• 대한방사선기술학회지:방사선기술과학
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• 제32권2호
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• pp.213-218
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• 2009

감마선 원천핵을 두 개의 금속 평행판 사이, 실린더나 구와 같은 유한한 공간 내부에 놓았을 때 에너지 준위폭이 어떻게 변경되는가에 대하여 이론적으로 조사했다. 2개의 금속판에 의해 뒤쪽 산란된 감마선이 원천핵에 재흡수됨으로써 $^{133}Cs$핵의 여기상태(81 keV)의 에너지폭은 평행판 간격이 1.0 mm일 때 4.2 K에서 3.7% 정도 감소됨을 볼 수 있었다. 여기서 금속판은 두께 0,5 mm, 반경이 3 cm이며 금으로 만들었다. 반경 1.0 mm, 두께 0.5 mm, 길이 5 cm의 실린더 금판에 의해 뒤쪽 산란되었을 때에는 4.2 K에서 6.5% 에너지 폭이 좁아지는 결과를 얻었다. 또한, 두께 0.5 mm, 반경 1.0 mm의 금으로 된 구 안에 원천핵을 놓았을 때 4.2 K에서 18.2% 에너지준위 폭이 감소하였다. 이러한 에너지 준위 폭의 감소는 그 준위의 반감기가 연장된 것을 의미한다.

• 천문학회보
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• 제41권2호
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• pp.57.3-58
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• 2016

A symbiotic star is a wide binary system consisting of a hot white dwarf and a mass losing giant, where the giant loses its material in the form of a slow stellar wind resulting in accretion onto the white dwarf through gravitational capture. Symbiotic stars are known to exhibit unique spectral features at 6825 and 7082, which are formed from O VI 1032 and 1038 through Raman scattering with atomic hydrogen. In this Monte Carlo study we investigate the flux ratio of 6825 and 7082 in a neutral region with a geometric shape of a slab, cylinder and sphere. By varying the amount of neutral hydrogen parametrized by the column density along a specified direction, we compute and compare the flux ratio of Raman scattered O VI 6825 and 7082. In the column density around 1020 cm-2, flux ratio changes in a complicated way, rapidly decreasing from the optically thin limit to unity the optically thick limit as the column density increases. It is also notable that when the neutral region is of a slab shape with the O VI source outside the slab, the optically thick limit is less than unity, implying a significant fraction of O VI photons escape through Rayleigh scattering near the boundary. We compare our high resolution CFHT data of HM Sge and AG Dra with the data simulated with finite cylinder models confirming that 'S' type symbiotic tend to be characterized by thicker HI region that 'D' type counterparts. It is expected that this study will be useful in interpretation of the clear disparity of Raman O VI 6825 and 7082 profiles, which will shed much light on the kinematics and the asymmetric distribution of O VI material around the hot white dwarf.

• Structural Engineering and Mechanics
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• 제74권5호
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• pp.679-698
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• 2020

In this paper, the meshless local Petrov-Galerkin (MLPG) method is developed for dynamic analysis of non-symmetric nanocomposite cylindrical shell equations of elastic wave motion with nonlinear grading patterns under shock loading. The mechanical properties of the nanocomposite cylinder are obtained based on a micro-mechanical model. In this study, four kinds of grading patterns are assumed for carbon nanotube mechanical properties. The displacements can be approximated using shape function so, the multiquadrics (MQ) Radial Basis Functions (RBF) are used as the shape function. In order to discretize the derived equations in time domains, the Newmark time approximation scheme with suitable time step is used. To demonstrate the accuracy of the present method for dynamic analysis, at the first a problem verifies with analytical solution and then the present method compares with the finite element method (FEM), finally, the present method verifies by using the element free Galerkin (EFG) method. The comparison shows the high capacity and accuracy of the present method in the dynamic analysis of cylindrical shells. The capability of the present method to dynamic analysis of non-symmetric nanocomposite cylindrical shell is demonstrated by dynamic analysis of the cylinder with different kinds of grading patterns and angle of nanocomposite reinforcements. The present method shows high accuracy, efficiency and capability to dynamic analysis of non-symmetric nanocomposite cylindrical shell, which it furnishes a ground for a more flexible design.

