• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.10 no.4
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• pp.295-303
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• 2012

Based on the data observed and analyzed on a groundwater flow system in the KURT (KAERI Underground Research Tunnel) site, the transport of radionuclides, which were assumed to be released at the supposed position, was calculated on the time-domain. A groundwater pathway from the release position to the surface was identified by simulating the groundwater flow model with the hydrogeological characteristics measured from the field tests in the KURT site. The elapsed time when the radionuclides moved through the pathway is evaluated using TDRW (Time Domain Random Walk) method for simulating the transport on the time-domain. Some retention mechanisms, such as radioactive decay, equilibrium sorption, and matrix diffusion, as well as the advection-dispersion were selected as the factors to influence on the elapsed time. From the simulation results, the effects of the sorption and matrix diffusion, determined by the properties of the radionuclides and underground media, on the transport of the radionuclides were analyzed and a decay chain of the radionuclides was also examined. The radionuclide ratio of the mass discharge into the surface environment to the mass released from the supposed repository did not exceed $10^{-3}$, and it decreased when the matrix diffusion were considered. The method used in this study could be used in preparing the data on radionuclide transport for a safety assessment of a geological disposal facility because the method could evaluate the travel time of the radionuclides considering the transport retention mechanism.

• Journal of Korean Society for Atmospheric Environment
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• v.12 no.1
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• pp.29-41
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• 1996

The two-stage numerical model was used to study the relation between three-dimensional local wind model, advection/diffusion model of random walk method and second moment method on Pusan coastal area. The first stage is three dimensional time-dependent local wind model which gives the wind field and vertical dirrusion coefficient. The second stage is advection/diffusion model which uses the results of the first stage as input data. First, wind fields on Pusan coastal area for none synoptic scale wind showed typical land and sea breeze circulation, and convergence zone occured at 1200LST in northern of domain, in succession, moved northward of domain. Emissions from Sinpyeong industrial district were trasnported toward the inland by sea breeze during daytime, and reached the end part of domain about 1800LST. During nighttime, emissions return to sea by land breeze and vertical diffusion also contributes to upward transport. In order to use this model for forecast of air pollution concentration on the Pusan coastal area, it is necessary that computed value must be compared with measured value and wind fields model must also be dealt in detail.

• Journal of the Korean GEO-environmental Society
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• v.4 no.3
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• pp.79-88
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• 2003

This paper presents a numerical study of the spatial behavior of a linear absorbing solute in a heterogeneous porous medium. The spatially correlated log-normal hydraulic conductivity field is generated in a given two-dimensional domain by using the geostatistical method (Turning Bands algorithm). The velocity vector field is calculated by applying the two-dimensional saturated groundwater flow equation to the Galerkin finite element method. The simulation of solute transport is carried out by using the random walk particle tracking model with CD(constant displacement) scheme in which the time interval is automatically adjusted. In this study, the spatial behavior of a solute is analyzed by the longitudinal center-of-mass displacement, longitudinal spatial spread moment and longitudinal plume skewness.

• Journal of Mechanical Science and Technology
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• v.19 no.12
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• pp.2312-2320
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• 2005

The aerodynamic effects of leading-edge accretion can raise important safety concerns since the formulation of ice causes severe degradation in aerodynamic performance as compared with the clean airfoil. The objective of this study is to develop a numerical simulation strategy for predicting the particle trajectory around an MS-0317 airfoil in the test section of the NASA Glenn Icing Research Tunnel and to investigate the impingement characteristics of droplets on the airfoil surface. In particular, predictions of the mean velocity and turbulence diffusion using turbulent flow solver and Continuous Random Walk method were desired throughout this flow domain in order to investigate droplet dispersion. The collection efficiency distributions over the airfoil surface in simulations with different numbers of droplets, various integration time-steps and particle sizes were compared with experimental data. The large droplet impingement data indicated the trends in impingement characteristics with respect to particle size ; the maximum collection efficiency located at the upper surface near the leading edge, and the maximum value and total collection efficiency were increased as the particle size was increased. The extent of the area impinged on by particles also increased with the increment of the particle size, which is similar as compared with experimental data.

