• Journal of computational fluids engineering
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• v.17 no.4
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• pp.41-48
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• 2012

A numerical analysis of thermal stratification in the upper plenum of the MONJU fast breeder reactor was performed. Calculations were performed for a 1/6 simplified model of the MONJU reactor using the commercial code, CFX-13. To better resolve the geometrically complex upper core structure of the MONJU reactor, the porous media approach was adopted for the simulation. First, a steady state solution was obtained and the transient solutions were then obtained for the turbine trip test conducted in December 1995. The time dependent inlet conditions for the mass flow rate and temperature were provided by JAEA. Good agreement with the experimental data was observed for steady state solution. The numerical solution of the transient analysis shows the formation of thermal stratification within the upper plenum of the reactor vessel during the turbine trip test. The temporal variations of temperature were predicted accurately by the present method in the initial rapid coastdown period (~300 seconds). However, transient numerical solutions show a faster thermal mixing than that observed in the experiment after the initial coastdown period. A nearly homogenization of the temperature field in the upper plenum is predicted after about 900 seconds, which is a much shorter-term thermal stratification than the experimental data indicates. This discrepancy is due to the shortcoming of the turbulence models available in the CFX-13 code for a natural convection flow with thermal stratification.

• Journal of KIISE:Information Networking
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• v.27 no.1
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• pp.98-107
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• 2000

This paper compares the performance of feedback consolidation algorithms with or without a fast overload indication function which has been recently proposed in the ATM Forum for multicast ABR (Available Bit Rate) services in ATM networks. We use the performance criteria such as the ACR (Allowed Cell Rate) of a source, queue length at a switch, link utilization, fairness, and BRM (Backward Resource Management) cell overhead. Simulation results show that performance of the feedback consolidation algorithms with a fast overload indication function can be improved sign.ificantly than that of algorithms without this function in terms of ramp-down delay, particularly in initial start-up period or under a severe overload situation. The fast overload indication function can be combined with any feedback consolidation algorithm, whereas its performance is highly dependent on an underlying basic feedback consolidation algorithm.

• Journal of KIISE:Information Networking
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• v.35 no.1
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• pp.1-10
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• 2008

Once the sensor node in wireless sensor networks is installed, it usually operates without human intervention for a long time. The remote code update scheme is required because it is difficult to recall the sensor node in many situations. Therefore, studies on the reliable and efficient transport protocol for code propagation in wireless sensor networks have been increasingly done. However, by considering only the stability aspect of transmission, most of previous works ignore the consideration on the fast code propagation. This results the energy inefficiency by consuming unnecessary energy due to the slow code propagation. In this paper, in order to overcome limitation of the previous code propagation protocols, we propose a new code propagation protocol called "FCPP(Fast Code Propagation Protocol)". The FCPP aims at improving the reliability at well as performance. For this purpose, the FCPP accomplishes the fast code propagation by using the RTT-based transmission rate control and NACK suppression scheme, which provides a better the network utilization and avoids a unnecessary transmission delay. Based on the ns-2 simulation result, we prove that the FCPP Improves significantly both reliability and performance.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.7 no.6
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• pp.1194-1201
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• 2003

Measurements of network traffic have shown that self-similarity is a ubiquitous phenomenon spanning across diverse network environments. We have advance the framework of multiple time scale congestion control and show its effectiveness at enhancing performance for fast TCP prototype control. In this paper, we extend the fast TCP prototype control framework to window-based congestion control, in particular, TCP. This is performed by interfacing TCP with a large time scale control module which adjusts the aggressiveness of bandwidth consumption behavior exhibited by TCP as a function of "large time scale" network state. i.e., conformation that exceeds the horizon of the feedback loop as determined by RTT. Performance evaluation of fast TCP prototype is facilitated by a simulation bench-mark environment which is based on physical modeling of self-similar traffic. We explicate out methodology for discerning and evaluating the impact of changes in transport protocols in the protocol stack under self-similar traffic conditions. We discuss issues arising in comparative performance evaluation under heavy-tailed workload. workload.

• Journal of information and communication convergence engineering
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• v.10 no.2
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• pp.187-193
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• 2012

A 1 GHz CMOS fast-lock phase-locked loop (PLL) is proposed to support the quick wake-up time of mobile consumer electronic devices. The proposed fast-lock PLL consists of a conventional charge-pump PLL, a frequency-to-digital converter (FDC) to measure the frequency of the input reference clock, and a digital-to-analog converter (DAC) to generate the initial control voltage of a voltage-controlled oscillator (VCO). The initial control voltage of the VCO is driven toward a reference voltage that is determined by the frequency of the input reference clock in the initial mode. For the speedy measurement of the frequency of the reference clock, an FDC with a parallel architecture is proposed, and its architecture is similar to that of a flash analog-to-digital converter. In addition, the frequency-to-voltage converter used in the FDC is designed simply by utilizing current integrators. The circuits for the proposed fast-lock scheme are disabled in the normal operation mode except in the initial mode to reduce the power consumption. The proposed PLL was fabricated by using a 0.18-${\mu}m$ 1-poly 6-metal complementary metal-oxide semiconductor (CMOS) process with a 1.8 V supply. This PLL multiplies the frequency of the reference clock by 10 and generates the four-phase clock. The simulation results show a reduction of up to 40% in the worstcase PLL lock time over the device operating conditions. The root-mean-square (rms) jitter of the proposed PLL was measured as 2.94 ps at 1 GHz. The area and power consumption of the implemented PLL are $400{\times}450{\mu}m^2$ and 6 mW, respectively.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.14 no.1
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• pp.93-99
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• 2014

