• 한국전산유체공학회지
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• 제21권4호
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• pp.11-18
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• 2016

A numerical study is conducted on the optical fiber coating flow in a primary coating nozzle consisting of three major parts: a resin chamber, a contraction and a coating die of small diameter. The flow is driven by the optical fiber penetrating the center of the nozzle at a high speed. The axisymmetric two-dimensional flow and heat transfer induced by viscous heating are examined based on the laminar flow assumption. Numerical experiments are performed with varying the convergent angle of nozzle contraction and the optical fiber drawing speed. The numerical results show that for high drawing speed greater than 30 m/s, there is a transition in the essential flow features depending on the convergent angle. For a large convergent angle greater than $30^{\circ}$, unfavorable multicellular flow structures are monitored, which could be associated with wall boundary-layer separation. In the regime of small convergent angle, as the angle increases, the highest resin temperature at the exit of die and the coating thickness decrease but the sensitivity of coating thickness on drawing speed and the maximum shear strain of resin on the optical fiber increase. The effects of the convergent angle are discussed in view of compromise searching for an appropriate angle for high-speed optical fiber coating.

• 터널과지하공간
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• 제12권4호
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• pp.277-283
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• 2002

본 연구는 안쪽축이 회전하고 바깥쪽 실린더는 정지해있는 반경비가0.52이고 30$^{\circ}$ 경사진 동심 환형관내의 헬리컬 유동 특성에 관한 것이다. 비뉴튼 유체인 UC 수용액과 벤토나이트 수용액을 사용하여 안쪽축이 0~400pm으로 회전할 때 축방향 유동을 완전히 발달시킨 후 축방향 압력손실값을 측정하였다. 또한, 헬리컬 유동의 가시화는 불안정한 파를 관찰하기 위해 수행되었다. 현재 연구 결과는 표면마찰계수에 대해 로스비순(Ro)와 레이놀즈수(Re)의 관계를 나타내었다. 또한, 그 결과들은 유동 불안정성 메카니즘의 존재를 보인다. 축회전수가 증가함에 따라 압력손실이 증가하지만, 그 증가폭은 천이 및 난류영역에서는 레이놀즈수가 증가할수록 감소하며, 회전의 영향으로 유동교란이 증진되어 천이가 촉진된다. 또, 이런 유동교란의 증진은 표면마찰계수값의 증가와 함께 임계 레이놀즈수(Re$_{c}$)를 작게 만든다.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2007년도 추계학술대회
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• pp.134-137
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• 2007

The temporal evolution of wake behind a circular cylinder oscillating rotationally with a relatively high forcing frequency has been investigated experimentally using a dynamic PIV technique. Experiments were carried out with varying the frequency ratio $F_R\;(=f_f/f_n)$ in the range from 0.0 (stationary) to 1.6 at oscillation amplitude of ${\theta}_A=30^{\circ}$ and Reynolds number of $Re=4.14{\times}10^3$. Depending on the forcing condition ($F_R$), the flow was divided into three regimes; non-lock-on ($F_R=0.4$), transition ($F_R=0.8$, 1.6) and lock-on regimes ($F_R=1.0$) with markedly different flow structure in the near-wake region behind the cylinder. When the frequency ratio was less than 1.0 ($F_R{\le}1.0$), the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline. The entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. However, the flow characteristics changed markedly beyond the lock-on flow regime ($F_R=1.0$) due to high-frequency forcing. At $F_R=1.6$, the mutual interactions between the vortices shed from both sides of the cylinder were not so strong. Thereby, the flow entrainment and momentum transfer into the wake center region were reduced. In addition, the size of the large-scale vortices decreased since the lateral extent of the wake was suppressed.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2001년도 Proceedings of 2001 Korea-Japan Joint Seminar on Particle Image Velocimetry
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• pp.86-99
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• 2001

