• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.05a
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• pp.63-70
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• 2002

Wear on the tube-to-spring contact is investigated experimentally. The wear is caused by the vibration of the tube while the springs support it. As for the supporting conditions, applied are the contacting normal force (P) of 5 N, just-contact (P = 0 N) and the gap of 0.1 mm. The gap condition is tried far considering the influence of simultaneous impacting and sliding on wear. Results show that the wear volume increases in the order of the gap, the just-contact and the 5 N conditions. This is explained from the contact geometry of the spring, which is convex of smooth contour. The contact shear force is regarded smaller in the case of the gap existence compared with the other conditions. Wear mechanism is considered from SEM observation of the worn surface. The variation of the normal contact traction is analysed using the finite element analysis to estimate the slip displacement range on the contact with consulting the fretting map previously obtained.

• Transactions of Materials Processing
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• v.26 no.1
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• pp.11-17
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• 2017

In this study, process analysis of elbow-shaped tubes using a mandrel has been performed. To reach the final shape within the dimensional tolerance, the process analysis has been performed at various processing parameters such as tube dimensions, the curved cutting surface and the radius of curvature. The area outside the boundary of the target shape was expressed as a quantitative index to analyze the formability. The validation experiments have also been performed in order to increase the reliability of the process analysis. For the processing of elbow-shaped tubes, it is preferable to make the angle of the portion where the punch touches the tube smaller than the opposite angle. And the convex cutting surface is advantageous due to the increased contacts between the punch and the tube ends during the bending process. Elbow tube having larger radius of curvature shows higher dimensional accuracy due to the relatively uniform strain distribution.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.05a
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• pp.73-79
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• 2002

Fretting wear test in room temperature air was performed to evaluate the wear mechanism of fuel rod using a fretting wear tester, which has been developed for experimental study. The main focus was to compare the wear behaviors of fuel rod against support springs at different contact geometries (i.e. concave and convex) and slip directions (axial and transverse). The wear on the tube was examined by the surface roughness tester, which measures the volume. The result indicated that with change of contact geometry from 5N of normal load to 0.1mm gap, wear volume of tube Increased in the condition of concave spring, but slowly decreased in convex spring. From the results of SEM observation, wear mechanism of each test condition was also depend on the above contact parameters. The wear mechanism of each test condition in room temperature air is discussed.

• Nuclear Engineering and Technology
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• v.26 no.3
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• pp.440-452
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• 1994

Nuclear fuel bundles are designed such that the heat flux at a-fuel pin surface should not exceed the critical heat flux (CHF) during normal operation and anticipated transient. Therefore, evaluation of the CHF for fuel bundle is demanded in an exact and reliable manner. One of the major concerns with the current application of CHF correlations is that the CHF based on circular tubes is applied to the fuel bundle subchannel analysis, mainly in terms of the hydraulic diameter with correction factors which may result in a source of possibly large uncertainties in CHF prediction. The hydraulic diameter does not recognize the local properties of fluid nor such effect as the surface curvature; the turbulence action on the convex surface is much more pronounced than that on the concave surface. Even for the tube having concave curvature, the effect of tube diameter on CHF becomes important with decreasing diameter. These facts imply that the convex curvature effect is significant and crucial to the reliable CHF prediction. This paper reviews and discusses analytical and experimental aspects of effect of transverse convex curvature in incompressible turbulent flow and heat transfer, and on CHF. Flow models to quantify this effect are briefly mentioned and future works are recommended.

• Journal of the Korean Society of Visualization
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• v.1 no.1
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• pp.82-91
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• 2003

The study of swirl flow has been of technical and scientific interest because it has an internal recirculation field and its tangential velocity is related to the curvature of the streamline. The fluid flow for ducts or elbows of an internal engine has been much studied through numerical methods and experiments, but studies about swirl flow has been insufficient. Using the PIV (Particle Image Velocimetry) method, this study found the time-mean velocity distribution, time-mean turbulent intensity, with swirl and without swirl flow for Re=10,000, 15,000, 20,000, and 25,000 along longitudinal sections and the results appear to be physically reasonable. In addition, axial velocity distribution is compared with that of Jeong's, Kodadadi's and Murakami's. It was found that the highest velocity of swirl and non-swirl flow occurs in the opposite position at the center of a round tube, $\phi$=45$^{\circ}$