• 대한기계학회논문집A
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• 제32권6호
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• pp.465-473
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• 2008

Type II compressed natural gas(CNG) storage vessels for automobiles have been acknowledged for their excellence and have recently become established in local regions. Their supply is not only to automakers in Korea such as Hyundai Motors but they are being increasingly exported. Although the available products have undergone safety evaluations and are certified by an authorized institution they are still short of the optimal design that is possible for such storage vessels. This research investigates the shape and thickness of the dome with the aim of optimizing the type II CNG storage vessels by using a finite element analysis technique. CNG storage vessels can be largely divided into 3 parts namely, the hear part, the cylinder part and the dome part. The head part is designed by means of a hot spinning process and this method is safer than that used in the design of the dome part even though its shape is similar. The thickness of the liners and reinforcing materials was optimized based on the requirements of the cylinder and dome parts. In addition, the shape of the dome, which is most suitable for Type II CNG storage vessels, is proposed by a process of review and analysis of various existing shape, and then conducting a structural stability evaluation to ensure the optimal design plan.

• 대한조선학회논문집
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• 제56권6호
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• pp.507-514
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• 2019

Computational studies of accelerating flow around 2D Circular Cylinder was performed to investigate characteristics of wake field and drag forces. Previous studies had revealed that drag on the cylindrical body in accelerating flow is much greater than that in the flow with constant velocity; however, the underlying physics on the drag increase has not been clearly investigated. In order to investigate the drag increase and its relationship with wake development, this study employed a finite-volume based CFD code, Fluent 13.0 with k-ω SST model for turbulence effects. Inflows are modeled with varied accelerations from 0.4905 to 9.81m/s². The drag computed in the present study is in good agreement with previous studies, and clearly shows the increase compared to the drag on the body in the flow with constant velocity. The results also show that drag crisis observed at high Reynolds number in the case of the flow with constant velocity is also found in the case of accelerating flow. The analysis for wake and recirculation length shows that conventional vortex shedding does not occur even at high Reynolds number and the drag increase is larger at higher acceleration.

• 한국가스학회지
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• 제18권5호
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• pp.6-11
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• 2014

차량용 LNG 연료 용기의 내진동 단열 지지구조 최적화 설계 개발을 위하여 종래의 해외특허구조 설계를 기본으로 한 특성요인도 분석으로 용기의 내조 및 외조 지지부 구조설계의 주요 설계 인자를 도출하였다. 도출된 설계인자 중에서 우선적으로 지지 봉재의 직경과 단열 격판 연결부 곡률을 대상으로 하여 최적화를 수행하였다. 차량용 LNG 연료 용기 설계안에 대한 평가를 위해 설계안을 MSC/MARC 상용유한 요소해석 패키지를 활용하여 유한요소 모델링하여 진동모드해석과 열전달 및 열응력해석을 수행하였다. 최적화 설계 결과를 통하여 도출된 설계안은 고유진동해석을 통한 1차 모드 고유진동수(1st Mode Natural Frequency), 열전달해석을 통한 초저온 용기 내조 외조간 지지부를 통한 총전열량 및 열응력해석을 통한 최대 Von-Mises 응력이 모두 설계 목표치를 만족하였으며, 개발된 설계안에 따라 차량용 LNG 연료 용기의 제작하여 3차원 진동 시험과 단열성능 시험을 통해 설계를 검증하였다.

• 대한조선학회지
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• 제11권1호
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• pp.9-16
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• 1974

To contribute towards more accurate estimation of the virtual inertia coefficient for the horizontal vibration of ships, three dimensional correction factor $J_H$ for the added mass of finitely long elliptic prismatic bars in horizontal vibration in a free surface of an ideal fluid are calculated. In the problem formulation Dr. T. Kumai's quasi-finite length concept[1,11,12] is employed. Now that, in Dr. Kumai's work[1] for the horizontal vibration the mathematical model was a circular cylinder, the principal aim of the authors' work is to investigate the influence of the beam-draft ratio B/T on $J_H$. The numerical results of this work are shown in Fig.3 graphically, from which we may recognize that the influence of B/T on $J_H$ is remarkable as much as that of the length-draft ratio L/T(refer to Fig.1 also). In Fig.3 the curves for B/T=2.00 are of those based on Dr. Kumai's result[1]. On the other hand, the experimental data obtained by Burril et al.[9] for the horizontal vibration of finitely long prismatic bars of various cross-section shapes are compared with the theoretical added mass coefficients defined by combination of the authors' $J_H$ from Fig.3 and two dimensional coefficients $C_H$ obtained by Lewis form approximation for the corresponding sections. They are in reasonable correspondence with each other as shown in Fig.2. Finally, considering that the longitudinal profile of full-form ship's hull is well resembled to that of an elliptic cylinder and that the influences of other factors such as the sectional area coefficient and the shape of section contour itself can be well merged in the two dimensional added mass coefficient, the authors recommend that the data given in Fig.3 may be successfully adopted for the three dimensional correction factor the added mass in the horizontal vibration of hull-form ships.

