• 한국군사과학기술학회지
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• 제9권4호
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• pp.128-136
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• 2006

The purpose of this paper is to examine the dynamic stall characteristics of an elliptic airfoil when subject to constant pitch motions. In this study, which was motivated by the pressing need for a greater understanding of the reduced frequency$({\kappa})$ effects on flow patterns of elliptic airfoil, the various reduced frequencies were considered. The result confirms that the reduced frequency has a profound effects on the flow patterns. The increase of ${\kappa}$ accelerate the separation bubble bursting process up to ${\kappa}=0.10$, then diminish with further increase in ${\kappa}$. Compared with static condition, the dynamic pitching airfoil delays stall angle approximate $4{\circ}{\sim}5{\circ}$ during pitch-up stroke for ${\kappa}=0.10$. Results from this qualitative analysis provided valuable insight Into the control of dynamics stall.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 1999년도 유체기계 연구개발 발표회 논문집
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• pp.81-89
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• 1999

The major source of noise in the process of transporting liquids is related to the cavitation phenomenon. The control valve noise is mostly dominated by bubble dynamics under cavitating conditions. In this investigation, an orifice configuration is set-up to correlate its flow-field and acoustic signatures with those from a control valve device. The performance and noise characteristics form the orifice configuration in anechoic surroundings were measured to reveal the noise sources depending on pressure differences across the orifice configuration. The sound powers from the orifice configuration are effectively normalized using proposed scaling parameters. Flow-excited dynamic systems for which there is no strong coupling between the flow and the system response can be described using a linear source-filter model. On this assumption, the normalized sound powers can be decomposed of noise source function and a response function. To find noise sources, pressure spectra measured over a range of pressure differences are transformed into the product of two non-dimensional frequency function : $P_{ss}(He,f_{ca},x/D) = F(f_{ca})\;G(He,x/D)$. This scheme of finding noise sources is shown to be applicable to the cavitation noise from the control valve effectively Two kinds of cavitating modes based on our experimental data are found and discussed.

• Tribology and Lubricants
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• 제38권3호
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• pp.109-114
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• 2022

This paper reports the development of an oil flushing system combined with a microbubble generator. Oil flushing plays a crucial role in regulating the lubricant's performance during the lubricant replacement process. Moreover, harmful contaminants, such as sludge, wear particles, and rust, from piping systems or lubrication system can be removed by oil flushing. Oil flushing aims to increase the system's efficiency using a dedicated flushing oil, increasing of the supply pressure and generating a vortex. In addition, it helps the mechanical system or equipment achieve peak performance and reduces the potential for premature failure. However, the contaminant-removal applications of existing oil flushing system are limited. In this research, we aim to improve the performance of oil flushing system by incorporating a microbubble generator, which uses the venture effect to generate microbubbles and mixes them with lubricant. The microbubbles in the blended lubricant remove contaminants from the lubrication system more effectively. Structural mechanics and fluid dynamics are analyzed through fluid-structure interaction (FSI) analysis, and the numerical analysis results are used for the designing the system. The magnitude of the maximum stress is investigated based on the pressure results obtained by the CFD analysis; through the CFD analysis, the mixing ratio of air (bubble) and lubricant is evaluated using the volume of fluid (VOF) model according to the working conditions.

• 한국항해항만학회지
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• 제40권4호
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• pp.173-181
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• 2016

2016년부터 배출통제지역(ECA : Emission Control Atea)을 운항하는 선박에 대하여 배출되는 NOx(질소산화물) 및 SOx(황산화물)의 배기량 감소규제가 강화되었다. 상기의 규제 물질 중 NOx를 제거하는 탈질장비 중 선택적 촉매 환원(SCR : Selectivity Catalytic Reduction) 시스템은 효율이 높고 상업적으로 많이 활용되고 있으나, 높은 온도에서 요소수가 활성화되는 단점이 있다. 이에 초미세기포를 이용하여 낮은 온도에서도 반응할 수 있는 요소수 및 요소수 활성화 기기를 개발하여 상기의 문제점들을 최소화 할 수 있도록 하였다. 또한 SCR 시스템의 효율성을 향상시키는 방안을 마련하기 위하여, ANSYS-CFX package를 이용한 전산유체역학(CFD : Computational fluid dynamics)기법을 사용하였다. Navier-Stokes 방정식을 해석의 지배방정식으로 적용하여 SCR 시스템의 점성유동해석 시뮬레이션을 수행하였다. SCR 시스템의 형상은 CATIA V5를 사용하여 3D 모델링을 하였고, SCR 시스템의 효율성을 비교하기 위해 요소수 분사 노즐의 위치를 요소수 분사 노즐은 배기관의 입구로부터 1/3, 1/2, 2/3로 변경하며 확인하였다. 또한, 노즐의 분사구 수가 SCR 시스템의 효율에 미치는 영향을 확인하기 위하여 분사구 수가 4, 6, 8개일 경우를 시뮬레이션 하여 비교 분석하였다. 시뮬레이션 결과 배기관 입구에 가까울수록, 분사구 수가 많을수록 효율이 향상됨을 확인하였다.

