Jo Jan 29, 2025
A text can be represented by a vector of binary weights indicating the presence or absence of terms inside documents. Therefore, the correlation of terms is quantified by the frequency of co-occurrence or the internal product of the index term vector, but it has the limitation that it does not capture the joint relationship. As each element of the binary document vector is set to be 0 or 1, 0 and 1 are dealt with quantitatively and methods of multidimensional incidence tables and various statistics are used.
Maximum entropy is one method applicable to such problems, and it can represent a variety of models depending on which interaction terms are involved. The maximum entropy model is the basic principle of the maximum entropy classifier, and it constructs a model for all known factors and excludes the unknown factors. The basic characteristic is that the features are independent of each other, so the features useful to the final classification can be added freely, regardless of mutual influence.
The maximum entropy model with constraints on binary features is widely used in natural language processing, speaker identification, information retrieval, text filtering, machine translation, etc.
In lexeme equivocality removal using maximum entropy, when the context, i.e., neighboring words are given, the probability of each outcome (each meaning of the target word) is estimated and the variance of the neighboring words for each meaning is used to compute the probability. Either log-linear (log-linear) model or exponential (exponential) model is used to combine features. However, where the dependence structure between variables using maximum entropy is included, except for the relationship where the joint probability is zero, which would reduce the computational effort in training and prediction still remains a challenge.
Pak Il Chol, a researcher at the Faculty of Information Science and Technology, has proposed a joint distribution estimation method via marginal distribution constraint based on the maximum entropy principle in document data represented by binary weight vectors, and how to apply it to document classification and ranking. The proposed method allows determination of the target joint distribution using the marginal distribution obtained from the given data as a constraint. He has also proposed a method to search for the maximum dimension of the marginal distribution, except for the case of marginal probability 0.
The simulation results show that the proposed method for classification through marginal distribution estimation underperforms the simple Bayesian approach assuming conditional independence when the dataset size is small, but outperforms the simple Bayesian approach when the dataset size gets larger. He has also found that the ranking shows an improvement of about 4% in MAP over the two-valued independent retrieval model.
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Jo Jan 27, 2025
The adaptive algorithm plays an important role in ensuring the stability and performance of adaptive systems. In particular, in the case of time-varying systems where parameter invariance cannot be assumed, the performance of the adaptive algorithm becomes more important. The least mean square (LMS) algorithms and the recursive least square (RLS) algorithms have been widely used for the adaptive identification of time-varying systems. In order to apply them to time-varying systems, there have been many studies on the variation types of LMS and RLS algorithms.
Despite the efforts of many researchers, the study on improved adaptive algorithms with faster convergence rates, lower computational complexity and more improved tracking performance still remains an important task for scholars.
Based on the concept of distance in the parameter space, Kim Kwang Ho, a section head at the Faculty of Automation Engineering, has proposed a real-time identification algorithm and compared it with NLMS (Normalized LMS) and RLS. Through the comparison, he has found that the proposed algorithm shows desirable convergence and tracking performance as an adaptive algorithm for rapidly time-varying systems.
The numerical simulation results demonstrate that the proposed algorithm is more effective than other algorithms in adaptive identification for rapidly time-varying systems.
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Jo Jan 25, 2025
Efficiency and maximum output are the most important two goals to be analyzed in hydraulic turbines. Turbines normally operate in variable head conditions, so the tests to analyze their performance are frequently conducted for a selected number of power plant heads. Usually, they are limited to three heads: low, medium and high. The efficiency of water turbines is often expressed as the weighted average efficiency or arithmetic mean efficiency which is calculated from the results measured in the test heads. For the calculation of efficiency, it is essential to know several parameters such as kinetic and potential energy of water in its position and it is also necessary to know the flow rate entering the turbine. The flow rate of water through the turbine is determined as the volume of water flowing in the unit time and its unit is ㎥/s. The measurement of this quantity is one of the most difficult tasks for water turbine tests.
Measuring methods of flow rate of water turbines mainly include pressure-time method (Gibson), Winter Kennedy method, ultrasonic method and tachometric method. Winter-Kennedy method utilizes the static differential pressure between the outside and the inside of the turbine spiral due to the centrifugal force acting on the curved streams of liquid in the spiral case. This method is accepted as one of the simplest and the most convenient measurement methods of hydraulic power plant measurement technology and it is the most widely used in hydraulic power plants recently. This method is simple in the installation of measurement equipment. In addition, it does not disturb fluid flow and supports real-time measurement. The accuracy is about 1%.
Mun Yong Guk, a section head at the Electric Power System Institute, based on the investigation into the principles of flow measurement for hydraulic power plants and the literature on flow meters, has designed and manufactured a volute differential pressure flow meter and proved its effectiveness through simulations and field application.
The simulation results show that the flow coefficient K was 0.057 and that the accuracy was 0.616%, higher than the standard flow meter.
