Jo Aug 30, 2024
Stress sensitivity and the elastic outer boundary (EOB) condition have a great effect on the analysis of the characteristics of fluid flow in a reservoir. When analyzing the characteristics of fluid flow, researchers considered the stress sensitivity and the EOB condition separately, not simultaneously. Therefore, errors were inevitable during the analysis of well testing.
Kim Song Chol, a section head at the Faculty of Earth Science and Technology, has established a well-testing model for the DPR, considering the stress sensitivity and the EOB condition simultaneously and presented its semianalytical solution.
First, on the basis of the consideration of the EOB condition and stress sensitivity of permeability (SSP), he built a seepage model for the DPR with the EOB by considering wellbore storage and skin. To solve this model, he changed a nonlinear partial differential equation into a linear form by introducing an effective well radius and applying Pedrosa’s transformation and perturbation transformation. Then, by applying the Laplace transformation, he obtained an analytical solution in the Laplace space, and by numerically inverting it, he obtained a solution in the real space, thus drawing the curves of pressure and pressure derivative (PPD).
By comparing the proposed model with the one with the EOB without consideration of SSP, he verified its effectiveness.
For more information, please refer to his paper “Well-Testing Model for Dual-Porosity Reservoir considering Stress-Sensitivity and Elastic Outer Boundary Condition” in “Geofluids” (SCI).
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Jo Aug 29, 2024
Developing high quality learning contents (LCs) and improving their interactions in online learning with e-learning contents is important for greater cognitive abilities of a learner.
Studies on the interactions in online learning have increased and focus has been placed on its increased effectiveness. However, there has been less discussion on the relationship of LC design to learner’s cognition. In addition, there are few qualitative data on the interactions between a learner and LC, and less has been known about the effectiveness of self-feedback in learning processes on increasing the cognitive abilities of a learner. On the other hand, non-structural interaction brings about negative results to learners and cannot improve learner’s cognition, which means less cost-effective. This clearly shows that the interactions with LCs should be improved for successful online learning and, to this end, LCs with a self-feedback structure be designed to suit the intrinsic features of a learner.
Kim Jang Hak, a section head at the Faculty of Distance Education, has designed LCs with new self-feedback structure by considering the personalized characteristics of a learner and described a method for improving learners’ cognition abilities through improved interactions between a learner and LCs.
Based on the previous research findings on feedback and interaction, he divided the learning object into knowledge unit learning nodes and managed the learning control and appropriate interactions through self-feedback. Then, he conducted an experiment to investigate what improvement was made in the learner’s cognition by putting the proposed method into practical application.
The result showed that the self-feedback is a significant factor that gives positive influence on the learner’s cognition.
You can find the details in his paper “Improving Learners' Cognition through Designing Learning Contents with Self-feedback Structure and Advanced Interaction in Online Learning” in “Informatica” (SCI).
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Jo Aug 28, 2024
In general, temperature and structural loads may generate deformation of optomechanical systems and this deformation has a negative effect on the optical performance.
To analyze and minimize the effects of this deformation on the optical performance is a very important step in optomechanical structure design.
Kim Chol Hyon, a section head at the Faculty of Physical Engineering, has presented an integrated design process for an optomechanical system based on multidisciplinary optimization.
The proposed integrated optimal design process consists of finite element analysis (FEA) by ANSYS Workbench, MATLAB optomechanical transfer program, optical analysis by ZEMAX and multidisciplinary optimization solver by Isight.
By using optimum bipod flexures, he decreased the wavefront error of the primary mirror of Cassegrain Telescope from 29.9 nm to 16.1 nm.
For more information, please refer to his paper “Multidisciplinary integrated optimal design process for Optomechanical System” in “Applied optics” (SCI).
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Jo Aug 27, 2024
In order to increase the life of blast furnaces, the wall of blast furnaces must be equipped with cooling equipment. The cooling stave is commonly manufactured with cast iron, steel and copper as cooling materials. Among them, copper is widely used as a cooling stave material in blast furnaces because it has good thermal conductivity and high cooling capacity.
The structure and size of the copper cooling stave depend on the structure, size and operation characteristics of the blast furnace. A HIsmelt (high-intensity smelt) furnace has some differences from conventional ones in the fuel used and in the structure.
In order to manufacture a copper cooling stave for the HIsmelt furnace, O Song Ryong, a student at the Faculty of Materials Science and Technology, has designed its optimal structure for safe operation and its casting process.
For the design of the copper cooling stave, he determined the diameter of a copper tube installed in it and its arrangement, the wall thickness and the flow rate of cooling water according to the size. He examined the stability of the copper cooling stave by simulating the cooling capacity under the HIsmelt furnace operating conditions using Fluid Flow (Fluent) of Ansys Workbench programme (version 15).
