• Nuclear Engineering and Technology
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• v.44 no.8
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• pp.889-896
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• 2012

Recent progress in the computational fluid dynamics methods of two- and multiphase phase flows has already started opening up new exciting possibilities for using complete multidimensional models to simulate boiling systems. Combining this new theoretical and computational approach with novel experimental methods should dramatically improve both our understanding of the physics of boiling and the predictive capabilities of models at various scale levels. However, for the multidimensional modeling framework to become an effective predictive tool, it must be complemented with accurate mechanistic closure laws of local boiling mechanisms. Boiling heat transfer has been studied quite extensively before. However, it turns out that the prevailing approach to the analysis of experimental data for both pool boiling and forced-convection boiling has been associated with formulating correlations which normally included several adjustable coefficients rather than based on first principle models of the underlying physical phenomena. One reason for this has been the tendency (driven by practical applications and industrial needs) to formulate single expressions which encompass a broad range of conditions and fluids. This, in turn, makes it difficult to identify various specific factors which can be independently modeled for different situations. The objective of this paper is to present a mechanistic modeling concept for both pool boiling and forced-convection boiling. The proposed approach is based on theoretical first-principle concepts, and uses a minimal number of coefficients which require calibration against experimental data. The proposed models have been validated against experimental data for water and parametrically tested. Model predictions are shown for a broad range of conditions.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1528-1532
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• 2000

The conventional optics and near field optics are compared numerically in the view points of the spot size and propagation characteristics. The decaying characteristics of near field light require the optics to access the object within several tens of nanometers. Therefore the gap control is one of the main issues in the near field optics area. In this paper the gap control is done by using the shear force of the NF(Near Field) probe and the characteristics are examined. The probe is modeled as a 2'nd order mass-spring-damper system driven by a harmonic force. The primary cause of the decrease in vibration amplitude is due to the damping force - shear force - between the surface and the probe. Using the model, damping constant and resonance frequency of the probe is calculated as a function of probe-sample distance. Detecting the amplitude and phase shift of the NF probe attached to the high Q-factor piezoelectric tuning fork, we can control the position of the NF probe about 0 to 50nm above the sample. The feedback signal to regulate the probe-sample distance can be used independently for surface topography imaging. 3-D view of the shear force image of a testing sample with the period of $1{\mu}m$ will be shown.

• Journal of Ocean Engineering and Technology
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• v.34 no.2
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• pp.147-154
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• 2020

Underwater acoustics that is the study of the phenomenon of underwater wave propagation and its interaction with boundaries, has mainly been applied to the fields of underwater communication, target detection, marine resources, marine environment, and underwater sound sources. Based on the scientific and engineering understanding of acoustic signals/data, recent studies combining traditional and data-driven machine learning methods have shown continuous progress. Machine learning, represented by deep learning, has shown unprecedented success in a variety of fields, owing to big data, graphical processor unit computing, and advances in algorithms. Although machine learning has not yet been implemented in every single field of underwater acoustics, it will be used more actively in the future in line with the ongoing development and overwhelming achievements of this method. To understand the research trends of machine learning applications in underwater acoustics, the general theoretical background of several related machine learning techniques is introduced in this paper.

• Proceedings of the KIEE Conference
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• 2008.07a
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• pp.257-258
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• 2008

The inrush current of a transformer is a high-magnitude and harmonic-rich current generated when the transformer core is driven into saturation during energizing. The inrush current usually leads to undesirable effects, for example potential damage to the transformer, misoperation of a protective relay, and power quality deterioration in the distribution power system. Inrush current reduction is therefore important for power system operation. In this paper, to reduce the inrush current, the insertion resistance of the Superconducting Fault Current Limiter (SFCL) that is connected in series with the transformer in the distribution system is used. This paper implements the SFCL by using the Electromagnetic Transient Program-Restructured Version (EMTP-RV) to model the SFCL in the distribution system. The simulation results show the beneficial effects of the SFCL for reduction of the inrush current.

• Wind and Structures
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• v.11 no.3
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• pp.209-220
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• 2008

Flutter derivatives provide the basis of predicting the critical wind speed in flutter and buffeting analysis of long-span cable-supported bridges. Many studies have been performed on the methods and applications of identification of flutter derivatives of bridge decks under wind action. In fact, strong wind, especially typhoon, is always accompanied by heavy rain. Then, what is the effect of rain on flutter derivatives and flutter critical wind speed of bridges? Unfortunately, there have been no studies on this subject. This paper makes an initial study on this problem. Covariance-driven Stochastic Subspace Identification (SSI in short) which is capable of estimating the flutter derivatives of bridge decks from their steady random responses is presented first. An experimental set-up is specially designed and manufactured to produce the conditions of rain and wind. Wind tunnel tests of a quasi-streamlined thin plate model are conducted under conditions of only wind action and simultaneous wind-rain action, respectively. The flutter derivatives are then extracted by the SSI method, and comparisons are made between the flutter derivatives under the two different conditions. The comparison results tentatively indicate that rain has non-trivial effects on flutter derivatives, especially on and $H_2$ and $A_2$thus the flutter critical wind speeds of bridges.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.10
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• pp.2712-2723
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• 1994

