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Determination of Proper Time Step for Molecular Dynamics Simulation
조종철,김병철,Jo, Jong Cheol,Kim, Byeong Cheol Korean Chemical Society 2000 Bulletin of the Korean Chemical Society Vol.21 No.4
In this study we have investigated the determination of proper time step in molecular dynamics simulation.Since the molecular dynamics is mathematically related to nonlinear dynamics, the analysis of eigenvalues isused to explain the relationship between the time step and dynamics. The tracings of H2 and CO2 molecular dynamics simulation agrees very well with the analytical solutions. For H2, the time step less than 1.823 fs pro-vides stable dynamics. ForCO2, 3.808 fs might be the maximum time step for proper molecular dynamics. Al-though this results were derived for most simple cases of hydrogen and carbon dioxide, we could quantitatively explain why improperly large time step destroyed the molecular dynamics. From this study we could set the guide line of the proper time step for stable dynamics simulation in molecular modeling software.
Numerical prediction of a fl ashing fl ow of saturated water at high pressure
조종철,정재준,윤병조,Frederick J. Moody 한국원자력학회 2018 Nuclear Engineering and Technology Vol.50 No.7
Transient fluid velocity and pressure fields in a pressurized water reactor (PWR) steam generator (SG)secondary side during the blowdown period of a feedwater line break (FWLB) accident were numericallysimulated employing the saturated water flashing model. This model is based on the assumption thatcompressed water in the SG is saturated at the beginning and decompresses into the two-phase regionwhere saturated vapor forms, creating a mixture of steam bubbles in water by bulk boiling. The numericalcalculations were performed for two cases of which the outflow boundary conditions aredifferent from each other; one is specified as the direct blowdown discharge to the atmosphere and theother is specified as the blowdown discharge to an extended calculation domain with atmosphericpressure on its boundary. The present simulation results obtained using the two different outflowboundary conditions were discussed through a comparison with the predictions using a simple nonflashingmodel neglecting the effects of phase change. In addition, the applicability of each of thenon-flashing water discharge and saturated water flashing models for the confirmatory assessments ofnew SG designs was examined.
Fluidelastic Instability Characteristics of Helical Steam Generator Tubes
조종철,김웅식,최영환,정명조 한국원자력학회 2004 Nuclear Engineering and Technology Vol.36 No.4
This study investigates the fluidelastic instability characteristics of helical steam generator typetubes used in operating nuclear power plants. To obtain a natural frequency, correspondingmode shape, and participation factor, modal analyses using various conditions are performedfor helical type tubes. Investigated are the effects of the number of turns, the number ofsupports, and the status of the inner fluid on the modal and fluidelastic instability characteristicsof the tubes, which are expressed in terms of the natural frequency, the corresponding modeshape, and the stability ratio.
FINITE ELEMENT BASED FORMULATION OF THE LATTICE BOLTZMANN EQUATION
조종철,KYUNG WAN ROH,YOUNG W. KWON 한국원자력학회 2009 Nuclear Engineering and Technology Vol.41 No.5
The finite element based lattice Boltzmann method (FELBM) has been developed to model complex fluid domain shapes, which is essential for studying fluid-structure interaction problems in commercial nuclear power systems, for example. The present study addresses a new finite element formulation of the lattice Boltzmann equation using a general weighted residual technique. Among the weighted residual formulations, the collocation method, Galerkin method, and method of moments are used for finite element based Lattice Boltzmann solutions. Different finite element geometries, such as triangular, quadrilateral, and general six-sided solids, were used in this work. Some examples using the FELBM are studied. The results were compared with both analytical and computational fluid dynamics solutions.
조종철,정명조,김웅식,김효정,김태형 한국소음진동공학회 2003 한국소음진동공학회 논문집 Vol.13 No.5
This paper investigates the vibration characteristics of steam generator (SG) U-tubes with defect. The operating SG shell-side flow field conditions for determining the fluidelastic instability parameters such as added mass are obtained from three-dimensional SG flow calculation. Modal analyses are performed for the U-tubes either with axial or circumferential flaw with different sizes. Special emphases are on the effects of flaw orientation and size on the modal and instability characteristics of tubes, which are expressed in terms of the natural frequency, corresponding mode shape and stability ratio. Also, addressed is the effect of the internal pressure on the vibration characteristics of the tube.
재순환식 증기발생기 U-튜브군에 대한 유체탄성 불안정 해석
조종철,이상균,김웅식,신원기,은영수 대한기계학회 1993 대한기계학회논문집 Vol.17 No.1
This paper presents the results of fluidelastic instability analysis performed for the U-tube bundle of a Westinghouse model 51 steam generator, one of the recirculating types designed at an early stage, in which the principal region of external cross-flow is associated with the U-bend portion of tube. The prerequisites for this analysis are detailed informations of the secondary side flow conditions in the steam generator and the free vibration behaviours of the U-tubes. In this study, the three-dimensional two-phase flow field in the steam generator has been calculated employing the ATHOS3 steam generator two-phase flow code and the ANSYS engineering analysis code has been used to calculate the free vibration responses of specific U tubes under consideration. The assessment of the potential instability for the suspect U-tubes, which is the final analysis process of the present work, has been accomplished by combining the secondary side velocity and density distributions obtained from the ATHOS3 prediction with the relative modal displacement and natural frequency data calculated using the ANSYS code. The damping of tubes in two-phase flow has been deduced from the existing experimental data by taking into account the secondary side void fraction effect. In operation of the steam generator, the tube support conditions at the tube-to-tube support plate intersections due to either tube denting degradation or deposition of tube support plate corrosion products or ingression of dregs. Thus, various hypothetical cases regarding the tube support conditions at the tube-to-tube support plate intersections have been considered to investigate the clamped support effects on the forced vibration response of the tube. Also, the effect of anti-vibration bars support in the curved portion of tube has been examined. 본 연구에서는 Westinghouse Model 51 증기발생기의 U-bend 영역에서 2차측 유체의 횡단유동으로 유발될 수 있는 튜브군의 유체탄성불안정을 예측하기 위한 해석 을 수행하고 그 대표적인 결과들을 제시하였다. 그리고 U-bend 영역에서 AVB에 의한 튜브의 지지상태와 형태 및 최상부 TSP에서 Denting 또는 이물질 고착으로 인하여 변 경된 튜브의 고정지지조건 등이 튜브의 유체탄성불안정 응답에 미치는 영향을 조사하 였다. 유체탄성불안정 해석과정에서 필수적으로 선행되어야 하는 2차측 3차원 2상 유동장 계산은 증기발생기 열수력 해석용인 ATHOS3 코드로써 수행되었으며, U-튜브의 고유진동수와 모우드 형상은 공학해석용 유한요소 프로그램인 ANSYS코드로써 계산되었 다.