For the design of the casting process, he made a casting plan and simulated on ProCAST the occlusion of flow channel of the cooling water due to the melting of the copper pipe in the mold during casting and solidification of the molten metal and the supply of cooling agent to prevent the separation of the cooling stave and the copper pipe. The cooling agent was determined to be water (94%) + air (6%).
For more information, please refer to his paper “Structure and Casting Process Optimization of Inclined Copper Cooling Stave for Hismelt Furnace Using Process Simulation Based on Taguchi Method” in “International Journal of Metal casting” (SCI).
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Jo Aug 25, 2024
During hot working, material exhibits a complex nonlinear relationship between flow stress and thermo-mechanical processing parameters, due to the simultaneous occurrence of work hardening and softening. In order to express more accurately the nonlinear relationship between flow stress and thermo-mechanical processing parameters, a large number of constitutive models have been developed, which can be divided into three categories: a phenomenological constitutive model, a physically-based model and an artificial neural network model.
Phenomenological constitutive models are widely used to predict the hot deformation behavior of metallic materials including alloy steels and titanium alloys, due to their simple equations and small computational quantities. Physically-based constitutive models consider not only the thermo-mechanical processing parameters, but also the physical mechanism such as dislocation movement and thermal activation during hot working process. Compared with phenomenological constitutive models, physically-based models are more complex to establish. However, due to their high prediction accuracy, physically-based models are widely used in many commercial finite element analysis programs.
The Ti-6Al-2Sn-2Zr-3Mo-1.5Cr-2Nb (TC21) alloy is a 1 100MPa damage tolerance titanium alloy, with ultimate tensile strength of 1 130MPa, yield strength of 1 020MPa, elongation of 12%, toughness value of 80MPa·m1/2, and crack extension rate of 1.47×10-5 mm·cycle-1. Due to a good combination of strength and toughness, the application value of TC21 alloy is much higher than that of TC4 alloy.
So far, there have been many studies on the hot deformation behavior of TC21 alloy. However, few studies have been conducted or reported on other constitutive models for TC21 alloy including physically-based constitutive models.
Pak Hun, a student at the Faculty of Materials Science and Technology, has constructed phenomenological and physically-based models for predicting more accurately the hot deformation behavior of TC21 alloy, and compared their predictability. To this end, he carried out isothermal uniaxial compression tests at different strain rates in α + β two-phase and β phase fields. Based on the experimental data (flow stress curves), he developed a modified Johnson-Cook (m-JC) model, a strain-compensated Arrhenius type (sc-AT) model and a modified Zerilli-Armstrong (m-ZA) model for Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy. In addition, he compared the prediction accuracies of the three constitutive models using standard statistical parameters.
The results showed that the proposed m-ZA model is the preferred constitutive model to predict the hot deformation behavior of Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy.
You can find the details in his paper “Comparisons of phenomenological and physically-based constitutive models for Ti-6Al-2Zr-2Sn-3Mo-1.5Cr-2Nb alloy” in “Applied Physics” (SCI).
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Jo Aug 23, 2024
Today, electricity is not only the main power of industry but also the main energy source indispensable for the people’s lives, and the demand is in a steady rise. Therefore, it is important for the power industry to reliably ensure the operation of power systems in a scientific and technological way.
The trend of global power system development is to establish and operate a flexible AC transmission system (FACTS) that can rapidly control the main characteristics of power system like voltage, frequency, active and reactive power, etc.
One of the important issues in establishing and operating such FACTS is to reduce the total harmonic distortion rate in power system equipment including reactive power compensators, for power quality.
Ri Tong Chol, a researcher at the Faculty of Electrical Engineering, has established a fuzzy logic controller for controlling the DC bus voltage of the converter to improve the response in a distribution grid static synchronous compensator (DSTATCOM) which is widely used in FACTS and to ensure normal operations of the compensator under asymmetric conditions and grid voltage variations, and verified its effectiveness through computer simulation tests.
For effective operation of DSTATCOM, the DC-side capacitor voltage of the voltage-type converter should be kept constant, and for this purpose, a fuzzy PI controller was used.
The simulation model of DSTATCOM was designed in a three-phase distribution system using SIMULINK tool in MATLAB environment and the performance of DSTATCOM with fuzzy logic control algorithms was simulated and analyzed under distorted PCC voltage due to unbalanced nonlinear load.
The harmonic distortion factors (THD) of the power supply current, load current and PCC voltage on A were measured to be 3.6%, 26.0% and 4.4%, respectively, using the proposed fuzzy logic controller of DSTATCOM.
The results confirmed that the controller performance improved with a power supply current of 3.6%, while the THD of PCC voltage was 4.4%.
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