Numerical simulation of an axisymmetric ethylene-air jet diffusion flame has been carried out in order to investigate flame dynamics and soot formation. The model solves the time-dependent Navier-Stokes equations and includes models for soot formation, chemical reaction, molecular diffusion, thermal conduction, and radiation. Numerically FCT(Flux Corrected Transport) and DOM(Discrete Ordinate Method) methos are used for convection and radiation trasport respectively. Simulation was conducted for a 5 cm/sec fuel jet flowing into a coflowing air stream. The maximum flame temperature was found to be approximately 2100 K, and was located at an axial position of approximately 5 cm from the base of the flame. The maximum soot volume fraction was about $7{\times}10^{-7}$, and was located within the high temperature region where the fuel mole fraction ranges from 0.01 to 0.1. The buoyancy-driven low-frequency(12~13 Hz) structures convected along the outer region of the flame were captured. In case without radiation trasport, the maximum temperature was higher by 150 K than in case with radiation. Also the maximum soot volume fraction reached about $8{\times}10^{-6}$. As the the hydrocarbon fuel forms many soot particles, the radiation transport becomes to play a more important role.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.720-724
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• 2000

In this paper, we introduce a seamlessly integrated CAD-based design system (DS) for CAD modeling, engineering analysis, and optimal design which has been developed in CCED at KAIST, The key points of this integrating philosophy are to make full use of a parametric CAD program as the platform of integration and to adopt finite difference method for design sensitivity analysis in optimization process to get robustness and versatility. Design variables are directly selected by clicking CAD model parameters and all the analysis and design activities are menu-driven. This integrated program, named as DS/FDM, runs on Windows NT or Unix and FE analyses are performed at a remote Unix-workstation for multiple users. Application examples include shape optimal design of a belt clip that fits onto a portable electronic device and a bracket to show performance of DS/FDM with shell and tetra solid elements. This software is found efficient and effective fur shape design and size design of engineering structures.

• Journal of the Korea Computer Industry Society
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• v.7 no.3
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• pp.155-162
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• 2006

Because of their lower cost, higher speed, and longer travel, a belt drive system are quite desirable over screw driven system. However, a belt drive system are inherently difficult to control due to belt flexibility, friction, vibration, backlash and other non-linearities. This thesis presents servo control algorithm and the designing method of controller appliable to a belt drive system. In this paper, a LQ servo controller for a belt drive system is proposed to accomplish an optimal design of improved control system. In this scheme a mathematical model for the control system is obtained in state space form. Finally, the effectiveness of the proposed servo controller was verified through the computer simulation results.

• The Journal of Asian Finance, Economics and Business
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• v.7 no.9
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• pp.477-488
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• 2020

This study provides the empirical testing to test the influences of contemporary leadership styles (i.e., transactional and transformational) on job satisfaction and customer satisfaction as the mediating variable between job satisfaction and customer loyalty. The approach utilized in this study was a quantitative research design using a survey model. The participants of this study were recruited from 160 front-liners in the airline's industry in Indonesia. This study employed 160 front liners (flight attendants, reservation/ticketing, and check-in-counter officers) who worked in Branch Office/General Sales Agent of Indonesian commercial airlines. The studied airlines serve domestic routes based at Soekarno-Hatta International Airport. Furthermore, the data were examined the four developed hypotheses by using Structural Equation Modeling (SEM). The primary results are: First, in the context of job satisfaction, both leadership styles (i.e., transactional and transformational) have similar influences; Second, job satisfaction affects customer satisfaction positively and significant, Third, customer loyalty is driven by passengers' satisfaction when they are utilizing the airline services. The findings imply that the working-role encouragement is essential to maximize the productivity of front-liners to serve their customer well. This research supports on developing the general knowledge based on leadership styles in the context of the airline's industry in managing the human resources strategically.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2001.05a
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• pp.408-413
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• 2001

DC motor has widely been used in the field of variable-speed driving unit since it is easy to control flux and torque precisely but it is troublesome to check and maintain periodically. In addition, there are difficulties in hish power and high speed running due to rectifying limit of commutator, and are a lot of restrictions in installation. Therefore, speed control in BLDC(Brushless DC) motor has seriously been studied for a long while. In this paper, a mathematical model of BLDC motor driven by PWM inverter is developed. Dynamics and steady-state characteristics of BLDC motor are simulated and analyzed with a series of experiment for the parameter estimation : torque, speed, phase voltage and current.