• 한국통신학회논문지
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• 제23권5호
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• pp.1270-1281
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• 1998

주파수 옵셋은 OFDM 신호의 부채널간 간섭을 유발하며, 수신 신호의 진폭과 위상을 왜곡시켜 전체적인 시스템 성능에 심각한 영향을 미친다. 본 논문에서는 이러한 주파수 옵셋을 추정하기 위해서 특정 부채널을 동기채널로 할당하고 파일럿 심벌을 전송하여 주파수 옵셋을 추정하는 개선된 최우 추정 방식을 제안하며, 제안된 주파수 옵셋 추정기의 오차 성능을 해석적으로 유도한다. 제안된 방식에서는 coherent 위상 변화에 의해 발생된 주파수 옵셋 부분을 증가 시키고 랜덤 위상 오차에 의해 발생된 주파수 옵셋 부분을 감소시킴으로써 주파수 옵셋 추정기의 성능을 향상시킨다. 모의 실험을 통하여 본 논문에서, 해석적으로 유도된 제안된 주파수 옵셋 추정기의 오차 분산의 upper bound가 올바르며, 제안된 추정기가 기존의 방식에 비해 오차 분산, 추적 범위, 시변 채널 영향의 면에서 우수하다는 것을 확인한다.

• 한국광학회지
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• 제18권1호
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• pp.44-49
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• 2007

광섬유 망을 통해 수 kHz의 좁은 선폭을 가지는 $1.5{\mu}m$ 레이저광원을 높은 전송 안정도로 전송하였다. 525 m 길이의 단일모드 광섬유를 통과하면서 발생하는 광 위상 잡음을 능동적으로 제거하는 실험 장치를 구성하였고, 이렇게 하여 전송된 광 주파수의 전송 안정도는 1 초의 평균시간에서 $2{\times}10^{-17}$로 측정되었다. 전송된 광 주파수의 품질을 주파수 영역과 시간 영역에서 정량적으로 분석하였다.

• 대한조선학회논문집
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• 제39권3호
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• pp.41-47
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• 2002

선박용 프로펠러 후류의 유동 특성을 적응형 하이브리드 2-frame PTV(Particle Tracking Velocimetry)기법을 적용하여 실험적으로 해석하였다. 프로펠러 위상각에 대해 위상평균하여 하류로 나아감에 따른 후류 유동의 발달과정을 연구하였으며, 주 유동 방향으로 날개의 뒷날로부터 프로펠러 직경만큼의 거리까지를 측정하였다. 하나의 날개에 대해 4개의 다른 위상각 각각에서 얻은 위상평균 속도장 결과는 프로펠러 날개의 압력 차이로 인해 발생하는 주기적인 날개끝 보오텍스가 하류로 이동해 나감을 보여주고 있다. 또한, 프로펠러 날개 표면을 따라 발달하는 경계층에 기인한 점성 후류는 축방향 속도성분의 결손을 가진다. 프로펠러 날개 뒷날에서 발생하는 후연 보오텍스는 하류로 나아감에 따라 수축되며 점성 소산으로 인해 그 세기 및 크기는 점차 작아졌다.

• 한국전자파학회논문지
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• 제13권8호
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• pp.797-803
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• 2002

본 논문에서는 라이시안(Rician) 감쇄가 존재하는 채널 환경에서 협대역 잡음이 더하여진 경우에 대역 여파기를 통과한 수신신호의 위상오류에 대한 확률 밀도 함수를 유도하고 0차 모먼트가 1이 됨을 보임으로써 확률밀도 함수로서의 타당성을 검증한다. 일반적으로 감쇄 환경에서 시스템의 오류 확률은 먼저 가산성 백색 가우시안 잡음(AWGN : Additive White Gaussian Noise)만이 존재할 때의 오류 확률을 구한 후, 그 결과 식을 해당 감쇄에 대한 확률 밀도 함수로 평균을 취하여 구한다. 하지만 본 논문에서는 감쇄 환경에서의 수신 신호에 대한 위상 오류식을 구한 다음, 그 식을 한번의 이중 적분을 취함으로써 오류 확률을 구하게 된다.

• 대한기계학회논문집B
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• 제21권3호
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• pp.427-435
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• 1997

Conditional sampling techniques are utilized to investigate the relation between the wall skin-friction and stream wise velocity fluctuations in a turbulent boundary layer. Conditionally averaged results using a peak detection and the VITA (variable-interval time-averaging) technique show that a high skin friction is associated with high frequency components of the wall skin-friction fluctuations. The conditionally averaged wall skin-friction fluctuations obtained by using the VITA technique have a positively-skewed characteristics compared with the conditionally averaged stream wise velocity fluctuations. It is confirmed that there exists a phase shift between the wall skin-friction and stream wise velocity fluctuations, which was also found from the long-time averaged space-time correlations. The amount of phase shift between the wall skin-friction and stream wise velocity fluctuations is the same as that from the long-time averaged space-time correlations and does not change despite the variation of the detection threshold.

