Jo Aug 22, 2024
While water consumption increases steadily due to various socio-economic factors such as population concentration in cities, increase in industrial wastewater along with the rapid development of industry and the improvement of people’s living and cultural level, a large amount of water resources are contaminated, so it is vital to increase the amount and efficiency of wastewater treatment at sewage treatment plants.
The trend in the development of global wastewater treatment processes is the introduction of a large number of aeration facilities that provide biochemical oxidation of organic pollutants to the plant. The problem is that with the increase of sewage treatment plants, its operating cost, especially the power consumption of the fan, increases. Aeration is one of the most energy-intensive processes in the wastewater treatment plants and any improvement in it is likely to enhance the efficiency of the overall process.
Of course, 15~20% of electric power consumption can be reduced by controlling the blast volume according to the influx of sewage and its composition, but upgrading of the aerating facilities for improvement of the oxidation effect is more stable and effective.
Ji Chol Hyok, a researcher at the Nano Physics Engineering Institute, has proposed a sewage clarification method by micro-bubble aeration for improving the clarification efficiency and reducing its cost, and compared its effectiveness with conventional ones, thus demonstrating its superiority.
He chose an 80㎥ tank of a pig farm as an experimental reaction tank and measured some sewage treatment parameters.
The results showed that the proposed method ensured up to 90.8% of removal efficiency of COD (chemical oxygen demand) while the conventional ones guaranteed only 86.4%, and that the overall removal efficiency is over 20% higher than bottom aeration when the temperature is low.
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Jo Aug 20, 2024
The main purpose of laser surface melting is to obtain great strength of alloy surface and to ensure high productivity. The laser surface melting of AZ31B Mg alloy is very important in determining the microstructures and strength of alloy surface. The laser power and laser scanning speed in the laser surface melting play important roles in the temperature change and the morphology of the melting region. However, massive amounts of data on laser surface melting process take a lot of time and cost for detailed experiments of investigation.
To date, there has been an impressive amount of research on this topic, especially to investigate the melting flow and its thermal transfer by laser.
Kim Yong Il, a researcher at the Nano Physics Engineering Institute, has estimated the effects of laser power and laser scanning speed on the temperature distribution of AZ31B Mg alloy using COMSOL Multiphysics 5.3.
Firstly, based on finite element analysis software, he built a finite element model of laser surface melting Mg alloy. Then, he conducted a numerical computation with the variables used in the experiments of previous works. The numerical computation by simulation was done with different laser powers at the laser scanning speed of 360mm/min, and with different laser scanning speeds at the laser power of 2 000W.
The simulation results seem to be in good agreement with the experimental results based on infrared temperature measurement.
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Jo Aug 18, 2024
Determination and description problems are two basic problems of Formal Concept Analysis (FCA). Since most methods of determining the concept lattice are based on generation of non-neighbor concepts, the process of analyzing the neighboring relation between generated concepts is required to solve the description problem.
Pak Chol Hong, a researcher at the Faculty of Applied Mathematics, has proposed some new efficient algorithms for simultaneously describing the concept lattice and its hierarchy-matrix without this process. The hierarchy-matrix is a successful description of the concept lattice, by which any software can autonomously understand the information of hierarchy of the concepts. The focus of his algorithms is on generation of neighbor concepts tested for canonicity and registering the neighbor information in the hierarchy-matrix sequentially.
He drew the following conclusions.
First, the concepts of subcontexts induced by a given concept are lower (or upper) ones of the concept and all lower (or upper) neighbor concepts of the concept are denoted by the antitone Galois connections on the subcontexts.
Second, all lower (or upper) neighbor concepts of a given concept are generated by the antitone Galois connections on the subcontexts and restricted by the rank of the subcontex-matrices.
Third, the key aspects of the efficiency of the concept lattice and its hierarchy-matrix based on the generation of lower neighbor concepts (BCLHMLN) and the concept lattice and its hierarchy-matrix based on the generation of upper neighbor concepts (BCLHMUN) are generation of neighbor concepts on the subcontext, the equivalence class of the object or the attribute with respect to adjoin mappings of Galois connections, the design of canonicity test and the utilization of the histories of generated lower (or upper) concepts.
