Jo May 8, 2026
The rotor of a steam turbine is a main element for converting thermal energy into mechanical energy, and its lifetime affects the safety of the whole unit. The units in power plants face several constraints during operation. For instance, the start-up time of a steam turbine is affected by thermal stress. Especially, due to the heavy variation of operation environments, some features of a rotor such as thermal stress, apparent expansion and thermal strain get worse. If they exceed the limitation, the corresponding elements are damaged or even destroyed, causing fatal accidents.
For rapid start-up of a unit, it is necessary to find out its appropriate start-up curve, and it should be operated accordingly to improve the safety and economic efficiency of the plant.
Optimization of turbine start-up in power plants has been studied in many aspects so far. Most of the models were built for real-time simulations. They reflected the features of stage groups of turbine using real-time operation database and various equations for quickness, and in most cases, the efficiency of stage groups were considered to be invariable. In order to determine the thermal stress exerted on the rotor during the start-up and shut-down of a turbine, it is necessary to accurately evaluate the inlet and outlet parameters of the nozzle and blades at each stage, not stage groups. Therefore, detailed calculations for each stage should be conducted to determine the pressure and enthalpy at the nozzle and blades, and their relative internal efficiency.
Rim Ju Yong, a researcher at the Faculty of Thermal Engineering, built a model of a 210MW subcritical condensing steam turbine for a power plant to determine the parameters mentioned above, and composed a special program with VC. Then, according to the operation regulation of turbine, he framed various cold start-up schedules. After that, he calculated the steam parameters on the parts of all stages by using the special program, and studied the temperature field and stress field by the finite element method (FEM).
Taking the calculation results as sample data, he established a regression model to determine the stress at the risk point of rotor. Based on the regression model, he determined the optimal cold start-up schedule using genetic algorithm (GA).
The results show that the fatigue lifetime increases by 20% with the decrease in the maximum von Mises stress by 1.4%, and the start-up period of the optimized schedule is reduced by 124min compared to the original one.
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Jo May 7, 2026
In information security systems, a single authentication mechanism plays a very important role in information processing systems as it provides convenient and secure authentication in multi-Web service environments today when service systems are growing rapidly, and thus there is a lot of research on it around the world. In particular, the access control schemes in single authentication mode have been studied a lot worldwide.
Verifying user’s trust in systems and performing a variety of access controls are very important as a large number of attacks on service systems are attributable to the malicious behavior of registered users.
The characteristics of SSO system make it very difficult to apply the traditional RBAC (Role Based Access Control) model directly because the roles of each service system may be the same but the corresponding rights may be different. Thus, there has been a lot of research on applying RBAC models that can facilitate user management to single authentication systems.
However, the previous dynamic access control schemes based on RBAC model failed to propose a detailed access control scheme according to user’s behavior changes.
Having analyzed the characteristics of the single authentication system and the reliability-based access control scheme, Myong Kwang Min, a section head at the Faculty of Information Science and Technology, has proposed a secure and flexible access control system for SSO system by combining the entropy weighting and the reliability-based access control scheme, and evaluated its performance.
The evaluation results show that the proposed access control model is superior to all single authentication schemes and trust-based access control schemes.
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Jo May 5, 2026
Field Programmable Gate Array (FPGA) is an important component for modern digital systems. FPGA is a semi-custom integrated circuit for implementing arbitrary logic functions of users by using look-up tables. FPGA has two main resources. One is logic resource and the other is interconnect resource. The logic resources implement the logic functions of user circuits and the interconnect resources connect logic resources to each other. Logic resource includes look-up table, flip-flop and multiplexer, and a net is an example of interconnect resource. When you write the user circuit into FPGA, FPGA is said to be configured. It is possible to write circuits into FPGA several times, i.e., FPGA has reconfigurability.
FPGA test is a processing for finding out whether there is a fault in FPGA’s resources or not, while FPGA diagnosis is for finding out what fault is in which resource of FPGA. There are two methods for testing and diagnosing FPGAs. One is application-independent method and the other is application-dependent method. The application-independent method can test all resources of FPGA, and the application-dependent method can test only the resources used in the circuit mapped into FPGA.
Factories increase the manufacturing yield of FPGAs by performing application-dependent tests and diagnoses. Application-dependent tests and diagnoses of FPGAs are also important to users because there may be FPGAs that have not passed application-independent tests. In FPGAs, the interconnect resource occupies more chip area than other resources. Therefore, faults in FPGA are mostly interconnect faults.
Two methods have been mainly employed for application-dependent tests and diagnoses of interconnect faults of FPGA. These previous methods are all for testing and diagnosing interconnect of circuits with look-up tables and flip-flops when mapped into FPGA. The circuits mapped into FPGA often include multiplexers in addition to look-up tables and flip-flops. The previous methods are not enough for testing and diagnosing the interconnect of circuits that include multiplexers when mapped into FPGA because the function of multiplexers cannot be changed arbitrarily, unlike look-up tables.
