• Journal of Korea Water Resources Association
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• v.44 no.4
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• pp.275-283
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• 2011

A local or site-specific water temperature is downscaled from the nation-wide air temperature that represents simulation by General Circulation Model (GCM). Both two-step and one-step method are tested and compared in three sites: Masan Bay, Lake Sihwa, and Nakdong River Estuary. Two-step method uses a linear regression model as the first step that converts nation-wide air temperature into local air temperature, and the corresponding coefficient of determination is in the range of 0.98~0.99. The second step that converts air temperature into water temperature uses a nonlinear curve, so called S-curve, and the corresponding root mean squared error (RMSE) is 2.07 for rising limb in Masan Bay, 1.93 for falling limb in Masan Bay, 2.59 for Lake Sihwa, and 1.58 for Nakdong River Estuary. In a similar way, one-step method is performed to directly convert nation-wade air temperature into local water temperature, and the corresponding RMSE is 2.28 for rising limb in Masan Bay, 1.89 for falling limb in Masan Bay, 2.55 for Lake Sihwa, and 1.52 for Nakdong River Estuary. Consequently both methods show a similar level of performance, and one-step method is recommendable in that it is simple and practical in relative terms.

• Journal of the Korean Society for Precision Engineering
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• v.34 no.2
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• pp.83-87
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• 2017

A multi-step warm forging process for subminiature screws is investigated. Due to the low formability of Titanium alloys, bit forming of Titanium screws is difficult by cold forging. In order to overcome this low formability of Titanium alloys, two candidate processes, i.e., multi-step forging and warm forging are introduced. First, a multi-step (two-step) forging process is investigated. The punch shape and stroke of forging during the first step is designed via various analyses. Finally, the bit formability is investigated at different forging temperatures. Analyses are carried out for two-step forging at various temperatures and the formability under these thermal conditions is compared.

• Journal of the Korean Ceramic Society
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• v.50 no.1
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• pp.70-74
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• 2013

Dependence of microwave dielectric properties on the crystallization behaviors of $CaMgSi_2O_6$ (diopside) glass-ceramics was investigated with different heat treatment methods (one and/or two-step). The crystallization behaviors of the specimens, crystallite size and degree of crystallization, were evaluated by differential thermal analysis (DTA), scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis by combined Rietveld and reference intensity ratio (RIR) methods. With an increase in heattreatment temperature, the dielectric constant (K) and the quality factor (Qf) increased due to the increase of the crystallite size and degree of crystallization. The specimens heat-treated by the two-step method had a higher degree of crystallization than the specimens heat-treated by the one-step method, which induced improvement in the quality factor (Qf) of the specimens.

• The Korean Journal of Applied Statistics
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• v.34 no.2
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• pp.239-254
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• 2021

This paper studies time series analysis with estimation and forecasting for Korean COVID-19 confirmed cases, based on the approach of a heterogeneous autoregressive (HAR) model with two-piece t (TP-T) distributed errors. We consider HAR-TP-T time series models and suggest a step-by-step method to estimate HAR coefficients as well as TP-T distribution parameters. In our proposed step-by-step estimation, the ordinary least squares method is utilized to estimate the HAR coefficients while the maximum likelihood estimation (MLE) method is adopted to estimate the TP-T error parameters. A simulation study on the step-by-step method is conducted and it shows a good performance. For the empirical analysis on the Korean COVID-19 confirmed cases, estimates in the HAR-TP-T models of order p = 2, 3, 4 are computed along with a couple of selected lags, which include the optimal lags chosen by minimizing the mean squares errors of the models. The estimation results by our proposed method and the solely MLE are compared with some criteria rules. Our proposed step-by-step method outperforms the MLE in two aspects: mean squares error of the HAR model and mean squares difference between the TP-T residuals and their densities. Moreover, forecasting for the Korean COVID-19 confirmed cases is discussed with the optimally selected HAR-TP-T model. Mean absolute percentage error of one-step ahead out-of-sample forecasts is evaluated as 0.0953% in the proposed model. We conclude that our proposed HAR-TP-T time series model with optimally selected lags and its step-by-step estimation provide an accurate forecasting performance for the Korean COVID-19 confirmed cases.

