• 대한기계학회논문집B
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• 제31권8호
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• pp.653-662
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• 2007

Due to the difficulty raised from the coupling of cavitation modeling with turbulent flow, numerical simulation for two phase flow remains one of the challenging issues in the society. This research focuses on the development of numerical code to deal with incompressible two phase flow around 2D hydrofoil by combing the cavitation model suggested by Kunz et al. with $k-{\varepsilon}$ turbulent model. The simulation results are compared to experimental data to verify the validity of the developed code. Also, the comparison of the calculation results is made with LES results to evaluate the capability of $k-{\varepsilon}$ turbulence model. The calculation results show very good agreement with experimental observations even though this code can not grasp the small scaled bubbles in the calculation wheres LES can hold the real physics. This code will be extended to 3D compressible two phase flow for the study on the fluid dynamics in the inducers and impellers.

• Nuclear Engineering and Technology
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• 제56권1호
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• pp.9-18
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• 2024

In-vessel retention through external reactor vessel cooling (IVR-ERVC) is a severe accident management (SAM) strategy that has been adopted and used in many nuclear reactors such as AP1000, APR1400, and light water reactor etc. Some reactor accidents have raised concerns about nuclear reactors among residents, leading to a decrease in residents' acceptability and many studies on SAM are being conducted. Experiments on IVR-ERVC are almost impossible due to its specificity, so fluid characteristics are analyzed through BALI experiments with similar condition. In this study, computational fluid dynamics (CFD) via Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) for BALI experiments were performed. Steady-state CFD analysis was performed on three turbulence models, and SST k-ω model was in good agreement with the experimental measurement temperature within the maximum error range of 1.9%. LES CFD analysis was performed based on the RANS analysis results and it was confirmed that the temperature and wall heat flux for depth was consistent within an error range of 1.0% with BALI experiment. The LES CFD analysis results were compared with those of the Lagrangian-based solver. LES matched the temperature distribution better than SOPHIA, but SOPHIA calculated the position of boundary between stratified layer and convective layer more accurately. On the other hand, Lagrangian-based solver predicted several small eddy behaviors of the convective layer and LES predicted large vortex behavior. The vibration characteristics near the cooling part of the BALI experimental device were confirmed through Fast Fourier Transform (FFT) investigation. It was found that the power spectral density for pressure at least 10 times higher near the side cooling than near the top cooling.

• 항공우주시스템공학회지
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• 제13권1호
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• pp.1-10
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• 2019

교반기에서의 유동 특성은 광범위한 산업 분야에서 매우 유용하다. 일반적으로 교반되는 용기에서의 유동 패턴, 전력 소비 및 혼합 시간은 임펠러의 설계뿐만 아니라 용기 형상 및 내부 구조에 달려 있다. 본 연구에서는 베플 형상과 임펠러의 상호 작용에 의해 생성되는 불안정하고 비정상상태의 복잡한 유동 특성 분석을 ANSYS FLUENT LES 난류 모델을 사용하여 수행하였다. Axial Flow 와 Radial Flow 두 가지 타입의 회전 임펠러와 3가지 베플의 형상 사이의 상호 작용과 영향을 전산유체역학(CFD)으로 예측 비교함으로써 교반 시 임펠러와 베플 주변에서의 유동 특성과 혼합 유동장에서 상대적으로 효율적인 경향을 보이는 설계 모델을 검증할 수 있었다.

• 한국화재소방학회논문지
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• 제26권1호
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• pp.31-37
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• 2012

본 연구는 주거용 건축물의 열역학적 수치해석모델을 검토하는데 목적이 있다. 화재하중과 화재강도는 성능설계의 사용이 증가함에 따라 건축물 화재안전 설계에 중요한 요소으로 대두되고 있으며, 컴퓨터 성능의 발전으로 수치해석을 통한 예측이 가능해 지고 있다. 주거 가연물의 연소특성을 예측하기 위해 각가연물의 수치해석용 모델을 설계하였다. 해석된 결과를 검증하기 위해 수치해석의 결과를 실물 화재실험의 결과와 비교하였다. 수치해석을 위해 FDS5를 사용하였으며, 난류해석을 위해 LES모델이 적용하였다. 검증결과 화염전파 현상 및 온도곡선은 실험결과와 정성적으로 잘 일치함을 확인하였다.

• 한국항공우주학회지
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• 제38권2호
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• pp.99-110
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• 2010

강한 압축성을 갖는 유동의 DES 해석에서, 일반적인 경험상수 $C_{DES}$ 값 0.65를 사용할 경우 경계층 내에서 인위적으로 LES 모드로 수행된다. 본 연구에서는 S-A DES 모델에서 RANS 모드 보호를 위하여 사용되는 난류 길이와 벽거리 비의 분포 함수를 이용한 $C_{DES}$의 동적 결정 방법을 제시하였다. 동적 $C_{DES}$ 결정식을 초음속 기저 유동장에 적용한 결과 다른 모델 상수를 사용한 기존의 연구 결과에 비하여 우수한 예측을 보여주었다.

