• Proceedings of the Korean Magnestics Society Conference
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• 2001.10a
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• pp.34-35
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• 2001

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.3
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• pp.342-351
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• 1999

A jet injected normally into a cross flow has been found to have the cross section of a horseshoe shape. It occurs by a twin vortex motion in the region downstream of the jet injection. Such a flow is inherently and highly three-dimensional and numerical calculations should play an important role. The three-dimensional momentum equations with buoyancy effect and energy equation are solved to obtain the velocity distributions, center-line trajectories, cross sectional shape and entrainment. The density difference is sufficiently small, so that the Boussinesq approximation is considered to be valid. The SIMPLE algorithm is applied in a staggered grid system of a calculational domain for the numerical method.

• Nuclear Engineering and Technology
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• v.44 no.8
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• pp.977-984
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• 2012

The cross-correlation technique has been widely used for leakage detection of buried pipes, and this technique can be successfully applied when the leakage signal has a high signal-to-noise ratio. In the case of a power plant, the measured leakage signals obtained from the sensors may contain background noise and mechanical noise generated by adjacent machinery. In such a case, the conventional method using the cross-correlation function may fail to estimate the leakage point. In order to enhance the leakage estimation capability of a buried pipe in a noisy environment, an improved cross-correlation technique is proposed. It uses a noise rejection technique in the frequency domain to effectively eliminate the tonal noise due to rotating machinery. Experiments were carried out to verify the validity of the proposed method. The results show that even in a tonal noisy environment, the proposed method can provide more reliable means for estimating the time delay of the leakage signals.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.646-653
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• 2001

LQG/LTR Control Methology is recently used for the analysis of multi-variable control in frequency domain. Target filter loop is designed by the demanding requirements such as cross-over frequency, disturbance rejection in low frequency domain, zero steady-state error, identification of maximum and minimum singular values and sensor noise rejection in high frequency domain. Loop transfer recovery is accomplished by solving the cheap control and then simulation close to the target filter loop. In this study, LQG/LTR Control Methodology is applied to the seat suspension system. It is found that this technique is very effective to control the system and improve the ride quality of human body.

• Journal of the Korea Computer Graphics Society
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• v.16 no.4
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• pp.17-23
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• 2010

Effective visualizing is an important issue when one processing real number domain volume data such as distance fields, or volume texture. In this paper, we introduce a framework for inspecting, magnifying, cross-section viewing of real number domain volume data from an implementation of a simple interface. The interface can be freely implemented from any kind of existing algorithm, so that we can easily view the result and evaluate the algorithm.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.1 s.256
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• pp.55-61
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• 2007

Recently simulation model becomes an essential tool for analysis and design of a system but it is often expensive and time consuming as it becomes complicate to achieve reliable results. Therefore, high-fidelity simulation model needs to be replaced by an approximate model, the so-called metamodel. Metamodeling techniques include 3 components of sampling, metamodel and validation. Cross-validation approach has been proposed to provide sequnatially new sample point based on cross-validation error but it is very expensive because cross-validation must be evaluated at each stage. To enhance the cross-validation of metamodel, sequential sampling method using candidate points and representative cross-validation is proposed in this paper. The candidate and representative cross-validation approach of sequential sampling is illustrated for two-dimensional domain. To verify the performance of the suggested sampling technique, we compare the accuracy of the metamodels for various mathematical functions with that obtained by conventional sequential sampling strategies such as maximum distance, mean squared error, and maximum entropy sequential samplings. Through this research we team that the proposed approach is computationally inexpensive and provides good prediction performance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.3A
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• pp.195-202
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• 2005

The third generation cellular system requires the seamless macro/micro mobility support. Mobile IP provides a simple and scalable macro mobility solution but lacks the support for fast handoff control in micro-domain. However, A lot of micro-mobility protocols have been proposed to complement the Mobile IP capability by providing the fast, seamless, and local handoff control. Cellular If also provides the seamless mobility support in limited geographical area. But semi-soft handoff mechanism of Cellular IP produces the packet loss and the duplication problem due to the difference of propagation delay between the new path and the old path. In this paper, we present an efficient handoff mechanism in micro-domain. The proposed handoff mechanism uses the SCD (Suitable Cross Delay) in order to minimize the packet loss and the duplication problem during the handoff. Also, the proposed mechanism is verified by the performance evaluation through the NS-2 Simulation.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.9B
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• pp.1527-1533
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• 2000

