• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2010년도 학술발표회
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• pp.189-193
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• 2010

RMA-2, Telemac-2D, River2D 등 기존의 대부분 국내외 상용모형에서는 내부 경계인 하천구조물과 외부 경계인 측벽에서의 경계조건을 활동조건(slip condition)으로 가정하여 흐름장을 계산하였다. 그러나 실제로 벽면에서는 마찰력에 의해 흐름이 존재하지 않는 무활조건(no slip condition)이 물리적으로 타당하다. 따라서 본 연구에서는 내부구조물인 교각이 존재하는 영역에서의 수평 2차원 유속분포를 구하기 위해 천수방정식을 Galerkin법과 Newton-Raphson법에 의해 이산화한 수치모형을 개발하였다. Yulistiyanto 등(1998)이 수치모의 및 수리실험에 사용한 조건을 채택하여 벽면에서의 접선방향 유속이 존재하는 활동조건과 벽면에서 유속이 없는 무활조건을 부여하고 교각을 포함한 수로에서 유속, 수위 및 전단분포를 해석하였다. 활동경계조건을 적용한 경우 교각 표면을 따라 큰 유속이 분포하고 후면에서도 관성력에 의해 흐름방향 유속이 두드러지게 나타났으나 무활조건을 적용한 경우 교각 후면에서 와가 형성되고 후류현상이 두드러지게 나타났다. 무활조건을 적용한 경우 교각 전면부에서는 선수파(bow wave)가 4 cm정도 높게 나타났으며 교각 측면부에서는 2 cm 높게 나타났다. 반면 교각 후면에서는 후류의 영향으로 수면이 2 cm 낮게 분포하였다. 교각부에 작용하는 전단력을 분석한 결과 무활조건을 적용한 경우가 활동조건을 적용한 경우에 비해 최대 6배 높은 전단력이 나타났다.

• International Journal of Fluid Machinery and Systems
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• 제9권4호
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• pp.287-299
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• 2016

In China, agricultural irrigation water often contains a lot of suspended sediment which may cause the nozzle wear. In this study, a new numerical simulation combing the Discrete Phase Model and a remeshing algorithm was conducted. The geometric boundary deformation caused by the erosion wear, was considered. The weight loss of the nozzle, the node displacement and the flow field were investigated and discussed. The timestep sensitivity analysis showed that the timestep is very critical in the erosion modeling due to the randomness and the discreteness of the erosion behavior. Based on the simulation results, the major deformation of the boundary wall due to the erosion was found at the corners between outlet portion and contraction portion. Based on this remeshing algorithm, the simulated erosion weight loss of the nozzle is 4.62% less compared with the case without boundary deformation. The boundary deformation changes the pressure and velocity distribution, and eventually changes the sediment distribution inside the nozzle. The average turbulence kinetic energy at the outlet orifice is found to decrease with the erosion time, which is believed to change the nozzle's spray performance eventually.

• 한국지반환경공학회 논문집
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• 제2권1호
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• pp.81-89
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• 2001

본 연구에서는 차수벽 재료로 자주 사용되는 흙-벤토나이트 혼합물의 투수계수가 해수로 인하여 어떻게 영향을 받는지 고찰하였으며 혼합물로 축조된 가상의 제방에 해수가 침입할 때 어떻게 영향을 받는지를 고찰하였다. 유한요소법을 이용한 RFP 방법을 사용하였으며 투수계수변화를 이 해석에 도입하였다. 해석을 위하여 정상흐름상태와 비정상 흐름 상태에서 경계면을 구한 결과 투수계수변화를 고려하지 않았던 과거의 방법과 비교할 때 해수쪽으로는 낮게 담수영역에서는 경계면이 높게 나타났다.

