• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.2
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• pp.123-130
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• 2008

The effects of characteristic condition number on the convergence of preconditioned Navier-Stokes equations were investigated. The two-dimensional preconditioned Navier-Stokes adopting Choi and Merkle's preconditioning and the temperature preconditioning are considered. Preconditioned Roe's FDS scheme was adopted for spatial discretization and preconditioned LU-SGS scheme was used for time integration. It is shown that the convergence characteristics of the Navier-Stokes equations are strongly affected by the characteristic condition number. Also it is shown that the optimal characteristic condition numbers for viscous flows are larger than that in inviscid flows.

• Journal of computational fluids engineering
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• v.17 no.1
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• pp.8-15
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• 2012

A code developed using the flux-difference splitting scheme on the hybrid Cartesian/immersed boundary method is applied to simulate three-dimensional internal waves. The material interface is regarded as a moving contact discontinuity and is captured on the basis of mass conservation without any additional treatment across the interface. Inviscid fluxes are estimated using the flux-difference splitting scheme for incompressible fluids of different density. The hybrid Cartesian/immersed boundary method is used to enforce the boundary condition for a moving three-dimensional body. Immersed boundary nodes are identified within an instantaneous fluid domain on the basis of edges crossing a boundary. The dependent variables are reconstructed at the immersed boundary nodes along local normal lines to provide the boundary condition for a discretized flow problem. The internal waves are simulated, which are generated by an pitching ellipsoid near an material interface. The effects of density ratio and location of the ellipsoid on internal waves are compared.

• Journal of computational fluids engineering
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• v.6 no.4
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• pp.43-53
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• 2001

본 논문은 충격파 불안정성이 나타나지 않는 충격파 안정적인 수치기법의 개발을 목표로 하고 있다. Roe의 수치기법은 유동의 수치계산에 있어 높은 정확도를 보장하지만 carbuncle 현상과 같은 충격파 불안정성이 나타나는 것으로 알려져 있다. Roe의 수치기법과 HLLE 수치기법의 수치점성을 비교하여 충격파 불안정성의 원인을 살펴보았으며, Roe의 수치기법에 나타나는 반감쇠항에 마하수의 함수인 조절함수 f와 g를 도입하여 충격파 안정성을 획득하였다. 본 논문에서 제안된 수치기법을 다양한 유동문제에 적용하여 수치기법의 충격파 안정성과 정확성을 검증하였다

• Proceedings of the KSR Conference
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• 2008.06a
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• pp.1773-1780
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• 2008

When the fire occur in the deeply underground subway station, the difficulties of passenger evacuation are expected because of many stairs to the exit. In this study, SOONGSIL-University station (7 line, 47m depth) is the one of the deepest subway stations of the each line in the Seoul metro. The numerical computational-simulation was performed for the fire driven flow in the subway station. Hot and smoke flow was analyzed from the simulation results. The proper plan of evacuation against fire was considered through the results. The fire driven flow was simulated using FDS code in which LES method was applied. The Heat Release Rate was 10MW and the ultrafast model was applied for the growing model of the fire source. The proper mesh size was determined from the characteristic length of fire size. The parallel computational method was employed to compute the flow and heat eqn's in the meshes, which are about 10,000,000, with 6cpu of the linux clustering machine.

• Journal of the Korean Society of Propulsion Engineers
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• v.8 no.1
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• pp.27-37
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• 2004

The convergence characteristics of preconditioned Euler equations were studied. A perturbation analysis was conducted to understand the behavior of the preconditioned Euler equations. Various speed flows in a two-dimensional channel with a 10％ circular arc in the middle of the channel were calculated. Roe's FDS scheme was used for spatial discretization and the LU-SGS scheme was used for time integration. It is shown that the convergence characteristics of pressure and velocity were maintained regardless of the Mach numbers but that the convergence characteristics of temperature were strongly related to the Mach number and became worse as the Mach number decreased. The perturbation analysis well explained the trend of the convergence characteristics and showed that the convergence characteristics are strongly related with the behavior o( the Preconditioning matrix.

• Fire Science and Engineering
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• v.21 no.2 s.66
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• pp.111-117
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• 2007

Full scale fire suppression test by water mist system were performed in machinery engine room ($20m{\times}15m{\times}10m$) according to IMO MSC/circ. 1165. The K-factor and operating pressure were 2.4 and 80 bar respectively. To assess the prediction capability of numerical simulation, FDS simulation was performed at the same operating condition with the full scale experiment. It was found that FDS simulation had the limitation for the fire extinguishing time prediction but was able to predict the spatial temperature distribution.