Vortex lock-on or resonance in the flow behind a circular cylinder is investigated from a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K\acute{a}rm\acute{a}n$ and streamwise vortices in the wake-transition regime at the Reynolds number 360. Streamwise vortices at the lock-on and natural shedding states are observed, as well as the changes in the wake region with the change of the shedding frequency of lock-on state. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. At the lock-on state, the $K\acute{a}rm\acute{a}n$ vortices are observed to be more disordered by the increased strength and spanwise wavelength of the streamwise vortices, which leads to a strong three-dimensional motion. Recirculation and vortex formation region at the lock-on state is reduced as the oscillating frequency is increased. By comparing the Reynolds stresses at the lock-on and natural shedding states, $\bar{u'u'}\;and \;\bar{u'u'}$ at the lock-on state are concentrated on the shear layer around the cylinder. The $\bar{u'u'}\;at\;f_o/f_n=2.0$ has a large value near the centerline, compared with that of other cases. Considering the traces of maximum of u', in the wake region near the cylinder, wake width at the lock-on state is wider than that at the natural shedding state.

• 한국사회복지학
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• 제57권2호
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• pp.231-252
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• 2005

외국인가족들은 대체로 고령화되고 있는 산업사회에서 사회경제적 체제유지를 위해 많은 기여를 하고 있다. 그러나 이들은 이주사회에서 사회경제적으로 안정적인 지위를 확보하기가 어려우며, 문화적 적응과정에서 본국이나 이주사회 어느 곳에도 친화감을 느끼지 못하고 주변화될 위험을 안고 있다. 본 논문은 독일에서 사민당정부가 집권하게 되면서 급속히 변화된 외국인 가족복지정책의 배경요인들을 사회통합적 측면에서 분석해 보고, 이를 통해 독일의 사례가 우리나라의 외국인 가족복지정책에 시사하는 함의를 도출해 보았다. 외국인가족을 위한 사회복지서비스가 미흡하고, 저출산과 인구의 고령화문제에 당면해 있는 우리사회에 독일의 사례는 외국인가족을 위한 자립정착과 지역사회 내 서비스의 확충을 위한 함의를 제공한다.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2014년도 추계학술대회 논문집
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• pp.182-183
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• 2014

Hemispherical dimples with diameter, ø=$60{\mu}m$ and depth, d= $30{\mu}m$ were created on the metal and ceramics surfaces using INYA 10 watt Laser of 1064 nm wavelength. This study reports the influence of dimple pitch on friction and wear behavior rather than dimple size, depth and density. LST was performed on the specimens with dimple pitch and density in the range of 80 to-$200{\mu}m$ and 44 to 7 %, respectively. Surface topography was analyzed by using roughness measurement, scanning electron microscopy (SEM), and optical microscopy. Friction and wear characteristics were analyzed on textured surfaces at lubricating environment to observe the effect of surface texturing on reduction of friction and wear. Reduction on coefficient of friction was achieved by more than 70% due to the dual behavior of dimples as wear (debris) traps and lubricant reservoirs. Wear reduced significantly for the textured surface as compared to the polished surface. Moreover, the friction coefficient of the textured specimens reduced with increasing load and speed which may be attributed to the transition of lubrication regime.

• 한국가시화정보학회지
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• 제18권3호
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• pp.23-34
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• 2020

Two-phase flow and heat transfer characteristics during the reflood phase of a single heated rod in the KHU reflood experimental facility were examined. Two-phase flow behavior during the reflooding experiment was carefully visualized along with transient temperature measurement at a point inside the heated rod. By numerically solving one-dimensional inverse heat conduction equation using the measured temperature data, time-resolved wall heat flux and temperature histories at the interface of the heated rod and coolant were obtained. Once water coolant was injected into the test section from the bottom to reflood the heated rod of >700℃, vast vapor bubbles and droplets were generated near the reflood front and dispersed flow film boiling consisted of continuous vapor flow and tiny liquid droplets appeared in the upper part. Following the dispersed flow film boiling, inverted annular/slug/churn flow film boiling regimes were sequentially observed and the wall temperature gradually decreased. When so-called minimum film boiling temperature reached, the stable vapor film between the heated rod and coolant was suddenly collapsed, resulting in the quenching transition from film boiling into nucleate boiling. The moving speed of the quench front measured in the present study showed a good agreement with prediction by a correlation in literature. The obtained results revealed that typical two-phase flow and heat transfer behaviors during the reflood phase of overheated fuel rods in light water nuclear reactors are well reproduced in the KHU facility. Thus, the verified reflood experimental facility can be used to explore the effects of other affecting parameters, such as CRUD, on the reflood heat transfer behaviors in practical nuclear reactors.