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.11
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• pp.1473-1480
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• 1999

In the case of Impingement of plane moving shock wave over concave or convex double wedges (pseudo-stationary flow) and cylindrical walls (truly non-stationary flow), it Is expected that there are transitions from regular reflection to Mach reflection or vice versa In shock wave reflections. In these connections, it is necessary to verify the various of reflection process and transition angle for the reflection problems In double wedges, and to verify the transition angle, effects of curvature radius and initial wall angle on it for the reflection problems In cylindrical walls. Especially, we focused our attention to confirm the existence of hysteresis phenomenon induced by the different transition processes, and Neumann paradox, which is a small discrepancy between theoretical and experimental transition angles. Experiments were carried out by using the shock tube of $6{\times}6cm^2$, and high speed photographic technique consisted of delay unit, triggering system, light source of Xe lamp and so on was used for flow visualization.

• Korean Journal of Optics and Photonics
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• v.6 no.1
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• pp.21-25
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• 1995

Q-switched Waveguide $CO_{2}$ laser using polygon mirror were constructed and their output characteristics were measured. Waveguide was made of the beryllium oxide tube of inner diameter of 2 mm, outer diameter of 20 mm, and length of 200 mm. A flat ZnSe output coupler of 80% reflectivity was placed on the 9 mm distance from the end of the waveguide. Convex lens was used to reduce the coupling loss between the polygon mirror and the waveguide. The polygon mirror was located behind of convex lens. With this condition, continuous output of the laser were measured as 1.2 W. In the Q-switched mode. pulse width 120 ns were measured with rotational speed 7,559 RPM. and peak power of up to 250 W. 250 W.

• Journal of the Ergonomics Society of Korea
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• v.22 no.3
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• pp.27-44
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• 2003

An experiment was carried out to compare the suitability in visual tasks between flat and conventional (convex) cathode-ray tube (CRT) displays. The subjects performed visual search tasks during 2-h for detecting target words among distracters presented on the screen. The subjects' visual performance was evaluated with average time and number of errors made to complete the tasks. Visual fatigue after the search tasks was also evaluated in terms of degradations in accommodative power and subjective ratings. Difference was not found in task time between the two displays, but flat CRT showed a lower number of errors than conventional CRT. The difference in number of errors was statistically significant at 0=0.05. Although there was no difference between the displays in degradations of accommodative power, results from the subjective ratings showed that flat CRT yields less fatigue than conventional CRT. The results partially support the hypothesis that panel convexity of CRT displays has a significant effect on the performance and fatigue during visual tasks and thus flat CRT is the better display than conventional one.

• Journal of Advanced Marine Engineering and Technology
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• v.27 no.5
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• pp.648-656
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• 2003

An experimental research was performed to study turbulent flow characteristic in a $90^{\circ}$ circular bend by using the PIV(Particle Image Velocimetry) method, this study found the time-mean velocity distribution, time-mean turbulent intensity with turbulent flow for Re = 10,000, 15,000, 20,000, and 25,000 along the test bend. It was found that the highest streamwise velocity of turbulent flow occurs near y/D = 0.5 and the flow moved to y/D =0.15. The peak turbulence intensity shifted toward the concave wall from $\theta= 45 and as \theta$ increased. the intensity decayed along the test tube.

• Bulletin of the Korean Mathematical Society
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• v.54 no.2
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• pp.543-557
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• 2017

In this paper, we investigate the notion of q-pseudoconvexity to discuss and describe some geometric characterizations of q-pseudoconvex domains ${\Omega}{\subset}{\mathbb{C}}^n$. In particular, we establish that ${\Omega}$ is q-pseudoconvex, if and only if, for every boundary point, the Levi form of the boundary is semipositive on the intersection of the holomorphic tangent space to the boundary with any (n-q+1)-dimensional subspace $E{\subset}{\mathbb{C}}^n$. Furthermore, we prove that the Kiselman's minimum principal holds true for all q-pseudoconvex domains in ${\mathbb{C}}^p{\times}{\mathbb{C}}^n$ such that each slice is a convex tube in ${\mathbb{C}}^n$.