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Jo Jan 22, 2025
Hydrogen production by water electrolysis is the most widely used because of its low energy consumption, environmental friendliness and high purity. In water electrolysis, electrode overpotential is the main reason for the increase in power consumption. Therefore, lowering overpotential at the anode where oxygen evolution reactions occur is one of the ways to reduce the cost of hydrogen production. Lowering the electrode overpotential is also important for other electrochemical devices such as fuel cells in addition to water electrolysis. To solve this problem, it is important to develop an electrode catalyst with low electrode overpotential, high conductivity, high corrosion resistance and low cost. In addition, it is necessary to enhance the catalytic activity and stability of the electrode by providing high bonding ability between the catalyst layer and the collector.
IrO2 and RuO2 have been recognized as the most efficient oxygen evolution reaction catalyst in alkaline water electrolysis, but they are scarce and expensive. Hence, there has been a study to develop a non-precious metal electrocatalyst with high oxygen evolution reaction activity and stability and to improve the electrode performance in water electrolysis by enhancing the bonding ability between the catalyst layer and the collector.
Kim Sol Song, a researcher at the Institute of Nano Science and Technology, on the basis of the previous studies, has proposed a method of preparing NiCo2O4 electrocatalytic nano powder of spinel structure and combining it with a nickel foam support using polytetrafluoroethylene (PTFE) binder so as to enhance the performance of the oxygen evolution reaction electrode in alkaline water electrolysis. In addition, he has determined the content of the binder for not reducing the oxygen evolution reaction catalyst performance of the electrodes while enhancing the bonding ability between the catalyst powder and the support.
He has found that the onset potential of oxygen evolution reaction of the catalytic electrode reaches the standard when the content of the PTFE binder is about 20%, and that the potential changes are small and the catalytic electrode is stable in the continuous operation test for 12 hours.
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Jo Jan 21, 2025
The most widely used materials for covering greenhouses these days are plastic films of various types. However, plastic films are partly transparent to long wave thermal radiation. This is why all the methods recently proposed take into consideration the influence of the divisional transparence of those materials relative to long wave radiation on the whole radiation heat exchange.
In these methods, the radiation heat exchange in the radiation system including partly transparent bodies like glass or films has been calculated in different ways, but detailed quantitative analysis of the radiative structure when radiation goes through the partly transparent bodies has not been conducted. Furthermore, since different studies developed formulas with individual radiation properties, the amount of determinant became large, and therefore, calculation methods by computer was the only solution to the calculation of the heat quantity of resultant radiation and transmitting radiation, which took a long time for its calculation. Meanwhile, the study on determining the radiation heat quantity by multiple reflections, ray tracing and radiant light methods became more complicated than the effective radiation method. Introducing computers to technical calculations made it possible to calculate the heat quantities in greenhouses by the repeated radiation method, but programming also required much labor. In a word, they are complicated and less intuitive than the effective radiation method.
Kim Chol Gon, a researcher at the Faculty of Heat Engineering, has derived a generalized formula for calculating radiation heat exchange by the effective radiation method when both grey bodies randomly placed in the space are non-transparent and partly transparent.
Then, he has proved that the analysis result of the calculated characteristic of the daily temperature variation in a single-covering plastic filmed solar greenhouse by the derived formula is just the same as the one by the repeated reflection method.
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Jo Jan 20, 2025
With the rapid development of science and technology, the production of a large number of high-efficiency heat pumps has led to the active use of shallow geothermal energy with lower development cost than deep geothermal heat in many countries.
U-type GSHE (ground source heat exchanger) is a system used for shallow geothermal energy development where the heat exchanger is embedded in the ground and the geothermal energy is extracted from the heat exchange between the ground and the water flowing through the pipe.
Heat transfer models for mathematically modeling U-type GSHE and several experimental and numerical simulation methods have been proposed.
Previous studies used laboratory and field experimental data for comparative analysis, focusing on simulation calculations to enhance the performance and heat exchange efficiency of single well modes such as U-type ground source heat pump (GSHP) system, standing column well (SCW) geothermal heat pump system, and forced external circulation single well (FECSW) geothermal heat pump system. In addition, the simulation calculations and experimental results were compared, limited to individual modes, and three modes have never been studied in relation to one another.
In order to study and introduce these modes to realistic conditions, the heat exchange characteristics of individual modes under different environmental conditions should be studied and the heat extraction efficiency of each mode should be compared.
Choe Tok Gi, an institute head at the Faculty of Earth Science and Technology, has designed a 1:20 scaled model and made 10 experimental scenarios considering different experimental conditions such as soil moisture, groundwater convection, heat conduction, abstraction and injection interval selection. Then, he has conducted heat extraction experiments according to the scenarios and compared the effectiveness.
The results show that in the U-type mode it is reasonable to use porous casing or gravel-filled materials because there is no possibility of groundwater contamination in the case of water without antifreeze, in the standing column well mode the convective heat exchange between the circulating water and surrounding ground should be increased as much as possible to increase the geothermal extraction, and in the forced external circulation single well (FECSW) mode it is effective to inject water into the top of the borehole during heating and into the bottom during cooling to generate convection.
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