Basically, a general characterization procedure based on the extraction of the characteristic impedance and propagation constant for analyzing a single MIS(Metal-Insulator-Semiconductor) transmission line is used. In this paper, an analysis for a new substrate shielding MIS structure consisting of grounded cross-bars at the interface between Si and SiO2 layer using the Finite-Difference Time-Domain (FDTD) method is presented. In order to reduce the substrate effects on the transmission line characteristics, a shielding structure consisting of grounded cross bar lines over time-domain signal has been examined. The extracted distributed frequency-dependent transmission line parameters and corresponding equivalent circuit parameters as well as quality factor have been examined as functions of cross-bar spacing and frequency. It is shown that the quality factor of the transmission line can be improved without significant change in the characteristic impedance and effectve dielectric constant.

• Wind and Structures
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• v.27 no.6
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• pp.381-397
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• 2018

Nonlinear behavior in fluid-structure interaction (FSI) of bridge decks becomes increasingly significant for modern bridges with increasing spans, larger flexibility and new aerodynamic deck configurations. Better understanding of the nonlinear aeroelasticity of bridge decks and further development of reduced-order nonlinear models for the aeroelastic forces become necessary. In this paper, the amplitude-dependent and neutral angle dependent nonlinearities of the motion-induced loads are further highlighted by series of computational fluid dynamics (CFD) simulations. An effort has been made to investigate a semi-analytical time-domain model of the nonlinear motion induced loads on the deck, which enables nonlinear time domain simulations of the aeroelastic responses of the bridge deck. First, the computational schemes used here are validated through theoretically well-known cases. Then, static aerodynamic coefficients of the Great Belt East Bridge (GBEB) cross section are evaluated at various angles of attack, leading to the so-called nonlinear backbone curves. Flutter derivatives of the bridge are identified by CFD simulations using forced harmonic motion of the cross-section with various frequencies. By varying the amplitude of the forced motion, it is observed that the identified flutter derivatives are amplitude-dependent, especially for $A^*_2$ and $H^*_2$ parameters. Another nonlinear feature is observed from the change of hysteresis loop (between angle of attack and lift/moment) when the neutral angles of the cross-section are changed. Based on the CFD results, a semi-analytical time-domain model for describing the nonlinear motion-induced loads is proposed and calibrated. This model is based on accounting for the delay effect with respect to the nonlinear backbone curve and is established in the state-space form. Reasonable agreement between the results from the semi-analytical model and CFD demonstrates the potential application of the proposed model for nonlinear aeroelastic analysis of bridge decks.

• Journal of the Korean GEO-environmental Society
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• v.15 no.5
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• pp.31-37
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• 2014

In situ investigations and laboratory tests using elastic wave have become popular in geotechnical and geoenvironmental engineering. Propagation velocity of elastic wave is the key index to evaluate the ground characteristics. To evaluate this, various methods were used in both time domain and frequency domain. In time domain, the travel time can be found from the two points that have the same phase such as peaks or first rises. Cross-correlation can also be used in time domain by evaluating the time shift amount that makes the product of signals of input and received waveforms maximum. In frequency domain, wave propagation velocity can be evaluated by computing the phase differences between the source and received waves. In this study, wave propagation velocity evaluated by the methods listed above were compared. Bender element tests were conducted on the specimens cut from the undisturbed hand-cut block samples obtained from Block 37 excavation site in Chicago, IL, US. The evaluation methods in time domain provides relatively wide range of wave propagation velocities due to the noise in signals and the sampling frequency of data logger. Frequency domain approach provides relatively accurate wave propagation velocities and is irrelevant to the sampling frequency of data logger.