• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.30 no.2
• /
• pp.58-67
• /
• 2018

This study investigated the effect of variation in the angle of the elliptic cylinder as well as the presence of circular cylinder on natural convection inside a square enclosure. The Rayleigh number was varied between $10^3$ and $10^6$, and the Prandtl number was fixed to 0.7. In the present study, the angle of the elliptic cylinder was changed from $0^{\circ}$ to $90^{\circ}$, and the perimeter of the elliptic cylinder was same as that of the circular cylinder. The immersed boundary method was used to capture the virtual wall boundary of the inner cylinder. With the increasing angle of the elliptic cylinder, the surface-averaged Nusselt numbers on the cylinder and the enclosure increased. In the Rayleigh number range considered in the present study, the surface-averaged Nusselt number on the elliptic cylinder over = $45^{\circ}$ was higher than that of the circular cylinder. The effect of elliptic cylinder's angle on natural convection in the enclosure was analyzed according to the flow and thermal fields, and the distributions of the Nusselt number.

• Journal of Korean Society for Atmospheric Environment
• /
• v.27 no.6
• /
• pp.681-691
• /
• 2011

The size distributions of smoke particles from fire are prerequisite for the studies on fire detection and adverse health effects. Above the flame of the fire, coagulation dominates and the smoke particles grow from 1 to 50 nm up to 100 to 3,000 nm, sizes ranging from the free-molecular regime to the continuum regime. The characteristics of the agglomeration of the smoke particles are well known, independently for each of the free-molecular and continuum regimes. However, there are not many systematic studies in the entire regime by the complexity of the mechanisms. The purpose of this work is to find the characteristics of the development of the size distribution of smoke particles by agglomeration in the entire size range covering the free-molecular regime, via transition regime, to the near-continuum and continuum regime for each variation of parameters such as fractal dimension, primary particle size and dimensionless coagulation time. In this work, the dynamic equation for the discrete-size spectrum of the particles was solved using a nodal method based on the modification of a sectional method. In the calculation, the collision frequency function for the entire regime, which is derived by using the concept of collision volume and general enhancement function, was applied. The self-preserving size distribution for the entire regime is compared with the ones for the free-molecular or continuum regimes for each variation of the parameters.

• Korean Journal of Metals and Materials
• /
• v.48 no.5
• /
• pp.436-444
• /
• 2010

The hot tear susceptibility of Al alloys was investigated by using a constrained-rod mold designed to quantify 8 types of tear tendency. The severity of the crack was scored by 5 grades on a scale of 0 to 4, with 0 being "no crack formed" and 4 being "complete separation by crack". The Hot Tear Susceptibility index (HTS) which consists of crack type scores and position scores, was proposed to compare the hot tear tendency of Al alloys. A356.0 cast alloy and AA6061 wrought Al alloy showed an HTS value of 27.5 and 53 respectively. The effects of Si, Cu, and Mg content on hot tear tendency were also investigated with a constrained-rod mold. The variation of HTS values with alloying elements represents similar behavior in the variation of the solidification range in a pseudo binary phase diagram.

• International Journal of Fluid Machinery and Systems
• /
• v.10 no.2
• /
• pp.164-175
• /
• 2017

Francis turbines are often operated over a wide load range due to high flexibility in electricity demand and penetration of other renewable energies. This has raised significant concerns about the existing designing criteria. Hydraulic turbines are not designed to withstand large dynamic pressure loadings on the stationary and rotating parts during such conditions. Previous investigations on transient operating conditions of turbine were mainly focused on the pressure fluctuations due to the rotor-stator interaction. This study characterizes the synchronous and asynchronous pressure and velocity fluctuations due to rotor-stator interaction and rotating vortex rope during load variation, i.e. best efficiency point to part load and vice versa. The measurements were performed on the Francis-99 test case. The repeatability of the measurements was estimated by providing similar movement to guide vanes twenty times for both load rejection and load acceptance operations. Synchronized two dimensional particle image velocimetry and pressure measurements were performed to investigate the dominant frequencies of fluctuations, vortex rope formation, and modes (rotating and plunging) of the rotating vortex rope. The time of appearance and disappearance of rotating and plunging modes of vortex rope was investigated simultaneously in the pressure and velocity data. The asynchronous mode was observed to dominate over the synchronous mode in both velocity and pressure measurements.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.21 no.12
• /
• pp.1083-1089
• /
• 2008

We studied the dielectric characteristics of low-k SiOCH thin films by Ellipsometry. The SiOCH thin films were prepared by deposition of BTMSM precursors on p-Si wafer by CCP-PECVD method. The nano-porous structural organic/inorganic hybrid-type of SiOCH thin films correlated directly to the formation of low dielectrics close to pore(k=1). The structural groups including highly dense pores in SiOCH thin films originated the anisotropic geometry type of network structure directing to complex refractive characteristics of SiOCH single layer on the p-Si wafer. The linearly polarized beam of Xe-ramp in the range from 190 nm to 2100 nm introduced to the surface of SiOCH thin film, and the reflected beam was Elliptically polarized by complex refractive coefficients of SiOCH dipole groups. The amplitude variation $\Psi$ and phase variation $\Delta$ of the relative reflective coefficients between perpendicular and parallel components to the incident plane were measured by Ellipsometry. The complex optical constants n and k as well as the dielectric constant and thickness of SiOCH thin films were driven by the measured value of $\Psi$ and $\Delta$.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
• /
• 2004.05a
• /
• pp.117-122
• /
• 2004

