• 한국지반공학회논문집
• /
• 제26권12호
• /
• pp.51-60
• /
• 2010

자중에 의한 토압은 일반적으로 깊이에 따라 선형비례하여 증가되지만, 상재하중에 의해 발생되는 토압은 분포형태와 지반상태 및 경계조건에 따라 다르게 분포한다. 본 연구에서는 변위가 구속된 강성벽체에 연직상재하중에 의한 토압분포를 실험적으로 연구하기 위하여 실내모형토조[30 cm(폭) ${\times}$ 110 cm(길이) ${\times}$ 88 cm(높이)]에 각 2개의 로드셀이 배치된 8개의 토압판으로 구성된 강성벽체를 설치하여 연직상재하중의 크기와 위치(벽체에서 이격거리)를 변화시켜 가면서 토압분포와 토압의 영향범위를 관찰하였다. 실험결과, 연직상재하중에 의해 발생되는 수평토압은 일정위치에서 최대값을 보이고 깊어질수록 감소하여 한계깊이를 초과하면 무시할 만큼 작아졌다. 수평토압이 최대가 되는 깊이와 한계깊이는 연직상재하중의 크기와는 무관하게 벽체와의 이격거리에 따라 다르게 나타났다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2007년도 춘계학술대회B
• /
• pp.3003-3008
• /
• 2007

Critical nozzle has been frequently employed to measure the flow rate of various gases, but hydrogen gas, especially being at high-pressure condition, was not nearly dealt with the critical nozzle due to treatment danger. According to a few experimental data obtained recently, it was reported that the discharge coefficient of hydrogen gas through the critical nozzle exceeds unity in a specific range of Reynolds number. No detailed explanation on such an unreasonable value was made, but it was vaguely inferred as real gas effects. For the purpose of practical use of high-pressure hydrogen gas, systematic research is required to clarify the critical nozzle flow of high-pressure hydrogen gas. In the present study, a computational fluid dynamics(CFD) method has been applied to predict the critical nozzle flow of high-pressure hydrogen gas. Redlich-Kwong equation of state that take account for the forces and volume of molecules of hydrogen gas were incorporated into the axisymmetric, compressible Navier-Stokes equations. A fully implicit finite volume scheme was used to numerically solve the governing equations. The computational results were validated with some experimental data available. The results show that the coefficient of discharge coefficient is mainly influenced by the compressibility factor and the specific heat ratio, which appear more remarkable as the inlet total pressure of hydrogen gas increases.

• Journal of Magnetics
• /
• 제15권4호
• /
• pp.155-158
• /
• 2010

The HDDR (hydrogenation, disproportionation, desorption, and recombination) process can be used as an effective way of converting a no coercivity Nd-Fe-B ingot material, with a coarse $Nd_2Fe_{14}B$ grain structure, to a highly coercive one with a fine grain structure. Careful control of the HDDR process can lead to an anisotropic powder with good $Nd_2Fe_{14}B$ grain texture; the most critical step for inducing texture is disproportionation. The critical conditions (hydrogen pressure and temperature) for the disproportionation reaction of fully hydrogenated $Nd_{12.5}Fe_{81.1-(x+y)}B_{6.4}Ga_xNb_y$ (x = 0 or 0.3, y = 0 or 0.2) alloys, in different atmospheres of pure hydrogen and a mixed gas of hydrogen and argon, was investigated with TPA (thermopiezic analyser). From this, the hydrogen pressure-temperature diagram showing the critical conditions was established. The critical disproportionation temperature of the fully hydrogenated $Nd_{12.5}Fe_{81.1-(x+y)}B_{6.4}Ga_xNb_y$ alloys was slightly increased as the hydrogen pressure decreased in both pure hydrogen and mixed gas. The critical disproportionation temperature of the hydrogenated alloys was higher in the mixed gas than in pure hydrogen. Addition of Ga and Nb increased the critical disproportionation temperature of the fully hydrogenated Nd-Fe-B alloys.

