• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.12A
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• pp.1356-1366
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• 2004

Recently, the IEEE 802.11 working group has announced a new distributed MAC called EDCF to provide service differentiation among traffic classes. With the increasing demand for supporting Quality of Service in IEEE 802.11 wireless LANs, the EDCF is now attracting many researchers' attention due to its practical worth as a standard mechanism. In this paper, we focus on the analytical approach to evaluate the performance of the EDCF. An analytical model is developed to estimate the throughput of the EDCF in saturation (asymptotic) conditions. Extensive simulation studies have been carried out for the validation of the analysis, and they show that it estimates the throughput of the EDCF accurately By utilizing the analytical model, we evaluate the performance of the EDCF. Specifically, we concentrate on discovering the characteristics of the EDCF Parameters, such as CW/Sub min/, CW/Sub max/ and AIFS, in the way that they influence on the performance of the EDCF.

• Progress in Superconductivity and Cryogenics
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• v.19 no.2
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• pp.25-28
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• 2017

In superconducting coil applications particularly in wet wound coils, coated conductor (CC) tapes are subjected to different type of stresses that could affect its electromechanical transport property. These include hoop stress acting along the length of the CC tape and the Lorentz force acting perpendicular to the CC tape's surface. Since the latter is commonly associated with the delamination problem of multi-layered REBCO CC tapes, more understanding and attention on the delamination phenomena induced in the case of coil applications are needed. Difference on the coefficient of thermal expansion (CTE) of each constituent layer of the CC tape, the bobbin, and the impregnating materials is the main causes of delamination in CC tapes when subjected to thermal and mechanical cycling. In the design of degradation-free superconducting coils, therefore, characterization of the delamination behaviors including mechanism and strength in the multi-layered REBCO CC tapes becomes a critical issue. Various trials to increase the delamination strength by improving interface characteristics at interlayers have been performed. In this study, in order to investigate the influences of laser cleaning and Ag annealing treated at the substrate side surface, transverse tensile tests were conducted under different sample configurations using $4.5mm{\times}8mm$ upper anvil. The mechanical delamination strength of differently processed CC samples was examined at room temperature (RT). As a result, the Sample 1 with the additional laser cleaning and Ag annealing processes and the Sample 2 with additional Ag annealing process only showed higher mechanical delamination strength as compared to the Sample 3 without such additional treatments. Sample 3 showed quite different behavior when the loading direction is to the substrate side where the delamination strength much lower as compared to other cases.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.48 no.12
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• pp.761-767
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• 1999

Poly-Si TFTs(polycrystalline silicon thin film transistors) fabricated on a low cost glass substrate have attracted a considerable amount of attention for pixel elements and peripheral driving circuits in AMLCS(active matrix liquid crystal display). In order to apply poly-Si TFTs for high resolution AMLCD, a high operating frequency and reliable circuit performances are desired. A new poly-Si TFT with CLBT(counter doped lateral body terminal) is proposed and fabricated to suppress kink effects and to improve the device stability. And this proposed device with BC(buried channel) is fabricated to increase ON-current and operating frequency. Although the troublesome LDD structure is not used in the proposed device, a low OFF-current is successfully obtained by removing the minority carrier through the CLBT. We have measured the dynamic properties of the poly-Si TFT device and its circuit. The reliability of the TFTs and their circuits after AC stress are also discussed in our paper. Our experimental results show that the BC enables the device to have high mobility and switching frequency (33MHz at $V_{DD}$ = 15 V). The minority carrier elimination of the CLBT suppresses kink effects and makes for superb dynamic reliability of the CMOS circuit. We have analyzed the mechanism in order to see why the ring oscillators do not operate by analyzing AC stressed device characteristics.

• Composites Research
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• v.12 no.6
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• pp.22-30
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• 1999

Since the surface diffusion processed $Nb_3Sn$ superconducting tape has the advantage of having large overall critical current density. it is used for the construction of open type MRI superconducting magnets. However one of the disadvantages of this tape is that $Nb_3Sn$ compound often exhibited multiple cracking due to its intrinsic brittleness when subjected to mechanical loading such as bending and winding during the fabrication process for superconducting coil. This will eventually cause the severe degradation of critical current density. Therefore it is important to understand the microscopic deformation behavior of this kind of superconducting tape under the mechanical loading.In this study, acoustic emission(AE) was used to clarify microscopic deformation behavior at room temperature for $Nb_3Sn$ superconducting tape which was strengthened and stabilized with copper. For this purpose, special attention was paid to AE characteristics including AE event, energy, and amplitude distribution which were associated with microscopic mechanism of deformation of $Nb_3Sn$ superconducting tape under tensile load.

