• Wind and Structures
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• 제4권2호
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• pp.85-100
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• 2001

The mechanism of wind-induced ovalling vibrations of cylindrical shells is numerically investigated by using a vortex method. The subject of this paper is limited to a two-dimensional structure in the subcritical regime. The aerodynamic stability of the ovalling vibrations in the second to fourth circumferential modes is discussed, based on the results of a forced-vibration test. In the analysis, two modal configurations are considered; one is symmetric and the other is anti-symmetric with respect to a diameter parallel to the flow direction. The unsteady pressures acting on a vibrating cylinder are simulated and the work done by them for one cycle of a harmonic motion is computed. The effects of a splitter plate on the flow around the cylinder as well as on the aerodynamic stability of the ovalling vibrations are also discussed. The consideration on the mechanism of ovalling vibrations is verified by the results of a free-vibration test.

• EDISON SW 활용 경진대회 논문집
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• 제1회(2012년)
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• pp.33-36
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• 2012

풍력발전기 성능은 유동의 안정성과 풍속에 의해 결정되는데, 이때 유동 불안정성은 풍력발전기의 성능뿐만 아니라 구조적 문제를 함께 유발시킨다. 본 연구에서는 풍력발전기 타워 후류에서의 불안정성을 최소화시키기 위하여 타워 단면의 기초 형상설계 연구를 수행하였다. 기존의 풍력발전기 타워 형상에 부가 구조물을 설치함으로써 Karman vortex의 생성을 지연시키고 와류 간섭현상을 줄여 풍력발전기의 안정성을 증대시키고자 하였다. 이를 위해 다양한 타워 단면 형상에 대하여 양력계수 및 항력계수를 비교 분석하였다. 그 결과 반지름의 1/2 길이의 자유류 방향 tip과 splitter plate를 후방에 설치하는 것이 후류 불안정성을 억제하는데 가장 효율적인 것으로 나타났다.

• 한국군사과학기술학회지
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• 제2권2호
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• pp.279-286
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• 1999

Wall-bounded 유동과 달리 자유 전단 유동은 Hyperbolic Tangent Profile을 가지고 비점성 불안정에 의해 지배된다. 따라서 자유 전단 유동에서 난류로의 천이과정은 비점성 불안정성 이론에 의해 해석되어 진다. 본 연구는 분리판(Splitter plate)에 의해 분리된 속도가 다른 자유 유속의 혼합에 의해 형성되는 혼합층에서 와류병합과정에 대한 속도비의 영향에 대하여 연구한다. 속도비는, R, $ \frac{U_1-U_2}{U_1+U_2}$ 로 정의되며, 여기서 $U_1$은 분리판 위에서의 자유 유속을 그리고 $U_2$ 는 분리판 아래에서의 자유 유속을 나타낸다. 본 연구에서 와류구조의 병합작용을 분석하기 위하여 2차원 비정상 Large-Eddy Simulation 방법을 적용하였다. 속도비의 변화에 따라 혼합층에서 불안정 Wave가 성장하게 되고, 유체는 2차원 와류구조에서 Roll-up한다. 이러한 2차원 와류구조는 주위의 다른 와류구조와 상호작용을 하게 되고 하나의 커다란 와류구조를 형성하는 것을 볼 수 있다. 혼합층에서 와류병합과정은 반복적으로 일어나는 것을 알 수 있었고, 이 결과를 이용하여 혼합층의 성장을 제어할 수 있다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 추계학술대회논문집
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• pp.379-383
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• 2000

This paper describes the design program of duct silencer. Duct silencer is used ventilating system. Variables in the program for predicting transmission loss are width of splitter, airway width, perforate plate, absorption material and frequency. Generally used expression for predicting transmission loss has something that don't think about frequency characteristic. Therefore we propose the new expression that considered frequency characteristic. In the 1/1 octave band center frequency, the expected weighting number(K) is fitted with absorption ratio and airway width. The fitted 2nd degree polynomial expression is based on the test data performed in YOUIL Industrial Corperation. This program's accuracy is about 90 percent.

• Wind and Structures
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• 제9권1호
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• pp.1-22
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• 2006

The response characteristics and suppression of flow-induced vibrations of rectangular prisms with various width-to-depth ratios were experimentally investigated. The prisms were rigid and elastically mounted at both ends to enable constrained torsional vibrations only. The present study focused on torsional vibrations, one of the three types of flow-induced vibrations generated in a rectangular prism. First, the response characteristics of torsional vibrations generated in rectangular prisms were investigated by free-vibration tests. It was found that the response characteristics of torsional vibrations generated in rectangular prisms could be classified into six patterns depending on the width-to-depth ratio. Next, the response characteristics of torsional vibrations observed in the free-vibration tests were reproduced by forced-vibration tests, and the mechanisms by which the three types of flow-induced vibrations, low-speed torsional flutter, vortex excitation and high-speed torsional flutter, are generated in the rectangular prisms were elucidated on the basis of characteristics of fluid forces and visualized flow patterns. Experiments were also carried out to establish an effective method for suppressing flow-induced vibrations generated in the rectangular prisms, and it was found that low-speed torsional flutter and high-speed torsional flutter could be suppressed by placing a small normal plate upstream of the prism, which results in suppression of the alternating rolling-up of the shear layers separating from the leading edges of the prism. It was also found that vortex excitation could be suppressed by placing a splitter plate downstream of the prism, which results in suppression of the generation of wake vortices.

