• Bulletin of the Korean Chemical Society
• /
• 제23권11호
• /
• pp.1595-1603
• /
• 2002

In this paper we have presented the results of diffusion behavior of model systems for eight liquid n-alkanes ($C_{12}$-$C_{44}$) in a canonical (NVT) ensemble at several temperatures using molecular dynamics simulations. For these n-alkanes of small chain length n, the chains are clearly <$R_{ee}^2$>/6<$R_g^2$>>1 and non-Gaussian. This result implies that the liquid n-alkanes over the whole temperatures considered are far away from the Rouse regime, though the ratio becomes close to the unity as n increases. Calculated self-diffusion constants $D_{self}$ are comparable with experimental results and the Arrhenius plot of self-diffusion constants versus inverse temperature shows a different temperature dependence of diffusion on the chain length. The global rotational motion of n-alkanes is examined by characterizing the orientation relaxation of the end-to-end vector and it is found that the ratio ${\tau}1/{\tau}2$ is less than 3, the value expected for a isotropically diffusive rotational process. The friction constants ${\xi}$of the whole molecules of n-alkanes are calculated directly from the force auto-correlation (FAC) functions and compared with the monomeric friction constants ${\xi}_D$ extracted from $D_{self}$. Both the friction constants give a correct qualitative trends: decrease with increasing temperature and increase with increasing chain length. The friction constant calculated from the FAC's decreases very slowly with increasing temperature, while the monomeric friction constant varies rapidly with temperature. By considering the orientation relaxation of local vectors and diffusion of each site, it is found that rotational and translational diffusions of the ends are faster than those of the center.

• 한국수자원학회논문집
• /
• 제51권10호
• /
• pp.895-903
• /
• 2018

국내의 하천에는 많은 수의 보가 설치되어 있으며, 이러한 특성은 국외에서는 흔하지 않은 편이다. 흐름이 보와 같은 구조물을 통과하는 경우에는 불연속 흐름이 발생하게 되며, 수치모의 측면에서는 흐름항과 생성항의 균형 등의 문제로 수치적 안정성에 많은 영향을 준다. 이러한 문제점을 해결하기 위해서 경험식이나 해석기법의 단순화 등에 의존해 왔으며, 최근에 들어서는 보다 정확한 수치해석기법을 이용하려는 연구가 꾸준히 수행되고 있다. K-River는 국내의 하천 특성을 반영하고, 불연속 흐름을 보다 정확히 계산하기 위한 목적으로 개발되었다. K-River의 검증을 위하여 1) 하상융기가 존재하는 개수로 수치실험 모의, 2) 도수현상 실내실험 모의, 3) 실제 하천의 수문 사상 모의를 수행하였다. 모든 모의에서 해석해 및 관측치와 유사한 결과를 모의하여 K-River의 적용성을 검증하였다.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제13권1호
• /
• pp.236-245
• /
• 2021

In this study, direct numerical and large eddy simulations of transitional flows around studs were conducted to investigate the effectiveness of turbulence stimulators at very low speeds for the minimum propulsion power condition of four knots. For simplicity, the studs were assumed to be installed on a flat plate, while the wake was observed up to 0.23 m downstream behind the second stud. For applicability to a model ship, we also studied the flow characteristics behind the first and second studs installed on a curved plate, which was designed to describe the geometry of a bulbous bow. A laminar-to-turbulent transition was observed in the wake at Re_D ≥ 921 (U_∞≥0.290 m/s), and the wall shear stress at Re_D = 1162 (U_∞ = 0.366 m/s) in the second wake was similar to that of the fully developed turbulent boundary layer after a laminar-to-turbulent transition in the first wake. At Re_D = 581 (U_∞ = 0.183 m/s), no turbulence was stimulated in the wake behind the first and second studs on the flat plate, while a cluster of vortical structures was observed in the first wake over the curved plate. However, a cluster of vortical structures was revealed to be generated by the reattachment process of the separated shear layer, which was disturbed by the first stud rather than directly initiated by the first stud. It was quite different from a typical process of transition, which was observed at relatively high Re_D that the spanwise scope of the turbulent vortical structures expanded gradually as it went downstream.

• 전자공학회논문지 IE
• /
• 제44권2호
• /
• pp.14-20
• /
• 2007

본 논문에서는 개선된 성능을 갖는 4치 D-플립플롭을 제안하였다. 제안된 4치 D 플립플롭은 뉴런모스를 기반으로 바이어스 인버터, 온도계 코드 출력회로, EX-OR 게이트, 전달 게이트를 이용하여 4치 항등 논리회로(Identity logic circuit)를 구성하고, 이를 2진의 RS 래치 회로와 결합하여 설계하였다. 설계된 회로들은 3.3V 단일 공급 전원에서 $0.35{\mu}m$ 1-poly 6-metal COMS 공정 파라미터 표준조건에서 HSPICE를 사용하여 모의실험 하였다. 모의실험 결과, 본 논문에서 제안된 4치 D 플립플롭은 100MHz 전후까지의 빠른 동작속도로 측정되었으며 PDP(Power dissipation-delay time product)와 FOM(Figure of merit)은 각각 59.3pJ과 33.7로 평가되어졌다.