• Proceedings of the Korea Water Resources Association Conference
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• 2002.05b
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• pp.1027-1033
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• 2002

The solute transport in a two-dimensional heterogeneous porous medium is numerically studied by using a random walk particle tracking (RWPT) method. Lognormally isotropic hydraulic conductivity fields are generated by using the turning band methods with mean zero and four different values of standard deviation. The numerical transport experiments are carried out to investigate the large time and spatial effects of the variable pore velocity field on solute plumes. The behavior of the solute plume through numerical simulations is presented in terms of longitudinal and transverse spatial moments: displacement of center-of-mass, plume spread variance and skewness coefficient. It was observed that the dispersive behavior of the solute plume is strongly affected by the degree of heterogeneity in the flow domain.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.11 no.4
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• pp.281-291
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• 2013

A hypothetical repository was assumed to be located at the KURT (KAERI Underground Research Tunnel) site, and the travel times of radionuclides released from three source positions were calculated. The groundwater flow around the KURT site was simulated and the groundwater pathways from the hypothetical source positions to the shallow groundwater were identified. Of the pathways, three pathways were selected because they had highly water-conductive features. The transport travel times of the radionuclides were calculated by a TDRW (Time-Domain Random Walk) method. Diffusion and sorption mechanisms in a host rock matrix as well as advection-dispersion mechanisms under the KURT field condition were considered. To reflect the radioactive decay, four decay chains with the radionuclides included in the high-level radioactive wastes were selected. From the simulation results, the half-life and distribution coefficient in the rock matrix, as well as multiple pathways, had an influence on the mass flux of the radionuclides. For enhancing the reliability of safety assessment, this reveals that identifying the history of the radionuclides contained in the high-level wastes and investigating the sorption processes between the radionuclides and the rock matrix in the field condition are preferentially necessary.

• Journal of KIISE:Information Networking
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• v.31 no.5
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• pp.490-500
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• 2004

In mobile/wireless environment, mobility management is widely being focused as one popular researches. But, disruption happens when messages are exchanged between nodes as registration is made after handoff, and unnecessary traffic occurs because of the use of the Random-walk model, in which the probability for MN to move to neighboring cells is equal. In order to solve these problems, this study proposes a technique and algorithm for composing Directional Shadow Registration Region (DSRR) that provides seamless mobility. The core of DSRR is to prevent disruption and unnecessary traffic by minimizing the number o) neighboring cells with a high probability of handoff (AAAF). This study sensed the optimal time for handoff through regional cell division by introducing a division scheme, and then decided DSRR, the region for shadow registration, by applying direction vector (DV) obtained through directional cell sectoring. According to the result of the experiment, the proposed DSRR processes message exchange between nodes within the intra-domain, the frequency of disruptions decreased significantly compared to that in previous researches that process in inter-domain environment. In addition, traffic that occurs at every handoff happened twice in DSRR compared to n (the number of neighboring cells) times in Previous researches. As an additional effect, divided regions obtained from the process of composing DSRR filter MN that moves regardless of handoff.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.18 no.2_spc
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• pp.305-315
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• 2020

For geological disposal of radioactive wastes, a method was proposed to evaluate the radionuclide transport in the biosphere by calculating the elapsed time of nuclide migration. The radionuclides were supposed to be introduced from a natural barrier and reached a large surface water body following a groundwater flow in a shallow subsurface. The biosphere was defined as a shallow subsurface environment that included aquifers on a host rock. Using the proposed method, a calculation algorithm was established, and a computer code that implemented the algorithm was developed. The developed code was verified by comparing the simulation results of the simple cases with the results of the analytical solution and a public program, which has been widely used to evaluate the radiation dose using the radionuclide transport near the surface. A case study was constructed using the previous research for radionuclide transport from the hypothetical geological disposal repository. In the case study, the code calculated the mass discharge rate of radionuclide to a stream in the biosphere. Because the previous research only demonstrated the transport of radionuclides from the hypothetical repository to the host rock, the developed code in the present study could help identify the total transport of radionuclide along the complete pathway.

