• 한국지구과학회지
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• 제38권1호
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• pp.1-10
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• 2017

공생별 AG Peg는 적색거성(GS)과 백색왜성(WD)으로 구성된 성운으로 둘러싸인 쌍성계이다. AG Peg의 분광자료는 1998년, 2001년, 그리고 2002년의 세 시기에 미국 Lick 천문대에서 관측한 자료로 HI 발머 방출선 자료를 분석하였다. AG Peg의 선세기와 폭은 각 시기에 따라 변하는데, $H{\alpha}$와 $H{\beta}$ 선에서 모두 청색편이, 적색편이, 넓은 폭 성분이 나타났다. 가스 성운의 운동학적 특성을 보여주는 방출선은 WD주변에 형성된 강착원반의 반경이 매우 큼을 보여준다. 관측자의 시선 방향을 고려하면, 1998년 관측은 AG Peg의 GS와 WD가 나란히 하늘에 있는 반면, 2002년에는 WD가 GS의 전면에, 2001년에는 WD가 GS의 뒷면에 위치하였다. 이러한 상대적인 위치와 분광선의 변화를 고려하여, 우리는 GS에서 WD로의 가스유입이 지속적으로 이루어지고, 그 결과 형성된 두꺼운 원반의 회전이 관측된 분광선 윤곽의 형성을 가져온 것으로 결론지었다.

• 대한기계학회논문집B
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• 제34권10호
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• pp.901-906
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• 2010

복잡한 지형에 위치한 풍력발전소의 유동장을 분석하기 위해 3차원 유동해석이 수행되었다. 본 논문의 목적은 복잡한 지형과 풍력발전기의 배치가 풍력발전소의 성능에 미치는 영향을 연구하는 것이다. 자세한 블레이드 형상을 고려한 총 49대의 풍력발전기가 계산 도메인에 포함되었다. 풍력발전기의 회전운동을 고려하기 위해 고정회전자 기법이 사용되었고, 블레이드에 작용하는 토크를 계산함으로써 풍력발전기의 성능을 평가하였다. 수치해석 결과를 통하여 풍력발전소 전체의 자세한 유동장과 지형적 영향으로 풍속이 감소되는 국부적인 영역을 예측하였고, 상류의 발전기에서 발생하는 후류가 하류에 위치한 발전기의 성능에 미치는 영향도 분석되었다. 본 연구의 해석기법은 추후 건설되는 풍력발전소의 부지와 풍력발전기의 최적 위치를 선정하는 데 사용될 수 있을 것으로 사료된다.

• 천문학논총
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• 제30권2호
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• pp.175-178
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• 2015

Due to the lack of an accretion disk in a polar (magnetic cataclysmic variable, MCV), the material transferred from the secondary is directly accreted onto the white dwarf, forming an accretion stream and a hot spot on the white-dwarf component. During the eclipses, different light components can be isolated. Therefore, the monitoring of eclipsing polars could provide valuable information on several modern astrophysical problems, e.g., CVs as planetary hosting stars, mass transfer and mass accretion in CVs, and the magnetic activity of the most rapidly rotating cool dwarfs. In the past five years, we have monitored about 10 eclipsing polars (e.g., DP Leo and HU Aqr) using several 2-m class telescopes and about 100 eclipse profiles were obtained. In this paper, we will introduce the progress of our research group at YNOs. The first direct evidence of variable mass transfer in a CV is obtained and we show that it is the dark-spot activity that causes the mass transfer in CVs. Magnetic activity cycles of the cool secondary were detected and we show that the variable mass transfer is not caused by magnetic activity cycles. These results will shed light on the structure and evolution of close binary stars (e.g., CVs and Algols).