• Composites Research
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• 제34권3호
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• pp.155-160
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• 2021

메타물질은 자연발생 물질에서 발견되지 않는 특성을 만들 수 있는 여러가지 요소들의 복합체로, 힘의 방향을 변환하거나, 음의 강성을 만들거나, 진동 및 충격 특성을 바꿀 수 있다. 제작이 용이하고, 수직방향의 진동과 충격을 저감시키는데 탁월한 성능을 지닌 우드파일 메타물질의 경우, 충격 저감을 위해 충격 전달에 영향을 끼치는 변수에 대한 기초 연구가 필요하다. 최근 기하학적 요소에 따른 충격저감에 대해 연구가 진행되고 있지만, 재료적 변수가 충격저감에 끼치는 영향에 대한 연구는 미흡하다. 본 논문에서는 우드파일 실린더의 기하학적 특성(적층각도, 직경, 길이)과 재료적 특성(탄성계수, 비중, 포아송 비)을 변수화하여 유한요소해석이 진행되었다. 유한요소해석을 통해 우드파일 실린더가 충격을 전달하는 양상을 확인하고, 주효과도 분석을 통해 충격 힘과 에너지의 저감에 대한 변수 별 영향이 고찰되었고, 고속 푸리에 변환(FFT)을 통해 주파수 대역에 대한 분석이 진행되었다. 충격 힘과 진동을 저감시키기 위하여 실린더의 접촉 면적에 영향을 주는 변수들이 크게 영향을 끼치는 것으로 나타났다.

• 한국도로학회논문집
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• 제6권1호
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• pp.47-56
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• 2004

신 구 콘크리트의 부착강도 시험시 두 재료의 계면에서 파괴가 유도되어 순수한 부착강도를 측정할 수 있도록 계면에 원형의 비부착면을 삽입하여 직접인발시험에 의해 부착강도를 측정하는 실험방법을 제시하였다. 먼저, 새로 제안한 실험방법에 의해 계면에서 응력이 집중되는 정도를 파악하기 위해 유한요소해석을 수행하여 두 재료의 탄성계수비 및 비부착면의 면적 (균열률)에 따른 계면에서의 파괴에너지를 산정하였으며, 부재의 크기 및 하중에 대한 보정을 감안하여 무차원함수로 환산하였다. 그리고 본 연구에서 제시된 부착강도 시험방법의 신뢰성을 입증하기 위해, 3가지 크기의 원형 비부착면(균열률 0.2, 0.4. 0.6)이 삽입된 신 구 콘크리트 복합시편(유황 폴리머 콘크리트+보통 콘크리트)을 사용하여 부착강도를 측정하였고 앞서 전개된 무차원함수로부터 계면 파괴에너지를 역산하였다. 시험결과, 모든 시편이 계면에서 파괴가 유도되었다. 또한 실험 데이터 및 해석결과를 분석하여 균열률이 0.4$\sim$0.6인 경우에 부착강도의 오차가 가장 적게 발생될 수 있음을 파악하였다.

• Wind and Structures
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• 제34권1호
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• pp.127-136
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• 2022

In this work a multi-fidelity non-intrusive polynomial chaos (MF-NIPC) has been applied to a structural wind engineering problem in architectural design for the first time. In architectural design it is important to design structures that are safe in a range of wind directions and speeds. For this reason, the computational models used to design buildings and bridges must account for the uncertainties associated with the interaction between the structure and wind. In order to use the numerical simulations for the design, the numerical models must be validated by experi-mental data, and uncertainties contained in the experiments should also be taken into account. Uncertainty Quantifi-cation has been increasingly used for CFD simulations to consider such uncertainties. Typically, CFD simulations are computationally expensive, motivating the increased interest in multi-fidelity methods due to their ability to lev-erage limited data sets of high-fidelity data with evaluations of more computationally inexpensive models. Previous-ly, the multi-fidelity framework has been applied to CFD simulations for the purposes of optimization, rather than for the statistical assessment of candidate design. In this paper MF-NIPC method is applied to flow around a rectan-gular 5:1 cylinder, which has been thoroughly investigated for architectural design. The purpose of UQ is validation of numerical simulation results with experimental data, therefore the radius of curvature of the rectangular cylinder corners and the angle of attack are considered to be random variables, which are known to contain uncertainties when wind tunnel tests are carried out. Computational Fluid Dynamics (CFD) simulations are solved by a solver that employs the Finite Element Method (FEM) for two turbulence modeling approaches of the incompressible Navier-Stokes equations: Unsteady Reynolds Averaged Navier Stokes (URANS) and the Large Eddy simulation (LES). The results of the uncertainty analysis with CFD are compared to experimental data in terms of time-averaged pressure coefficients and bulk parameters. In addition, the accuracy and efficiency of the multi-fidelity framework is demonstrated through a comparison with the results of the high-fidelity model.