energy equation are solved to predict oil film temperature and the 3D heat conduction equation is solved for pad temperature. The power turbine rotor is supported by two tilting pad bearings consisting of five pads with an oil supply block between the pads. Copper backing pads with higher thermal conductivity compared to steel backing pads are adopted to improve thermal management. The predicted maximum pad temperature is around $55^{\circ}C$ which is approximately $15^{\circ}C$ higher than oil supply temperature. In addition, the predicted minimum film thickness is 50 mm at a rotating speed of 5,000 rpm. These results indicate that there is no issue in the thermal behavior of the bearing. An operation test is performed with a power turbine module consisting of a power turbine, a reduction gear and a generator. Thermocouples are installed at the 75% position from the leading edge of the pad to monitor pad temperature. The power turbine uses compressed air at a temperature of $250^{\circ}C$ in its operation. The steady state pad temperatures measured in the test show good agreement with the predicted temperatures.

전력 효율적인 통신을 위한 저전력 통신에 대한 조사

이동현(Donghyun Lee),장건희(Gunhee Jang),이철(Cheol Lee),이윤성(Yunseong Lee),이충현(Chunghyun Lee),김남규(Namkyu Kim),조성래(Sungrae Cho) 한국통신학회 2020 韓國通信學會論文誌 Vol.45 No.5

마찰을 고려한 포일 저널베어링의 성능해석

이동현(Donghyun Lee),김영철(Youngchul Kim),김경웅(Kyungwoong Kim) 한국트라이볼로지학회 2006 한국트라이볼로지학회 학술대회 Vol.2006 No.6

In foil bearings, the friction between bumps and their mating surfaces is the major factor which exerts great influence on the performance. Especially, the Coulomb damping which is generated by friction increases the stability of bearing. Therefore, the need for developing analytical tool for friction in a foil bearing performance analysis has been well-recognized. In this paper, a method of considering friction in a foil bearing performance analysis is proposed. To simulate bump motion with friction the algorithm to consider stick-slip phenomenon occurred at a frictional interface is developed using finite element method. By using the proposed method, a nonlinear transient analysis is performed. The results of the analysis show that the relative motion between bumps and their mating surface provide Coulomb damping which is represented by hysteresis loop between applied load and vertical deflection of the bump. An increase in movability of bumps is an effective method of achieving Coulomb damping to enhance stability level of foil bearing. In addition, various static equilibrium positions are available by considering the friction.

Thermal properties of nano fumed silica added epoxy adhesives for micro-assembly

이동현(Donghyun Lee),임지윤(Jiyun Lim),남승웅(Seungwoong Nam),김대흠(Daeheum Kim) 한국생산제조학회 2007 한국공작기계학회 춘계학술대회논문집 Vol.2007 No.-

For synthesis of adhesive material for micro-assemblys, epoxy adhesives that especially compounded with epoxy resins(YD-128, YDPN-638), hardner(Hexahydro-4-methylphthalic anhydride), catalyst(2-ethyl-4-methylimidazole) and filler(nano fumed silica) with ratios of Tg's of epoxy adhesives that cured at 120℃, for 2 hours were analyzed by DSC(Differential Scanning Calorimetry) and TGA(Thermogravimetrical Analyzer). Thermal property increases of epoxy adhesives were confirmed with increasing nano fumed silica contents.

유한 요소법을 이용한 슬립제어용 토크 컨버터 섭동각 예측에 관한 연구

이동현(Donghyun Lee),장영기(Younggi Jang),장재덕(Jaeduk Jang),주인식(In-sik Joo) 한국자동차공학회 2008 한국자동차공학회 지부 학술대회 논문집 Vol.2008 No.11

The lock-up clutch in the torque converter can improve highly the efficiency of power transfer from an engine by locking up with the transmission gears directly at a high speed instead of fluid coupling. Upon locking up, NVH problems take place due to shudder, friction material characteristics, and so on. Front cover taper angle, friction material, contact surface roughness and damper characteristics are important design parameters for improvement of drivability. Front cover taper angle is based on the surface pressure between front cover and friction facing. In this study, the pressure distribution of the contact surface is analyzed by finite element method and Non-linear material condition. Also, we propose design optimization method for slip-control torque converter taper angle.

차량의 공력 성능 향상을 위한 Separation Fin 적용

이동현(Donghyun Lee),최상열(Sangyeol Choi),김용석(Yongsuk Kim) 한국자동차공학회 2012 한국자동차공학회 학술대회 및 전시회 Vol.2012 No.11

On vehicle Aerodynamic performance development, main interest is exterior design. After exterior design fixed, Aerodynamic performance improvement is difficult due to design constraint. Especially on Hatch Back type vehicle, due to vortex air flow on rear area, Aerodynamic performance improvement is harder than Notch Back type vehicle without surface design change. But in many cases, vehicle performance improvement required after design fixed. To improve Aerodynamic performance without exterior surface change, most effective way is attach additional Aerodynamic parts on vehicle. In this study, improvement of drag and lift performance accomplished by CFD analysis and wind tunnel test through additional parts.

마찰을 고려한 포일저널베어링의 동특성해석 : 회전불균형 응답

이동현(Donghyun Lee),김영철(Youngcheol Kim),김경웅(Kyungwoong Kim) 한국트라이볼로지학회 2007 한국트라이볼로지학회 학술대회 Vol.2007 No.6

The dynamic performance of air foil bearings relies on a coupling between a thin air film and an elastic foil structure. A number of successful analytical techniques to predict dynamic performance have been developed. However, the evaluation of its dynamic characteristic is still not enough because of the mechanical complexity of the foil structure and strong nonlinear behavior of friction force. This work presents a nonlinear transient analysis method to predict dynamic performance of foil bearings. In this method, time dependent Reynolds equation is used to calculate pressure distribution and a finite element method is used to model the bump foil structure. The analysis is treated with a direct implicit integration technique that can handle nonlinear problems and the stick-slip algorithm is used to consider friction force. Using this method the response to the mass unbalance excitation is investigated for various design parameters and operating conditions. The results of analysis show that foil bearing is very effective on the restriction of vibration at the resonance frequency compared to the rigid surface bearings and the effectiveness depends on the operating conditions, static load and a amount of mass unbalance. In addition, there exist optimum values of friction coefficient, bump foil sti예less and number of circumferential slit with regards to minimizing dynamic response at the resonance frequency. These optimum values are system dependent.