• 한국수자원학회논문집
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• 제53권1호
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• pp.1-13
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• 2020

높은 레이놀즈수를 갖는 공학적인 흐름을 예측하는 가장 일반적인 방법은 여전히 벽함수를 이용하는 난류모형에 근거한 RANS 수치모의이다. 최근 벽근처의 점성영역 관계식과 벽에서 떨어진 대수영역 관계식을 혼합하여 개발된 일반화된 벽함수들은 두 영역사이의 난류량과 유속이 부드럽게 천이하도록 한다. 이 연구는 난류운동에너지(TKE), 에너지 소산율, 비소산율, 와점성에 대해서 적용 가능한 벽함수들을 조합하여 일련을 수치 모의를 수행하여 널리 이용되고 있는 난류모형들의 성능과 수렴 특성을 분석하였다. 이 연구 결과는 RNG k-𝜖 모형의 경우 첫번째 계산격자가 완충층에 놓이게 될 때는 반복 계산시 작은 허용오차를 이용하여 주의 깊게 적용을 하여야 안정된 해를 구할 수 있음을 보여준다. 표준 k-𝜖과 RNG k-𝜖 모형은 TKE와 와점성에 대해서 적용 가능한 벽함수들 중 어느 것을 선택하여 적용하더라도 수치모의 결과가 민감하게 반응하지 않는 것으로 나타났다. 한편, k-ω SST 모형의 경우 TKE에 대해서는 kL-벽함수 그리고 와점성에 대해서는 nutUB-벽함수를 이용하여야 정확하고 안정된 경계 조건 설정을 보장할 수 있다. 레이놀즈수 155,000조건에서 적용한 후방계산흐름 수치모의 결과 격자 해상도에 상관없이 약 13% 정도 재부착 거리를 과소평가하는 모형을 제외하고 나머지 적용한 난류모형들 모두 적절히 세밀한 해상도의 격자에서 양호하게 재부착거리를 잘 예측하는 것으로 나타났다.

• 한국수자원학회논문집
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• 제33권4호
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• pp.461-469
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• 2000

본 연구에서는 부유입자를 포함하는 유동을 입자크기와 입자량을 달리한 몇 가지 경우에 대하여 수치적으로 해석하고 Coleman(1981, 1986)의 실험결과와 비교하였다. 완전 난류 유동장을 해석하기 위하여 k-$\omega$난류모형을 사용하였으며, 농도장 해석을 위해서는 침강속도를 고려한 일반화된 농도방정식을 적용하였다. 유동과 입자의 상호작용은 Einstein과 Chien(1955)의 모형을 도입하여 수치계산하였다. 기존의 대부분 연구에서는 소유사의 두께를 고려하지 않은 연구를 수행하였으나, 입자량이 많아지거나 입자크기가 클 경우 이를 무시할 수 없는 것으로 밝혀졌다. 소유사의 두께와 하상에 의한 표면 거칠기 효과를 고려하여 본 연구를 수행하였는데, 여기서 농도분포를 결정짓게 되는 $\beta$값이 입자의 크기와 입자량에 관련되어 있다는 사실을 확인할 수 있었다. 기존 연구결과는 $\beta$가 1.0보다 큰 값을 가진다고 보고되었으나, 본 연구를 통해 1.0보다 작아질수 있음이 확인되었고, 이는 최근에 보고된 연구 결과와 일치되는 결과이다.