• Wind and Structures
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• 제25권6호
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• pp.507-535
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• 2017

In the strong wind shutdown state, the blade position significantly affects the streaming behavior and stability performance of wind turbine towers. By selecting the 3M horizontal axis wind turbine independently developed by Nanjing University of Aeronautics and Astronautics as the research object, the CFD method was adopted to simulate the flow field of the tower-blade system at eight shutdown positions within a single rotation period of blades. The effectiveness of the simulation method was validated by comparing the simulation results with standard curves. In addition, the dynamic property, aerostatic response, buckling stability and ultimate bearing capacity of the wind turbine system at different shutdown positions were calculated by using the finite element method. On this basis, the influence regularity of blade shutdown position on the wind-induced response and stability performance of wind turbine systems was derived, with the most unfavorable working conditions of wind-induced buckling failure of this type of wind turbines concluded. The research results implied that within a rotation period of the wind turbine blade, when the blade completely overlaps the tower (Working condition 1), the aerodynamic performance of the system is the poorest while the aerostatic response is relatively small. Since the influence of the structure's geometrical nonlinearity on the system wind-induced response is small, the maximum displacement only has a discrepancy of 0.04. With the blade rotating clockwise, its wind-induced stability performance presents a variation tendency of first-increase-then-decrease. Under Working condition 3, the critical instability wind speed reaches its maximum value, while the critical instability wind speed under Working condition 6 is the smallest. At the same time, the coupling effect between tower and blade leads to a reverse effect which can significantly improve the ultimate bearing capacity of the system. With the reduction of the area of tower shielded by blades, this reverse effect becomes more obvious.

• Smart Structures and Systems
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• 제20권6호
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• pp.683-696
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• 2017

In this study, the reliability of nonlinear ultrasonic modulation based fatigue crack detection is improved using a feature-level data fusion approach. When two ultrasonic inputs at two distinct frequencies are applied to a specimen with a fatigue crack, modulation components at the summation and difference of these two input frequencies appear. First, the spectral amplitudes of the modulation components and their spectral correlations are defined as individual features. Then, a 2D feature space is constructed by combining these two features, and the presence of a fatigue crack is identified in the feature space. The effectiveness of the proposed fatigue crack detection technique is experimentally validated through cyclic loading tests of aluminum plates, full-scale steel girders and a rotating shaft component. Subsequently, the improved reliability of the proposed technique is quantitatively investigated using receiver operating characteristic analysis. The uniqueness of this study lies in (1) improvement of nonlinear ultrasonic modulation based fatigue crack detection reliability using feature-level data fusion, (2) reference-free fatigue crack diagnosis without using the baseline data obtained from the intact condition of the structure, (3) application to full-scale steel girders and shaft component, and (4) quantitative investigation of the improved reliability using receiver operating characteristic analysis.

• 한국해안·해양공학회논문집
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• 제32권1호
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• pp.11-16
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• 2020

인터로킹 케이슨에 의한 장대화된 방파제의 회전 안정성에 대해서 연구하였다. 무한 방파제에 경사지게 입사하는 선형파에 대한 해석해를 이용하여 방파제 기준선 방향으로의 파압 위상차 효과를 고려하였으며, 설계파의 비선형 특성을 고려하기 위하여 설계 기준 Goda 파압식을 차용하였다. 방파제의 회전 안전율을 케이슨 자중에 의한 회전 마찰저항모멘트와 수평 및 수직 파력에 의한 작용 회전모멘트의 비로 정의하고 최소 안전율을 보이는 회전 중심점 위치와 최소 안전율에 대한 해석해를 제시하였다. 규칙파, 불규칙파 및 다방향 불규칙파 등 현행 항만구조물 설계에 사용되고 있는 모든 설계파 조건에 대해서 적용 가능하도록 각 조건에 대한 평가식을 제안하였다.

• Journal of Biosystems Engineering
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• 제26권5호
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• pp.423-430
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• 2001

