• Journal of Ship and Ocean Technology
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• 제3권2호
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• pp.1-16
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• 1999

The incompressible Reynolds-Averaged Navier-Stokes(RANS) equations are solved using the standard $\textsc{k}-\varepsilon$ turbulence model and a finite volume method(FVM)with an O-type grid system. The computed results for its performance test are in good agreement with the published experimental data. The present method is applied to the study on the leading edge radius of a hydrofoil section Calculated results suggest that the leading edge radius has some effects on cavitation performances of a 2-D foil. A natural leading edge radius for the NACA66 section is determined from this study.

• 대한조선학회논문집
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• 제50권5호
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• pp.355-363
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• 2013

Panel methods are essential tools for analyzing a fluid-flow problem around complex three dimensional bodies, but they lack ability to solve viscous effects. On the other hand, CFD methods are considered as powerful tools for analyzing fluid-flow characteristics including viscosity. However, they also have short falls, requiring more computing time and showing different results depending on the selection of turbulence models and grid systems. In this paper a hybrid scheme combining a panel method and a CFD method is suggested. The scheme adopts a panel method for far-field solution where viscous effects are negligible and a CFD method for the solution of RANS equations in near-field where viscous effects are relatively strong. The intermediate region between the far-field and near-field is introduced where the calculated field point velocities by the panel method are given as boundary velocities for the CFD method. To verify the scheme, calculated results, by a panel method, a CFD method and the hybrid scheme, for a two dimensional foil section are compared. The suggested hybrid scheme gives reasonable results, while computation time and memory can be dramatically reduced. By using the hybrid scheme efforts can be concentrated for the local flow near the leading and trailing edges, by providing more dense grid system, where detailed flow characteristics are required.

• Journal of Advanced Marine Engineering and Technology
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• 제35권2호
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• pp.196-203
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• 2011

유체 유동에 수직인 방향으로 반복적 리브이 설치된 사각 채널에서 열전달 계수와 마찰계수를 측정하기 위해 실험을 수행하였다. 시험부의 치수는 198 mm(폭)과 40 mm(높이) 그리고 길이가 712mm인 직사각형 채널이다. 시험부 채널의 형상비는 4.95이고 수력 직경 $D_h$은 6.66 cm이었다. 4종류의 와이어 스크린 리브(rib)과 한 개의 일체형 리브을 사용하였다. 0.1 mm 두께의 스테인레스 강판 히터와 T형 열전대를 사용하였다. 레이놀즈 수의 범위는 20,000에서 60,000이었다. 수력직경($D_h$)과 리브의 높(e)이 비($e/D_h$)는 0.075이고 리브 간격(p) 대 높이(e)으 비(p/e)는 10이다. 연구결과 일체형 리브이 누셀트수와 마찰계수 모두 가장 컸다.

• 한국공작기계학회논문집
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• 제10권6호
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• pp.57-62
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• 2001

Recently, it has increased that the importance of gas pressure weldment of steel bars in large construction bars. But there has hardly been any studies about it. Therefore we need more research. SD40 steel bar (32mm in diameter) which has been practically used at construction sites are tested about tension, bonding, fatigue, a macro structure and micro structure at foil gitudinal section and hardness to fed out the mechanical property and best welding ranges in some cases of mechanical cut and gas cut before gas pressure welded. It is that a gas-pressure welded zone of steel bar where was cleaned of impurities in way of two-upsetted method is more excellent bending and tension property than the regulation of KD D 0244. Also gas cut bars gained hardness from the heat affected zone so. In conclusion, to improve the weldability of steel bars, it is considered best to clean mechanically cut sutra- faces and then weld them by a method of 2 step upset way.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.216-227
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• 2004

A new cavitating model by using bubble size distribution based on bubbles-mass has been proposed. Both liquid and vapor phases are treated with Eulerian framework as a mixture containing minute cavitating bubbles. In addition vapor phase consists of various sizes of vapor bubbles, which are distributed to classes based on their mass. The bubble number-density for each class was solved by considering the change of the bubble-mass due to phase change as well as generation of new bubbles due to heterogeneous nucleation. In this method, the bubble-mass is treated as an independent variable, and the other dependent variables are solved in spatial coordinates and bubble-mass coordinate. Firstly, we employed this method to calculate bubble nucleation and growth in stationary super-heated liquid nitrogen, and bubble collapse in stationary sub-cooled one. In the case of bubble growth in super-heated liquid, bubble number-density of the smallest class based on its mass is increased due to the nucleation. These new bubbles grow with time, and the bubbles shift to larger class. Therefore void fraction of each class is increased due to the growth in the whole class. On the other hand, in the case of bubble collapse in sub-cooled liquid, the existing bubbles are contracted, and then they shift to smaller class. It finally becomes extinct at the smallest one. Secondly, the present method is applied to a cavitating flow around NACA00l5 foil. Liquid nitrogen and liquid oxygen are employed as working fluids. Cavitation number, $\sigma$, is fixed at 0.15, inlet velocities are changed at 5, 10, 20 and 50m/s. Inlet temperatures are 90K in case of liquid nitrogen, and 90K and 1l0K in case of liquid oxygen. 110K of oxygen is corresponding to the 90K of nitrogen because of the same relative temperature to the critical one, $T_{r}$=$T/T_c^{+}$. Cavitating flow around the NACA0015 foils was properly analyzed by using bubble size distribution. Finally, the method is applied to a cavitating flow in an inducer of the LE-7A hydrogen turbo-pump. This inducer has 3 spiral foils. However, for simplicity, 2D calculation was carried out in an unrolled channel at 0.9R cross-section. The channel moves against the fluid at a peripheral velocity corresponding to the inducer revolutions. Total inlet pressure, $Pt_{in}$, is set at l00KPa, because cavitation is not generated at a design point, $Pt_{in}$=260KPa. The bubbles occur upstream of the foils and collapse between them. Cavitating flow in the inducer was successfully predicted by using the bubble size distribution.

• 한국해양환경ㆍ에너지학회지
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• 제14권4호
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• pp.273-279
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• 2011

최근 관심이 높아지고 있는 해상풍력 터빈의 성능평가를 위해 풍력터빈 날개에 대한 점성유동장 해석을 수행하였다. Fluent package를 사용하여 회전하는 비관성 좌표계에서 Reynolds-averaged Navier-Stokes 방정식의 해를 구하였다. 난 류 모형은 realizable k-$\varepsilon$ 모델을 사용하였으며, 격자계는 구조 격자계와 비구조 격자계를 혼합하여 사용하였다. 먼저 실험값이 공개되어 있는 2차원 날개에 대한 유동해석을 통해 수치계산 결과를 검증하였다. 다음으로 NACA 4-series 단면을 가지는 세 개의 날개가 2 MW의 정격출력을 내도록 설계된 풍력터빈 날개를 대상으로 회전수와 피치각을 고정시킨 상태에서 입사류의 속도를 변화하여 주속비 별로 성능해석을 수행하였다. 그리고 풍력터빈의 성능향상을 위해 날개의 끝에 세 가지 형태의 날개 끝판을 붙여 성능 변화를 추정하였다. 그 중에서 날개의 흡입면에 날개 끝판을 붙인 경우가 가장 우수한 성능을 보였다. 또한 풍력터빈 날개가 경사각과 레이크를 가진 경우에 대하여 출력계수를 각각 비교하였다.