• 한국전산유체공학회지
• /
• 제19권2호
• /
• pp.40-48
• /
• 2014

For rarefied gas flow regimes, physical phenomena such as velocity slip and temperature jump occur on the solid body surface. To predict these phenomena accurately, either the Navier-Stokes solver with a slip boundary condition or the direct simulation Monte Carlo method should be used. In the present study, flow simulations of a wedge were conducted in Mach-10 flow of argon gas for several different flow regimes using a two-dimensional Navier-Stokes solver with the Maxwell slip boundary condition. The results of the simulations were compared with those of the direct simulation Monte Carlo method to assess the present method. It was found that the values of the velocity slip and the temperature jump predicted increase as the Knudsen number increases. Also, the results are comparatively reasonable up to the Knudsen number of 0.05.

• Nuclear Engineering and Technology
• /
• 제47권3호
• /
• pp.267-283
• /
• 2015

The main objective of this study is to investigate experimentally the two-phase flow characteristics in terms of the direct contact condensation of a steam-water stratified flow in a horizontal rectangular channel. Experiments were performed for both air-water and steam-water flows with a cocurrent flow configuration. This work presents the local temperature and velocity distributions in a water layer as well as the interfacial characteristics of both condensing and noncondensing fluid flows. The gas superficial velocity varied from 1.2 m/s to 2.0 m/s for air and from 1.2 m/s to 2.8 m/s for steam under a fixed inlet water superficial velocity of 0.025 m/s. Some advanced measurement methods have been applied to measure the local characteristics of the water layer thickness, temperature, and velocity fields in a horizontal stratified flow. The instantaneous velocity and temperature fields inside the water layer were measured using laser-induced fluorescence and particle image velocimetry, respectively. In addition, the water layer thickness was measured through an ultrasonic method.

• 한국가시화정보학회지
• /
• 제19권2호
• /
• pp.26-31
• /
• 2021

The spanwise growth of turbulence structures in turbulent pipe flow was investigated using the direct numerical simulation data of Re_𝜏 = 544, 934 and 3008. Two-point correlations and pre-multiplied energy spectra of streamwise velocity fluctuations were examined along the spanwise direction. The arclength direction is defined as r𝛳, which is useful for an analogy with the spanwise direction for channels or boundary layers; here, r and 𝛳 are the radial distance from the core and the azimuthal angles, respectively. Both analyses showed that the arclength scales increased with increasing the wall-normal distance. It showed that the coherent structures were confined in the core region due to the crowding effect of a circular pipe geometry. The pipe flow simulation could describe a realistic geometrical flow along the azimuthal direction, unlike the simulations of turbulent channel or boundary layer flow using periodic boundary conditions along the spanwise direction. The present results provided the spanwise organization of energy-containing motions over a broad range of scales in turbulent pipe flow.

• Journal of Mechanical Science and Technology
• /
• 제14권4호
• /
• pp.441-447
• /
• 2000

Direct numerical simulation has been used to study turbulent boundary layers with convex curvature. A direct numerical simulation program has been developed to solve incompressible Navier-Stokes equations in generalized coordinates with the finite volume method. We considered two boundary layer thicknesses. When the curvature effect is small, mean velocity statistics show little difference with those of a plane channel flow. Turbulent intensity decreases as curvature increases. Contours suggest that streamwise vorticities are strong where large pressure fluctuations exist.

• 직업교육연구
• /
• 제29권3호
• /
• pp.265-289
• /
• 2010

이 연구의 목적은 중장기 복무 제대군인의 원활한 사회적응을 돕기 위하여 국가보훈처에서 전직지원교육을 실시하고 있는데 실제적으로 얼마나 효과가 있었는지를 알아보기 위함이다. 전직지원교육 과정에 입교하여 수료한 323명의 제대군인을 대상으로 설문을 실시하여 AMOS 7.0 으로 분석하였으며 연구결과는 다음과 같다. 첫째, 학습동기는 학습몰입, 교육 만족도에 직접적인 정(+)의 유의한 영향을 주었다. 둘째, 조직지원은 교육 만족도와 심리적 안정감에 직접적인 정(+)의 유의한 영향을 주었으나 학습몰입에는 부(-)의 직접적인 유의한 영향을 주었다. 셋째, 교육프로그램은 학습몰입, 교육 만족도, 구직효능감, 심리적 안정감에 직접적인 정(+)의 유의한 영향을 주었다. 넷째, 학습몰입은 구직 효능감, 심리적 안정감에 직접적인 유의한 영향을 주지 못하였으나 교육만족도는 정(+)의 유의한 영향을 주었다. 다섯째, 교육만족도는 학습동기, 교육프로그램과 구직효능감, 심리적 안정감 간의 관계에서 부분 매개역할을 하였으나 학습몰입은 매개효과가 없었다. 연구결과를 토대로 연구의 함의와 후속연구를 위한 제언을 하고자 한다. 먼저 연구결과에서 나타난 전직지원교육이 실질적으로 제대군인에게 구직효능감과 심리적 안정감을 가져다 준 것으로 나타났다. 따라서 전직지원교육 성과에 가장 크게 영향을 미치는 교육프로그램 설계시 제대군인에게 실질적으로 도움이 되는 사회적응에 필요한 내용으로 구성을 해야 한다. 이 연구는 특정시점에서 제대군인을 대상으로 횡단적인 연구를 실시하였는데 향후에는 취업이나 창업 성공 여부를 결과변수에 추가하여 시간경과에 따른 종단적인 연구를 할 필요가 있다.