• 동남아시아연구
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• 제23권1호
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• pp.235-276
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• 2013

Recent incidents of lethal violence in the Rakhine State of Myanmar between the majority Buddhist Rakhine and the Muslim Rohingya have been the source of much concern for the international community. Unlike the past, the killings and incendiary attacks by both communities have intensified to a critical level, proving to be a great liability for the forward-thinking Myanmar government, whose recent transition to civilian rule after a long military one has made it eager to move on. The roots of the conflict trace back to the military regime, who branded the Rohingyas living in Rakhine state as illegal immigrants and refused to confer upon them official recognition as Myanmar citizens. The discord then moved to an ethnic conflict, pitting the Rohingya not merely against the Myanmar government but rather the majority Buddhist Rakhine. The conflict, as it has developed into the present, is an immensely complicated one that simultaneously encompasses ethnic and religious issues, all intertwined together. This study aims to see how the two ethnic groups have come to resort to such violence, despite having lived in each other's presence for many centuries, and why the violence persists. It will attempt to reconcile the fact that Rakhine had historically been a place of convergence for two groups, the Buddhist Rakhine and the Rakhine Muslim(the Rohingya). Based on the argument, this study also seeks to uncover, identify, and understand the Rohingya identity with the extreme arguments exhibited by both sides, and from there, locate the underlying causes of the greater religio-ethnic conflict in Rakhine that has so ravaged the place as of recent.

• 한국수소및신에너지학회논문집
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• 제34권3호
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• pp.274-282
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• 2023

This study conducted direct numerical simulations of the natural convection phenomena of gaseous hydrogen in a physical storage container containing four circular cylinders. Rayleigh numbers (Ra) in the range of 10⁴≤Ra≤10⁶ and a Prandtl number (Pr)=0.69 (gaseous hydrogen) were considered. The main parameter is a horizontal distance of four circular cylinders and the values of ε_h=0.1, 0.2, 0.3, 0.4, and 0.5 are considered. The flow and thermal structures and corresponding heat transfer characteristics are investigated with respect to the transition of the flow regime. The time- and surface-averaged Nusselt number on the cylinder surface and the wall of physical storage container increased by about 57% and 69% according to the Ra and ε_h, respectively. Thus, the horizontal distance has an influence on the heat transfer characteristics on natural convection of gaseous hydrogen.

• 의정연구
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• 제27권2호
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• pp.41-70
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• 2021

바이든 행정부의 동북아 정책은 트럼프 행정부의 정책과 어떤 면에서 유사하고 어떤 면에서 다를 것인가? 본 논문은 트럼프 행정부로부터 바이든 행정부로의 이행에 따라 미국의 동북아 정책에 다음과 같은 변화와 연속성이 나타날 것이라고 제시한다. 미국 외교의 전통적 원칙과 관행을 중시하는 바이든 행정부의 성향상, 트럼프 시기와는 달리 미국의 동북아 정책에 있어 높은 안정성과 예측 가능성이 예상된다. 강력한 대중 압박 정책은 바이든 행정부에서도 지속될 것이며, 이에 따라 미중 대립은 더욱 지속·심화될 것이다. 한편 북한에 대해 바이든 행정부는 파격적 '그랜드 바겐'을 시도한 트럼프와는 달리 보수적 상향식 접근법을 취할 것으로 예상되며, 북한 정권의 협상 의도 및 태도에 회의적 관점을 가지고 있어 극적인 북미관계 진전은 당분간 어려워 보인다. 한국과 일본에 대해 바이든은 트럼프와 달리 양국을 동맹으로 중시하고 존중하는 태도를 보일 것이나, 심화되는 미중 갈등 속에서 한일 양국이 중국 견제에 있어 더 큰 역할을 수행하길 요구할 가능성이 높다.