• 한국음향학회지
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• 제40권4호
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• pp.297-303
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• 2021

천해 환경에서 음파가 장거리 전파되는 경우, 해저면의 비균질성으로 인해 일반적으로 사용하는 Rayleigh reflection 모델을 적용한 음파전달 모의 결과보다 더 큰 전달손실을 보이는 것으로 알려지고 있다. 이에 따라 미 해군은 경험식 기반의 해저면 반사손실(High-Frequency Bottom Loss, HFBL) 모델을 적용하여 음파 전달을 예측하고 있다. 본 연구에서는 여름철 동해 천해환경에서 중주파수(2.3 kHz, 3 kHz)를 이용한 해상실험 전달손실 측정 및 분석이 수행되었다. BELLHOP 모델을 통해 고유음선을 추적한 결과, 임계각보다 낮은 수평입사각에 대해서만 음파가 수 km 이상 장거리 전파되었으며, Rayleigh reflection 모델 기반의 전달손실 예측값과 실측 전달 손실 값과의 차이는 전달거리가 증가함에 따라 점차 증가하는 경향을 보였다. 큰 수평입사각 영역에서 Rayleigh reflection 모델과 HFBL 모델을 비교하여 HFBL의 입력값인 해저면 province 값을 추정한 후, 이를 적용한 전달 손실을 모의하여 실측 전달 손실 값과 비교하였다. 그 결과 BELLHOP 모델의 반사 손실 모델로 경험식 기반의 HFBL을 적용하여 전달 손실을 모의했을 때, 실측 전달 손실과 일치하는 것을 확인할 수 있었다.

• Structural Engineering and Mechanics
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• 제71권3호
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• pp.291-303
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• 2019

In the paper the analysis of natural vibrations of the transmission line with use of spectral elements and the laboratory experiments is performed. The purpose of the investigation is to analyze the natural vibrations of the transmission line and compare with the results obtained in the numerical simulations. Particular attention is paid to the hysteretic and aerodynamic damping analysis. Sensitivity of the wave number is performed for changing of the tension force, as well as for the different damping parameters. The numerical model is made using the Spectral Element Method. In the spectral model, for various parameters of stiffness, damping and tension force, the system response is checked and compared with the results of the accelerations obtained in the measurements. A frequency response functions (FRF) are calculated. The credibility of the model is assessed through a validation process carried out by comparing graphical plots of FRF and time history analysis and numerical values expressing differences in acceleration amplitude (MSG), phase angle differences (PSG) and differences in acceleration and phase angle total (CSG) values. The next aspect constituting the purpose of this paper is to present the wide possibilities of modelling and simulation of slender conductors using the Spectral Element Method. The obtained results show good accuracy in the range of both experimental measurements as well as simulation analysis. The paper emphasizes the ease with which the sensitivity of the conductor and its response to changes in density of spectral mesh division, tensile strength or material damping can be studied.

• 한국전자파학회논문지
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• 제17권3호
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• pp.267-271
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• 2006

본 논문에서는 공간적 전파전파 특성을 고려한 MIMO 안테나에 적용한 OFDM 전송 채널 모형을 사용하여, 채널의 에르고딕 용량을 계산하고 그 특성에 대해 논의한다. 특히 3GPP TR 25.966 V6.1.0에 정의되어 있는 case I의 LOS off 채널의 에르고딕 용량을 구하고, 클러스터 각 퍼짐과 도착 각도(Angle Of Arrival: AOA)의 차이에 따른 채널 용량 변화를 고찰한다. 연구 결과 클러스터 각 퍼짐의 증가에 따라 채널 용량이 증가하며, 35도 이상에서는 큰 증가가 없는 것을 볼 수 있었다.

• Applied Microscopy
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• 제45권3호
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• pp.115-118
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• 2015

In this review paper, I'd like to introduce the basics of angle-resolved photoemission spectroscopy (ARPES) and some of my results taken at the Pohang Accelerator Laboratory (PAL), the only synchrotron radiation in South Korea. The results show that ARPES is very useful, in particular, for studying two-dimensional materials. It looks like a microscope in momentum space similar to transmission electron microscope imaging atoms in real space.

• 한국광학회지
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• 제32권3호
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• pp.126-132
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• 2021

본 논문에서는 특이 광 투과 현상 센서에서 보조 파장 홀 패턴에 입사되는 광원의 각도 조절을 통해 센서의 민감도를 측정 및 분석하여 이를 극대화할 수 있는 방법을 고안하였다. 입사파 광원을 평행화된 광원과 각도가 서로 다른 두 개의 집속화된 광원 총 3개의 광원을 시스템적으로 설계하여 서로 다른 각도로 보조 파장 홀 패턴에 입사되게 제작하였다. 그리고 증류수(n=1.333)와 굴절률 정합액(n=1.360, 1.380)을 사용하여 투과 스펙트럼을 측정하고 센서의 민감도를 계산한 결과 평행화된 광원을 사용하였을 때 센서의 민감도가 향상됨을 확인하였다.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1991년도 한국자동제어학술회의논문집(국제학술편); KOEX, Seoul; 22-24 Oct. 1991
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• pp.1659-1663
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• 1991

