• 한국정보처리학회논문지
• /
• 제2권3호
• /
• pp.374-381
• /
• 1995

디지탈 무전 통신 시스템을 설계하는 경우 원하는 송신 전력, 전송 속도와 비트 오율등 원하는 통신 품질을 얻기 위해서는 무선 채널의 영향을 고려하여야 한다. 무선 채널에서 통신 품질을 열화시키는 주된 원인으로는 송신점과 수신점 사이를 연결하는 다중 경로에 의한 페이딩을 들 수 있으므로 시스템 설계시에는 페이딩 현상을 정확하 게 나타내기 위한 채널의 모델이 필요하게 된다. 본 논문은 채널에서 발생되는 다중 경로 페이딩 현상과 지연 시간을 고려하는 채널 시뮬레이터 모델의 구성에 관한 것이 다. 제안된 모델의 성능 분석을 위하여 송신을 이용한 시뮬레이션을 수행하였다. 시뮬 레이션 결과 페이딩 수신 신호를 확률밀도 함수, 레벨 교차율, 평균 페이딩 지속시간 분포에서 이론치와 유사함을 얻을 수 있었다.

• 대한전기학회논문지:전력기술부문A
• /
• 제55권1호
• /
• pp.37-44
• /
• 2006

In this paper we explore the susceptibility of common sensitive loads by voltage sags of power distribution systems. The experimental approach was used for obtaining the susceptibility of single-phase loads and the three-phase induction motor. The experimental result of single-phase loads was transformed to the ITIC(Information of Technology Industry Council) format and used for evaluating the adverse impacts of a individual and repetitive sags using the performance contour of the foreign standard data. In order to assess the impact of voltage sags on three-phase induction motor, also, the experiment was peformed. The experiment was focused on the current, torque, and speed loss of the motor during a voltage sag. For comparing the impacts of individual and repetitive voltage sags, the variations of motor torque is focused among the experimental results. The sensitive curves of instantaneous current peak are used to describe the susceptibility of three-phase induction motor and 진so it were used for the quantitative analysis of the impact of three-phase induction motor due to voltage sags. Through the results of experiment, we verified that some types loads have more severe impact at repetitive voltage sags than individual ones and proposed method can be effectively used to evaluate the actual impact of voltage sags.

• Smart Structures and Systems
• /
• 제17권6호
• /
• pp.1067-1085
• /
• 2016

Nonstationary features existing in tropical storms have been frequently captured in recent field measurements, and the applicability of the stationary theory to the analysis of wind characteristics needs to be discussed. In this study, a tropical storm called Nakri measured at Taizhou Bridge site based on structural health monitoring (SHM) system in 2014 is analyzed to give a comparison of the stationary and nonstationary characteristics. The stationarity of the wind records in the view of mean and variance is first evaluated with the run test method. Then the wind data are respectively analyzed with the traditional stationary model and the wavelet-based nonstationary model. The obtained wind characteristics such as the mean wind velocity, turbulence intensity, turbulence integral scale and power spectral density (PSD) are compared accordingly. Also, the stationary and nonstationary PSDs are fitted to present the turbulence energy distribution in frequency domain, among which a modulating function is included in the nonstationary PSD to revise the non-monotonicity. The modulated nonstationary PSD can be utilized to unconditionally simulate the turbulence presented by the nonstationary wind model. The results of this study recommend a transition from stationarity to nonstationarity in the analysis of wind characteristics, and further in the accurate prediction of wind-induced vibrations for engineering structures.

• Smart Structures and Systems
• /
• 제12권5호
• /
• pp.547-577
• /
• 2013

To suppress vibration and noise of mechanical structures piezoelectric ceramics play an increasing role as effective, simple and light-weighted damping devices as they are suitable for sensing and actuating. Out of the various piezoelectric damping methods this paper compares mode based active control strategies to passive shunt damping for thin plates. Therefore, a new approach for the optimal placement of the piezoelectric sensors/actuators, or more general transducers, is proposed after intense theoretical investigations based on the Kirchhoff kinematical hypotheses of plates; in particular, modal and nilpotent transducers are discussed in detail. Based on the proposed distribution a discrete design for modal transducers is implemented, tested and verified on an experimental setup. For active control the modal sensors clearly identify the eigenmodes, whereas the modal actuators impose distributed eigenstrains in order to reduce the transverse plate vibrations. In contrast to the modal control, passive shunt damping works without requiring additional actuators or auxiliary power and can therefore act as an autonomous system, but it is less effective compensating the flexible vibrations. Exemplarily, an acryl glass plate disturbed by an arbitrary force initialized by a loudspeaker is investigated. Comparing the different methods their specific advantages are highlighted and a significant broadband reduction of the vibrations of up to -20dB is obtained.

• 한국분무공학회지
• /
• 제10권2호
• /
• pp.18-31
• /
• 2005

This study was performed to explore the liquid breakup and atomization characteristics for the classification of drop formation mode and background of uniform droplets generation in electrohydrodynmaic atomization according to the change of experimental parameters such as nozzle material (stainless steel. teflon). fluid flow rate, applied electrical field and intensity, and frequency. In results, from the classification map of drop formation modes according to the variation of applied AC voltage and frequency at a stainless nozzle, the droplet size was smaller than the outer diameter of the nozzle tip relatively in the spindle mode. The transition points became clearly to be moved toward the high applied voltage by rising the applied AC frequency beyond 450Hz. Also the droplet radius can be observed quite small in the frequency bandwidth of $350{\sim}450Hz$. The droplet radiuses decrease as the applied voltage increases for a fixed applied AC frequency within the range from 50Hz to 400Hz Over 400Hz, the relation between the power intensity and the droplet size was not consistent with a continuous mechanism of liquid breakup. Thus, it is showed that the droplet size distribution using the teflon nozzle was analogous to the results of stainless steel, but the droplet size was bigger than that of stainless steel relatively in case of a teflon nozzle.