• 대한기계학회논문집B
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• 제38권10호
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• pp.865-871
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• 2014

본 연구는 벤츄리 시스템을 미세기포 생성을 위한 공기공급 장치로 개발하는데 그 목표를 두고 수행하였다. 이를 위해 상용유동해석 프로그램인 ANSYS CFX-15를 사용한 전산 유동해석을 통해 기하학적 형상변화가 벤츄리 관 내 유동특성들에 미치는 영향을 규명하였다 그리고 공급공기를 공급하는 공기 공급관의 위치, 크기, 개수 등을 변수로 2-유체 유동 해석을 수행하여 이들 설계 값들이 공기 공급 특성에 미치는 효과를 규명하였다. 최종적으로 직경 비 ${\beta}=0.75$의 벤츄리 확대관이 시작되는 위치에 공기 공급 구멍을 설치할 경우 가장 많은 공기가 벤츄리 관으로 유입되는 것을 확인할 수 있었으며, 유입공기 공급구멍 개수 및 직경과 벤츄리 관 내 공급되는 공기량 사이에는 선형적인 관계가 성립됨을 확인하였다.

• 대한기계학회논문집B
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• 제39권7호
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• pp.617-624
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• 2015

용존공기부상법이란 오염물에 미세기포를 부착하여 수표면으로 부상시킴으로써 이를 제거하는 수처리 방법이다. 본 연구에서는 난류모델에 따른 용존공기부상조 내부 유동해석의 변화를 고찰하기 위해 물과 기포의 혼합물에 대한 2상 유동을 모사하였다. 이때, 주어진 용존공기부상조 형상 및 조건에 대하여 다양한 난류모델에 따른 용존공기부상법 내부의 미세기포 분포량, 계산시간 및 수렴성 등을 비교하였으며, 그 결과 기존에 주로 사용되었던 표준 ${\kappa}-{\varepsilon}$ 모델이 타 난류모델과는 다른 거동을 예측하는 것으로 확인되었다.

• 에너지공학
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• 제2권2호
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• pp.179-186
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• 1993

평판형 태양열 집열기에 적용되는 열파이프는 증발부에 상대적인 낮은 열유속으로 길이가 길고 가느다란 형상을 갖는다. 이러한 열파이프는 증발부의 하단부에서 액체 Pool이 형성하는 경향이 있다. 그리고 이 Pool에서 과열, 급속한 기포의 생성, 기포의 폭발적인 성장과정과 Flooding 등의 복잡한 증발과 유체 역학적 현상이 발생한다. 본 논문에서는 4개의 열파이프와 3개의 열사이폰을 이용하여 주 설계변수인 작동유체의 충전량과 위크의 설치 영역을 조절함으로써 이러한 문제를 해결하도록 하였다. 이에 대한 결과들은 다음과 같이 요약할 수 있다$^{1)}$ . 열파이프의 유효 열전도도는 단열부와 응축부에 위크를 제거함으로써 상당히 개선할 수 있었다$^{2)}$ .액체의 충전량은 위크를 적실수 있는 양보다 약 40%정도 증가시켜야 한다$_{3)}$ . 증발부에서 위크는 핵비 등의 단속적 발생에 의한 불안정한 작동과 포기 시동과정에서 응답시간을 줄이는데 유용한 효과를 갖는다.