• 한국군사과학기술학회지
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• 제20권5호
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• pp.654-664
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• 2017

The temperature profiles upon high speed impact of reactive structural materials were analyzed. A two color pyrometer, which included high-speed camera, spectral splitter, and band pass filters, was utilized to measure transient temperature profiles during and after reactive metal samples impacted into steel plate with velocities of 1600~1700 m/s. The spatial temperature distribution was analyzed from the ratio of spectral radiances at two different wavelength in infrared zone, i.e. 700 and 900 nm. The measured temperatures were calibrated with black body source. Two different types of metal samples, namely aluminum and nickel, were employed to understand reaction behavior upon the impact of samples in ambient condition. According to our experiments, the Ni sample appeared to barely react with ambient air producing an instant small fireball, while Al sample reacts violently with air generating a relatively prolong fireball.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1992년도 하계학술대회 논문집 B
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• pp.794-798
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• 1992

We present an optical Voltage and current sensor using $BSO(Bi_{12}SiO_{20})$ monocrystal. The voltage and current sensor consist of PBS(Polarizing Beam Splitter), 1/4 wavelength plate, ZnSe, Selfoc lens, LED, and PIN-PD etc. Magnetic core was made using permalloy for applying magnetic field to current sensor effectively. Current was measured from 100 to 1,600 ampere and accuracy was about ${\pm}$5%. The accuracy could be improved to ${\pm}$l% after reducing the nonlinear property of BSO crystal using our own program in PC (IBM286). We noticed that these data were not influenced by 154,000 voltage at all. Applied voltage was reduced to 1/20 using capacitors. And experiment was carried out up to 450V of the reduced voltage. The data fran optical voltage sensor was similar to that from conventional voltage sensor. The accuracy of the data was within about ${\pm}$1%.

• 한국전산유체공학회지
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• 제18권1호
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• pp.36-42
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• 2013

The performance of the wind turbine is determined by wind speed and unsteady flow characteristics. Unsteady wake flow causes not only the decline in performance but also structural problems of the wind turbine. In this paper, conceptual designs for the wind turbine tower are conducted to minimize unsteady wake flow. Numerical simulations are performed to inspect the shape effect of the tower. Through the installation of additional structures at the rear of the tower, the creation of Karman vortex is delayed properly and vortex interactions are reduced extremely, which enhance the stability of the wind turbine. From the comparative analysis of lift and drag coefficients for each structure, it is concluded that two streamwise tips with a splitter plate have the most improved aerodynamic characteristics in stabilizing wake flow.

• 한국광학회지
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• 제26권1호
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• pp.38-43
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• 2015

반사도 가변형 출력경을 갖는 긴 공진기형 선편광 Yb:YAG 레이저를 제안하고 그 출력 특성을 조사하였다. 반사도 가변형 출력경은 편광 광 분할기와 1/4 파장판로 구성되어 있다. 제안된 레이저 공진기 길이는 약 3.7 m 이다. 제안된 레이저의 기울기 효율은 19%이고 빔질 ($M^2$)는 약 1.2이었다.

• Wind and Structures
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• 제16권5호
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• pp.517-540
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• 2013

This paper presents applications of proper orthogonal decomposition in both the time and frequency domains based on both cross spectral matrix and covariance matrix branches to analyze multi-variate unsteady pressure fields on prisms and to study spanwise and chordwise pressure distribution. Furthermore, modification of proper orthogonal decomposition is applied to a rectangular spanwise coherence matrix in order to investigate the spanwise correlation and coherence of the unsteady pressure fields. The unsteady pressure fields have been directly measured in wind tunnel tests on some typical prisms with slenderness ratios B/D=1, B/D=1 with a splitter plate in the wake, and B/D=5. Significance and contribution of the first covariance mode associated with the first principal coordinates as well as those of the first spectral eigenvalue and associated spectral mode are clarified by synthesis of the unsteady pressure fields and identification of intrinsic events inside the unsteady pressure fields. Spanwise coherence of the unsteady pressure fields has been mapped the first time ever for better understanding of their intrinsic characteristics.