• 한국연소학회지
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• 제22권1호
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• pp.46-52
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• 2017

High-frequency combustion instabilities may occur during the development of feasible engine combustors. These instabilities can result in irreparable damages to the wall of combustors or the degradation of engine performance. So, it is essential to identify injectors that have high stability characteristics during the early stages of development. The objective of present study was to assess the stability of coaxial injectors and an impinging injector with different recess lengths in order to develop stable injectors optimally. Stability margin was evaluated based on the distance from operating condition to the unstable regions. A simulating combustion test method was used to analyze the stability of injectors. A small-scale combustion chamber was designed to simulate the first tangential acoustic mode of the actual combustor. Gaseous oxygen and a mixture of methane and propane were used as simulant propellants to satisfy their flow similarity to the actual propellants of a combustor in a liquid rocket combustor. The results indicated that injectors having small recess lengths showed relatively large combustion stability margins. For the injectors of large recess lengths, instability regions with large and super-large amplitude oscillations were observed. Thus, injector with shorter recess lengths had a higher stability than that of longer one due to the different mixing processes.

• 동력기계공학회지
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• 제15권5호
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• pp.43-48
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• 2011

The article represents an experimental investigation on friction and turbulent flow characteristics of free airflow through a rectangular duct fitted with semicircular ribs of uniform height (e = 3.5 mm) on one principle wall. The aspect ratio of the rectangular duct was AR= 5 where the duct height (H) was of 30 mm. Four different rib pitches (P) of 28 mm, 35 mm, 42 mm and 49 mm were used for constant rib height to hydraulic diameter ratio (e/Dh = 0.07) and constant rib height to channel height ratio (e/H = 0.11). The experimental results show some significant effects on pressure drop as well as turbulent characteristics at various configurations among different numbers of rib arrangements varying Reynolds number in the range of 15000 to 30000. Pressure transducer and hot wire anemometer were used for data acquisition of this experiment.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2000년도 제18회 학술발표회 논문개요집
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• pp.210-210
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• 2000

Vertically well aligned multi-wall carbon nanotubes (CNT) were grown on nickel coated glass substrates by plasma enhanced hot filament chemical vapor deposition at low temperatures below 600$^{\circ}C$. Acetylene and ammonia gas were used as the carbon source and a catalyst. Effects of growth parameters such as pre-treatment of substrate, plasma intensity, filament current, imput gas flow rate, gas composition, substrate temperature and different substrates on the growth characteristics of CNT were systematically investigated. Figure 1 shows SEM image of CNT grown on Ni coated glass substrate. Diameter of nanotube was 30 to 100nm depending on the growth condition. The diameter of CNT decreased and density of CNT increased as NH3 etching time etching time increased. Plasma intensity was found to be the most critical parameter to determine the growth of CNT. CNT was not grown at the plasma intensity lower than 500V. Growth of CNT without filament current was observed. Raman spectroscopy showed the C-C tangential stretching mode at 1592 cm1 as well as D line at 1366 cm-1. From the microanalysis using HRTEM, nickel cap was observed on the top of the grown CNT and very thin carbon amorphous layer of 5nm was found on the nickel cap. Current-voltage characteristics using STM showed about 34nA of current at the applied voltage of 1 volt. Electron emission from the vertically well aligned CNT was obtained using phosphor anode with onset electric field of 1.5C/um.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2012년도 학술발표회
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• pp.700-700
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• 2012