• Ocean Systems Engineering
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• 제9권1호
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• pp.21-42
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• 2019

In this paper, the Finite-Analytic Navier-Stokes (FANS) code is coupled with an in-house finite-element code to study the dynamic interaction between a floating buoy and its mooring system. Hydrodynamic loads on the buoy are predicted with the FANS module, in which Large Eddy Simulation (LES) is used as the turbulence model. The mooring lines are modeled based on a slender body theory. Their dynamic responses are simulated with a nonlinear finite element module, MOORING3D. The two modules are coupled by transferring the forces and displacements of the buoy and its mooring system at their connections through an interface module. A free-decay model test was used to calibrate the coupled method. In addition, to investigate the capability of the present coupled method, numerical simulations of two degree-of-freedom vortex-induced motion of a CALM buoy in uniform currents were performed. With the study it can be verified that accurate predictions of the motion responses and tension responses of the CALM buoy system can be made with the coupling CFD-FEM method.

• 한국가시화정보학회지
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• 제19권1호
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• pp.88-93
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• 2021

For a flow normal to a circular disk, the flow separation occurs from the edge of the disk and the flow recirculation zone exists behind the disk. Many existing studies conducted simulations of flow normal to a circular disk under low Reynolds numbers. Some studies performed LES or DES simulations under high Reynolds numbers. However, comparative study for different RANS models for high Reynolds numbers is very limited. This study presents numerical simulations of a flow normal to a circular disk using Realizable k-ε model and SST k-ω model. The recirculation bubble length and drag coefficient were compared with the experimental data. The SST k-ω model showed the excellent predictions for the recirculation bubble length and drag coefficient.

• 대한기계학회논문집B
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• 제34권10호
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• pp.925-931
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• 2010

본 논문은 공동 주위 난류유동특성을 LES 기법으로 수치해석을 수행하여 알아보았다. 본 연구에 적용된 레이놀즈수는 공동 깊이만큼의 높이 h 에서의 유속을 기준으로 $1.0{\times}10^5$ 이며 3 차원 공동에서의 유동특성을 알아보았다. 적절한 비압축성 Filtered Navier-Stokes 방정식을 적용하기 위해, 계산격자를 공동 표면 근처에는 조밀하게 멀어질수록 성기게 생성하였으며, 이는 계산시간을 단축시키며 빠른 수렴을 도와준다. 또한, Boussinesq 가설을 subgrid-scale 난류모델에 적용하였고, Subgrid-scale 난류점성을 얻기 위해 smagorinsky-Lilly SGS 모델을 적용하였으며, 그 때의 CFL 수는 1.0 이다. 또한, 본 논문은 서로 다른 4 가지 형상의 공동의 및 입구조건의 변화에 따른 유동 특성도 함께 연구되었다.

• 대한토목학회논문집
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• 제28권1B호
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• pp.115-123
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• 2008

본 연구에서는 파 잠제 해빈의 상호간섭에 의한 파동장 및 흐름장의 변화를 검토하기 위하여, 2기의 잠제와 모래해빈을 설치한 경우를 대상으로 허와 이(2007)에 의해 개발된 모델(LES-WASS-3D)을 이용하여 잠제 주변의 3차원적 흐름특성과 파고분포에 대해 고찰하였다. 그 결과, 잠제단부를 중심으로 회전하는 순환류를 확인할 수 있었으며, 여러 단면의 평균류 및 평균와도를 검토함으로서 잠제주변의 3차원 흐름특성을 고찰하였다. 아울러 잠제 천단상의 쇄파점분포, 잠제주변의 파고분포 및 wave set-up에 대해서도 논의하였다.

• Wind and Structures
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• 제30권4호
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• pp.339-366
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• 2020

This study aims at modeling boundary layers (BLs) encountered in sparse and built environments (i.e. open, suburban and urban) at the subsonic Wind Tunnel (WT) at Ryerson University (RU). This WT has an insignificant turbulence intensity and requires a flow-conditioning system consisting of turbulence generating elements (i.e., spires, roughness blocks, barriers) to achieve proper turbulent characteristics. This system was developed and validated in the current study in three phases. In phase I, several Computational Fluid Dynamic (CFD) simulations of the tunnel with generating elements were conducted to understand the effect of each element on the flow. This led to a preliminary design of the system, in which horizontal barriers (slats) are added to the spires to introduce turbulence at higher levels of the tunnel. This design was revisited in phase II, to specify slat dimensions leading to target BLs encountered by tall buildings. It was found that rougher BLs require deeper slats and, therefore, two-layer slats (one fixed and one movable) were implemented to provide the required range of slat depth to model most BLs. This system only involves slat movement to change the BL, which is very useful for automatic wind tunnel testing of tall buildings. The system was validated in phase III by conducting experimental wind tunnel testingof the system and comparing the resulting flow field with the target BL fields considering two length scales typically used for wind tunnel testing. A very good match was obtained for all wind field characteristics which confirms accuracy of the system.