• Fire Science and Engineering
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• v.29 no.2
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• pp.8-12
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• 2015

Performance Based Design is evaluating after each calculatiuon ASET and RSET. Risk informaion values such as heat, smoke, toxic gas etc are extracted by FDS in this study. These Risk informaion values by FDS apply Artisoc (evacuation simulator). Building structure made $60m{\times}65m$, exit number is made 2 positions and people in this building are 50 ramdonly. 20 times (case1~case 20) simulated and analysis evacuees risk by evacuated route positions.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.586-592
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• 2000

터보머신 태부에 존재하는 정익 - 동익의 상호작용 유동현상을 수치모사 하는 코드를 병렬화 하였다 정익 - 동익의 상호작용을 해석하는 데에 편리하도륵 Multi-Block Grid System을 도입하여 계산영역을 형성하였고, 동익의 움직임으로 인해 발생하는 Sliding Interface부분은 Patched 알고리즘을 적용하여 해석하였다. 정익과 동익의 수를 1대 1로 단순화시켜 수치모사한 결과와 정익과 동익의 수를 실제 조건과 더 비슷하게 설정한 3대 4의 비율로 맞추어 수치모사한 결과를 비교하였다. 또한, 병렬컴퓨팅으로 인해 단축된 계산시간을 다른 연구에서의 계산시간들과 서로 비교하였다. 2차원 비정상 압축성 Navier-Stokes 방정식이 이용되었고, 난류모델링에는 K-w SST 모델링이 적응되었다. Roe의 FDS 기법을 사용하여 플럭스를 계산하였고, MUSCL 기법을 적용하여 3차의 공간정확도를 갖도록 하였다. 시간적분에는 이보성의 DP-SGS를 사용하였다. 해석결과의 분석에는 Time-averaged pressure distribution과 Pressure amplitude distribution 데이터를 사용했다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2000.11a
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• pp.20-20
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• 2000

2개의 경사 충격파와 하나의 수직 충격파로 초음속 유동을 압축하는 초음속 공기 흡입구의 수치적 연구를 수행하였다. 지배방정식으로는 Navier-Stokes방정식을 사용하였고 난류모델로는 SST 모델을 사용하였다. 지배방정식의 점성항 계산에는 중심차분법을 사용하였고 대류항 계산에는 풍상차분법인 Roe의 FDS기법을 MUSCL기법과 결합하여 이용하였다. 유한 체적법을 이용하여 차분된 방정식은 LU분할 기법을 이용한 완전 내재적 방법으로 2차 정확도 시간 적분으로 비정상 과정의 연구를 수행하였다. 흡입구 배압을 정해주어야 하는 어려움을 해결하기 위해 흡입구 후면에 노즐을 달고 노즐의 면적을 조절하여 배압이 형성되도록 하였다.(중략)

• Journal of Korea Water Resources Association
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• v.56 no.10
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• pp.631-639
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• 2023

Flash drought (FD), characterized by the rapid onset and intensification, can significantly impact ecosystems and induce immediate water stress. A more comprehensive understanding of the causes and characteristics of FD events is required to enhance drought monitoring. Therefore, we investigated the FD events took place over the Korean peninsula using Global Land Data Assimilation System (GLDAS) data from 2012 to 2022. We first detected FD events using the stress-based method (Standardized Evaporative Stress Ratio, SESR), and analyzed the frequency and duration of FDs. The FD events were classified into three cases based on the variations in Actual Evapotranspiration (AET) and potential Evapotranspiration (PET), and spatially analyzed. Results revealed that there are regional disparities in frequency and duration of FDs, with a mean frequency of 6.4 and duration of 31 days. When classified into Case 1 (normal condition), Case 2 (AET-driven), and Case 3 (PET-driven), we found that Case 2 FDs emerged approximately 1.5 times more frequently than those driven by PET (Case 3) across the Korean peninsula. Case 2 FDs were found to be induced under water-limited conditions, and led both AET and PET to be decreased. Conversely, Case 3 FDs occurred under energy-limited conditions, with increase in both. Case 2 FDs predominantly affected the northwestern and central-southern agricultural regions, while Case 3 occurred in the eastern region, characterized by forested land cover. These findings offers insights into our understanding of FDs over the Korean peninsula, considering climate factors, land cover, and water availability.