The purpose of this study is to develop high-speed rotary tillage system for a power tiller by improving the rotary blade and the power train of transmission. Mechanical structure of gear train of rotary drive of conventional power tiller was simplified so that power can be transmitted directly from second shaft to tilling speed change shaft by rotating freely the transfer gear which changes the direction of rotation of shafts using needle bearing installed into middle shaft. A new gear train suitable for the single-edged rotary blade and high-speed rotary drive was developed with the rotational speed of rotary shaft faster than 7.5% at 1st-speed and 1.4% at 2nd-speed the one of conventional system by changing the numbers of teeth of gears of middle shaft, tilling speed change shaft and PTO shaft. Using the developed gear train for high-speed rotary drive, field tests were performed to compare tillage performances by the developed single-edged blade and by the conventional double-edged blade. The results showed that the performances by the single-edged blade compared with the one by the double-edged blade was improved about 18% in field capacity, about 34% in fuel consumption, and 9.4% in soil crushing ratio. Therefore, it may be concluded that tillage performance by the single-edged blade was improved compared to the one by the conventional blade. Evaluation of the developed system consisting of single-edged blade and gear train for high-speed rotary drive in field revealed that tillage performance of the developed system was similar to the one of field test conducted using the system consisting of single-edged blade and gear train for rotary drive of conventional power tiller However, considering the higher cone index of the upland field where evaluation was carried out compare to the one of the ordinary paddy field, it may be concluded that tillage performance of the developed rotary tilling system better than the one of conventional system.

• 한국항공우주학회지
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• 제48권1호
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• pp.43-53
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• 2020

프리스월 노즐과 리시버 홀의 상대적 위치가 반경방향 분사방식 프리스월 시스템의 성능에 미치는 영향을 분석하였다. 본 연구에서는 5개의 프리스월 노즐 위치와 4개의 리시버 홀 위치 조합을 통해 20개의 설계점에 대한 분석을 진행하여 유량계수와 온도 강하 효율 변화 경향성을 연구하였다. 시스템 유량계수는 프리스월 노즐의 압력비와 비슷한 경향을 보였다. 캐비티 내부의 유동이 벽면의 영향을 크게 받을수록 시스템 성능 변화가 발생하였으며 회전면보다 정지면의 영향력이 더 큰 것을 확인하였다. 형상변수 변화에 따라 기준 설계점 대비 유량계수는 -1.39%~1.25%, 온도강하효율은 -5.41%~2.94% 변화하였다.

• 대한기계학회논문집B
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• 제38권12호
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• pp.1043-1050
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• 2014

물이 채워진 원통 용기를 회전시킨 후 배수하면 일정시간이 지난 후 공기기둥이 발생한다. 용기의 크기, 초기 회전 속도나 교반 속도, 배수구 모양 등의 배수조건에 따라 공기기둥의 발생 여부나 발생시간이 달라진다. 본 연구에서는 2 단으로 구성된 계단형 배수구를 가진 원통 용기에서 물이 배수되는 과정을 수치해석적으로 연구하였다. 하부에 위치한 배수구 1 단은 길이와 반경이 고정되어 있고, 상부에 위치한 2 단은 길이와 반경을 변화시켰다. 2 차원 격자계에 축대칭 조건을 적용하여 원통 용기 내부의 배수유동을 해석하였다. 물과 공기의 자유표면 모양을 추적하여 수위 변화를 관찰하였으며, 배수구 2단의 형상이 공기기둥 발생시간과 내부 유동구조 변화에 미치는 영향을 확인하였다.

• 한국기계가공학회지
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• 제18권10호
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• pp.83-92
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• 2019

A pump is considered to be submersible when a motor and a pump are integrated and operate while submerged in water. Submersible pumps mainly function as rejection pumps to prevent foods in densely populated areas, as cold water circulation pumps in large power plants, as pumps to supply irrigation water, as drainage pumps to prevent flooding of agricultural lands, as water supply intake pumps, and as inflow pumps for sewage treatment. The flow in such turbomachines (submersible pumps) inevitably involves various eddy currents. Since it is almost impossible to accurately grasp the complex three-dimensional flow structure and characteristics of a rotating turbomachine through actual testing, three-dimensional numerical analysis using computational fluid dynamics techniques measuring the flow field, velocity, and the pressure can be accurately predicted. In this study, the shape of the impeller was developed to reduce vibration and noise. This was done by increasing the efficiency of the existing submersible pump and reducing turbulence. In order to evaluate the pump's efficiency and turbulence reduction, we tried to analyze the flow using ANSYS Fluent V15.0, a commercial finite element analysis program. The results show that the efficiency of the pump was improved by 4.24% and the Reynolds number was reduced by 15.6%. The performance of a developed pump with reduced turbulence, vibration, and noise was confirmed.