• Tribology and Lubricants
• /
• 제36권6호
• /
• pp.324-331
• /
• 2020

Wedging in a frictional elastic system is defined if the state of stick exists after the external loading on the system is removed. This paper presents a method to determine the critical coefficient of wedging for an elastic frictional system by considering the separation state. Wedging is always possible if the coefficient of friction exceeds a critical value known as the critical wedging coefficient. This method requires two concepts: a necessary and sufficient condition for wedging, which can be interpreted as positive spanning sets of constraint vectors existing in the wedged system, and the minimal positive basis that enables a minimum wedging coefficient. The algorithm based on the positive spanning concept is repeatedly executed after eliminating nodes from the contact stiffness matrix, for which the separation states are impending. The simulation results show that once a node enters the separation state, it never returns to the contact state again and the critical wedging coefficient reduces during repeated algorithm execution. The benefit of this method is that the computation time permits handling models with large numbers of contact nodes. The algorithm can also numerically find the critical wedging coefficient, thereby contributing to fastening and assembly performance improvements in mechanical systems.

• Nuclear Engineering and Technology
• /
• 제53권2호
• /
• pp.446-454
• /
• 2021

Critical heat flux (CHF) has traditionally been evaluated using look-up tables or empirical correlations for nuclear power plants. However, under complex moving condition, it is necessary to reconsider the CHF characteristics since the conventional CHF prediction methods would no longer be applicable. In this paper, the additional forces caused by motions have been added to the annular film dryout (AFD) mechanistic model to investigate the effect of moving condition on CHF. Moreover, a theoretical model of the natural circulation loop with additional forces is established to reflect the natural circulation characteristics of the loop system. By coupling the system loop with the AFD mechanistic model, a CHF prediction program called NACOM for natural circulation loop under moving condition is developed. The effects of three operating conditions, namely stationary, inclination and rolling, on the CHF of the loop are then analyzed. It can be clearly seen that the moving condition has an adverse effect on the CHF in the natural circulation system. For the calculation parameters in this paper, the CHF can be reduced by 25% compared with the static value, which indicates that it is important to consider the effects of moving condition to retain adequate safety margin in subsequent thermal-hydraulic designs.

• 조명전기설비학회논문지
• /
• 제24권4호
• /
• pp.73-80
• /
• 2010

접지임피던스는 접지전극의 크기를 증가시켜도 낮아지지 않고, 토양의 특성과 주파수 조건에 따라 접지 임피던스가 최소값을 보이는 접지전극의 길이가 존재하며, 이 접지전극의 길이를 각 토양 특성과 주파수에 따른 임계길이라 한다. 본 논문에서는 대지구조의 조건을 고려하는 새로운 분포정수회로모델을 제안하였으며, 새로이 제안된 해석모델을 적용하여 MATLAB프로그램으로 수직접지전극의 임계길이와 접지임피던스를 해석하였다. 그 결과, 수직접지전극의 임계길이와 접지임피던스의 주파수의존성에 대한 대지구조의 영향이 크며, 접지시스템의 최적 설계를 위해서는 대지구조를 고려하는 것이 바람직한 것으로 밝혀졌다.