• Asia pacific journal of information systems
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• v.24 no.2
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• pp.233-253
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• 2014

Can the stock market really be predicted? Stock market prediction has attracted much attention from many fields including business, economics, statistics, and mathematics. Early research on stock market prediction was based on random walk theory (RWT) and the efficient market hypothesis (EMH). According to the EMH, stock market are largely driven by new information rather than present and past prices. Since it is unpredictable, stock market will follow a random walk. Even though these theories, Schumaker [2010] asserted that people keep trying to predict the stock market by using artificial intelligence, statistical estimates, and mathematical models. Mathematical approaches include Percolation Methods, Log-Periodic Oscillations and Wavelet Transforms to model future prices. Examples of artificial intelligence approaches that deals with optimization and machine learning are Genetic Algorithms, Support Vector Machines (SVM) and Neural Networks. Statistical approaches typically predicts the future by using past stock market data. Recently, financial engineers have started to predict the stock prices movement pattern by using the SNS data. SNS is the place where peoples opinions and ideas are freely flow and affect others' beliefs on certain things. Through word-of-mouth in SNS, people share product usage experiences, subjective feelings, and commonly accompanying sentiment or mood with others. An increasing number of empirical analyses of sentiment and mood are based on textual collections of public user generated data on the web. The Opinion mining is one domain of the data mining fields extracting public opinions exposed in SNS by utilizing data mining. There have been many studies on the issues of opinion mining from Web sources such as product reviews, forum posts and blogs. In relation to this literatures, we are trying to understand the effects of SNS exposures of firms on stock prices in Korea. Similarly to Bollen et al. [2011], we empirically analyze the impact of SNS exposures on stock return rates. We use Social Metrics by Daum Soft, an SNS big data analysis company in Korea. Social Metrics provides trends and public opinions in Twitter and blogs by using natural language process and analysis tools. It collects the sentences circulated in the Twitter in real time, and breaks down these sentences into the word units and then extracts keywords. In this study, we classify firms' exposures in SNS into two groups: positive and negative. To test the correlation and causation relationship between SNS exposures and stock price returns, we first collect 252 firms' stock prices and KRX100 index in the Korea Stock Exchange (KRX) from May 25, 2012 to September 1, 2012. We also gather the public attitudes (positive, negative) about these firms from Social Metrics over the same period of time. We conduct regression analysis between stock prices and the number of SNS exposures. Having checked the correlation between the two variables, we perform Granger causality test to see the causation direction between the two variables. The research result is that the number of total SNS exposures is positively related with stock market returns. The number of positive mentions of has also positive relationship with stock market returns. Contrarily, the number of negative mentions has negative relationship with stock market returns, but this relationship is statistically not significant. This means that the impact of positive mentions is statistically bigger than the impact of negative mentions. We also investigate whether the impacts are moderated by industry type and firm's size. We find that the SNS exposures impacts are bigger for IT firms than for non-IT firms, and bigger for small sized firms than for large sized firms. The results of Granger causality test shows change of stock price return is caused by SNS exposures, while the causation of the other way round is not significant. Therefore the correlation relationship between SNS exposures and stock prices has uni-direction causality. The more a firm is exposed in SNS, the more is the stock price likely to increase, while stock price changes may not cause more SNS mentions.

• Journal of the Korean Society for Marine Environment & Energy
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• v.11 no.1
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• pp.13-23
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• 2008

In coastal region, eutrophication, Do deficit and red tide are frequently occurred by influx of fresh water. When the fresh water containing pollutants is discharged into the sea, the surrounding water is contaminated by dispersion of freshwater flowing into coastal waters. The prediction and analysis about the dispersion process of the discharged fresh water should be conducted. A modeling system using GUI was developed to simulate hydrodynamic flow and fresh water dispersion in coastal waters and to analyze the results efficiently. The modeling module of the system includes a tide model using a finite element method and a fresh water dispersion model using a particle-tracking method. This system was applied to predict the tidal currents and fresh water dispersion in Mokpo coastal zone. To verify accuracy of the hydrodynamic model, the simulation results were compared with observed sea level and time variations of tidal currents showing a good agreement. The fresh water dispersion was verified with observed salinity distribution. The dispersion model also was verified with analytic solutions with advection-diffusion problems in 1-dimensional and 2-dimensional simple domain. The system is operated on GUI environment, to ease the model handling such as inputting data and displaying results. Therefore, anyone can use the system conveniently and observe easily and accurately the simulation results by using graphic functions included in the system. This system can be used widely to decrease the environmental disaster induced by inflow of fresh water into coastal waters.