• Proceedings of the Korea Water Resources Association Conference
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• 2004.05b
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• pp.381-385
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• 2004

본 인구에서는 레이놀즈응력모형을 이용하여 직사각형 개수로 흐름을 수치모의 하고 이차흐름의 생성 메커니즘을 제시하였다. 수치모의 결과 자유수면과 측벽의 접합부 근처에서 inner secondary flow가 발생하였다. 이는 최근 Grega 등(1995)과 Hsu 등(2000)에 의해 밝혀진 새로운 이차흐름이다. 또한 측벽에서의 전단력 분포를 계산한 결과 inner secondary flow에 의하여 수면 근처에서의 전단력 값이 증가하는 것으로 나타났다. 계산된 결과를 이용하여 와도 방정식에서 각 항의 크기를 비교하여 이차 흐름의 생성 메커니즘을 살펴보았다. 그 결과 벽 및 측벽 경계 부근에서는 난류의 비등방성에 의한 와도 생성항에 의해 이차 흐름이 생성되고, 경계와 멀리 떨어진 영역에서는 레이놀즈응력에 의한 와도 생성항이 이차흐름을 생성시키는데 중요한 역할을 하는 것으로 나타났다.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.2B
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• pp.179-189
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• 2010

This paper presents a numerical investigation of vorticity generation in fully vegetated open-channel flows. The Reynolds stress model is used for the turbulence closure. Open-channel flows with rough bed-smooth sidewalls and smooth bed-rough sidewalls are simulated. The computed vectors show that in channel flows with rough bed and rough sidewalls, the free-surface secondary currents become relatively smaller and larger, respectively, compared with that of plain channel flows. Also, open-channel flows over vegetation are simulated. The computed bottom vortex occupies the entire water depth, while the free-surface vortex is reduced. The contours of turbulent anisotropy and Reynolds stress are presented with different density of vegetation. The budget analysis of vorticity equation is carried out to investigate the generation mechanism of secondary currents. The results of the budget analysis show that in plain open-channel flow, the production by anisotropy is important in the vicinity of the wall and free-surface boundaries, and the production by Reynolds stress is important in the region away from the boundaries. However, this rule is not effective in vegetated channel flows. Also, in plain channel flows, the vorticity is generated mainly in the vicinity of the free-surface and the bottom, while in vegetated channel flows, the regions of the bottom and vegetation height are important to generate the vorticity.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2011.06a
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• pp.278-278
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• 2011

• Proceedings of the Korea Water Resources Association Conference
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• 2009.05a
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• pp.545-549
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• 2009

본 연구에서는 주흐름방향으로 식생 영역이 교차적으로 존재하는 개수로 흐름에 대한 3차원 수치 모의를 수행하였다. 지배방정식의 난류 폐합을 위해 $k-{\varepsilon}$ 난류모형을 이용하였다. 먼저, 하상의 일부만 식재된 부분 식생 수로를 수치모의 하고 기존의 실험 결과와 비교하였다. 그 결과 본 모형이 평균유속 분포를 매우 잘 예측하는 것으로 나타났으나, 레이놀즈응력 분포는 실험 결과에 비해 비식생영역에서는 다소 과소 산정하고 식생영역에서는 과대 산정하는 것으로 나타났다. 이는 본 모형이 등방성 모형이기 때문에 식생 경계부에서 발생되는 난류의 비등방성 효과를 정확히 예측 할 수 없기 때문인 것으로 판단된다. 또한 주흐름방향으로 식생 영역이 교차적으로 존재하는 대응 식생 수로를 수치모의하고, 계산 결과를 기존의 실험 결과와 비교하였다. 그 결과 본 모형이 대응 식생 수로에서의 유속 분포를 매우 잘 예측하는 것으로 나타났다. 또한 식생 밀도가 증가함에 따라 식생이 흐름 방향을 변화시켜 점차 만곡수로와 유사한 형태의 흐름이 형성되는 것으로 나타났다.

• Proceedings of the Korea Water Resources Association Conference
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• 2008.05a
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• pp.655-659
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• 2008

본 연구에서는 3차원 수치모의를 통하여 횡방향 하상형상 및 격자형 이차흐름 구조의 생성 메커니즘을 분석하였다. 이를 위해 곡선좌표계에서의 지배방정식을 구성하고, 난류 폐합을 위해 Speziale(1987)가 제안한 비등방성 k-$\varepsilon$모형을 이용하였다. 또한 Exner 방정식을 이용하여 시간에 따른 하상변동을 예측하였다. 그 결과 바닥과 측벽 사이에서 발생되는 바닥 이차흐름의 하향류에 의해 측벽 부근부터 하상이 침식되고, 침식된 유사량은 이차흐름의 횡방향 유속에 의해 이동되어 퇴적되어, 결국 횡방향으로 연속적인 언덕 및 저면과 같인 하상형상이 생성되는 것으로 나타났다. 또한 시간에 따른 이차흐름 및 바닥 전단력, 하상고의 변화에 대해 살펴보았다.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.28 no.5
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• pp.375-388
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• 1999