• 한국수자원학회논문집
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• 제48권4호
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• pp.281-290
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• 2015

표준 k-${\varepsilon}$, RNG k-${\omega}$ 그리고 k-${\omega}$ SST 난류 모형과 VOF (volume of fluid)기법을 이용하여 사각형 광정위어를 통과하는 난류 흐름의 수면 변화와 유속분포를 수치모의 하였다. 지배방정식은 2차 정확도의 유한체적기법을 이용하여 해석하였으며, 두 개의 서로 다른 격자해상도에서 계산을 수행하여 수치해석 결과의 격자 민감도를 분석하였다. 계산 결과를 Kirkgoz et al. (2008)의 실험 결과 그리고 Moss (1972) 및 Zachoval et al. (2012) 무차원화된 실험값과 비교 분석하여 적용한 수치모형의 정확도를 평가하였다. 수치모의 결과는 사각형 개수로에 설치된 광정위어 흐름의 실험결과들을 합리적으로 예측하고 있으면 적용한 난류모형에 따라서 두 개의 주요 흐름분리 영역에서 계산 결과에 차이가 있는 것으로 나타났다. 표준 k-${\varepsilon}$ 모형은 이들 두 개의 흐름분리영역의 크기를 과소산정하고 있으며, k-${\omega}$ SST 모형은 위어 전면부에서 발생하는 흐름분리 영역을 다소 과대 산정하는 것으로 나타났다. RNG k-${\varepsilon}$ 모형은 전반적으로 양호하게 두 흐름분리 영역을 예측하는 한편, k-${\omega}$ SST 모형은 위어 상류부 모서리에서 발생하는 박리거품의 발생 형태를 가장 잘 예측하는 것으로 나타났다.

• 대한조선학회논문집
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• 제49권3호
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• pp.247-253
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• 2012

Dam-break flows, a type of very shallow gravity-driven flow, are substantially influenced by resistance forces due to viscous friction and turbulence. Assuming turbulent flow, the main focus of this study is to validate the increase of drag forces caused by surface roughness and especially turbulence intensity. A Reynolds Averaged Navier-Stokes(RANS) approach with the standard k-${\varepsilon}$ turbulence model is used for this study, where the free surface motion is captured by using a volume of fluid(VOF) method. Surface roughness effects are considered through the law of the wall modified for roughness, while the initial turbulence intensity which determines the lowest level of turbulence in the flow domain of interest is used for the variation of turbulence intensity. It has been found that the numerical results at higher turbulence intensities show a reasonably good agreement with the physical aspects shown by two different dam-break experiments without and with the impact of water.

• Journal of Advanced Marine Engineering and Technology
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• 제29권1호
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• pp.49-59
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• 2005

The evaporation heat transfer experiments are conducted with the shell and plate heat exchanger (S&PHE) without oil in the refrigerant loop using R-410A. An experimental refrigerant loop has been established to measure the evaporation heat transfer coefficient h. of R-410A in a vertical S&PHE. Two vertical counter flow channels were formed in the S&PHE by three plates haying a corrugated trapezoid shape of a $45^{\circ}C$ chevron angle. UP flow of the boiling R-410A in one channel receives heat from the hot down flow of water in the other channel The effects of the refrigerant mass flux. average heat flux. refrigerant saturation temperature and vapor qualify are explored in detail. Similar to the case of a plate heat exchanger. even at a very low Reynolds number, the flow in the S&PHE remains turbulent. The Present data shows that the evaporation heat transfer coefficients of R-410A increased with the vapor qualify. The results indicate a rise in the refrigerant mass flux caused an increase in the h.. Raising the imposed wall heat flux is found to slightly improve h., while h, is found to be lower at a higher refrigerant saturation temperature. Based on the present data. empirical correlation of the evaporation heat transfer coefficient is proposed.

• Journal of Advanced Marine Engineering and Technology
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• 제39권2호
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• pp.136-142
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• 2015

The shell and tube heat exchanger is an essential part of a power plant for recovering transfer heat between the feed water of a boiler and the wasted heat. The baffles are also an important element inside the heat exchanger. Internal materials influence the flow pattern in the bed. The influence of baffles in the velocity profiles was observed using a three-dimensional PIV (Particle Image Velocimetry) around baffles in a horizontal circular tube. The velocity of the particles was measured before the baffle and between them in the test tube. Results show that the velocity vectors near the front baffle flow along the vertical wall, and then concentrate on the upper opening of the front baffle. The velocity profiles circulate in the front and rear baffle. These profiles are related to the Reynolds number (Re) or the flow intensity. Velocity profiles at lower Re number showed complicated mixing to obtain the velocities and concentrate on the lower opening of the rear baffle as front wall. Numerical simulations were performed to investigate the effects of the baffle and obtain the velocity profiles between the two baffles. In this study, a commercial CFD package, Fluent 6.3.21 with the turbulent flow modeling, k-${\epsilon}$ are adopted. The path line and local axial velocities are calculated between two baffles using this program.