This study investigates the spatial and temporal distribution characteristics and relationships among water quality parameters, which based on 6 years' data(from 1989 to 1994) measured at 16 stations on Jinhae Bay. The results of these analysis, monthly variations range between surface and bottom layer of water quality had a tendency to increase and decrease, and appeared to be at the maximum value in August. The relationships between concentration of COD ana nutrients(DIN and DIP), which obtained by correlation analysis of water quality, were shown $85\%$ and $74\%$, respectively. Using the cluster analysis to develop the division of the sea basin by the dendrogram, before and after dredging of Masan bay and operating a sewage disposal plant, the variation characteristics of water quality of Jinhae Bay were discussed. Through it, we can see the serious pollution of northen sea basin of Jinhae Bay(B2) although dredging Masan bay and operating the sewage disposal plant. As the results, it doesn't appear the improvement effect of water quality in spite of carrying out the effort of water quality improvement.

• Journal of the Korean Society of Visualization
• /
• v.12 no.1
• /
• pp.13-17
• /
• 2014

The growth of micro bubble has been simulated under the variation of ambient pressure. The Rayleigh-Plesset equation governs the dynamic growth and collapse of a bubble according to pressure and temperature conditions. The Rayleigh-Plesset equation was solved by 4th-order Runge-Kutta method for wide range of pressure variations. As numerical parameters, the pressure difference between initial and final pressures, and the temporal pressure gradient are changed. The results show that the pressure difference has little effect on the growth rate of the micro bubble in the inertia controlled growth region. On the other hand, the growth rate increases linearly with the increase of the pressure gradient.

• Korean Journal of Plant Taxonomy
• /
• v.52 no.3
• /
• pp.144-155
• /
• 2022

Daphne kiusiana is an evergreen shrub with dense head-like umbels of white flowers distributed in southern Korea, Japan, China, and Taiwan. Plants in China and Taiwan are recognized as var. atrocaulis by having a dark purple stem, elliptic leaves, and persistent bracts. Recently, plants on Jejudo Island were segregated as a separate species, D. jejudoensis, given their elliptic leaves with an acuminate apex, a long hypanthium and sepals, and a glabrous hypanthium. Morphological variations of three closely related taxa, the D. kiusiana complex, were investigated across the distributional range to clarify the taxonomic delimitation of members of the complex. Twelve characters of the leaf and flower were measured from digitized herbarium specimens using the image analysis program ImageJ and were included in a morphometric analysis, the results of which indicate that the level of variation in the characters is very high. The results of a principal component analysis weakly separated D. jejudoensis from D. kiusiana according to their floral characteristics, such as a longer, glabrous hypanthium, and larger sepals. However, some individuals of D. kiusiana, particularly those from Bigeumdo Island, were included in D. jejudoensis. Recognition of D. kiusiana var. atrocaulis based on the leaf shape was not supported in the analysis, and D. jejudoensis may be recognized as a variety of D. kiusiana. Our morphometric analysis shows that digitized images of herbarium specimens could be useful and an additional method by which to investigate more diverse specimens.

• International Journal of Korean Welding Society
• /
• v.2 no.2
• /
• pp.51-56
• /
• 2002

The weld seam tracking system for arc welding process uses various kinds of sensors such as arc sensor, vision sensor, laser displacement sensor and so on. Among the variety of sensors available, electro-magnetic sensor is one of the most useful methods especially in sheet metal butt-joint arc welding, primarily because it is hardly affected by the intense arc light and fume generated during the welding process, and also by the surface condition of weldments. In this study, a dual-electromagnetic sensor, which utilizes the induced current variation in the sensing coil due to the eddy current variation of the metal near the sensor, was developed for arc welding of sheet metal I-butt joints. The dual-electromagnetic sensor thus detects the offset displacement of weld line from the center of sensor head even though there's no clearance in the joint. A set of design variables of the sensor was determined far the maximum sensing capability through the repeated experiments. Seam tracking is performed by correcting the position of sensor to the amount of offset displacement every sampling period. From the experimental results, the developed sensor showed the excellent capability of weld seam detection when the sensor to workpiece distance is near less than 5 ㎜, and it was revealed that the system has excellent seam tracking ability for the I-butt joint of sheet metal.

• Computers and Concrete
• /
• v.11 no.2
• /
• pp.149-167
• /
• 2013

The complete stress-strain behavior of steel fiber reinforced concrete in compression is needed for the analysis and design of structures. An experimental investigation was carried out to generate the complete stress-strain curve of high-performance steel fiber reinforced concrete (HPSFRC) with a strength range of 52-80 MPa. The variation in concrete strength was achieved by varying the water-to-cementitious materials ratio of 0.40-0.25 and steel fiber content (Vf = 0.5, 1.0 and 1.5% with l/d = 80 and 55) in terms of fiber reinforcing parameter, at 10% silica fume replacement. The effects of these parameters on the shape of stress-strain curves are presented. Based on the test data, a simple model is proposed to generate the complete stress-strain relationship for HPSFRC. The proposed model has been found to give good correlation with the stress-strain curves generated experimentally. Inclusion of fibers into HPC improved the ductility considerably. Equations to quantify the effect of fibers on compressive strength, strain at peak stress and toughness of concrete in terms of fiber reinforcing index are also proposed, which predicted the test data quite accurately. Compressive strength prediction model was validated with the strength data of earlier researchers with an absolute variation of 2.1%.