Son Yong Jun, a researcher at the Faculty of Electronics, has proposed a new method for diagnosing interconnect faults of circuits with multiplexers when mapped into FPGA.
This method is an application-dependent method for diagnosing only interconnects of FPGA used in circuits. What is special about this method is to use the hierarchical structure of multiplexers and a single-term function.
The proposed method can be applied to diagnosing any single stuck-at fault and bridging fault in the net of circuits comprised of look-up tables and multiplexers.
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Jo Apr 28, 2026
Composite shell structures with varying thickness are often used in different industrial fields because it could exhibit structural advantages at minimum material cost.
Recently, many works have been reported for static and dynamic analyses of different structures with variable thickness. Various methods such as Haar wavelet method, Jacobi-Ritz method spectral-Tchebychev method, dynamic stiffness method, finite element method and meshfree method have been employed for numerical analysis of composite structures. In recent years, the meshfree method has attracted significant attention of many scholars.
Jo Ju Chol, a researcher at the Faculty of Mechanical Science and Technology, developed a meshfree Jacobi-radial point interpolation (Jacobi-RPI) method for free vibration and stochastic response analyses of laminated composite sectorial and annular plates with straight fibers and variable thickness.
First, he adopted the Hamilton’s principle to establish the equations of motion of the laminated composite sectorial plate with straight fiber and variable thickness in the framework of first-order shear deformation theory (FSDT). Then, he approximated the displacement components of the plate by using the meshfree Jacobi-RPI shape function. As the laminated annular plate with straight fiber is not symmetrical, he obtained its motion equations by combining the equations of several sectorial plates.
He validated the accuracy and reliability of the proposed method through a sufficient number of numerical studies.
For more details, you can refer to his paper “A meshfree formulation for free vibration and stationary stochastic response analyses of laminated composite annular plate with straight fibers and variable thickness” in “Acta Mech” (SCI).
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Jo Apr 26, 2026
As is known, in a ball screw–driven servomechanism, the servo motor and the ball screw can be connected directly or via a tooth belt drive, a gear pair or a commercial reducer. However, more complex systems cause more kinematic errors in the machines, so in precision machine tools, the former is more common, but in conventional machine tools it is more economical to use the latter. This is because the latter can improve the resolution in an open system, replace the output of a high-speed low-torque servo motor with the output of a low-speed high-torque unit, significantly reduce the load inertia torque in terms of the motor shaft to favor inertial matching and has the potential to significantly reduce electric consumption by reducing the size of the motor. Especially, tooth belt drives are commercially available and inexpensive but have a relatively small backlash compared to other speed reducers, which makes them a large part of industrial applications. On the other hand, since the former is a special case of the latter, the results of the latter study can also be applied to the former. Certainly, the study on optimal selection of the key components of ball screw–driven servomechanisms with tooth belt drives is a subject of great socio-economic significance.
Jang Rim Chol, a researcher at the Faculty of Mechanical Science and Technology, proposed a method for determining the optimal combination of the key components (servo motor, ball screw, and tooth pulley pair) of a ball screw–driven servomechanism with a tooth belt drive transmission. The proposed method deals with a more generalized optimization problem because it focuses on the optimization of a ball screw–driven servomechanism with a reducer (tooth belt drive transmission) unlike previous works that focused only on the optimization of a system where the servo motor and the ball screw are directly connected.
The proposed method enables optimal selection of the key components of a ball screw–driven servomechanism for different purposes including machine tools, without subjective selection or prediction of some components or some parameters for different objective functions.
You can find the details in his paper “Selection of key components in ball screw–driven servomechanisms with toothed belt drive transmission for machine tools through combinational optimization” in “The International Journal of Advanced Manufacturing Technology” (SCI).
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Jo Apr 24, 2026
Hydrogen, the cleanest future fuel, can replace fossil fuel based on carbon. During the past decades, many methods for hydrogen production and storage have been studied for practical use. In particular, storage and transport of hydrogen has recently become a focus of intensive research for large-scale application of hydrogen energy systems.
One of the major challenges in the quest for feasible hydrogen-fueled vehicles is to develop lightweight materials with high hydrogen densities (>5wt %) which can absorb and release hydrogen in the range of 1-10 bar and 298-473 K.
Recently, perovskite materials have emerged as a multifunctional material for photovoltaics, luminescence, photocatalytics and hydrogen storage applications.
Ri Sol Hyang, a lecturer at the Faculty of Online Education, theoretically investigated the materials properties such as structural, electronic and lattice dynamics properties and mechanical and dynamical stabilities of the hydride perovskites ACaH3 (A = Li, Na) in cubic phase for its application as a hydrogen storage material by using the first-principles calculations.
The results show that cubic LiCaH3 is regarded as a potential H2 storage material due to its high H2 storage capacity, stability and suitable dehydrogenation temperature.
For more information, please refer to her paper “Perovskite-type hydrides ACaH3 (A = Li, Na): computational investigation on materials properties for hydrogen storage applications” in “RSC Advances” (SCOPUS).
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