• 한국항공우주학회지
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• 제36권10호
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• pp.939-946
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• 2008

압축성 유동장 해석을 위해 개발된 전산유체역학 프로그램의 응용범위를 확장하여 비압축성 유동장을 해석하고자 할 경우 해의 수렴성에 문제가 발생할 수 있다. 이러한 수렴성 저하 문제는 지배방정식의 수학적 특성에 기인하며 예조건화 방법을 이용하면 지배방정식의 큰 수정 없이 해결할 수 있다. 본 논문은 예조건화 기법을 적용한 압축성 유동해석 프로그램을 개발하여 수렴성의 문제없이 저속의 비압축성 유동장 해석의 적용에 관한 내용을 다루고 있다. 개발된 프로그램의 정확성과 수렴특성을 확인하기 위해 정상상태의 비점성 및 층류 그리고 난류 문제에 대하여 해석하였으며 이를 실험치와 비압축성 계산치와 비교하였다

• Journal of Advanced Marine Engineering and Technology
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• 제37권3호
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• pp.266-273
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• 2013

With the worldwide trend of controlling the utilization of fossil fuels inducing global climate change, many efforts will have to be made on securing a sustainable energy supply. Tidal current is a concentrated form of gravitational energy, its resource is significant, but limited locations. To effectively capture tidal current energy from the sea, a group of tidal turbines should be formed and positioned with optimal size and spacing for absorbing from multiple points. Thus, the flow field including turbines becomes a huge domain, a so-called tidal farm. It can be very convenient technically and economically if a whole turbine farm is simulated by means of actuator disc thoery. So, the analysis method using actuator discs coupled with a solution of Reynolds Averaged Navier-Stokes (RANS) equations is adopted for actual tidal turbines. Actuator discs have regions where similar forces imposed by actual turbines are applied to a flow. As working in group formation, turbines naturally have interaction effects on one another. Therefore, the present paper investigate the evaluation on the operating performance of tidal farm in terms of the mutual influence among turbine units with various lateral and longitudinal spacing. Authors expect that results of the present study contribute to the development of tidal farm for the future potential energy.

• 한국전산유체공학회지
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• 제17권3호
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• pp.11-17
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• 2012

In this study, aerodynamic analyses have been conducted for 4-Bladed Vertical-Axis Wind Turbine (VAWT) configuration and the results are compared with experimental data. Reynolds-averaged Navier-Stokes equation with LES turbulence model is solved for unsteady flow problems. In addition, the computation results by standard k-${\omega}$ and SST k-${\omega}$ turbulence models are also presented and compared. An experiment model of 4-Bladed VAWT model has been designed and constructed herein. Experimental tests for aerodynamic performance of the present VAWT model are practically conducted using the vehicle mounted testing system. Comparison results between the experiment and the computational fluid dynamics (CFD) analyses are presented in order to show the accuracy of CFD analyses using the different turbulent models.

• Wind and Structures
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• 제23권6호
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• pp.505-521
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• 2016

The objective of this study is to investigate numerically the effect of building roof shaps on wind flow and pollutant dispersion in a street canyon with one row of trees of pore volume, $P_{vol}=96%$. A three-dimensional computational fluid dynamics (CFD) model is used to evaluate air flow and pollutant dispersion within an urban street canyon using Reynolds-averaged Navier-Stokes (RANS) equations and the Explicit Algebraic Reynolds Stress Models (EARSM) based on k-${\varepsilon}$ turbulence model to close the equation system. The numerical model is performed with ANSYS-CFX code. Vehicle emissions were simulated as double line sources along the street. The numerical model was validated by the wind tunnel experiment results. Having established this, the wind flow and pollutant dispersion in urban street canyons (with six roof shapes buildings) are simulated. The numerical simulation results agree reasonably with the wind tunnel data. The results obtained in this work, indicate that the flow in 3D domain is more complicated; this complexity is increased with the presence of trees and variability of the roof shapes. The results also indicated that the largest pollutant concentration level for two walls (leeward and windward wall) is observed with the upwind wedge-shaped roof. But the smallest pollutant concentration level is observed with the dome roof-shaped.

• 한국수자원학회논문집
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• 제38권3호
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• pp.189-198
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• 2005

본 연구에서는 조력발전소 배수갑문의 형상과 배치에 따른 방류능력을 해석하는데 3차원 수치모의가 효과적으로 이용될 수 있음을 보였다. 3차원 수치모형은 RANS를 지배방정식으로 하는 FLOW-3D 모형을 이용하였다. 본 연구결과 배수갑문의 방류능력은 물받이길이와 도류벽의 접근각도에 큰 영향을 받는 것으로 나타났다. 그리고 이의 개선 여부에 따라 $10\%$ 이상의 방류량 차이가 발생하였다. 또한 방류량은 배수문과 수차구조물을 연결하는 구조물의 형상과 물받이 끝 사면경사의 영향을 받는 것으로 나타났다. 본 연구에서는 배수갑문의 설계시 방류능력 개선을 위해서는 수리학적 검토가 필요하며, 수치모형실험이 수리모형실험과 더불어 유용한 해석도구로 이용될 수 있음을 보였다.

