Jo Oct 9, 2023
Generally, rotary angular sensors can be classified into two types ― sensors based on changes in an electric field and a magnetic field, and optical sensors ― according to the physical principle used for measurement. Among them, optical sensors that operate on visible light or infrared light offer advantages of contactless measurement and insensitivity to electric and magnetic fields.
Yu Nam Chol, a researcher at the Science Engineering Institute, has proposed an absolute rotary angular sensor with a nonlinear transparent disc between light source and light dependent resistors (LDR).
In this sensor, the absolute rotary angle is measured by the output resistance of double LDRs which has a linear change by nonlinear transparency of the disc in a range of 0~360° according to the characteristics of LDR’s resistance via irradiance.
This sensor’s advantage is that it is immune to shock and vibration as it has a nonlinear transparent disc instead of a binary coded disc and it has a large gap between the disc and the optical sensor. It means that it is easy to make a sensor. The disc is made of fiber glass and covered up by a self-adhesive tape whose transparency changes nonlinearly in a range of 0~360°.
Another advantage of this sensor is that its resolution is determined by an AD converter in the signal processing circuit as the output signal of LDRs is analogue. Therefore, this rotary angular sensor is provided with a high resolution in a range of 0~360°.
For more information, please refer to his paper “Improving of Characteristics of Rotary Angular Sensor Using Nonlinear Transparent Disc” in “International Journal of Sensors and Sensor Networks”.
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Jo Oct 5, 2023
The Shockley-Read-Hall (SRH) lifetime is a very important parameter in silicon bipolar power devices that determine switching characteristics.
As for carrier lifetime control, recombination center levels in impurities-doped silicon are dependent on the properties of impurities and silicon materials, but not on the amount of impurities.
Preceding researchers studied diffusion of impurities and radiation action for recombination center levels by means of Au, Pt, electron irradiation, proton irradiation, etc.
Yu Nam Chol, a researcher at the Science Engineering Institute, has determined new recombination center levels in n-type silicon with gold and platinum, and investigated the effectiveness of the levels for low-level and high-level injection lifetime control.
At injection levels below 3×1013cm-3, reverse recovery technique was used, while open-circuit carrier decay (OCCD) technique was employed at levels above 1015cm-3. He performed lifetime measurements in the temperature interval of 303~403K. He studied the deep levels introduced by diffusion using DLTS, and calculated the corresponding SRH lifetime, and finally compared it with the measured values. The comparison suggests that low-level lifetime is mainly controlled by defect B located at 0.54eV below the conduction band, and high-level lifetime by defect A located at 0.25 eV.
The result shows that the recombination center levels estimated from DLTS measurement are dominant levels in n-type silicon doped gold and platinum.
If further information is needed, please refer to his paper “Analysis of recombination center levels in gold and platinum doped n-type silicon” in “Journal of Power electronics and devices”.
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Jo Sep 28, 2023
High-intensity focused ultrasound (HIFU) is being widely used to treat tumors in prostate, liver, kidney, pancreas, bone, breast, and uterine fibroids.
Several numerical simulation methods have been proposed for ultrasound propagation. They include Rayleigh–Sommerfelt diffraction integral, Khokhlov–Zabolotskaya–Kuznetsov (KZK) method, FDTD method, etc. In most methods, FDTD algorithm was applied to compute the temperature field caused by ultrasound exposure. In some methods, HIFU heating model by the finite element method was proposed to solve the linear Westervelt equation and the Pennes bio-heat transfer equation.
Kim Sang Jin, a researcher at the Faculty of Physics Engineering, has proposed a new numerical simulation method to predict the temperature distribution in human tissues during ultrasound tumour therapy. First, in order to calculate the pressure distribution in human tissue, he solved the nonlinear Westervelt equation using finite difference time domain (FDTD) algorithm. Second, to calculate the temperature distribution in human tissue, he solved the Pennes bio-heat transfer equation using lattice Boltzmann method (LBM). To verify the theoretical analysis, he compared the results with experimental measurements of previous works.
The results showed that the FDTD-LBM provides a high degree of accuracy in the simulation on the sound pressure field and temperature field in human tissue during high-intensity focused ultrasound therapy, and that the temperature value at focal point clearly increases and the focal length gets smaller with the increase in the ultrasound frequency.
For more information, please refer to his paper “Numerical Simulation Method for Prediction of HIFU Induced Lesions in Human Tissue: FDTD-LBM” in “Physics of Wave Phenomena”.
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Jo Sep 27, 2023
During a CC process, molten steel is continuously poured into the water-cooled mold through SEN, which forms a solidified shell of sufficient thickness when slab is pulled out.