• 한국해양공학회지
• /
• 제31권2호
• /
• pp.91-102
• /
• 2017

Recently, a new type of LNG membrane Tank called the "KC-1 membrane LNG Tank" was developed by KOGAS (Korean Gas Corporation). It is necessary to estimate the temperature distribution of the hull structure and insulation system for this new LNG tank, as well as the BOR (Boil-Off Rate) when exposed to outside temperature conditions to ensure the integrity of the tank structure and limit LNG evaporation, from a safety evaluation point of view. In this study, temperature distribution calculations for the hull structure and insulation system of the KC1 membrane tank were compared by employing four numerical approaches under the IGC condition. Approaches 1-3 studied 2D simulations and approach 4 used a 3D numerical simulation. Approach 1 was calculated by in-house Excel VBA codes and the three other approaches utilized ANSYS Fluent. The BOR of approach 4, the 3D simulation case, for the IGC condition was 0.0986%/day.

• 소성∙가공
• /
• 제20권4호
• /
• pp.296-302
• /
• 2011

The objective of this paper is to determine the effect of process parameters on the behavior of a 18Cr-10Mn-$0.44N_2$ nitrogen steel sample deformed by hot rolling. Compression tests were carried out at high temperatures to determine the flow stresses needed for a finite element(FE) analysis. The strain rate, ranging from 0.1 to $1.0s^{-1}$, significantly affected the flow stress at temperatures higher than $1,000^{\circ}C$. Non-isothermal rolling simulations and laboratory rolling tests were performed with plate specimens 14.5mm thick, 135mm wide and 226mm long. A rolling reduction of 15% per pass leading to a cumulative rolling reduction of 60% was determined as optimal. The extension ratio of 176.5% in the length direction was about 30.4 times greater than the extension ratio of 5.8% in the width direction. Isotropic properties for tensile strength, microstructure and grain size were measured after mock-up hot rolling tests. The results from the mockup tests were found to be in good agreement with those of the simulations.

• 한국통신학회논문지
• /
• 제30권8C호
• /
• pp.762-768
• /
• 2005

본 논문은 QAM 변조방식과 준정지(quasi-static) Rayleigh 페이딩 채널 환경에서 3이상의 임의의 송신안테나를 사용할 경우 추가로 주파수 대역을 사용하지 않고 최대 다이버시티 이득을 획득하는 새로운 시공간블록부호들을 제안한다. 이 제안된 부호들은 기존의 A-ST-CR 부호$^{[6,7]}$와 같이 성상 회전 선부호기$^{[4,5]}$와 Alamouti 부호$^{[3]}$ 적절히 직렬 연접하여 구성된다. 전산 실험 결과, 제안된 부호는 ST-CR 부호$^{[4,5]}$보다 높은 코딩이득을 획득하며, QPSK 변조방식과 3, 4, 5개의 송신 안테나 개수를 사용할 경우 평균 심벌오율 $10^{-5}$에서 기존의 ST-CR $^{[4,5]}$보다 각각 약 1.3dB, 1.4dB 그리고 1.5dB의 코딩 이득의 성능 향상을 보이고, 16-QAM 변조방식과 3개의 송신 안테나 개수를 사용할 경우 약 3dB의 성능 향상을 보인다.

• 한국수자원학회논문집
• /
• 제47권8호
• /
• pp.729-742
• /
• 2014

본 연구에서는 유입유사에 따른 저수지 내 퇴사 분포를 정량적으로 분석하고 효율적인 저수지 관리를 위하여 2차원 수리 유동 모형인 RMA-2와 유사이송 모형 SED2D를 사용하여 76년 장기 퇴사분포를 예측하였다. 모형의 입력 자료인 수위-유량자료는 한국수자원공사에서 관측한 실시간 자료를 활용하였으며, 유사량 자료는 한국수자원공사에서 개발한 K-DRUM 모형을 사용하여 대상영역의 유사량 값을 산정하여 적용하였다. 또한 갈수기시 저수지내 단면 코어채취를 수행하여 퇴적깊이를 모형 결과와 비교하였다. 검증결과 저수지 내 퇴적깊이는 비교적 실측값과 유사하게 나타났다. 장기 퇴사분포예측은 2012년 실측한 지형자료를 시작단면으로 하여 2088년까지 76년간 모의를 수행하였다. 모의결과 합천댐 상류구간에서 1.63~1.26m, 하류구간에서 1.45~0.007m의 퇴적층이 형성되었으며, 전체적으로 침식보다 퇴적이 우세한 결과를 보였다.

• 대한전기학회:학술대회논문집
• /
• 대한전기학회 2003년도 추계학술대회 논문집 전기물성,응용부문
• /
• pp.220-222
• /
• 2003

In this paper, we analyze on the discharge characteristics through 1-D simulations in an at plasma display panel discharge cell. The model is based on a Poisson' equation, continuity and drift-diffusion equation. Results are presented in a 95% neon, 5% xenon gas mixture, for a gap length of 100us and a gas pressure of 400Torr at ambient temperature. Results for other gap length are also discussed. As a result, an increase of the gap cause increase of luminous efficiency but with larger sustaining voltage.

• 제어로봇시스템학회논문지
• /
• 제19권9호
• /
• pp.774-781
• /
• 2013

The purpose of this paper is to design a DVL-RPM based VKF (Velocity Kalman Filter) design for a performance improvement underwater integrated navigation system. The proposed approach relies on a VKF, augmented by a altitude from Echo-sounder based switching architecture to yield robust performance, even when DVL (Doppler Velocity Log) exceeds the measurement range and the measured value is unable to be valid. The proposed approach relies on two parts: 1) Indirect feedback navigation Kalman filter design, 2) VKF design. To evaluate proposed method, we compare the results of the VKF aided navigation system with simulation result from a PINS (Pure Inertial Navigation System) and conventional INS-DVL method. Simulations illustrate the effectiveness of the underwater navigation system assisted by the additional DVL-RPM based VKF in underwater environment.