1974년 6월 Rowland와 Molina가 CFCs등의 화학물질이 성층권의 오존층을 파괴시킨다는 가설의 발표는 많은 환경론자들의 관심과 함께 논쟁의 대상이 되었다. 이 가설은 1985년 10월 Farman 등의 영국 남극조사팀에 의해 남극상공의 성층권 오존이 1970년대에 비해 약 40% 정도 감소된 것이 확인되면서 지구전체의 심각한 환경문제로 대두되었다.

• Water for future
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• v.24 no.1
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• pp.109-117
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• 1991

The governing equation of flow in porous media is developed on the bases of the continuity equation of fluid for transient flow through a saturated-unsaturated zone, and substitution of Darcy's law. The numerical solutions are obtained by finite element method based on the Galerkin principles weighted residuals. The analysis are carried out by using the unsteady storm data observed and rainfall intensities which are obtained by using the rainfall excess model in considering of the initial losses. The functional relationships between the hydraulic conductivity, capillary pressure head and volumetric water content are applied to the flow of water through unsaturated soil varied with changes of water content.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.4
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• pp.242-251
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• 2001

Flow boiling heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A were measured for a horizontal plain tube. The test section was made of a copper tube of 8.8mm inner diameter and 1000mm length respectively. The refrigerant was heated by passing hot water through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of $5^{\circ}C$ with mass fluxes of 100~300 kg/$m^2$,/TEX>s. HTCs were measured by two methods: the direct wall temperature measurement method and the indirect Wilson plot method. Experimental results showed that the Wilson plot method was affected greatly by the external test conditions and yielded inconsistent results. For the mass flux of 100kg/$m^2$,/TEX>s, HTCs were almost constant regardless of the quality for a given refrigerant HTCs of R134a and R407C were similar to those of R22 while those of R410A were 60% higher than those of R22. For the mass fluxes of 200 and 300kg/$m^2$,/TEX>s, HTCs of R407C were almost the same as those of R22, while HTCs of R134a and R410A were 12-13% and 20~23% higher than those of R22 respectively. For pure refrigerant, Shah\`s correlation yielded a good agreement with the measured data both qualitatively and quantitatively.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6B
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• pp.643-651
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• 2008

In the present paper, turbulent open-channel flows over longitudinal bedforms are numerically simulated. The Reynolds- averaged Navier-Stokes equations in curvilinear coordinates are solved with the non-linear $k-{\varepsilon}$ model by Speziale( 1987). First, the developed model is applied to rectangular open channel flows for purposes of model validation and parameter sensitivity studies. It is found that the parameters $C_D$ and $C_E$ are important to the intensity of secondary currents and the level of turbulent anisotropy, respectively. It is found that the non-linear $k-{\varepsilon}$ model can hardly reproduce the turbulence anisotropy near the free surface. However, the overall pattern of the secondary currents by the present model is seen to coincide with measured data. Then, numerical simulations of turbulent flows over longitudinal bedforms are performed, and the simulated results are compared with the experimental data in the literature. The simulated secondary currents clearly show upflows and downflows over the ridges and troughs, respectively. The numerical results of secondary currents, streamwise mean velocity, and turbulence structures compare favorably with the measured data. However, it is observed that the secondary currents towards the troughs were significantly weak compared with the measured data.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.8
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• pp.692-700
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• 2001

Flow boiling heat transfer coefficients(HTCs) of R22, R134a, R407C, and R410A were measured experimentally for a horizontal plain and a microfin tube. Experimental apparatus was composed of 3 main parts: a refrigerant loop, a water loop and a water-glycol loop. The test section in th refrigerant loop was made of a copper tube of 9.52 mm outer diameter and 1 m length for both tubes. The refrigerant was heated by passing hot water through an annulus surrounding the test section. Tests were performed at a fixed refrigerant saturation temperature of $5^{\circ}C$ with mass fluxes of 100~300 kg/$m^2$s. Test results showed that at similar mass flux the flow boiling HTCs of R134a were similar to those of R22 for both plain and microfin tube. HTCs of R407C were similar to those of R22 for a plain tube but lower than those of R2 by 25~48% for a microfin tube. And HTCs of R410A were higher than those of R2 by 20~63% for a plain tube and were similar to those of R22 for a microfin tube. In general, HTCs of a microfin tube were 1.8~5.7 times higher than those of a plain tube.