• 한국신재생에너지학회:학술대회논문집
• /
• 한국신재생에너지학회 2009년도 추계학술대회 논문집
• /
• pp.176-179
• /
• 2009

The performance of direct methanol fuel cells is affected by operating conditions such as, methanol feeding temperatures, methanol concentrations, and methanol flow rates during the operation in different environmental conditions. In this study, effects of the environmental temperature on performance of direct methanol fuel cells have been investigated in order to test a applicability of direct methanol fuel cell to the vehicle. The environmental temperature (ET) was varied from $-20^{\circ}C$ to $+30^{\circ}C$. The inside fuel cell temperature (CT) during test at various operating conditions was monitored and the performance of fuel cell was measured in the I-V polarization curve. With increasing the ET, the performance of the fuel cell was significantly improved and the CT also almost linearly increased. However, at below $0^{\circ}C$ ET, the DMFC showed very poor performance and needed to control CT or methanol feeding temperature (MFT), methanol flow rate(MFR) to obtain enough power of the vehicle.

• 설비공학논문집
• /
• 제3권3호
• /
• pp.153-160
• /
• 1991

Heat transfer characteristics of a direct contact heat exchanger utilizing sieve trays and spray nozzles for steam condensation for the purpose of combining with a LNG evaporator have been investigated with various cooling water flow rates and different vacuum pressures within the heat exchanger for the purpose of steam condensation. Temperature profiles and the volumetric overall heat transfer coefficients in a direct contact heat exchanger have been obtained for comparisons. The results show that the temperature differences between cooling water and steam along the direct contact heat exchanger height are rapidly decreasing and the volumetric overall heat transfer coefficients of the exchanger improves greatly as the inside vacuum pressure increases. The values of the overall heat transfer coefficients at P=-680mmHg have been increased significantly compared with at atmospheric pressure. At given pressure conditions, it is found that the values of average volumetric overall heat transfer coefficients for the sieve tray are found to be approximately 10% higher than those of the spray nozzle.

• 한국자동차공학회논문집
• /
• 제14권1호
• /
• pp.129-137
• /
• 2006

This paper studies the effects of the swirl for the variation of intake port configuration that is key parameters in the flow field of direct injection diesel engines. In-cylinder flow characteristics is known to have significant effects on air-fuel mixing, combustion, and emissions. To investigate the effects of the swirl flow, various rpm(250, 500, 750) and two different intake port were used. And to evaluate the swirl motion in the flow field visualization engine, steady state flow test was conducted. Helical port intake port and SCV(Swirl Control Valve) were selected as the design parameters to increase the swirl flow and parametric study was performed. In the case of non-SCV, intake flow rate and non-dimensional swirl ratio were higher than those of SCV for the swirl head type. So, we could strengthen the swirl in the flow field with the swirl head type and don't using SCV. From the results of steady state flow test, non-swirl head type has the most good advantage for intake flow rate, and also the flow rate could be increased by using the SCV slightly. The effects of the type of engine head on intake air flow capability are dominant with respect to the existence of the SCV. We could measure the qualitative grade of swirl by capturing the scattering signal of microballoon from ICCD camera in the visualization diesel engine.

• 디지털융복합연구
• /
• 제10권1호
• /
• pp.293-300
• /
• 2012

본 연구에서는 몰입경험, 자기주도학습 준비도, 인터넷중독과 웹기반 컴퓨터 교육의 학업성취도 간의 인과관계를 분석하였으며, 이 인과관계 모형의 남녀 성별 차이를 조사하였다. 연구결과에 따르면, 자기주도학습 준비도는 몰입경험과 학업성취도에 양의 방향으로 직접적이며 유의한 영향을 미치는 것으로 나타났으며, 인터넷 중독에는 음의 방향으로 유의한 직접 영향을 미치는 것으로 나타났다. 여학생의 경우, 학업성취도는 몰입경험에 양의 방향으로 유의한 직접적 영향을 받으며, 자기주도학습 준비도에 의해 몰입경험을 매개로 유의한 직 간접적 영향을 받는 것으로 조사되었다. 하지만, 남학생은 여학생과는 달리, 몰입경험이 학업성취도에 미치는 직접영향과 자기주도학습이 학업성취도에 미치는 간접 영향은 통계적으로 유의하지 않은 것으로 나타났다.

• 한국통신학회논문지
• /
• 제29권1A호
• /
• pp.99-106
• /
• 2004

본 논문에서는 DSSS (Direct Sequence Spread Spectrum) 수신기에서 초기동기 탐색에 사용될 수 있는 정합필터에 대해서 연구하였다. 하드웨어기술언어 (HDL)로 표현될 수 있는 단일 데이터 흐름 경로를 갖는 정합필터가 설명되었다. 필터 연산의 처리시간을 개선하기 위해 데이터의 흐름이 이중으로 표현될 수 있도록 식이 정리되고 이와 연관된 하드웨어 모델이 제시되었다. 제안된 모델은 고속 처리를 위해 병렬처리와 파이프라인을 기반으로 하고 일련의 메모리, 곱셈기, 누산기로 구성된 두 개의 데이터 흐름 경로가 평행하게 배열된 구조이다. 제안된 모델에 대해 성능을 분석하였고 단인 데이터 흐름 경로 구조의 정합필터와 비교하였다.