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Jo Aug 16, 2024
In recent years, a number of modal identification methods based on continuous wavelet transform (CWT) have been developed. The time variation of the instantaneous amplitude and phase of each mode component within measured signals can be considered by time-frequency representation. In the time-frequency plane, wavelet analysis decomposes the measured signals into a series of curves called ridges which directly express the amplitude and phase of each mode component within the measured signals. Thus, by the extraction of ridges and the value of CWT along the ridges, modal parameters are identified from the measured signals. The measured signals are free-decay responses of mechanical structures which are processed by time-frequency transform. In this process, the challenge is to determine ridges exactly. When the frequency window of CWT includes closely spaced natural frequencies, the ridge is distorted and a pseudo mode is generated.
Existing methods for modal identification by CWT give correct results when the frequency window includes one natural frequency. Thus, it is necessary to build a theoretical basis for correctly determining closely spaced natural frequencies by considering the distortion of ridges and determining the reasonable bandwidth when the frequency window includes two closely spaced natural frequencies.
Ri Yong Ho, a researcher at the Science Engineering Institute, has proposed a formula for mathematically considering the distortion of characteristics of a ridge when the frequency window includes two close mode components, and conducted a numerical simulation on MATLAB to verify it.
The simulation verified the accuracy of the proposed formula.
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Jo Aug 15, 2024
Most rare earth minerals contain uranium, thorium and their decay products, so the technology for removing radioactivity plays a key role in the production of rare earths, especially in reduction of production cost, environmental protection and application of rare earth products. Furthermore, complete removal of trace radioactive substances existing in rare earth products is highly significant for wide application of rare earths and in medicine for health protection and cure.
Up to now, many methods for separating and removing radioactive substances have been reported: ion exchange method, solvent extraction method, co-precipitation method, etc. among which co-precipitation method is very effective for infinitesimal amount of radioactive materials such as radium and actinium. In an extremely dilute system, trace radioactive substances are usually separated by co-precipitation with carriers. Porous materials such as diatomaceous earth, activated carbon, fire brick, silica gel and alumina, barium sulfate, strontium sulfate , iron hydroxide(Ⅲ) and aluminum hydroxide are used as carriers.
In an attempt to remove trace radioactive substances existing in industrially produced rare earth carbonate, Kim Chol Ju, a researcher at the Faculty of Chemical Engineering, has conducted an experimental study of the radioactivity removal characteristics of [Ba-Fe] co-precipitation method, the combination of BaSO4 and Fe(OH)3 co-precipitation methods.
In the [Ba-Fe] co-precipitation process, the total removing rate is over 92% and loss rate of rare earths is less than 0.1% in the case of industrially produced rare earth carbonate.
As this method enables prompt and effective removal of radioactive elements such as uranium, thorium, radium, radiolead, etc., remaining in extremely small quantities in rare earth compounds, it could be applied in the field of hydrometallurgy in which rare earth compounds are produced from several kinds of rare earth minerals and in other fields of production and application of rare earth materials from rare earth compounds.
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Jo Aug 12, 2024
It is very important to correctly model and identify damping in all vibration systems of physical devices that involve damping.
A variety of methods and techniques of damping identification have been developed, most of which can be classified into frequency domain (circle-fitting method, half-power bandwidth method, wavelet transform method, etc.) and time domain (logarithmic decrement method, least squares complex exponential (LSCE) method, Smith least squares method, Hilbert transform method, etc.). Some of the damping identification methods in frequency domain have high accuracy, but the algorithm is very complicated, which restricts its wide application in practice.
Matrix method and modal method are also used for damping identification. In the modal method, modal identification is a must as the damping characteristics are decided by means of modal damping ratio. Modal identification is complicated and it is difficult to ensure the correctness of modal parameters. In the matrix method, which directly identifies the damping matrix by experimentally measured FRF, the imaginary part of experimentally measured FRF is decided for damping identification. This method is highly accurate and it is possible to decide the damping ratio of mechanical structures of lower DOF. However, it is very difficult for all numbers of DOF as mechanical structures are of high DOF in practice.
Kim Won Ju, a researcher at the Faculty of Mechanical Science and Technology, has proposed a method for correctly and simply deciding the damping characteristics of the whole system using the experimentally measured FRF and the FE model. Using the experimentally measured frequency response function matrix, he built an experimental model and combined it with the FE model to enlarge the experimental identity data to the whole system.
Comparing his method with preceding ones through simulations and error analysis, he verified its precision and effectiveness.
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