• Journal of the Korean Mathematical Society
• /
• v.38 no.1
• /
• pp.1-23
• /
• 2001

This paper is concerned with an optimal shape control problem for the stationary Navier-Stokes system. A two-dimensional channel flow of an incompressible, viscous fluid is examined to determine the shape of a bump on a part of the boundary that minimizes the viscous drag. by introducing an artificial compressibility term to relax the incompressibility constraints, we take the penalty method. The existence of optima solutions for the penalized problem will be shown. Next, by employing Lagrange multipliers method and the material derivatives, we derive the shape gradient for the minimization problem of the shape functional which represents the viscous drag.

• Journal of Communications and Networks
• /
• v.8 no.2
• /
• pp.220-227
• /
• 2006

In this paper, we present a novel approach to analyze the throughput of direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system. In the unslotted system, the departure rate of interfering transmissions is proportional to the number of current interferers that can be regarded as the system state. In order to model this state-dependency, we introduce a two-dimensional state transition model that describes the state transition of the system. This model provides a more rigorous analysis tool for the DS/SSMA unslotted ALOHA systems with both fixed and variable packet lengths. Numerical results reveal that this analysis yields an accurate system performance that coincides with the simulation results. Throughout the analysis we have discovered that the state-dependency of the departure rate causes interference averaging effect in the unslotted system and that this effect yields a higher throughput for the unslotted system than for the slotted system when supported by a strong channel coding.

• Journal of the Korean Institute of Telematics and Electronics A
• /
• v.31A no.1
• /
• pp.54-60
• /
• 1994

In this paper, we present the analytical models for the change of the lateral electric field distribution and the velocity saturation region length with the electron trapping of stressed SC-PMOSFET in the saturation region. To derive the hot-electron-induced lateral electric field of stressed SC-PMOSFET. Ko's pseudo two dimensional box model in the saturation region which illustrates the analysis of the velocity saturation region is modified under the condition of electron trapping in the oxide near the drain region. From the results, we have the following lateral electric field in the y-direction, that is, E(y) ES1satT.cosh(y/l) qNS1tT.sinh(y/l)/lCox. It is shown that the trapped electrons influence the field in the drain region. decreasing the lateral electric field. Calculated velocity saturaion length increases with the trapped electrons. increasing the drain current of stressed SCPMOSFET. This results well explain the HEIP phenomenon of PMOSFET's.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.17 no.8
• /
• pp.799-805
• /
• 2004

Two dimensional MEDICI simulator is used to study the characteristics of latch-up current of Dual Gate Emitter Switched Thyristor. The simulation is done in terms of the current-voltage characteristics, latch-up current density, ON-voltage drop and electrical property with the variations of p-base impurity concentrations. Compared with the other power devices such as MOS Controlled Cascade Thyristor(MCCT), Conventional Emitter Switched Thyristor(C-EST) and Dual Channel Emitter Switched Thyristor(DC-EST), Dual Gate Emitter Switched Thyristor(DG-EST) shows to have the better electrical characteristics, which is the high latch-up current density and low forward voltage-drop. The proposed DG-EST which has a non-planer p-base structure under the floating $N^+$ emitter indicates to have the better characteristics of latch-up current and breakover voltage.

• Journal of IKEEE
• /
• v.17 no.4
• /
• pp.531-536
• /
• 2013

The impact of UV irradiation process on the AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistor was investigated. Due to the high intensity UV irradiation before the gate dielectric deposition, the conductivity of AlGaN/GaN structure and the drain saturation current of the transistor increased by about 10 %. However, the pinch off characteristics of transistor was severely deformed by the process. By comparing the electrical characteristics of the transistors, it was proposed that the high intensity UV irradiation formed a sub-channel under the two dimensional electron gas of AlGaN/GaN structure even without additional impurity injection.

• 유체기계공업학회:학술대회논문집
• /
• 2006.08a
• /
• pp.493-496
• /
• 2006

Rarefied gas flows through two-dimensional micro channels are studied numerically for the performance optimization of a nanomembrane-based Knudsen compressor. The effects of the wall temperature distributions on the thermal transpiration flow patterns are examined. The flow has a pumping effect, and the mass flow rates through the channel are calculated. The results show that a steady one-way flow is induced for a wide range of the Knudsen number. The DSMC(direct simulation Monte Carlo) method with VHS(variable hard sphere) model and NTC(no time counter) techniques has been applied in this work to obtain numerical solutions.

• Journal of the Korean Institute of Telematics and Electronics A
• /
• v.33A no.2
• /
• pp.131-139
• /
• 1996

Inthis apper wa analyzed the channel-hot-electron programming characteristics of the single-poly EEPROM with different control gate and drain structures. The single-poly EEPROM uses the p$^{+}$/n$^{+}$-diffusion in the n-well as a control gate instead of the second poly-silicon. The program and erase characteristics of the single-poly EEPROM were verified using the two-dimensional device simulator, MEDICI. The single-poly EEPROM was fabricated using 0.8$\mu$m ASIC CMOS process, and its CHE programming characteristics were measured using HP4155 parameteric analyzer and HP8110 pulse gnerator. Especially we investigated the CHE programming characteristics of the single-poly EEPROM with the p$^{+}$-diffusion or n$^{+}$-diffusion in the n-well as a control gate and the LDD or single-drain structure. The single-poly EEPROM with p$^{+}$-diffusion in the n-well as a control gate and single-drain structure was programmed to about VT$\thickapprox$5V with VDS=6V, VCG=12V(1ms pulse width).th).

• 한국전산유체공학회:학술대회논문집
• /
• 2003.10a
• /
• pp.237-239
• /
• 2003

The present study investigates on design optimization of rib-roughened two-dimensional channel to enhance turbulent heat transfer. Response surface method with Reynolds-averaged Navier-Stokes analysis is used as an optimization technique. Standard $k-{\varepsilon}$model with wall functions is adopted as a turbulence closure. The objective function is defined as a linear combination of heat transfer and friction drag coefficients with weighting factor. Computational results for overall heat transfer rate show good agreements with experimental data. Four design variables are optimized for weighting factor of 0.02.

• Nuclear Engineering and Technology
• /
• v.44 no.1
• /
• pp.79-88
• /
• 2012

Migration tests were performed with conservative tracers in a fractured zone that had a single fracture of about 2.5 m distance at the KURT. To interpret the migration of the tracers in the fractured rock, a solute transport model was developed. A two dimensional variable aperture channel model was adopted to describe the fractured path and hydrology, and a particle tracking method was used for solute transport. The simulation tried not only to develop a migration model of solutes for open flow environments but also to produce ideas for a better understanding of solute behaviours in indefinable fracture zones by comparing them to experimental results. The results of our simulations and experiments are described as elution and breakthrough curves, and are quantified by momentum analysis. The main retardation mechanism of nonsorbing tracers, including matrixdiffusion, was investigated.

• Journal of the Korean Society of Visualization
• /
• v.6 no.2
• /
• pp.28-32
• /
• 2008

Contrary to the flow rate in fully filled pipe flows, the flow rate in partially filled pipe flows is significantly influenced by the variation of water level, channel slop, and so on. The major difference in these two flows results from the existence of a free surface. To make it clear, in the present study, a similarity of the velocity profile in a partially filled circular pipe has been investigated according to the water level. A particle image velocimetry (PIV) technique was applied to measure the three-dimensional velocity profiles. As a result, there is found a similarity of the velocity profile near the central region. However, near the side wall, the similarity is broken due to the interaction between the wall and the free surface.