• Journal of Energy Engineering
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• v.24 no.2
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• pp.84-90
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• 2015

Hydrogen has some remarkably high values of the key properties for transport processes, such as kinematic viscosity, thermal conductivity and diffusion coefficient. Hydrogen, as an energy medium, has some distinct benefits for its high efficiency and convenience in storage, transportation and conversion. Hydrogen has much wider limits of flammability in air than methane, propane or gasoline and the minimum ignition energy is about an order of magnitude lower than for other combustibles. Statistical thermodynamics provides the relationships that we need in order to bridge this gap between the macro and the micro. Our most important application will involve the calculation of the thermodynamic properties of the ideal gas.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.47-55
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• 2003

This paper Presents active control algorithm using probability density function of structural energy. It is assumed that the structural energy under excitation has Rayleigh probability distribution. This assumption is based on the fact that Rayleigh distribution satisfies the condition that the structural energy is always positive and the occurrence probability of minimum energy is zero. The magnitude of control force is determined by the probability that the structural energy exceeds the specified target critical energy, and the sign of control force is determined by Lyapunov controller design method. Proposed control algorithm shows much reduction of peak responses under seismic excitation compared to LQR controller, and it can consider control force limit in the controller design. Also, chattering problem which sometimes occurs in Lyapunov controller can be avoided.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1995.04b
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• pp.183-186
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• 1995

A simulation method basee on upper bound method is developed in order to characterize forging characters in forging of spur gears. In this paer, utlizing a kinematically admissible velocity field and applying it to study the effect of inner diameter of holow billet. To predict the variation of inner diameter of hollow billet, neutal surface has been introduced into forging of hollow gears from hollow billes with flat punch. The neutral surface of each step is assumed as a circle and determined in order to have minimum forging energy by golden section method. According to the analysis, the magnitude of inner diameter of initial billet is vary important to reduce the relative pressure and forging load. And the variation of inner diameter of billet during spur gear forging is successfully predicted.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05a
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• pp.771-776
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• 1995

Tensile and J-R tests were carried out to estimate the effects of dynamic strain aging(DSA) on SA106Gr.C piping steel. Tensile tests were performed under temperature range RT to $400^{\circ}C$ md strain rates from $1.39{\times}10^{-4}\;to\;6.95{\times}10^{-2}/s$. Fracture toughness was tested in the temperature range RT to $350^{\circ}C$ and load-line displacement rates 0.4 and 4mm/min. The effects of DSA on the tensile properties were clearly observed for phenomena such serrated flow, variation of ultimate and yield stress, and negative stram rate sensitivity. However, the magnitude of serration and strength increase by DSA was relatively small. this may be due to high ratio of Mn to C. In addition, crack initiation resistance, Ji and crack growth resistance, dJ/da were reduced in the range of $200-300^{\circ}C$, where DSA appeared as serrated flow and UTS hardening. The temperature corresponding to minimum fracture resistance was shifted to higher temperature with increasing loading rate.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.12
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• pp.3127-3135
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• 1994

An approach to the simultaneous optimal design of structure and control system for large free-free flexible beam is presented. The flexible beam is modeled by the finite element method. And the reduced model of small degree of freedom is constructed by use of modal analysis. The tapered beam is considered so that the number of design variables is not dependent on the increasing number of finite elements. The width of several points of tapered beam and control gain are taken as design variables. The shape of beam and control gain are optimized simultaneously for the minimum weight of total structure including control system subject to the constraints of the magnitude of displacement of beam. It is shown that the simultaneous optimal design of structure and control systems is indeed useful.

• Journal of electromagnetic engineering and science
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• v.17 no.1
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• pp.39-43
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• 2017

In this paper, a systematic and statistical calibration technique was implemented to calibrate a high-speed signal converting system containing multiple digital-to-analog converters (DACs). The systematic error (especially the imbalance between DACs) in the current combining network of the multi-DAC system was modeled and corrected by calculating the path coefficients for individual DACs with wideband reference signals. Furthermore, by applying a Kalman filter to suppress noise from quantization and clock jitter, accurate coefficients with minimum noise were identified. For correcting an arbitrary waveform generator with two DACs, a co-simulation platform was implemented to estimate the system degradation and its corrected performance. Simulation results showed that after correction with 4.8 Gbps QAM signal, the signal-to-noise-ratio improved by approximately 4.5 dB and the error-vector-magnitude improved from 4.1% to 1.12% over 0.96 GHz bandwidth.

• Journal of Hydrogen and New Energy
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• v.30 no.6
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• pp.614-620
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• 2019

In the present study, the wind characteristics were analyzed according to the time averages to evaluate the performance of small wind turbines required for the development of energy independent village. Measuring data of wind speed were recorded between January 2016 and April 2016 every second. Experimental data is averaged out using 5, 10, 15, 20 and 30 minute time steps. Throughout the experimental data analysis, 5 minutes averaged data is used to analyze the performance of the wind turbine, because it produces a minimum turbulence intensity in wind speed. The measuring power of the wind turbine is less than the designed value due to the unsteady nature wind of sudden changes in magnitude of wind speed and wind angle. Detailed wind conditions are also analysed using two variable Weibull probability density functions.

• Proceedings of the KIEE Conference
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• summer
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• pp.1106-1108
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• 2004

The arcing contact wear of circuit breakers can be the major cause for the maintenance requirement. The control of arcing times as short as possible but larger than the given minimum arcing time may be the effective way to mitigate the contact wear and to lessen the expenditure of a maintenance. In this paper the method for minimizing the arcing time is described. The basic concept used in the proposed method is the estimation of the current interruption instant from the informations such as the phase angle at the instant of trip commands and the magnitude of interrupted currents.

• Korean Journal of Plant Resources
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• v.11 no.3
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• pp.326-334
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• 1998

Vascular cambium in 16 species of Salicaceae is nonstoried in all of them and is made up of fusiform and ray initials. The growth of cambial girth occurred by pseudotransverse division of fusiform initials and subsequent apical intrusive growth of daughter initials. The measurements of fusiform initials in tangerntial sections have revealed that the average length varies from 252.2-437.1${\mu}{\textrm}{m}$ in investigated species, with the maximum being in Populus euramericana and the minimum in Salix purprea var. Multinervis. The length of fusiform initials depends on the length of the tapering end wall , therefore, the longer the length of the tapering end wall, therefore, the longer the length of the taperding end wall, the longer the length of fusiform initials. Ray initials have their own size, magnitude, and frequency of occrrence in each species. The relative proportion of ray initials fall about 15.0-24.5% in Salicaceae. Shrubby species of Salix have a lower ratio of ray initials than the species which possess wider trunks. New ray initials are originated from cut off the side or the end of fusiform initials and central partition of a fusiform initials.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.59.2-59.2
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• 2021

On March 6 2021, Apophis made a close approach to the Earth with a minimum distance of 0.11 AU when the apparent magnitude reached up to V~16. This was the most favorable condition to observe this asteroid until its 2029 encounter. The observations during this apparition were extremely important to determine major physical properties, such as size, rotational state, 3D shape model, surface mineral properties. So, we organized the observation campaign during the 2021 apparition. The main goals of our campaign are to refine the spin state and 3D shape model and check the surface composition variations. The campaign involved dozens of countries and included ground-based photometry and spectroscopy, and spacecraft observations. Our timely observation campaign will provide essential data in planning the operation scenario for the space mission. In this presentation, we will report the preliminary result of the Apophis observation campaign during the 2021 apparition.