• 한국작물학회지
• /
• 제47권2호
• /
• pp.127-131
• /
• 2002

Critical nitrogen concentration (Nc), which is defined as the minimum % N in shoots required to maintain the maximum growth rate of top dry weight (W) at any time, was determined for rice plant. Using two rice varietal groups, japonica varieties and an indica $\times$ japonica "Dasanbyeo", 18 data points fulfilling the statistical criteria for determining Nc were obtained through eight N-fertilization experiments over two years at Suwon (37$^{\circ}$16'N), Korea. Nc dilution curve for each variety was obtained by fitting the Nc-W relationship to power function. However, The critical nitrogen curves for the two variety groups were not different statistically. Thus, a Nc dilution curve was fitted for the Nc data points pooled over the two variety groups and proposed in rice as: Nc=4.08, where W＜1.73 t h $a^{-1}$ , Nc=5.197 $W^{0.425}$3/ ($R^2$=0.964), where 1.73 t h $a^{-1}$ ＜W＜12 t h $a^{-l}$. The Nc for W＜1.73 t h $a^{-l}$ were estimated as a constant value of 4.08%, the mean value of the maximum N concentration for N-limiting condition and the minimum N concentration for N non-limiting condition. The model for Nc is applicable to diagnosing the nitrogen nutrition status during the rice growth period from emergence to heading stage. The Nc curve well discriminated the 144 data points between the N limiting and the N non-limiting groups regardless of varieties, cultural methods, and years.-limiting groups regardless of varieties, cultural methods, and years.

• Korean Journal of Mathematics
• /
• 제19권3호
• /
• pp.331-342
• /
• 2011

We consider the multiplicity of the solutions for a class of a system of critical growth beam equations with periodic condition on t and Dirichlet boundary condition $$\{u_{tt}+u_{xxxx}=av+\frac{2{\alpha}}{{\alpha}+{\beta}}u_{+}^{{\alpha}-1}v_{+}^{\beta}+s{\phi}_{00}\;\;in\;(-\frac{\pi}{2},\;\frac{\pi}{2}){\times}R,\\u_{tt}+v_{xxxx}=bu+\frac{2{\alpha}}{{\alpha}+{\beta}}u_{+}^{\alpha}v_{+}^{{\beta}-1}+t{\phi}_{00}\;\;in\;(-\frac{\pi}{2},\;\frac{\pi}{2}){\times}R,$$ where ${\alpha}$, ${\beta}$ > 1 are real constants, $u_+=max\{u,0\}$, ${\phi}_{00}$ is the eigenfunction corresponding to the positive eigenvalue ${\lambda}_00=1$ of the eigenvalue problem $u_{tt}+u_{xxxx}={\lambda}_{mn}u$. We show that the system has a positive solution under suitable conditions on the matrix $A=\(\array{0&a\\b&0}\)$, s > 0, t > 0, and next show that the system has another solution for the same conditions on A by the linking arguments.

• 한국초전도ㆍ저온공학회논문지
• /
• 제16권4호
• /
• pp.36-39
• /
• 2014

We report on the superconducting and structural characteristics of Pb-based alloy ($Pb_{0.9}In_{0.1}$, $Pb_{0.8}In_{0.2}$ and $Pb_{0.85}Bi_{0.15}$) thin films, depending on the film deposition rate. The maximum critical magnetic field strength of $Pb_{0.85}Bi_{0.15}$ is almost six times larger than that of $Pb_{0.9}In_{0.1}$, and more rapid growth of the film enhances the critical magnetic field strength even for the same alloy material. Scanning electron microscopy inspection indicates that lower deposition rate condition is vulnerable to the formation of void structure in the film. Topographic images using atomic force microscopy are useful to optimize the deposition condition for the growth of smooth superconducting film. Our work can be utilized for future studies on hybrid superconducting devices using low-dimensional nanostructures.

• Korean Chemical Engineering Research
• /
• 제50권1호
• /
• pp.61-65
• /
• 2012

이성분 용융액의 시간의존형 응고계에서 mush 층의 대류발생을 선형 안정성 이론으로 해석하였다. 본 연구에서는 근공융물 mush 층을 다공성 블록으로 가정한 단순화된 모델에 전파이론을 적용하여 대류발생 임계조건을 구하였다. 본 연구 모델에서는 기존의 실험결과 및 mush층 위의 액체층을 포함하여 고려한 이론적 연구의 결과보다 더 높은 임계 Rayleigh 수가 얻어졌다. mush 층의 윗 경계면에 일정압력(투과)조건을 적용하는 경우가 비투과조건에 비해 임계 Rayleigh 수를 더 작게 하며 염화암모늄 수용액의 응고실험결과와 더 근접한 것으로 조사되었다.