• Journal of Drive and Control
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• v.15 no.2
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• pp.9-21
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• 2018

The aim of this study was to develop and test a model of an auto-sensing hydraulic breaker that can automatically change its 4-step impact mode according to the rock strength using SimulationX. The auto-sensing hydraulic breaker with a 4-step variable impact mode has the advantage of obtaining optimal impact energy and impact frequency under various rock conditions compared to an auto-sensing hydraulic breaker with a 2-step variable impact mode, which has already been developed overseas. Several steps were necessary to conduct this study. First, the operation principle of the auto-sensing hydraulic breaker with the 2-step variable impact mode was analyzed. Based on the findings, an analysis model of the auto-sensing hydraulic breaker with the 4-step variable impact mode was developed (and compared with the 2-step variable impact mode) Finally, an analysis of the results established that the stepwise variable of the impact mode was implemented according to the rock strength and the difference of each impact mode was confirmed. This study is expected to contribute to the development of auto-sensing hydraulic breakers that are superior to those developed by advanced companies in foreign countries.

• Journal of Engineering Education Research
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• v.9 no.3
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• pp.22-36
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• 2006

This paper is aimed to design for qualification structure and criteria for preparation of qualification test of computer operation engineer. In this study, a DACUM(Development A curriculum) method job analysis to extract contents such as task range, grades or ranks, eligibility, examination standards, selection criteria of examination questions, examination methods, etc was done by an job analysis committee composed of 7 specialists in the industry and academia. In the first place, this report presents job analysis models by procedures for developing selection criteria of examination questions of computer operation engineer. The first step is preparation for job analysis(data research, interviews and selection of experts), the second step the establishment of job models(first expert council and site study), the third step the drawn-out of knowledge, skills, and tools(the second expert council and site study), the fourth step the selection of courses and subjects(the third expert council), the fifth step the establishment of guidelines for making questions(the third site study).

• 한국전산유체공학회:학술대회논문집
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• 2007.10a
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• pp.194-199
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• 2007

IB (immersed boundary) method is one of the prominent tool in computational fluid dynamics for the analysis of flows over complex geometries. The IB technique simplyfies the solution procedure by eliminating the requirement of complex body fitted grids and it is also superior in terms of memory requirement. In this study we have developed numerical code (FOTRAN) for the analysis of 2D flow over a cylinder using IB technique. The code is validated by comparing the wake lengths and separation angles given by Guo et. al. We employed fractional-step procedure for solving the Navier-Stokes equations governing the flow and discrete forcing IB technique for imposing boundary conditions. Also we have developed a 3D code for the backward-facing-step flow and flow over a sphere. The reattachment length in backward-facing-step flow was compared with the one given by Nie and Armaly, which has proven the validity of our code.

• Journal of Electrical Engineering and Technology
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• v.4 no.2
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• pp.229-233
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• 2009

This paper proposes a correction method for the error inherently created by time-step approximation in finite element analysis (FEA). For a simple RL and RLC linear circuit, the error in time-step analysis is analytically investigated, and a correction method is proposed for a non-linear system as well as a linear one. Then, for a practical inductor model, linear and non-linear time-step analyses are performed and the calculation results are corrected by the proposed methods. The accuracy of the corrected results is confirmed by comparing the electric input and output powers.

• KIEE International Transaction on Systems and Control
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• v.2D no.2
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• pp.97-107
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• 2002

In this paper, we propose the concept of orthonormalized compressed measurement for the stability analysis of discrete linear time-varying Kalman filters. Unlike previous studies that deal with the homogeneous portion of Kalman filters, the proposed Lyapunov method directly deals with the stochastically-driven system. The orthonorrmalized compressed measurement provides information on the a priori state estimate of the Kalman filter at the k-th step that is propagated from the a posteriori state estimate at the previous block of time. Since the complex multiple-step propagations of a candidate Lyapunov function with process and measurement noises can be simplified to a one-step Lyapunov propagation by the orthonormalized compressed measurement, a stochastic radius of attraction can be derived that would be impractically difficult to obtain by the conventional multiple-step Lyapunov method.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.05a
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• pp.130-135
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• 2003

This paper presents the method for structure borne noise analysis of a flexible body in multibody system. The proposed method is the superposition method using flexible muitibody dynamic analysis and finite element one. This method is executed in 3 steps. In the la step, time dependent quantities such as dynamic loads, modal coordinates ana gross body motion of the flexible body are calculated efficiently through flexible multibody dynamic analysis. And frequency response functions are computed using Fourier transforms of those time dependent quantities. In the 2$\^$nd/ step, acoustic pressure coefficients are obtained through structure-acoustic coupling analysis by finite element analysis. In the final step, frequency responses of acoustic pressure at the acoustic nodes are recovered through linear superposition of frequency response functions with acoustic pressure coefficients. The accuracy of the proposed method is verified in the numerical example of a simple car model.