• BMB Reports
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• v.39 no.5
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• pp.560-570
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• 2006

Mistletoe (Viscum album) lectins, which are classified as a type II ribosome-inactivating protein (RIP) due to their unique biological function and the potential medical and therapeutic application in cancer cells, receive a rising attention. The heterodimeric glycoproteins contain the A-chain with catalytic activity and the B-chain with sugar binding properties. In recent years, studies involving the lectins from the white berry European mistletoe (Viscum album) and the yellow berry Korean mistletoe (Viscum album coloratum) have been described. However, the detailed mechanism in exerting unique cytotoxic effect on cancer cells still remains unclear. Here, we aim to understand and define the molecular basis and biological effects of the type II RIPs, through the studies of the recombinant Korean mistletoe lectin. To this end, we expressed, purified the recombinant Korean mistletoe lectin (rKML), and investigated its molecular characteristics in vitro, its cytotoxicity and ability to induce apoptotic cell death in cancer cells. To gain structural basis for its catalytic activity and sugar binding properties, we performed homology modeling studies based on the high degree of sequence identity and conserved secondary structure prediction between Korean and European, Himalayan mistletoe lectins, and Ricin.

• Journal of Physiology & Pathology in Korean Medicine
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• v.34 no.2
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• pp.81-87
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• 2020

Cognitive impairment is symptoms of dementia, a degenerative brain disease that is drawing attention in a rapidly aging society. This study was conducted to investigate the improvement of cognitive function of Leonurus japonicus on scopolamine-induced memory impairment in mice and the effect and mechanism of memory recovery. In vivo studies were conducted on mice orally pretreated with L. japonicus in doses of 50, 100 and 200 mg/kg (p.o.) and scopolamine (1 mg/kg, i.p.) were injected 30 min before the behavioral task. Antioxidant activity was assessed by 2,2-diphenyl-1-picryl hydrazyl (DPPH) assay and 2,2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay, and acetylcholinesterase (AChE) inhibition activity evaluated by Ellman's method. In behavior studies showed that L. japonicus has an improved the memory of scopolamine-treated mice in Y-maze, passive avoidance and Morris water maze test. In addition, L. japonicus was also exerted free radical scavenging activity and inhibited acetyl cholinesterase activity. These results suggest that L. japonicus improves short-term and long-term memory in scopolamine-induced memory decline model and prevents scopolamine-induced memory impairments through in reduced oxidative stress and acetyl cholinesterase inhibition effect. Thus, L. japonicus is related to functional medicinal materials for prevention and treatment of human dementia patients.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.5
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• pp.742-749
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• 2002

Numerical Study with detailed chemistry has been conducted to investigate the flame structure and NOx formation characteristics in oxygen -enhanced(CH$_4$/O$_2$-$N_2$) and oxygen-enhanced-EGR(CH$_4$/O$_2$-$CO_2$) counter diffusion flame with various strain rates. A small amount of $N_2$is included in oxygen-enhanced-EGR combustion, in order to consider the inevitable $N_2$contamination by $O_2$production process or air infiltration. The results are as follows : In CH$_4$/O$_2$-$CO_2$flame it is very important to adopt a radiation effect precisely because the effect of radiation changes flame structure significantly. In CH$_4$/O$_2$-$N_2$flame special strategy to minimize NO emission is needed because it is very sensitive to a small amount of $N_2$. Special attention is needed on CO emission by flame quenching, because of increased CO concentration. Spatial NO production rate of oxygen-enhanced combustion is different from that of air and oxygen-enhanced-EGR combustion in that thermal mechanism plays a role of destruction as well as production. In case CH$_4$/O$_2$-$CO_2$flame contains more than 40% $CO_2$it is possible to maintain the same EINO as that of CH$_4$/Air flame with accomplishing higher temperature than that of CH$_4$/Air flame. EINO decreases with increasing strain rate, and those effects are augmented in CH$_4$/O$_2$flame.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.185-190
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• 2001

