• 한국전산유체공학회지
• /
• 제2권1호
• /
• pp.1-7
• /
• 1997

A FA-FD hybrid method, developed for solving three-dimensional incompressible Navier-Stokes equations, is applied to calculate three-dimensional laminar flows through a square duct with a 90° bend. The method discretizes the convective terms in the primary flow direction with 3rd-order upwind finite-differences and the convective and diffusive terms in the transverse directions with the two-dimensional finite analytic method. The non-staggered grid system is used and the pressure-velocity coupling is achieved by a global iteration procedure based on the PISO algorithm. Detailed comparisons between the computed solutions and the available experimental data are given mainly for the velocity distributions at cross-sections in a 90° bend of a square duct with both fully developed and developing entry flows. Although the computational result shows generally a good agreement with the experimental data, there are some significant discrepancies underlining the necessity of more accurate numerical methods as well as reliable experimental data for their validation.

• 대한기계학회논문집B
• /
• 제28권2호
• /
• pp.174-180
• /
• 2004

Failure of tube bundles due to excessive flow-induced vibrations continues to affect the performance of nuclear power plant Early experimental studies concentrated on rigid structures and later investigators dealt with elastic structures because of their importance in many engineering fields. On the other hand, much less numerical work has been carried out, because of the numerical complexity associated with the problem. Conventional approaches usually decoupled the flow solution from the structural problem. The present numerical study proposes the methodology in analyzing the fluidelastic instability occurring in tube bundles by coupling the Computational fluid Dynamics (C%) with the tube equation of motions. The motion of the structures is modeled by a spring-damper-mass system that allows transnational motion in two directions (a two-degree-of-freedom system). The fluid motion and the cylinder response are solved in an iterative way, so that the interaction between the fluid and the structure can be accounted for property. The aim of the present work is to predict the fluidelstic instability of tube bundles and the associated phenomena, such as the response of the cylinder, the unsteady lift and drag on the cylinder, the vortex shedding frequency.

• Wind and Structures
• /
• 제14권3호
• /
• pp.221-238
• /
• 2011

The estimated response of large-scale engineering structures to severe wind loads is prone to modelling uncertainties that can only ultimately be assessed by full-scale testing. To this end ambient vibration data from full-scale monitoring of the historic Clifton Suspension Bridge has been analysed using a combination of a frequency domain system identification method and a more elaborate stochastic identification technique. There is evidence of incipient coupling action between the first vertical and torsional modes in strong winds, providing unique full-scale data and making this an interesting case study. Flutter derivative estimation, which has rarely previously been attempted on full-scale data, was performed to provide deeper insight into the bridge aerodynamic behaviour, identifying trends towards flutter at higher wind speeds. It is shown that, as for other early suspension bridges with bluff cross-sections, single-degree-of-freedom flutter could potentially occur at wind speeds somewhat below requirements for modern designs. The analysis also demonstrates the viability of system identification techniques for extracting valuable results from full-scale data.

• 대한전자공학회논문지SD
• /
• 제39권1호
• /
• pp.58-66
• /
• 2002

Tapered 방향성 가중 결합 음향파 도파로를 이용하여 음향광학형 파장가변 광 필터를 LiNbO₃와 Ti 이중 확산 공정 기술을 이용하여 제작하였다. 1551.1㎚ 파장의 TE, TM 입사 편광모드와 173.58㎒ 35㎽ RF구동 신호에 대해서 각각 -14.29㏈, -14.99㏈ 부 모드 억압과 61%, 86% 변환효율이 측정되었다. 파장변화율과 통과 대역폭은 8.6㎚/㎒ 1.8㎚로 측정되었으며, 스위칭 속도는 2.82㎲로 측정되었다. 2.5㎚ 채널 간격에 대해서 약 -l4㏈누화 레벨이 측정되었으며, 부 모드와 채널 간격이 누화에 매우 큰 영향을 미치고있음을 확인하였다.

• 한국ITS학회 논문지
• /
• 제4권2호
• /
• pp.73-79
• /
• 2005

본 논문에서는 3가지 형태의 CMOS 직교 신호 발생 전압 제어 발진기를 설계 및 제작하여 위상 잡음 특성을 비교하였다. 첫 번째 구조는 PMOS를 이용하여 두개의 Colpitts 발진기를 구성한 후 이를 상호 결합시킨 구조이다. 두 번째와 세번째 구조는 각각 일반적인 LC VCO와 펼형 콜피츠 구조에 주파수 2분주기를 연결하여 직교신호를 얻는 구조이다. 모의실험 결과 콜피츠 구조를 사용한 구조가 LC 발진기를 사용하였을 때에 비해 위상 잡음 특성이 6 dB 이상 개선되었다.

