Jo Feb 25, 2024
In magneto-telluric (MT) method, data processing for transfer functions (TF) estimation is routinely carried out based on the regression model of 4-element impedance under the assumption of plane electromagnetic wave. However, observed EM field contains not only MT signals subjected to the assumption of plane wave but also remarkable amounts of various noises with non-planar nature.
Jo Kyong Rae, a researcher at the Faculty of Earth Science and Technology, has introduced a regression model of impedance and admittance with 6 elements. He has also derived a mathematical formula for their estimation.
To decrease the influence of industrial noise as well as long-period noise, he employed 6-element regression models of impedance and admittance tensors in MT data processing to improve the predicted coherences of TFs estimation by about 0.02, both in single station and remote reference modes. He tested the proposed method through the processing of MT data, from four stations in the northern part of our country.
The results showed that the 6-element model makes it possible to improve the predicted coherences in TFs estimation and to decrease the scattering of the curves of apparent resistivity and phases, and that the difference between common 4-element and 6-element models gradually gets bigger as frequency decreases. It was also demonstrated that when the vertical magnetic component has amplitude as great as or greater than horizontal ones in auto-power curves, there is no doubt that the 6-element regression model is more appropriate for MT data processing than commonly used 4-element regression models.
...
Jo Feb 23, 2024
Drawing up the most reasonable charging schedule is one of the most important challenges to be solved in wireless rechargeable sensor networks (WRSNs).
However, in existing on-demand charging schemes using multicriteria, it leaves space for efficient combination of multicriteria in the proactive charging process as well as in determination of charging location and charging time.
To solve this problem, Ri Man Gun, an institute head at the Faculty of Communications, has proposed a novel charging scheduling scheme called eIFVT (exploiting an Integrated FAHP-VWA-TOPSIS).
The eIFVT first calculates the exact weights of multicriteria characterizing the sensor nodes including charging request nodes using FAHP-VWA. These weights are then used to select the most suitable next-to-be-charged nodes and future potential-to-be-bottlenecked nodes with TOPSIS and to determine the partial charging time of the on-demand charging scheme adaptively.
The extensive simulation results show that the eIFVT greatly improves the charging and network performance for various metrics compared with existing schemes.
For more information, please refer to his paper “eIFVT: Exploiting an Integrated FAHP-VWA-TOPSIS in Whole-Process of On-Demand Charging Scheduling for WRSNs” in “IEEE SYSTEMS JOURNAL” (SCI).
...
Jo Feb 20, 2024
Quality index of cast aluminum alloy is a measure to evaluate its quality based on tensile mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), elongation to fracture (Ef) and strain energy density W.
Quality index Q0 evaluates material quality on the basis of the balance between YS and W, and it evaluates material’s potential in combination of tensile strength, ductility and toughness.
Its calculation formula involves strain energy density W as a term and W is calculated by integration of true stress–true strain tensile flow curve. So Q0 can be evaluated only if the tensile flow curve of the alloy is known and it may make the use of Q0 difficult.
Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, has developed an approximate formula for calculating Q0 according to YS, UTS and Ef without requiring W.
He developed approximate formulas for Q0 which are expressed as a function of YS, UTS and Ef using multiple linear and quadratic regression models, and evaluated their performances from the viewpoints of mean absolute error (MAE), mean relative error (MRE), determination coefficient (R2) and simplicity. The results demonstrated that they have excellent performances.
The MAE, MRE, R2 and F-statistic of the multiple linear regression model for Q0 are respectively 19.981 MPa, 3.792%, 0.944 and 523.344, and the MAE, MRE, R2 and F-statistic of the multiple quadratic regression model for Q0 are respectively 3.322MPa, 0.566%, 0.998 and 7 277.034.
These approximate formulas could be widely used to evaluate the quality of cast aluminum alloys for materials designers and engineers in practice.
You can find more information in his paper “Approximate formulas for quality index Q0 of cast aluminum alloys according to mechanical tensile properties using multiple linear and quadratic regression models” in “ International Journal on Interactive Design and Manufacturing” (EI).
...
Jo Feb 16, 2024
When the dimension of data is larger than the number of samples, linear discriminant analysis (LDA) cannot be applied directly to high-dimensional data. This case is called small-sample-size (SSS) or under sampled problem.
To solve this problem, some local learning based various image clustering models are proposed.