Slab quality, particularly regarding surface and internal cracks, is closely related to the turbulent flow in the mold and the heat transfer through the heat face on the mold copper plate during solidification in a CC process.
If thermal stress by temperature gradient in the mold copper plate is excessive, strain occurs in the mold copper plate, and with increase in casting time and constant iteration of heating and cooling processes on the copper plate, microcracks are generated, which might cause an irretrievable accident in the copper plate.
In the past, for most numerical simulations on temperature field in the mold copper plate, empirical formulas on already-developed heat flux density or the value obtained by applying the temperature measured from the thermocouple inserted in the mold copper plate to the inverse finite-element model were used, and the heat exchange coefficient between the cold face on the copper plate and cooling water calculated by means of Dittus-Boelter’s formula was applied to the boundary condition of the cold face on the copper plate.
These methods have the advantages of saving time for simulation calculation, but since the temperature value measured from the thermocouple inserted in the mold is not precise enough, it is difficult to ensure the accuracy of simulation results and to reflect the effect of as many factors as when using empirical formulas.
Moreover, few studies have been found on the temperature field in the mold copper plate with heat contact resistance like mold flux, allowance, and coating layer under consideration by a one-quarter model of Full SEN-3D FEMM.
Om Sang Chol, a section head at the Faculty of Materials Science and Technology, has simulated the temperature field in a Full SEN-3D FEMM considering the flux character of molten steel through SEN, the mold flux, the coating layer and the stainless back plate. In addition, he has carried out a simulation on the temperature field and thermal stress and strain on mold copper and stainless back plates by applying the maximum heat flux density on the heat face of copper plate obtained from the simulation to the element model of the mold copper plate.
Thus, he was able to determine reasonable design factors for water slot structure on a mold copper plate.
If further information is needed, please refer to his paper “A simulation method for the optimization of cooling water slot structure in slab continuous casting mold combined with submerged entry nozzle” in “The International Journal of Advanced Manufacturing Technology” (SCI).
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Jo Sep 25, 2023
Mechanical power loss in pneumatic hammers is caused by the friction between parts in relative motion, and wear is among failure mechanisms of the top hammer. Therefore, it is important for a high performance and a longer service life of pneumatic hammers to reduce the friction force between parts in relative motion.
Yang Un Hyok, a section head at the Faculty of Mining Engineering, has presented a novel approach to quantitatively determine the friction force and considered it in the simulation model of a pneumatic hammer.
First, the friction force between a piston and a cylinder in a small pneumatic hammer was measured using an experimental setup at different inlet pressures. Then, numerical analysis of a small pneumatic hammer performance was performed by a model with the friction force in consideration using AMESim software.
The result showed that the friction force was about 0.8N under the horizontal installation when there was no pressure supply, but it increased significantly, 20 times greater than that without pressure supply, due to aerodynamic action by compressed air leaking from the annular gaps between the cylindrical matching surfaces of the components. In addition, it indicated that friction increased from 10.27 to 16.7 N due to the increase in inlet pressure and mechanical power loss in the pneumatic hammer was about 10% of impact energy.
The proposed approach could significantly reduce errors between the simulated and the measured values for the impact energy due to the ignoring of friction force.
The details of this are found in his paper “Numerical analysis of a small pneumatic hammer performance based on evaluation method of friction force” in “Shock and vibration” (SCI).
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Jo Sep 24, 2023
Offshore structures such as oil platforms, wind turbines and marine renewable energy devices are constantly exposed to a harsher environment, which requires a deep understanding of the wave interaction with marine structures such as bottom-mounted ones to make them reliable, safe and cost-effective.
Rim Un Ryong, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has considered the interaction between a monochromatic incident wave and multiple bodies mounted on an undulated seabed using an exact Dirichlet-to-Neumann (DtN) boundary condition on an artificial circular cylindrical surface by which the fluid domain is divided into an interior subdomain and an exterior one.
The proposed method was validated by precedent results with a bottom-mounted circular cylinder and showed good agreement. Then, it was extended to the cases of a single rounded-rectangular cylinder, a 2*2 array and a 2*1 array with a paraboloidal shoal.
The proposed model can be applied to the study of the wave interaction with multiple bottom-mounted marine structures with irregular configurations whether the seabed is flat or undulated.
If further information is needed, please refer to his paper “Wave interaction with multiple bodies bottom-mounted on an undulated seabed using an exact DtN artificial boundary condition” in “Journal of Ocean Engineering and Marine Energy” (EI).
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