• Proceedings of the Korea Water Resources Association Conference
• /
• 2005.05b
• /
• pp.434-438
• /
• 2005

본 연구에서는 개수로 흐름에서 전단속도의 분포를 조사하였다. 이를 위해 직사각형 개수로 흐름과 복단면 개수로 흐름에 대한 DNS 자료를 이용하였으며, 계산된 전단속도는 기존의 실험자료 및 수치해석결과와 비교$\cdot$분석되었다. 평균흐름장에서는 직사각형 개수로 흐름의 경우, 자유수면 부근에서 내측이차흐름이 발생하는 것을 볼 수 있었으며, 복단면 개수로 흐름에서는 주수로와 홍수터의 접합부 부근에서 쌍와가 생성되는 것으로 나타났다. 이차흐름의 최대크기는 체적평균된 주흐름방향 유속의 약 $5\%$로서, 직사각형 개수로 흐름에서 발생되는 것보다 더 강하다는 것을 알 수 있었다. 이러한 이차흐름의 영향으로 주흐름방향 평균유속의 등속선이 편향된 것으로 나타났다. 본 DNS에서 전단속도가 최대값을 갖는 지점은 직사각형 개수로 흐름의 경우 자유수면이며, 복단면 개수로 흐름의 경우 접합부인 것으로 나타났다. 이는 기존 LES 결과와 동일하지만, 실험연구와는 다른 결과이다. 접합부에서 이러한 차이를 보이는 이유는 실험에서 마찰속도를 산정하기 위해 사용하는 방법의 부적합성에 있는 것으로 나타났다.

• Magazine of the Korean Society of Agricultural Engineers
• /
• v.43 no.2
• /
• pp.78-84
• /
• 2001

The daily streamflow in the Yaluhe watershed located in the north-eastern part of China was simulated by DAWAST model and the water balance parameters of the model were calibrated by simplex method. Model verification tests were carried out. The range of root mean square error was 0.34∼1.50mm, that of percent error in volume was -16.9∼-62.0% and that of correlation coefficient was 0.727∼0.920. DAWAST model was revised to consider the phreatic evaporation from the ground water in the frozen soil by adjusting soil moisture content in the unsaturated layer at the end of the melting season. The results of estimation of the daily streamflow by the revised model were statistically improved, that is, the range of root mean square error was 0.31∼1.49mm, that of percent error in volume was -11.7∼-12.1%, and that of correlation coefficient was 0.810∼0.932. The accuracy of DAWAST model was improved and the applicability of DAWAST model was expanded to the frozen region.

• Journal of computational fluids engineering
• /
• v.20 no.1
• /
• pp.57-64
• /
• 2015

This paper describes a volume of fluid(VOF) method, mRHRIC for the simulation of free surface flows around ship hulls and provides its validation against benchmark test cases. The VOF method is developed on the basis of RHRIC method developed by Park et al. that uses high resolution differencing schemes to algebraically preserve both the sharpness of interface and the boundedness of volume fraction. A finite volume method is used to solve the governing equations, while the realizable ${\kappa}-{\varepsilon}$ model is used for turbulence closure. The present numerical results of the resistance performance tests for DTMB5415 and KCS hull forms show a good agreement with available experimental data and those of other free surface methods.

• Journal of computational fluids engineering
• /
• v.20 no.4
• /
• pp.51-57
• /
• 2015

In the present study, two-dimensional numerical investigation of flow past a square cylinder beneath the free surface has been performed to identify the effects of presence of the free surface. An immersed boundary method was adopted for implementation of the cylinder cross-section in a Cartesian grid system. Also, a level-set method was used to capture the interface of two fluids. To prevent transition to three-dimensional flow, Reynolds number chosen for this simulation was 150. The cases for Froude number 0.2 and gap ratio(h/D) between 0.25 and 5.00 were examined. At the specific Reynolds number, we study the effects of gap ratio on flow characteristics around a square cylinder by computing flow fields, force coefficients and Strouhal number.

