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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<sub>r</sub>, permeability μ<sub>r</sub>, 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<sub>r</sub>, μ<sub>r</sub> 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<sub>r</sub> and μ<sub>r</sub>. As the ratio μ<sub>r</sub>/ε<sub>r</sub> 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<sub>r</sub> and μ<sub>r</sub>. 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<sub>r</sub>), 투자율(μ<sub>r</sub>), 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<sub>r</sub>/ε<sub>r</sub>' 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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.
플로팅 링 베어링으로 지지된 터보차저 로터의 안정성 해석
이동현(Donghyun Lee),김영철(Youngcheol Kim),김병옥(Byungok Kim) 한국트라이볼로지학회 2015 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.31 No.6
The use of turbocharger in internal combustion engines has increased as it is a key components for improving system efficiency without increasing engine size. Because of increasing demand, many studies have evaluated rotordynamic performance so as to increase rotation speed. This paper presents a linear and nonlinear analysis model for a turbocharger rotor supported by a floating ring bearing. We constructed rotor model by using the finite element method and approximated bearings as being infinitely short. In the linear model, we considered fluid film force as stiffness and damping element. In nonlinear analysis, calculation of the fluid film force involved solving the time dependent Reynolds equation. We verified the developed model by comparing the results to those of previous research. The analysis results show that there are four unstable modes, which are rigid body modes combining ring and rotor motion. As the rotating speed increases, the logarithmic decrement shows that certain unstable modes goes into the stable area or the stable mode goes into the unstable area. These unstable modes appear as sub-synchronous vibrations in nonlinear analysis. In nonlinear analysis frequency jump phenomenon demonstrated in several experimental studies appears. The analysis results also showed that frequency jump phenomenon occurs when the vibration mode changes and the sequence of unstable mode matches the linear analysis result. However, the natural frequency predicted using linear analysis differs from those obtained using nonlinear analysis.
Active Aero Shutter를 통한 차량의 Drag 성능 개선
이동현(Donghyun Lee),하종백(Jongpaek Ha),김영호(Youngho Kim) 한국자동차공학회 2010 한국자동차공학회 학술대회 및 전시회 Vol.2010 No.11
After vehicle design fixed, aerodynamic performance development has many restriction. It is due to major factor of Aerodynamic performance is vehicle styling. Among of restriction, cooling performance is one of biggest restriction. For vehicle cooling, large amount of airflow should be go through heat exchanger which called cooling drag. The cooling drag takes biggest part of overall vehicle drag. So, we always try to optimize vehicle front grille opening position, size and texture to reduce cooling drag within cooling requirement at early vehicle development stage. But, after style fixed and even grille opening optimized, there are need to reduce vehicle drag due to better fuel economy. For this many additional part applied to vehicle exterior, but drag performance improvement by additional part is not effective. At this study, applying active aero shutter, minimize cooling drag without any influence cooling performance, effectively improve vehicle drag performance.
초임계 CO₂ 발전용 파워터빈을 지지하는 틸팅패드 베어링의 열윤활 해석 및 패드 온도 측정
이동현(Donghyun Lee),김병옥(Byungok Kim),임형수(Hyungsoo Lim) 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 CO₂ 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 °C which is approximately 15 °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 °C in its operation. The steady state pad temperatures measured in the test show good agreement with the predicted temperatures.