• 설비공학논문집
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• 제7권1호
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• pp.132-141
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• 1995

Characteristics of pressure-drop oscillations(PDO) in a boiling channel were studied numerically and compared with experimental data. Effects of initial and boundary conditions on PDO were investigated in terms of oscillation period and amplitude. The period and amplitude of PDO increased with increasing of the compressible volume in the surge tank and the heat input. PDO occurred within the specific range of the fluid temperature, at which oscillation period and amplitude diminished rapidly with the increase of the fluid temperature. The increase of the loss coefficient in fluid supply line resulted in slightly longer oscillation period and larger amplitude. Numerical results showed good agreement with the experimental data.

• 설비공학논문집
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• 제5권3호
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• pp.179-186
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• 1993

The characteristics of pressure-drop oscillations(PDO) in boiling channel are studied experimentally. The effects of initial and boundary conditions on PDO are investigated in terms of oscillation period and amplitude. The period and amplitude of PDO are increased with the increase in the compressible volume in surge tank and heat input. However the amplitude of PDO is decreased with fluid temperature under low subcooling condition. Higher initial insurge flowrate resulted in almost invariant oscillation period but lower amplitude. At higher heat input the oscillation of heater wall temperature is significant, whose period is the same as that of pressure-drop instability.

• Molecules and Cells
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• 제43권3호
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• pp.276-285
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• 2020

Circadian rhythm is an endogenous oscillation of about 24-h period in many physiological processes and behaviors. This daily oscillation is maintained by the molecular clock machinery with transcriptional-translational feedback loops mediated by clock genes including Period2 (Per2) and Bmal1. Recently, it was revealed that gut microbiome exerts a significant impact on the circadian physiology and behavior of its host; however, the mechanism through which it regulates the molecular clock has remained elusive. 3-(4-hydroxyphenyl)propionic acid (4-OH-PPA) and 3-phenylpropionic acid (PPA) are major metabolites exclusively produced by Clostridium sporogenes and may function as unique chemical messengers communicating with its host. In the present study, we examined if two C. sporogenes-derived metabolites can modulate the oscillation of mammalian molecular clock. Interestingly, 4-OH-PPA and PPA increased the amplitude of both PER2 and Bmal1 oscillation in a dose-dependent manner following their administration immediately after the nadir or the peak of their rhythm. The phase of PER2 oscillation responded differently depending on the mode of administration of the metabolites. In addition, using an organotypic slice culture ex vivo, treatment with 4-OH-PPA increased the amplitude and lengthened the period of PER2 oscillation in the suprachiasmatic nucleus and other tissues. In summary, two C. sporogenes-derived metabolites are involved in the regulation of circadian oscillation of Per2 and Bmal1 clock genes in the host's peripheral and central clock machineries.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2003년도 하계종합학술대회 논문집 V
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• pp.2835-2838
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• 2003

We study the characteristics of oscillating in non-autonomous condition and the conducted noise generation in a RL-photocoupler circuit. This circuit may be shown a period-doubling and a chaos dynamics under any specific conditions of input circuit. But, the relationship between input signals and output signals is different according to the amplitude of driving input voltage. Then, the oscillation noise was analyzed with respect to both the frequency and the amplitude of an external ac signal and do values. The results show that the noise-induced oscillations for falling and rising cycles induced by kick-back effect in an inductor, nonlinear capacitance, nonlinear resistance and charge storage time in a diode and an LED. We also compared the simulation with the experimental results.

• 한국정보처리학회논문지
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• 제3권4호
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• pp.877-882
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• 1996

Bonhoeffer-Van der Pol(BVP)모델을 실제 소자값을 이용하여 하드웨어를 구현하고 A1 coswt를 인가하여 주기 운동과 카오스 운동을 조사하였다. BVP모델의 하드웨어를 구성하기 위해 컴퓨터 시뮬레이션〔11〕에 의해 구현된 결과를 이용하여 실제 소자값 으로 Rescaling 하였으며 각 계수의 값을 a=0.7, b=0.8, c=0.1로 정하고 주기적 자극 전류의 진폭을 0에서 1.3까지 변화시켜 주기운동에서 카오스 운동으로 천이됨을 위 상공간, 시계열 데이터로 확인하였다.

• 한국결정성장학회지
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• 제8권1호
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• pp.18-28
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• 1998

초크랄스키 대류에서의 온도진동 억제에 대한 실험 및 수치해석 연구가 수행되었다. 결정봉 회전 가속화기법을 초크랄스키 성장에 적용시키기 위해 결정봉 회전가속도를($\Omega=\Omega_0(1+A sin 2{\pi}ft/t_p)$)로 변화시켰다. 여기서 A는 가진증율, f는 가진주파수인자를 나타낸다. 제어이전에 나타나는 고유한 온도진동 주기 ($t_p$)를 근거하여, 무차원 혼합대류인자($0.217{\leq}Ra/PrRe^2{\leq}1.658$)에 온도진동 감소율을 조사하였다. 또한 제어인자 A와 f에 대한 효과를 조사하였다. 용융유동 내의 온도진동 억제 현상을 이해하기 위해 자오면상에 나타나는 온도 및 와도분포가 면밀히 검토되었다.