• Smart Structures and Systems
• /
• 제9권2호
• /
• pp.105-125
• /
• 2012

A modified Pagano method is developed for the three-dimensional (3D) free vibration analysis of simply-supported, multilayered functionally graded material (FGM) circular hollow cylinders with a constant rotational speed with respect to the meridional direction of the cylinders. The material properties of each FGM layer constituting the cylinders are regarded as heterogeneous through the thickness coordinate, and then specified to obey a power-law distribution of the volume fractions of the constituents, and the effects of centrifugal and Coriolis accelerations, as well as the initial hoop stress due to rotation, are considered. The Pagano method, which was developed for the static and dynamic analyses of multilayered composite plates, is modified in that a displacement-based formulation is replaced by a mixed formulation, the complex-valued solutions of the system equations are transferred to the real-valued solutions, a successive approximation method is adopted to extend its application to FGM cylinders, and a propagator matrix method is developed to reduce the time needed for its implementation. These modifications make the Pagano method feasible for multilayered FGM cylinders, and the computation in the implementation is independent of the total number of the layers, thus becoming less time-consuming than usual.

• 설비공학논문집
• /
• 제19권1호
• /
• pp.86-93
• /
• 2007

The PEM (polymer electrolyte membrane) fuel cell is one of the promising fuel cell systems as a new small power generating device for automobiles and buildings. The optimal design of cooling plates installed between MEA (membrane electrode assembly) is very important to achieve high performance and reliability of the PEMFC because it is very sensitive to temperature variations. In this study, six types of cooling plate models for the PEMFC including basic serpentine and parallel shapes were designed and their cooling performances were analyzed by using three-dimensional fluid dynamics with commercial software. The model 3 designed by revising the basic serpentine model represented the best cooling performance among them in the aspect of uniformity of temperature distribution and thermal reliability, The serpentine models showed higher pressure drop than the parallel models due to a higher flow rate.

• 조명전기설비학회논문지
• /
• 제17권1호
• /
• pp.103-111
• /
• 2003

교육용 건물에는 개인용 컴퓨터와 같은 비선형 부하가 증가하여, 중성선에는 많은 고조파 전류가 흐른다. 3상 4선식 배전 계통을 채용하는 중성선에 과다한 고조파 전류가 흐르면 중성선 도체 과열과 보호 시스템의 오동작등 많은 고조파 장해를 일으킨다. 본 논문에서는 현장에서 실측한 고조파 전류와 전압으로 고조파 해석용 3상 등가회로를 구성하였다. 실측한 값과 MATLAB으로 시뮬레이션한 값을 수치와 그래프로 비교 검토 하였다. 또한 중성선 고조파 전류 저감 대책으로 동조 수동 필터를 사례연구 시스템에 적용하여 고조파 저감 효과도 알아 보았다.

• International Journal of Aeronautical and Space Sciences
• /
• 제18권3호
• /
• pp.474-484
• /
• 2017

This paper studies the applicability of an efficient numerical model based on artificial neural networks (ANNs) to predict the dynamic responses of the wing structure of an airplane due to atmospheric turbulence in the time domain. The turbulence velocity is given in the form of a stationary Gaussian random process with the von Karman power spectral density. The wing structure is modeled by a classical beam considering bending and torsional deformations. An unsteady vortex-lattice method is applied to estimate the aerodynamic pressure distribution on the wing surface. Initially, the trim condition is obtained, then structural dynamic responses are computed. The numerical solution of the wing structure's responses to a random turbulence profile is used as a training data for the ANN. The current ANN is a three-layer network with the output fed back to the input layer through delays. The results from this study have validated the proposed low-cost ANN model for the predictions of dynamic responses of wing structures due to atmospheric turbulence. The accuracy of the predicted results by the ANN was discussed. The paper indicated that predictions for the bending moments are more accurate than those for the torsional moments of the wing structure.

• Journal of information and communication convergence engineering
• /
• 제11권1호
• /
• pp.1-6
• /
• 2013

Suppressing or mitigating signal distortion due to group velocity dispersion and optical Kerr effects is necessary in ultra-high speed and long-haul wavelength division multiplexing (WDM) transmission systems. Dispersion management (DM), optical phase conjugation (OPC), and the combination of these two are promising techniques to compensate for signal distortion. In this paper, to implement a flexible optical WDM network, a new optical link configuration with a randomly distributed single-mode fiber (SMF) length and fixed residual dispersion per span in the combination of DM and OPC is proposed and investigated. The simulation results show that the best net residual dispersion (NRD) in the proposed optical links is +10 ps/nm, which is independent of pre- and postcompensation. The effective launch power of the WDM channel is increased more in the optical links with NRD = +10 ps/nm controlled by only precompensation. Furthermore, the system performance difference between the proposed optical link configuration with the best NRD and the conventional optical link with uniform distribution of the SMF length had little significance. Consequently, it is confirmed that the proposed optical link configuration with the best NRD is effective and useful for implementing a reconfigurable long-haul WDM network.