Pak Kwang Jun, a lecturer at the Faculty of Applied Mathematics, has proposed a local learning based exponential regularized discriminant clustering model.
In the proposed local exponential regularized discriminant clustering (LERDC) model, local scatter matrices of regularized discriminant model are projected in the exponential domain in order to handle the SSS problem of LDA. In the proposed LERDC model, for each image, the local image matrix is constructed comprising k nearest neighbor images, and the local exponential regularized discriminant model (LERDM) is devised to evaluate the clustering results for the images in the local image matrix.
To verify his method, he compared it with existing state-of-the-art local learning based clustering approaches. The results showed that the proposed LERDC model achieved a comparable clustering performance to that of the near competitor LDMGI model which is based on LDA.
...
Jo Feb 15, 2024
Rare earth neodymium and neodymium-iron alloys are the main materials for production of Nd-Fe-B magnet.
The density, electric conductivity and melting point are the important properties of molten neodymium electrolyte. Knowledge of density is helpful for electrolyte selection with reasonable composition, which contributes to better separation of melts and metallic neodymium.
There have been some reports on the physical properties of neodymium fluoride electrolyte, where alkali fluorides were supposed to replace LiF and the changes of these physical properties with temperature and composition of the electrolyte were considered.
In an attempt to reduce the cost of producing metallic neodymium and neodymium-iron alloys by molten salt electrolysis, Jang Pok Nam, a section head at the Faculty of Metal Engineering, has investigated the melting point, density, electric conductivity and viscosity of ternary (NdF3-LiF-BaF2, NdF3-LiF-CaF2) and quaternary (NdF3-LiF-BaF2-CaF2) neodymium fluorides.
He drew the following conclusions.
The melting point of 90wt% (NdF3-LiF)-10wt% (BaF2-CaF2) melt is the lowest, about 702℃ and the density of molten salts linearly changes with the temperature. In ternary system, the melting points of 85wt%(NdF3-LiF)-15wt%BaF2 and 90wt%(NdF3-LiF)-10wt%CaF2 fall relatively, and in quaternary system, 90wt%(NdF3-LiF)-10wt%(BaF2-CaF2) has the lowest melting point among the considered samples. The relation between temperature and density is linear. Addition of BaF2 and CaF2 increased the viscosities of the considered molten salts. However, 90wt%(NdF3-LiF)-10wt%(BaF2-CaF2) melt has the lowest viscosity. Electrical conductivity of molten salts increases with the temperature and the content of LiF. At the same temperature, quaternary electrolyte has a greater value of electrical conductivity than ternary electrolyte. From the above measurements, it is concluded that 90wt%(NdF3-LiF)-10wt%(BaF2-CaF2) melt can reduce the cost of producing neodymium or neodymium iron alloys.
...
Jo Feb 12, 2024
Ryu Chang Sik, a section head at the Faculty of Distance Education, has developed a large-scale online lecture system in relay mode.
The relay system is an information system capable of providing thousands to tens of thousands of simultaneous subscribers with real-time online education on a nationwide scale in three-tier relay mode through central server, intermediate server, terminal server and terminal on a computer network.
The system aimed to narrow the difference in the level of education between the capital and other provinces and between cities and countries by providing real time or non-real time lectures given by competent lecturers from the capital and other cities to all the schools across the country. Therefore, it has to be designed to transfer real-time lecture data from the capital to provinces, from provinces to counties and from counties to schools, not from the central or provincial server to all terminals.
First, he newly designed and realized a three-stage relaying system via centre-province-county-terminals. Then he solved some technical problems like increasing compression ratio and reducing network load to deal with increase in network traffic per subscriber of three to five times more than the existing video conferencing system, and enhancing the real-time transmission of video and audio data in the relaying mode for perfect performance of online lectures.
Second, he applied load balancing to avoid network overload at provincial, city and county network stations.
Next, as relay servers are installed in all provinces and hundreds of counties across the country, he correspondingly solved the problem of transferring and synchronizing history data between central, provincial and county servers so that a detailed report on attendance in the provinces, cities and counties can be made to the center.
Finally, he made it possible to hold multiple conferences including academic uniformity as well as online lectures.
The proposed system will make a positive contribution to narrowing the differences in education levels between the capital and other provinces by stepping up IT-based education.
...