• 대한조선학회논문집
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• 제55권1호
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• pp.56-65
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• 2018

Pressure oscillation caused by the compressibility of entrapped air in dam break flow is analyzed using an open source code, which is a two-phase compressible code for non-isothermal immiscible fluids. Since compressible flows are computed based on a pressure-based method, the code can handle the equation of state of barotropic fluid, which is virtually incompressible. The computed time variation of pressure is compared with other experimental and computational results. The present result shows good agreements with other results until the air is entrapped. As the entrapped air bubbles pulsate, pressure oscillations are predicted and the pressure oscillations damp out quickly. Although the compressibility parameter of water has been varied for a wide range, it has no effects on the computed results, because the present equation of state for water is so close to that of incompressible fluid. Grid independency test for computed time variation of pressure shows that all results predict similar period of pressure oscillation and quick damping out of the oscillation, even though the amplitude of pressure oscillation is sensitive to the velocity field at the moment of the entrapping. It is observed that as pressure inside the entrapped air changes quickly, the pressure field in the neighboring water adjusts instantly, because the sound of speed is much higher in water. It is confirmed that the period of pressure oscillation is dominated by the added mass of neighboring water. It is found that the temperature oscillation of the entrapped air is critical to the quick damping out of the oscillations, due to the fact that the time averaged temperature inside the entrapped air is higher than that of surrounding water, which is almost constant.

• 수산해양기술연구
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• 제50권4호
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• pp.435-446
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• 2014

Tidal turbulence was examined using three-dimensional tidal velocity data observed at a trench offshore of Tongyoung, Korea. The kinetic energy and intensity, including the variation period of the flow velocity and direction, were used to investigate the relationships between tidal turbulence and fishing gear dynamics, including the effects of swimming fish during fishing operations. As the resultant velocity increased from 0.2 to 0.9 m/s, the kinetic energy also significantly increased, while the turbulence intensity decreased from 50 to 10%. Tidal flow in strong flow fields displayed shorter periods of between 4 and 10 s, as determined by fast Fourier transform, the global wavelet method, and peak event analysis, and the periods were compared with the period of response to swimming fish and to oscillation of fishing gear. As mean velocity increased, velocity amplitude also increased from 0.1 to 0.6 m/s, and its directional amplitude changed markedly from 20 and $90^{\circ}$. Our study suggests that tidal turbulence can influence fish behavior or fishing gear geometry during fishing operations, although our analysis considered only a limited area. In future work, observations should be carried out over a more extensive depth and area.

• 한국해양공학회지
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• 제32권5호
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• pp.341-350
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• 2018

A series of model tests was carried out to investigate the vortex-induced vibration (VIV) characteristics of a free hanging PVC pipe under forced oscillation conditions. The prescribed displacement with a period and amplitude was forced at the top of the riser. The motion of the riser along its length was measured with underwater cameras in three dimensions. The top-excited responses in the inline direction and vortex-induced vibration in the cross-flow direction were examined in the time and frequency domains. Multi-peak frequencies in the VIV were demonstrated to be strongly dependent upon the Keulegan-Carpenter number, corresponding with the results of Blevin. It was found that the Reynolds numbers (excitation period) was a critical parameter for the dominant VIV characteristics, even under the condition of using the same Keulegan-Carpenter number, under the top-excited condition. In the resonance at the nth natural frequency by the forced-motion induced VIV frequency, the riser responded with a large amplitude and forced frequency, dominantly in the VIV CF direction.

• 천문학회보
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• 제44권1호
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• pp.53.2-53.2
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• 2019

$H{\alpha}$ surges (i.e. cool/dense collimated plasma ejections) may act as a guide for a propagation of magnetohydrodynamic waves. We report a high-resolution observation of a surge observed with 1.6m Goody Solar Telescope (GST) on 2009 August 26, from 18:20~UT to 18:45UT. Characteristics of plasma motions in the surge are determined with the normalizing radial gradient filter and the Fourier motion filter. The shape of the surge is found to change from a 'C' shape to an inverse 'C' shape after a formation of a cusp, a signature of reconnection. There are apparent upflows seen above the cusp top and downflows below it. The upflows show rising and rotational motions in the right-hand direction, with the rotational speed decreasing with height. Near the cusp top, we find a transverse oscillation of the surge, with the period of ~2 min. There is no change of the oscillation phase below the cusp top, but above the top a phase change is identified, giving a vertical phase speed about 86kms-1. As the height increases, the initial amplitude of the oscillation increases, and the oscillation damping time decreases from 5.13 to 1.18min. We conclude that the oscillation is a propagating kink wave that is possibly excited by an x-point oscillation.