산지유역은 하천을 따라서 도로가 발달되어 있어서 대부분의 도로가 홍수시 하천의 영향을 많이 받는다. 산지하천은 경사가 급하고 만곡수충부가 많이 발달되어 있기 때문에 홍수시 유속이 빠르고 만곡수충부의 편수위가 매우 크다. 이는 만곡부 호안 파괴와 도로 유실의 피해를 일으키는 경우가 많다. 따라서 대부분의 산지하천 만곡수충부에는 홍수피해 방지를 위해 콘크리트 옹벽호안으로 되어 있다. 그러나 콘크리트 옹벽은 조도가 작기 때문에 유속이 더 빠르게 되고 편수위를 한층 증대시킬 수 있다. 산지하천 만곡수충부의 편수위를 줄이기 위해서는 접근유속을 줄여야 하나 산지하천 특성으로 볼 때 접근유속 저감을 위한 공학적 방법은 제한적이다. 따라서 만곡수충부의 유속을 줄이는 방법으로 콘크리트 옹벽호안의 조도계수를 증대시키는 것이 효과적일 수 있다. 본 연구에서는 개수로 측벽에 수직돌출줄눈이 설치되었을 때 흐름에 미치는 수리효과를 개수로 수리실험으로 파악하고자 한 것이다. 실험결과 돌출줄눈의 간격이 수직돌출줄눈의 무차원 폭이 9일 때 평균유속이 가장 작게 나타났다. 이는 돌출줄눈의 간격이 개수로 내부흐름의 유속분포, 최대유속발생 위치, 유수단 면적의 크기에 영향이 미치기 때문이다. 따라서 개수로 측벽 수직돌출줄눈의 간격을 조절함으로써 개수로 유수저항의 크기를 조절할 수 있다.

• Journal of Advanced Marine Engineering and Technology
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• 제20권3호
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• pp.91-100
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• 1996

Experimental results for forced convection condensation of non-azeotropic refrigerant mixtures inside a horizontal smooth tube are presented. The mixtures of R-22+R-134a and pure refrigerants R-22 and R-134a are used as the test fluids and a double pipe heat exchanger of 7.5mm ID and 4800mm long inside tube is used. The range of parameters are 100-300kg/h of mass flow rate, 0-1.0 of quality, and 0, 33, 50, 67, and 100 weight percent of R-22 mass fraction in the mixtures. The heat flux, vapor pressure, vapor temperature and tube wall temperature were measured. Using the data, the local and average heat transfer coefficients for the condensation have been obtained. In the same given experimental conditions, the liquid heat transfer coefficients for NARMs were considerally lower than that of the pure refrigerant of R-22 and R-134a. Local heat transfer characteristics for NARMs were different from pure refrigerant R-22 and R-134a. In some regions, local heat transfer coefficients for NARMs were increased in the following order ; Bottom$\rightarrow$Top$\rightarrow$Side. The condensation heat transfer coefficients for NARMs increased with mass velocity, heat flux, and quality, but were considerably lower than that of pure refigerant R-22 and R-134a.

• 한국정밀공학회지
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• 제14권2호
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• pp.102-112
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• 1997

This paper describes the method that can construct kinematically admissible velocity fields for forging of gear-like components which have tooth shape around the cylinder. The kinematically admissible velo- city fields for the various gear-like components, involute spur gear, trapezoidal spline, square spline, ser- ration and trochoidal gear, were constructed by pilling up the velocity components according to the shape of tooth and billet. The billets, of hollow and solid, were Al 2218 and 2024. To verify the method, the analyses and experiments were carried out and compared with each other. For analyses, the half pitches of com- ponents were divided into several deformation regions based on their tooth profile. A neutral surface was used to represent the inner flow of material during forging. Its location varied with the energy optimazation and its contour varied with the number of teeth. In experiment, the contour of material filling up the tooth zone is hyperbolic curve caused by the frictional drag on the interface of die-wall/workpiece but, in the analysis, it is an arc which retains the same contour during all forging operation.

• 대한두경부종양학회지
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• 제13권1호
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• pp.90-93
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• 1997

Aneurysm of extracranial carotid artery which usually originated from trauma, but it can be developed by atheroscrelosis, infection and congenital vascular disease, is defined as abnormal dilatation of intimal wall of carotid artery. The proper management should not be delayed due to occurrence of high neulorogic complication. Recently the authors experienced a case of nontraumatic extracranial internal carotid artery aneurysm, which was successfully resected using intraoperative EEG monitoring for the prevention of ischemic attack and was reconstructed with end to end anastomosis using nylon 9 - 0. On postoperative day #13, we could confirm well healed aneurysmal site and normal blood flow in the view of arteriography.