A new control design methodology is presented here which is based on a nonlinear time-series reference model. It is indicated by highly nonlinear simulations that such designs successfully stabilize troublesome aircraft maneuvers undergoing large changes in angle of attack as well as large electric power transients due to line faults. In both applications, the nonlinear controller was significantly better than the corresponding linear adaptive controller. For the electric power network, a flexible a.c. transmission system (FACTS) with series capacitor power feedback control is studied. A bilinear auto-regressive moving average (BARMA) reference model is identified from system data and the feedback control manipulated according to a desired reference state. The control is optimized according to a predictive one-step quadratic performance index (J). A similar algorithm is derived for control of rapid changes in aircraft angle of attack over a normally unstable flight regime. In the latter case, however, a generalization of a bilinear time-series model reference includes quadratic and cubic terms in angle of attack. These applications are typical of the numerous plants for which nonlinear adaptive control has the potential to provide significant performance improvements. For aircraft control, significant maneuverability gains can provide safer transportation under large windshear disturbances as well as tactical advantages. For FACTS, there is the potential for significant increase in admissible electric power transmission over available transmission lines along with energy conservation. Electric power systems are inherently nonlinear for significant transient variations from synchronism such as may result for large fault disturbances. In such cases, traditional linear controllers may not stabilize the swing (in rotor angle) without inefficient energy wasting strategies to shed loads, etc. Fortunately, the advent of power electronics (e.g., high-speed thyristors) admits the possibility of adaptive control by means of FACTS. Line admittance manipulation seems to be an effective means to achieve stabilization and high efficiency for such FACTS. This results in parametric (or multiplicative) control of a highly nonlinear plant.

• Tribology and Lubricants
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• 제30권2호
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• pp.71-76
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• 2014

As the importance of transmission efficiency to reducing fuel consumption and conserving the environment rapidly increases, reducing the drag torque in an automotive wet clutch is emerging as an important issue in the automotive industry. The drag torque in a clutch occurs from viscous drag generated by automatic transmission fluid in the narrow gap between separate friction plates. In this study, the drag torques in an automotive wet clutch are investigated with respect to the angle and density of the grooves between separate friction plates by three-dimensional finite element simulation of a single set of wet clutch disks considering the two-phase flow of air and oil. The simulation results shows that the drag torque generally increases with the rotational speed to a critical point and then decreases at the high-speed regime. The grooves between the plates plays an important role in reducing the drag peak, and the inclined angle of the grooves affects the oil flow. The grooves with an angle of $50^{\circ}$ shows the lowest drag torques at both low and high speeds. The flow vectors inside the $50^{\circ}$ grooves shows clear evidence that the fluid flows out more easily from the grooves compared with the flow vectors inside grooves with lower angles. The simulation results shows that increasing the number of grooves (density of grooves) decreases the drag torque.

• 대한기계학회논문집A
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• 제24권4호
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• pp.881-893
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• 2000

An analytical synthesis technique for offset slider-rocker 4-bar linkages is devised where transmission angles are used as essential synthesis parameters. Finding the location where both the maximum and the minimum transmission angles arise leads to establishing the algorithm for limiting transmission angles of the slider-rocker linkage which has different offsets according to the direction of the slider. Position analysis by the algorithm gives formula for the dimensional synthesis for the linkage. Application to examples shows that the algorithm and the formula yield proper slider-rocker linkages with regard to limiting transmission angles to the maximum value.

• Wind and Structures
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• 제34권2호
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• pp.173-183
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• 2022

The galloping of iced conductors has long been a severe threat to the safety of overhead transmission lines. Compared with normal transmission lines, the ultra-high-voltage (UHV) transmission lines are more prone to galloping, and the damage caused is more severe. To control the galloping of UHV lines, it is necessary to conduct a comprehensive analysis of galloping characteristics. In this paper, a large-span 1000-kV UHV transmission line in China is taken as a practical example where an 8-bundled conductor with D-shaped icing is adopted. Galerkin method is employed for the time history calculation. For the wind attack angle range of 0°~180°, the galloping amplitudes in vertical, horizontal, and torsional directions are calculated. Furthermore, the vibration frequencies and galloping shapes are analyzed for the most severe conditions. The results show that the wind at 0°~10° attack angles can induce large torsional displacement, and this range of attack angles is also most likely to occur in reality. The galloping with largest amplitudes in all three directions occurs at the attack angle of 170° where the incoming flow is at the non-iced side, due to the strong aerodynamic instability. In addition, with wind speed increasing, galloping modes with higher frequencies appear and make the galloping shape more complex, indicating strong nonlinear behavior. Based on the galloping amplitudes of three directions, the full range of wind attack angles are divided into five galloping regions of different severity levels. The results obtained can promote the understanding of galloping and provide a reference for the anti-galloping design of UHV transmission lines.

• Wind and Structures
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• 제31권6호
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• pp.509-522
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• 2020

Wind load is the principal cause for a large number of the collapse of transmission lines around the world. The transmission line is traditionally designed for wind load according to a linear equivalent method, in which dynamic effects of wind are not appropriately included. Therefore, in the present study, incremental dynamic analysis is utilized to investigate the stability behavior of a 400 kV transmission line under wind load. In that case, the effects of vibration of cables and aerodynamic damping of cables were considered on the stability behavior of the transmission line. Superposition of the harmonic waves method was used to calculate the wind load. The corresponding wind speed to the beginning of the transmission line collapse was determined by incremental dynamic analysis. Also, the effect of the yawed wind was studied to determine the critical attack angle by the incremental dynamic method. The results show the collapse mechanisms of the transmission line and the maximum supportable wind speed, which is predicted 6m/s less than the design wind speed of the studied transmission line. Based on the numerical modeling results, a retrofitting method has been proposed to prevent failure of the tower members under design wind speed.