• Journal of computational fluids engineering
• /
• v.19 no.1
• /
• pp.15-20
• /
• 2014

Accurate simulation of free-surface wave flows around a ship is very important for better hull-form design. In this paper, a computational fluid dynamics (CFD) code, termed SNUFOAM, which is based on the open source libraries, OpenFOAM, was developed to predict the wave patterns around a ship. Additional anti-diffusion source term for minimizing a numerical diffusion, which was caused by convection differencing scheme, was considered in the volume-fraction transport equation. The influence of the anti-diffusion source term was tested by applying it to free-surface wave flow around the Wigley model ship. In results, the band width of the volume fraction contours between 0.1 to 0.9 at the hull surface was narrowed by considering the anti-diffusion term.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2004.05b
• /
• pp.376-380
• /
• 2004

측벽이 존재하는 개수로 난류흐름에 대한 DNS 자료를 이용하여 난류의 비등방성을 해석하였다. 측벽의 2등분선(sidewall bisector)에서 난류강도의 분포를 통해 바닥과 자유수면에서의 비등방성을 분석하고, AIM을 도입하여 흐름장 전반에 걸쳐 세부적인 비등방성 해석을 수행하였다. 측벽의 2등분선에서의 난류강도의 분포를 통해 바닥과 자유수면 근처에서 난류강도가 강한 비등방성을 갖는 것을 볼 수 있었다. 또한 3개의 다른 영역에서 AIM의 도입을 통해 측벽이나 바닥근처에서는 난류의 비등방성이 구형 관수로 흐름과 유사한 것으로 나타났으나, 개수로 난류흐름의 주된 특성이 velocity-dip phenomena가 존재하는 영역에서는 구형 관수로 흐름과는 달리 천이영역이 존재한다는 것을 알 수 있었다.

• Journal of the Society of Naval Architects of Korea
• /
• v.36 no.2
• /
• pp.40-49
• /
• 1999

A Finite Volume Method for the two-dimensional incompressible, two-fluids Navies-Stokes equation and level-set scheme are used to analyse the interface of two fluids, free-surface flow. The numerical characteristics and the applicability of level-set scheme are brief1y investigated and appraised by solving oscillating small surface wave in a water tank and dam break problems. In the numerical results, a method for improving the convergence of the solution is presented.

• Journal of Ocean Engineering and Technology
• /
• v.29 no.6
• /
• pp.427-435
• /
• 2015

This paper provides the numerical results of a simulation of the flow around a propeller working beneath the free surface. A finite volume method is used to solve the unsteady Reynolds averaged Navier-Stokes (URANS) equations, where the wave-making problem is solved using a volume-of-fluid (VOF) method. The numerical analysis focuses on the propeller wake structure affected by the free surface, where we consider another free surface boundary condition that treats the free surface as a rigid wall surface. The propeller wake under the effect of these two free surface conditions shows a reduction in the magnitude of the longitudinal and vertical flow velocities, and its vortical structures strongly interact with the free surface. The thrust and torque coefficient under the free surface effect decrease about 3.7% and 3.1%, respectively. Finally, the present numerical results show a reasonable agreement with the available experimental data.

• Journal of Korea Ship Safrty Technology Authority
• /
• s.33
• /
• pp.12-31
• /
• 2012

수치 계산으로 고속 활주선의 저항 성능을 평가하기 위해서는 활주 상태에서의 항주자세 예측이 무엇보다도 중요하다고 할 수 있다. 항주자세의 변화가 큰 고속 활주선의 경우에는 활주 자세에 따라 선저 바닥면에서 나타나는 압력 변화에 의한 동적 부양력 변화가 크므로 단순히 정지 중 흘수를 기준으로 계산되어진 유체력으로 항주자세를 예측하기 보다는 자세 변화에 따른 동적 부양력의 변화와 이에 의한 자세 변화를 반복 계산을 통해 수렴시키는 것이 요구되어진다. 본 연구에서는 선형화된 자유수면 조건의 포텐셜 수치 계산으로 유체 동압력인 부양력을 계산해내고 이를 유체 정역학적 힘으로 간주하여 부력과 선체중량과의 힘과 트림 모멘트 평형 관계를 만족시키는 방법으로 반복적인 계산을 통해 수렴된 활주 자세를 얻어내었다.

• Journal of the Society of Naval Architects of Korea
• /
• v.42 no.2 s.140
• /
• pp.88-97
• /
• 2005

A new numerical scheme solving two-phase flow, the Hybrid VOF method for improved free surface capturing has been developed by combining a volume capturing VOF method with the Level-Set reinitialization procedure. For validation, the proposed method is applied to 3-D bubble rising problem, dam breaking and the free surface flow around a commercial container ship. The calculated results by using the Hybrid VOF method with the two previously applied VOF formulations are compared with available numerical and experimental data. It is found that the new method provides more reasonable results than the two previous ones.