• 한국해양공학회지
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• 제24권1호
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• pp.10-19
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• 2010

ANSYS multi-physics software was applied to solve the coupled dynamic problem related to a full-scale TLP foundation for floating wind turbines. In this coupled dynamics simulation, the forced oscillation imposed on the tethers' top resulting from the sway of the wind turbine platform and the self-excited vortex-induced vibration (VIV) along the tether span have been taken into account. The stability of this tensioned tether system has been validated in the form of separate static and dynamic analyses. The dynamic characteristics of the tensioned tether linked to the floating wind turbine were analyzed by the resultant modal form and its corresponding vortex shedding pattern. The calculated result shows that even a slight forced oscillation imposed on the tethers' top leads to the VIV amplification and enhances the risk of instability in the case of low pretension. It is also found that the "synchronization" would be aggravated when the top tension decreases and the "2P" vortex shedding mode takes place. The increased top tension imposed on the tethers contributes to the stability of the tensioned legs by diminishing the oscillation amplitude markedly.

• 대한기계학회논문집B
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• 제30권4호
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• pp.343-349
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• 2006

Systematic experiments in $CH_4/Air$ counterflow diffusion flames diluted with He have been undertaken to study the oscillatory instability in which lateral flame size was less than burner nozzle diameter and thus lateral heat loss could be remarkable at low global strain rate. The oscillatory instability arises for Lewis numbers greater than unity and occurs near extinction condition. The oscillation is the direct outcome from the advancing and retreating edge flame. The dynamic behaviors of extinction in this configuration can be classified into three modes; growing, harmonic and decaying oscillation mode near extinction. As the global strain rate decreases, the amplitude of the oscillation becomes larger. This is caused by the increase of lateral heat loss which can be confirmed by the reduction of lateral flame size. Oscillatory edge flame instabilities at low global strain rate are shown to be closely associated with not only Lewis number but also heat loss (radiation and lateral heat loss).

• 한국광학회지
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• 제10권6호
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• pp.512-517
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• 1999

이완발진 현상을 이용하는 단공진기 색소레이저(QDL:quenched dye laser)는 극단펄스 광원으로서 많은 장점을 갖고 있다. 본 연구에서는 파장 590nm에서 발진 스펙트럼의 최대값을 갖는 Rhodamine 6G를 레이저 매질로 하고, 파장 308nm의 XeCl 레이저를 펌핑원으로 하여, 이득의 길이가 5mm인 QDL의 이완발진에 의한 극단펄스 발생특성을 컴퓨터 시뮬레이션에 의해 분석하였다. 분석결과, 펌핑광의 펄스폭에 대하여 QDL 출력빔은 1/100 이하로 그 펄스폭이 짧아짐을 밝혀 QDL 이 극단펄스 발생용으로 매우 유용한 레이저임을 밝혔으며, 펌핑광의 공간적 크기를 조절함에 의하여 출력 펄스폭의 연속적인 가변동작이 가능함을 제시하였다.

• 대한기계학회논문집
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• 제18권2호
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• pp.456-466
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• 1994

Experimental investigation was made to study the mechanism of fluid and thermal oscillation phenomena of surface-tension driven flow in a cylindrical liquid column heated from above which is the low-gravity floating zone simulated on earth. Hexadecane, octadecane, silicon oil (10cs), FC-40 and water are used as the test liquids. The onset of the oscillatory thermocapillary convection appears when Marangoni number exceeds its criteria value and is found to be due to the coupling among velocity and temperature field with the free surface deformation. The frequency of temperature oscillation decreases with increasing aspect ratio for a given diameter and Marangoni number and the oscillation level increases with Marangoni number. The flow pattern in the liquid column appears either as symmetric or asymmetric 3-D flow due to the oscillatory flow in the azimuthal direction. The free surface deformation also occurs either as symmetric or asymmetric mode and its frequency is consistent with those of flow and temperature oscillations. The amplitude of surface deformation also increases with Marangoni number.

• Journal of Astronomy and Space Sciences
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• 제36권3호
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• pp.149-157
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• 2019

In this study, we investigate the associations between the solar variability and teleconnection indices, which influence atmospheric circulation and subsequently, the spatial distribution of the global pressure system. A study of the link between the Sun and a large-scale mode of climate variability, which may indirectly affect the Earth's climate and weather, is crucial because the feedbacks of solar variability to an autogenic or internal process should be considered with due care. We have calculated the normalized cross-correlations of the total sunspot area, the total sunspot number, and the solar North-South asymmetry with teleconnection indices. We have found that the Southern Oscillation Index (SOI) index is anti-correlated with both solar activity and the solar North-South asymmetry, with a ~3-year lag. This finding not only agrees with the fact that El $Ni{\tilde{n}}o$ episodes are likely to occur around the solar maximum, but also explains why tropical cyclones occurring in the solar maximum periods and in El $Ni{\tilde{n}}o$ periods appear similar. Conversely, other teleconnection indices, such as the Arctic Oscillation (AO) index, the Antarctic Oscillation (AAO) index, and the Pacific-North American (PNA) index, are weakly or only slightly correlated with solar activity, which emphasizes that response of terrestrial climate and weather to solar variability are local in space. It is also found that correlations between teleconnection indices and solar activity are as good as correlations resulting from the teleconnection indices themselves.