This paper deals with the FC-MMA (Fast Convergence-Multi Modulus Algorithm) which is improving the convergence characteristics of the MMA (Multiple Modulus Algorithm) adaptive equalization algorithm that is used for the minimization of the intersymbol interference which occurs in the time dispersive communication channel. In the time varying charateristics and the abnormal situation like as outage of the communication channel, the adaptive equalizer needs to adapting the new environment more rapidly. For this problem, the residual isi and the maximum distortion performance index which are meaning the convergence characteristics are widely adapted in the adaptive equalizer. The 16-QAM signal is transmitted and it was confirmed that the proposed algorithm, the FC-MMA has the fast convergence performance such as in the 1.75 times fast in residual isi and 2.5 times fast in the maximum distortion in order to reaching the steady state compare to the MMA algorithm in the same channel environment by the computer simulation.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.46 no.7
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• pp.39-46
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• 2009

A fast-switching current-pulse driver for light emitting diode (LED) backlight is proposed. It uses a regulated drain current mirror (RD-CM) [1] and a high-voltage NMOS transistor (HV-NMOS). It achieves the fast-response current-pulse switching by using a dynamic gain-boosting amplifier (DGB-AMP). The DGB-AMP does not discharge the large HV-NMOS gate capacitance of the RD-CM when the output current switch turns off. Therefore, it does not need to charge the HV-NMOS gate capacitance when the switch turns on. The proposed current-pulse driver achieves the fast current switching by removing the repetitive gate discharging and charging. Simulation results were verified with measurements performed on a fabricated chip using a 5V/40V 0.5um BCD process. It reduces the switching delay to 360ns from 700ns of the conventional current-pulse driver.

• Nuclear Engineering and Technology
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• v.51 no.8
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• pp.1871-1885
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• 2019

The deterministic MOC code STREAM of the Computational Reactor Physics and Experiment (CORE) laboratory of Ulsan National Institute of Science and Technology (UNIST), was initially designed for the calculation of pressurized water reactor two- and three-dimensional assemblies and cores. Since fast reactors play an important role in the generation-IV concept, it was decided that the code should be upgraded for the analysis of fast neutron spectrum reactors. This paper presents a coupled code - TULIP/STREAM, developed for the fast reactor assembly and core calculations. The TULIP code produces self-shielded multi-group cross-sections using a one-dimensional cylindrical model. The generated cross-section library is used in the STREAM code which solves eigenvalue problems for a two-dimensional assembly and a multi-assembly whole reactor core. Multiplication factors and steady-state power distributions were compared with the reference solutions obtained by the continuous energy Monte-Carlo code MCS. With the developed code, a sensitivity study of the number of energy groups, the order of anisotropic P_N scattering, and the multi-group cross-section generation model was performed on the k_eff and power distribution. The 2D core simulation calculations show that the TULIP/STREAM code gives a k_eff error smaller than 200 pcm and the root mean square errors of the pin-wise power distributions within 2%.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.37 no.3
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• pp.33-43
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• 2000

The guarantee of the fast dynamics stability is essential for successful application of singular Perturbation technique to control systems design. Even though the fast dynamics of the control systems is rendered stable by an analog controller, the fast dynamics stability of the control systems resulted from an digital implementation of the analog controller can be impaired severely. In this paper, we first investigate the time sampling effects on singular perturbation based control systems by centering on a design example of recently developed singular perturbation based STT missile autopilot with high performance. The investigation shows that the stability margin the fast dynamics of the STT misile autopilot system decreases rapidly as the sampling interval of discretizing the analog autopilot increases. Under this analysis, we propose a composite digital controller with compensation for the decreasing stability margin of the fast dynamics due to time sampling to achieve better performance with respect to sampling time. The improved performance of the proposed composite digital controller is verified by simulation. This result shows that one needs to investigate time sampling effects in the digital implementation of singular perturbation based controllder, and then can have benefit from the investigation.

• New & Renewable Energy
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• v.7 no.4
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• pp.50-57
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• 2011

A wind turbine simulation program for the coupled dynamics of aerodynamics, elasticity, multi-body dynamics and controls of turbine is newly developed by combining an aero-elastic code and a multi-body dynamics code. The aero-elastic code, based on the blade momentum theory and generalized dynamic wake theory, is developed by NREL(National Renewable Energy Laboratory, USA). The multi-body dynamics code is commercial one which is capable of accounting for geometric nonlinearity and twist deflection. A turbulent wind load case is simulated for the NREL 5-MW baseline wind turbine model by the developed program and FAST. As a result, the two results agree well enough to verify the reliability of the developed program.