• 전력전자학회논문지
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• 제19권5호
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• pp.407-414
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• 2014

This study proposes a control design for the grid output current and for reducing the neutral-point voltage oscillation through the small-signal modeling of the three-phase grid connected with a three-level neutral-point-clamped (NPC) inverter with LCL filter. The three-level NPC inverter presents an inherent problem: the neutral-point voltage fluctuation caused by the neutral-point current flowing in or out from the neutral point. The small signal modeling consists of averaging, dq0 transformation, perturbing, and linearizing steps performed on a three-phase grid connected to a three-level NPC inverter with LCL filter. The proposed method controls both the grid output and neutral-point currents at every switching period and reduces the neutral-point voltage oscillation. The validity of the proposed method is verified through simulation and experiment.

• 전력전자학회논문지
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• 제19권5호
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• pp.422-430
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• 2014

This study proposes a small-signal model and control design for a two-stage DC-DC-AC converter to investigate its dynamic characteristics in relation to battery energy storage system. When the circuit analysis of the two-stage DC-DC-AC converter is attempted simultaneously, the mathematical procedure of deriving the dynamic equation is complex and difficult. The main idea of modeling the two-stage DC-DC-AC converter states that this topology is separated into a bidirectional DC-DC converter and a single-phase inverter with an equivalent current source corresponding to that of the inverter or converter. The dynamic equations for the separated converter and inverter are then derived using the state-space averaging technique. The procedures of building the small-signal model of the two-stage DC-DC-AC converter are described in detail. Based on the derived small-signal model, the individual controllers are designed through a frequency-domain analysis. The simulation and experimental results verify the validity of the proposed modeling approach and controller design.

• 대한기계학회논문집B
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• 제24권10호
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• pp.1343-1350
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• 2000

The evolutionary structure of tip vortices has been investigated with a two-dimensional LDV system for a plain and a slotted blade, respectively. To analyze the effect of slots which bypasses a part of main stream into the tip face, velocity profiles, vortex sizes, their displacements and turbulence intensities during one revolution of the rotor were measured by the phase averaging process. For the comparison of circumferential velocity components of the plain blade and the slotted blade, the peak values of the slotted blade were lower than those of the plain blade, and axial velocity components of the slotted blade were considerably larger than those of the plain blade. The slotted rotor blade enlarged the core size and made the vortex delayed compared with those of the plain blade at the same wake ages. Turbulence profiles had peaks inside the core radii and decayed gradually in the radial direction of vortex coordinate. Also, using a quasi 3-D LDV measurement technique the budget of turbulence kinetic energy was analyzed in radial direction of the vortex core.

• 대한기계학회논문집
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• 제19권5호
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• pp.1340-1350
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• 1995

The effects of thermal stratification on the flow past a heated circular cylinder with various heating rates were examined in a wind tunnel. Turbulent intensities, r.m.s.values of temperature and turbulent convective heat flux distributions in the cylinder wakes with and without thermal stratification were measured by using a hot-wire and cold-wire combination probe. The phase averaging method was also used to estimate coherent contributions to the turbulent flow field in the near wake. The results show that the scalar mixing process is very different according to the mean temperature fields especially in the upper part of the wake. The coherent structure of the temperature field makes a large contribution to the time mean value like velocity components. However, the coherency of the temperature fluctuation is very different with the change of mean temperature fields, though the velocity coherent motions are quite similar in all experimental conditions.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.185-188
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• 2002

The objective of present paper is to apply a stereoscopic PIV(Particle Image Velocimetry) techiique for measuring the 3 dimensional flow structure of turbulent wake behind a marine propeller with 5 blades. It is essential to measure 3-components velocity fields for the investigation of complicated near-wake behind the propeller. The out-of-plane velocity component was measured using the particle images captured by two CCD cameras in the angular displacement configuration.400 instantaneous velocity fields were measured for each of few different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}\;and\;54^{\circ}$. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranged from the trailing edge to the region of one propeller diameter(D) downstream. The phase-averaged velocity fields show the viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were formed periodically and the slipstream contraction occurs in the near-wake region. The out-of-plane velocity component has large values at the tip and trailing votices. With going downstream, the axial turbulence intensity and the strength of tip vortices were decreased due to the visous dissipation, turbulence diffusion and blade-to-blade interaction. The blade wake traveling at higher speed with respect to the tip vortex overtakes and interacts with tip vortices formed from the previous blade. Tip vortices are separated from the wake and show oscillating trajectory