• Journal of Power Electronics
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• 제19권1호
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• pp.307-315
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• 2019

Condition monitoring has been recognized as an effective and low-cost method to enhance the reliability and improve the maintainability of power electronic converters. In power electronic converters, high-frequency oscillation occurs during the switching transients of power transistors, which is known as ringing. The ringing frequency mainly depends on the values of the parasitic capacitance and stray inductance in the oscillation loop. Although circuit stray inductance is an important factor that leads to the ringing, it does not change with transistor aging. A shift in either the inside inductance or junction capacitance is an important failure precursor for power transistors. Therefore, ringing frequency can be used to monitor the health of power transistors. However, the switching actions of power transistors usually result in a dynamic behavior that can generate oscillation signals mixed with background noise, which makes it hard to directly extract the ringing frequency. A frequency extraction method based on empirical mode decomposition (EMD) and Fast Fourier transformation (FFT) is proposed in this paper. The proposed method is simple and has a high precision. Simulation results are given to verify the ringing analysis and experimental results are given to verify the effectiveness of the proposed method.

• 천문학회보
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• 제44권2호
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• pp.40.3-40.3
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• 2019

Our recent observations of the Sun through strong spectral lines have revealed several important properties of the three-minute umbral oscillations inside sunspots -- the oscillations of intensity and Doppler velocity with periods of 2 to 3 minutes. The oscillations usually occur in the form of a time series of oscillation packets each of which lasts 10 to 20 minutes, not as continuous trains. Each oscillation packet is characterized by a singly peaked power spectrum of velocity oscillation. The oscillations propagate in the vertical direction from the photosphere to the corona. In the upper chromosphere, they develop into shocks that eventually collide with the transition region. When shocks propagate along a highly inclined direction, the merging of two successive shocks can take place. Once they enter the corona, they change to linear compressional waves. In the image plane, the three-minute oscillations propagate with high speeds in the transverse direction as well, usually propagating radially outwards from a point, and sometimes accompanying spiraling patterns of Doppler velocity. These observational properties can be theoretically explained by postulating the spatio-temporally localized source of fast MHD waves at a depth of about 2000 km below the surface, the excitation of slow MHD waves via mode conversion near the photosphere, and the resonance of the slow waves in the photospheric layer below the temperature minimum, and the nonlinear development of slow waves in the chromosphere.

• Journal of Electrical Engineering and Technology
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• 제10권5호
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• pp.1989-2000
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• 2015

In this paper, digital peak current mode control for single phase H-bridge inverters is developed and implemented. The digital peak current mode control is achieved by directly controlling the PWM signals by cycle-by-cycle current limitation. Unlike the DC-DC converter where the output voltage always remains in the positive region, the output of DC-AC inverter flips from positive to negative region continuously. Therefore, when the inverter operates in negative region, the control should be changed to valley current mode control. Thus, a novel control logic circuit is required for the function and need to be analyzed for the hardware to track the sinusoidal reference in both regions. The problem of sub-harmonic instability which is inherent with peak current mode control is also addressed, and then proposes the digital slope compensation in constant-sloped external ramp to suppress the oscillation. For unipolar PWM switching method, an adaptive slope compensation in digital manner is also proposed. In this paper, the operating principles and design guidelines of the proposed scheme are presented, along with the performance analysis and numerical simulation. Also, a 200W inverter hardware prototype has been implemented for experimental verification of the proposed controller scheme.

• 전기학회논문지
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• 제63권7호
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• pp.875-882
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• 2014

In this paper, we propose a new parameter estimation method that can deal with the data of multiple time intervals simultaneously. If there are common modes in the multiple time intervals, it is possible to create a new polynomial by summing the coefficients of the prediction error polynomials of each time interval. By calculating the roots of the new polynomial, it is possible to estimate the common modes that exist in each time interval. The accuracy of the proposed parameter estimation method has been proven by using appropriate test signals.

• 대한전자공학회논문지
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• 제10권2호
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• pp.7-15
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• 1973

전압제어형 부성저항소자를 사용하여 말티바이브레이터를 설계할 경우의 안정점 설정과 회로동작 안정성에 관하여 논한다. 즉 직류적으로 한개 혹은 두개의 안정점을 가지도록 설계된 회로가 일시적 흑은 영구적 발진을 일으키는 현상에 대하여 아나로그 계산기의 모의에 의하여 해석하고, 실험적으로 확인함으로써, 부성저항스윗칭회로 설계상의 유의점을 제시한다.