• 대한전자공학회논문지TC
• /
• 제41권8호
• /
• pp.119-124
• /
• 2004

본 논문에서는 무선LAN 및 ISM용을 위한 5［GHz］대역 원편파 마이크로스트립 배열안테나를 설계 제작하였다. 원편파를 위한 기존의 십자형 슬롯 안테나 구조를 수정하여 두 개의 슬롯이 직각으로 교차하는 부분을 없애고 분리된 4개의 슬롯으로 구성된 안테나 구조를 제안하였다. 기존의 구조에서 급전선로와 슬롯간의 비대칭으로 전자기적 결합의 약화와 슬롯의 비대칭으로 발생되는 기생요소의 증가로 안테나 효율이 약화되는 문제가 있는 반면, 제안된 구조의 안테나는 대역폭 증가와 후방 방사의 감소를 확인하였다. 제안된 안테나는 공진 주파수가 5［GHz］대역의 2${\times}$2 마이크로스트립 배열안테나를 제작하여 최대 12.5［dBi］의 이득과 280［MHz］의 대역폭(VSWR < 1.5)를 얻었다.

• 복합신소재구조학회 논문집
• /
• 제2권2호
• /
• pp.20-25
• /
• 2011

본 논문에서는 2차원 콘크리트 슬래브 철도교량을 특별직교이방성 복합적층판 이론으로 해석하였다. 슬래브의 해석에 있어 단면의 기하학적, 물리적 특성이 중립면에서 휨-연계강성 $B_{ij}$ = 0 이고, $D_{16}=D_{26}=0$임을 고려하였다. 교량상판은 특별직교이방성 판으로 거동한다. 이러한 경계조건을 갖는 단면 혹은 불규칙한 단면을 갖는 시스템은 해석적 해를 구하기가 매우 어렵다. 이러한 문제에 대한 해석을 위해서 유한차분법이 이용되었다. 본 논문에서는 보 이론과 유한차분법이 해석을 위해 사용되었다.

• 소성∙가공
• /
• 제4권4호
• /
• pp.375-389
• /
• 1995

In H shape rolling, accurate predictions of deformation and temperature distribution in a billet are quite important because they are the main factors in determining roll calibers and roll pass schedules. Many researches have been performed to achieve the predictions, but most of them are limited to single pass or isothermal assumptions. In the present investigation, it is attempted to develop a method to predict the deformation and temperature distributions which is applicable to a complete rolling process that usually consists of several rollings under different rolls for a period of time. The method works by coupling two analyses : one is an approximate analysis for temperature distribution prediction and the other is the slab-FEM hybrid analysis for deformation prediction. The method is applied to analyze a "H" shape rolling process consisting of nine passes under four different rolls. In the present paper, basic ideas of the method are presented. Also, shapes of cross sections, strain and temperature distributions, roll separating force and roll torque predicted by the method are discussed.

• Journal of the Optical Society of Korea
• /
• 제5권1호
• /
• pp.25-28
• /
• 2001

By using an XPM (Cross Phase Modulation) wavelength converter, an all-optical AND gate, which is one of six fundamental logic gates, has been demonstrated. The wavelengths for probe and pump signals are 1553.8 and 1545 nm, respectively. First, characteristics of the XPM wavelength converter have been studied. When both probe and pump signals are driven by high power, the output power of the XPM wavelength is high. Based on this fact and the experiment, the all-optical AND gate has been porved. Probe and pump signals are transformed to pulse signals by using Mach-Zehnder modulator, which is induced by a pulse generator. Square pulse signals that are similar to the format of NRZ signals have been generated. By coupling two pulse signals into the XPM wavelength converter, AND characteristics in substantiated.

• 한국재료학회지
• /
• 제12권4호
• /
• pp.254-258
• /
• 2002

To realize channel cross-connecting in optical communications systems, a high speed optical matrix switch was fabricated using z-cut $LiNbO_3$. For switch fabrication was design bending structure and coupling length and four $2{\times}2$ directional couplers were integrated on one substrate far construction of a $4{\times}4$ switch. Single-mode optical waveguides were formed by Ti-diffusion at a wet $O_2$ atmosphere. Ti-diffusion profile, refractive index variation and waveguide morphology were analyzed by Prism coupler and optical microscopy, respectively.