Numerical Study with detailed chemistry has been conducted to investigate the flame structure and NOx formation characteristics in oxygen-enhanced$(CH_4/O_2-N_2)$ and oxygen-enhanced-EGR$(CH_4/O_2-CO_2)$ counter diffusion flame with various strain rates. A small amount of $N_2$ is included in oxygen-enhanced-EGR combustion, in order to consider the inevitable $N_2$ contamination by $O_2$ production process or air infiltration. The results are as follows : In $CH_4/O_2-CO_2$ flame it is very important to adopt a radiation effect precisely because the effect of radiation changes flame structure significantly. In $CH_4/O_2-N_2$ flame special strategy to minimize NO emission is needed because it is very sensitive to a small amount of $N_2$. Special attention is needed on CO emission by flame quenching, because of increased CO concentration. Spatial NO production rate of oxygen-enhanced combustion is different from that of air and oxygen-enhanced-EGR combustion in that thermal mechanism plays a role of destruction as well as production. In case $CH_4/O_2-CO_2$ flame contains more than 40% $CO_2$ it is possible to maintain the same EINO as that of $CH_4/Air$ flame with accomplishing higher temperature than that of $CH_4/Air$ flame. EINO decreases with increasing strain rate, and those effects are augmented in $CH_4/O_2$ flame. Complementary study is needed with extending the range of strain rate variation.

• Wind and Structures
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• v.23 no.2
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• pp.157-169
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• 2016

The structural integrity of tube bundles represents a major concern when dealing with high risk industries, such as nuclear steam generators, where the rupture of a tube or tubes will lead to the undesired mixing of the primary and secondary fluids. Flow-induced vibration is one of the major concerns that could compromise the structural integrity. The vibration is caused by fluid flow excitation. While there are several excitation mechanisms that could contribute to these vibrations, fluidelastic instability is generally regarded as the most severe. When this mechanism prevails, it could cause serious damage to tube arrays in a very short period of time. The tubes are therefore stiffened by means of supports to avoid these vibrations. To accommodate the thermal expansion of the tube, as well as to facilitate the installation of these tube bundles, clearances are allowed between the tubes and their supports. Progressive tube wear and chemical cleaning gradually increases the clearances between the tubes and their supports, which can lead to more frequent and severe tube/support impact and rubbing. These increased impacts can lead to tube damage due to fatigue and/or wear at the support locations. This paper presents simulations of a loosely supported multi-span U-bend tube subjected to turbulence and fluidelastic instability forces. The mathematical model for the loosely-supported tubes and the fluidelastic instability model is presented. The model is then utilized to simulate the nonlinear response of a U-bend tube with flat bar supports subjected to cross-flow. The effect of the support clearance as well as the support offset are investigated. Special attention is given to the tube/support interaction parameters that affect wear, such as impact and normal work rate.

• Wind and Structures
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• v.23 no.2
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• pp.91-107
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• 2016

Flow-excited acoustic resonance in ducted cavities can produce high levels of acoustic pressure that may lead to severe damage. This occurs when the flow instability over the cavity mouth, which is created by the free shear layer separation at the upstream edge, is coupled with one of the acoustic modes in the accommodating enclosure. Acoustic resonance can cause high amplitude fluctuating acoustic loads in and near the cavity. Such acoustic loads could cause damage in sensitive applications such as aircraft weapon bays. Therefore, the suppression and mitigation of these resonances are very important. Much of the work done in the past focused on the fluid-dynamic oscillation mechanism or suppressing the resonance by altering the edge condition at the shear layer separation. However, the effect of the downstream edge has received much less attention. This paper considers the effect of the impingement edge geometry on the acoustic resonance excitation and Strouhal number values of the flow instabilities in a ducted shallow cavity with an aspect ratio of 1.0. Several edges, including chamfered edges with different angles and round edges with different radii, were investigated. In addition, some downstream edges that have never been studied before, such as saw-tooth edges, spanwise cylinders, higher and lower steps, and straight and delta spoilers, are investigated. The experiments are conducted in an open-loop wind tunnel that can generate flows with a Mach number up to 0.45. The study shows that when some edge geometries, such as lower steps, chamfered, round, and saw-tooth edges, are installed downstream, they demonstrate a promising reduction in the acoustic resonance. On the other hand, higher steps and straight spoilers resulted in intensifying the acoustic resonance. In addition, the effect of edge geometry on the Strouhal number is presented.