• Journal of Microbiology and Biotechnology
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• 제11권3호
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• pp.369-375
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• 2001

Urushiol, a natural monomeric oil, was used to prepare a detergentless micro-emulsion with water and 2-propanol The formation of micro-emulsion was verified by conductivity measurements and dynamic light scattering. The conductivity data showed phase change dynamics, a characteristics of micro-emulsions, and subsequent dynamic light scattering study further confirmed the phenomenon. Average water droplet diameter was 10 nm to 500 nm when the molar ratio of 2-propanol ranged from 0.40 to 0.44 . Earlier studies were performed on toluene and hexane, in which the insoluble substrate in water phase was added to the solvents to be reacted on by enzymes. However, in the present urushiol system, urushiol was used as both solvent and substrate in the laccase polymerization of urushiol. The laccase activity in the system was examined using polymerization of urushiol. The laccase activity in the system was examined using syringaldezine as a substrate, and the activity increased rapidly near the molar ratio of 2-propanol at 0.4, where micro-emulsion started. The activity rose until 0.46 and fell dramatically thereafter. The study of laccase activity in differing mole fractions of 2-propanol showed the existence of an ‘optimal zone’, where the activity of laccase was significantly higher. In order to analyze urushiol polymerization by laccase, a bubble column reactor using a detergentless micro-emulsion system was constructed. Comparative study using other organic solvents systems were conducted and the 2-propanol system was shown to yield the highest polymerization level. The study of laccase activity at a differing mole fraction of 2-propanol showed the existence of an ‘optimal zone’ where the activity was significantly higher. Also, 3,000 cP viscosity was achieved in actual urushi processing, using only 1/100 level of laccase present in urushi.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2010년도 한국우주과학회보 제19권1호
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• pp.40.4-41
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• 2010

Using magnetic field and plasma moment data obtained by THEMIS satellites(A, D, and E), we selected 203 fast flow events accompanied by dipolarization in the near-Earth region( X(GSM) = -7 ~ -12 RE) and statistically examined their properties. It was found that most of the fast flows show the maximum velocity between 1 minute before dipolarization onset and 2 minutes after onset and proceed earthward and duskward. We also found that only the flows with low velocity of less than 400 km/s are observed at X > -8 RE, while the high velocity flows(as well as low velocity flows) are observed at the further tailward region(X < -8 RE). And most of the tailward flows are slow regardless of distance at X(GSM) = -7 ~ -12 RE. On the other hand, if we consider the fast flow as a bubble (Pontius and Wolf, 1990), the entropy parameter, PV5/3 is an important factor to describe the plasma sheet dynamics. Thus we investigated the relationship between the flow velocity and the amount of change in PV5/3 before and after dipolarization onset and found out that the dipolarizations with more depleted entropy parameter tend to show higher flow velocity. Also we examined how the magnetic field at geosynchronous orbit responds to the fast flow accompanied by dipolarization in the near-earth plasma sheet, using the measurements from GOES 11 and 12 statellites. We found that most of the fast flows do not reach geosynchronous orbit as suggested by Ohtani et al. (2006).

• Nuclear Engineering and Technology
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• 제42권6호
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• pp.620-635
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• 2010

In this paper we describe current activities on the project Multi-Scale Modeling and Analysis of convective boiling (MSMA), conducted jointly by the Paul Scherrer Institute (PSI) and the Swiss Nuclear Utilities (Swissnuclear). The long-term aim of the MSMA project is to formulate improved closure laws for Computational Fluid Dynamics (CFD) simulations for prediction of convective boiling and eventually of the Critical Heat Flux (CHF). As boiling is controlled by the competition of numerous phenomena at various length and time scales, a multi-scale approach is employed to tackle the problem at different scales. In the MSMA project, the scales on which we focus range from the CFD scale (macro-scale), bubble size scale (meso-scale), liquid micro-layer and triple interline scale (micro-scale), and molecular scale (nano-scale). The current focus of the project is on micro- and meso-scales modeling. The numerical framework comprises a highly efficient, parallel DNS solver, the PSI-BOIL code. The code has incorporated an Immersed Boundary Method (IBM) to tackle complex geometries. For simulation of meso-scales (bubbles), we use the Constrained Interpolation Profile method: Conservative Semi-Lagrangian $2^{nd}$ order (CIP-CSL2). The phase change is described either by applying conventional jump conditions at the interface, or by using the Phase Field (PF) approach. In this work, we present selected results for flows in complex geometry using the IBM, selected bubbly flow simulations using the CIP-CSL2 method and results for phase change using the PF approach. In the subsequent stage of the project, the importance of effects of nano-scale processes on the global boiling heat transfer will be evaluated. To validate the models, more experimental information will be needed in the future, so it is expected that the MSMA project will become the seed for a long-term, combined theoretical and experimental program.