• 한국수소및신에너지학회논문집
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• 제31권4호
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• pp.387-394
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• 2020

A parametric study of a 2.5 kW class propeller type micro hydraulic turbine was performed. In order to analyze the internal flow characteristics in the hydraulic turbine, three dimensional Reynolds-averaged Navier-Stokes equations with shear stress transport turbulence model were used and the hexahedral grid system was used to construct computational domain. To secure the reliability of the numerical analysis, the grid dependency test was performed using the grid convergence index method based on the Richardson extrapolation, and the grid dependency was removed when about 1.7 million nodes were used. For the parametric study, the axial distance at shroud span (L) between the inlet guide vane and the runner, and the inlet and outlet blade angles (β₁, β₂) of the runner were selected as the geometric parameters. The inlet and outlet angles of the runner were defined in the 3 spans from the hub to tip, and a total of 7 geometric parameters were investigated. It was confirmed that the outlet angles of the runner had the most sensitive effect on the power and efficiency of the micro hydraulic turbine.

• 한국수자원학회논문집
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• 제38권3호
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• pp.179-188
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• 2005

본 연구에서는 화북다목적댐을 대상으로 2차원, 3차원 수치모형실험이 수리모형실험과 더불어 설계자에게 유용한 해석도구로 사용될 수 있음을 보였다. 2차원, 3차원 수치모형으로 천수방정식을 지배방정식으로 하는 RMA2와 RANS를 지배방정식으로 하는 FLOW-3D 모형을 이용하였다. 수몰지역과 접근수로에서의 유황, 월류웨어의 방류능력, 월류부, 도류부, 감세지에서의 수면형상, 월류부에서의 압력분포에 대한 수치모의 결과를 수리모형실험 결과와 비교하였다. 비교 결과, 대부분의 경우에 수치모의 결과는 수리모형실험 결과를 잘 재현하는 것으로 나타났다 그러나, 감세지에서의 도수현상에 대한 수치모의 결과는 공기연행효과로 인해 수리모형실험 결과와 다소 차이가 발생하는 것으로 나타났다.

• Nuclear Engineering and Technology
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• 제55권10호
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• pp.3874-3897
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• 2023

A high-fidelity computational fluid dynamics (CFD) analysis was performed using the Large Eddy Simulation (LES) model for the lower plenum of the High-Temperature Test Facility (HTTF), a ¼ scale test facility of the modular high temperature gas-cooled reactor (MHTGR) managed by Oregon State University. In most next-generation nuclear reactors, thermal stress due to thermal striping is one of the risks to be curiously considered. This is also true for HTGRs, especially since the exhaust helium gas temperature is high. In order to evaluate these risks and performance, organizations in the United States led by the OECD NEA are conducting a thermal hydraulic code benchmark for HTGR, and the test facility used for this benchmark is HTTF. HTTF can perform experiments in both normal and accident situations and provide high-quality experimental data. However, it is difficult to provide sufficient data for benchmarking through experiments, and there is a problem with the reliability of CFD analysis results based on Reynolds-averaged Navier-Stokes to analyze thermal hydraulic behavior without verification. To solve this problem, high-fidelity 3-D CFD analysis was performed using the LES model for HTTF. It was also verified that the LES model can properly simulate this jet mixing phenomenon via a unit cell test that provides experimental information. As a result of CFD analysis, the lower the dependency of the sub-grid scale model, the closer to the actual analysis result. In the case of unit cell test CFD analysis and HTTF CFD analysis, the volume-averaged sub-grid scale model dependency was calculated to be 13.0% and 9.16%, respectively. As a result of HTTF analysis, quantitative data of the fluid inside the HTTF lower plenum was provided in this paper. As a result of qualitative analysis, the temperature was highest at the center of the lower plenum, while the temperature fluctuation was highest near the edge of the lower plenum wall. The power spectral density of temperature was analyzed via fast Fourier transform (FFT) for specific points on the center and side of the lower plenum. FFT results did not reveal specific frequency-dominant temperature fluctuations in the center part. It was confirmed that the temperature power spectral density (PSD) at the top increased from the center to the wake. The vortex was visualized using the well-known scalar Q-criterion, and as a result, the closer to the outlet duct, the greater the influence of the mainstream, so that the inflow jet vortex was dissipated and mixed at the top of the lower plenum. Additionally, FFT analysis was performed on the support structure near the corner of the lower plenum with large temperature fluctuations, and as a result, it was confirmed that the temperature fluctuation of the flow did not have a significant effect near the corner wall. In addition, the vortices generated from the lower plenum to the outlet duct were identified in this paper. It is considered that the quantitative and qualitative results presented in this paper will serve as reference data for the benchmark.