• 대한기계학회논문집B
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• 제25권9호
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• pp.1219-1226
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• 2001

Pressure drops were measured for the flow boiling process within horizontal rectangular channels. The gap between the upper and the lower plates of each channel ranges from 0.4 to 2mm while the channel width being fixed to 20mm. Refrigerant 113 was used as the test fluid. The mass flux ranges from 50 to 200kg/㎡s and the channel walls were uniformly heated up to 15kW/㎡. The quality range covers from 0.15 to 0.75. The present experimental conditions coincide with the operating conditions of compact heat exchangers in which the liquid and gas flows are laminar and turbulent. The measured results were well represented by the two-phase frictional multiplier of Lee (2001) which has been developed for air-water two-phase flows within the deviation of $\pm$20%.

• 설비공학논문집
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• 제15권3호
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• pp.239-245
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• 2003

In the nuclear power plant, emergency core coolant system (ECCS) is furnished at reactor coolant system (RCS) in order to cool down high temperature water in case of emergency. However, in this coolant system, thermal stratification phenomenon can occur due to coolant leaking in the check valve. The thermal stratification produces excessive thermal stresses at the pipe wall so as to yield thermal fatigue crack (TFC) accident. In the present study, effects of turbulence penetration on the thermal stratification into T-branches with square cross-section in the modeled ECCS are analysed numerically. $textsc{k}$-$\varepsilon$ model is employed to calculate the Reynolds stresses in momentum equations. Results show that the length and strength of thermal stratification are primarily affected by the leak flow rate of coolant and the Reynolds number of the main flow in the duct. Turbulence penetration into the T-branch of ECCS shows two counteracting effects on the thermal stratification. Heat transport by turbulence penetration from the main duct to leaking flow region may enhance thermal stratification while the turbulent diffusion may weaken it.

• 대한조선학회논문집
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• 제42권6호
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• pp.593-600
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• 2005

The finite volume based multi-block RANS code, WAVIS developed at KRISO, is used to simulate the turbulent flow field around the KRISO container ship (KCS) and the modified KRISO tanker (KVLCC2M). The realizable k-$\varepsilon$ turbulence model with a wall function is employed for the turbulence closure. The free surface flow with and without propeller is mainly investigated for the KCS and the double model flow is concerned for the KVLCC2M which is obliquely towed in still water. The computed results are compared with the experimental data provided by CFD Tokyo Workshop 2005 in terms of wave profiles, hull surface pressure and wake distribution with and without propeller for the HCS and wake distribution and hydrodynamic forces and moments with various drift angles for the KVLCC2M.

• 한국항만학회지
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• 제9권2호
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• pp.25-38
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• 1995

The wind generated circulation model describes the phenomenon based on the following physical assumptions: a) As the horizontal dimension of the flow domain is several orders of magnitude larger than vertical dimension, nearly horizontal flow is realistic. b) The time taken for circulation to develop may effect on the flow domain of the earth's rotation, the contribution of the Coriolis force. c) A flow domain of large dimension results in quite large Reynolds number and the Reynolds stresses are approximated by the turbulent mean velocity gradient. d) The circulation is forced by the shear stresses on the water surface exercised by the wind. Modification made to the depth average approximation of the convective terms and the bed shear stress terms by adopting a certain distribution of current over the depth and laboratory measurements for the bed shear expression. Modification circulation patterns, energy evolution and surface profile gave the significant differences comparing with the classical model results. The modified model results in higher free surface gradients balancing both the free surface shear and the bed shear and consequently to higher surface profiles along the coast.