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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Jo Sep 19, 2023
Ti2AlNb-based alloys are under continuous development as promising candidates for advanced automotive and aerospace applications due to their good creep resistance, low density and optimal balance of strength and elongation at the elevated temperature.
Ti2AlNb-based alloys can be deformed not only into sheet or rod, but also into complex types of parts such as turbine engine blades. However, deformation processing of Ti2AlNb-based alloys is inevitably performed at high temperatures because of their limited plasticity at room temperature.
Therefore, investigation into the flow behavior and reasonable deformation criteria of Ti2AlNb-based alloys is quite crucial for design and control of industrial hot working processes.
In recent years, some scholars have developed constitutive models and processing maps of as-cast Ti2AlNb-based alloys and powder metallurgy Ti2AlNb-based alloys. These powder metallurgy Ti2AlNb-based alloys showed microstructure with coarse grain due to higher sintering temperature and longer sintering time.
Unfortunately, few researchers have endeavored to construct the constitutive model and processing maps of the fine-grained (FG) Ti-22Al-25Nb alloy by mechanical alloying (MA) and subsequent spark plasma sintering (SPS) using elemental powders.
Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has established suitable constitutive models to predict the high–temperature flow behavior of FG Ti–22Al–25Nb alloy fabricated by MA and subsequent SPS. He has also developed processing maps to describe the reasonable deformation criteria for hot working of FG Ti–22Al–25Nb alloy.
First, in order to obtain true stress-strain curves of FG Ti-22Al-25Nb alloy, isothermal uniaxial compression tests were conducted at different deformation conditions of 950 – 1 070℃ and 0.001 – 1 s-1.
Second, constitutive models for FG Ti–22Al–25Nb alloy were developed by using the modified Johnson–Cook model and the strain–compensated Arrhenius type model based on the corrected experimental data.
Finally, DMM–based processing maps were constructed to determine reasonable parameters of hot working processes for FG Ti–22Al–25Nb alloy.
Conclusively, the following conclusions were drawn:
(i) The modified Johnson–Cook model for the FG Ti–22Al–25Nb alloy showed good prediction accuracy at the reference temperature and strain rate. However, the predictability was lowered in other deformation conditions. The AARE and R2 values of the predicted and friction–corrected flow stress were 9.78 % and 0.985 8, respectively.
(ii) The strain–compensated Arrhenius type model for FG Ti–22Al–25Nb alloy exhibited excellent predictability under most deformation conditions. The AARE and R2 values of the predicted and friction–corrected flow stress were 4.19 % and 0.992 7, respectively. Compared with the modified Johnson–Cook model, the strain–compensated Arrhenius type model is more suitable for describing the high–temperature flow behavior of FG Ti–22Al–25Nb alloy.
(iii) Based on the processing maps of FG Ti–22Al–25Nb alloy, flow instability was predicted to occur at temperatures lower than 990℃ and strain rates higher than 0.1s–1. The reasonable parameters of hot working processes for FG Ti–22Al–25Nb alloy were in the temperature range of 1 020 – 1 070℃ and the strain rate range of 0.001 – 0.32s–1.
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Jo Sep 18, 2023
A sharp rise in electric power transmitting capacity and distance makes it important to ensure the security of power transmission lines.
The major cause of most cutting accidents of transmission lines is vibration by wind. Transmission lines vibrate continuously owing to the Kalman effect by wind. This effect generates endurance fracture in the cross sections of transmission lines, resulting in the decrease in the life time of transmission lines.
Therefore, a lot of researchers installed anti-vibrators on transmission lines to reduce vibration amplitude largely.
Recent researches are not enough with vibration of a tower-transmission line-dampers system as a whole.
Choe Sun Bok, a researcher at the Faculty of Mechanical Science and Technology, has studied an analytical method for determining the correct solution for the forced vibration of a power transmission line-anti-vibrators system with flexible supports by wind, regarding a steel tower as a flexible body. On this basis, she has performed numerical calculations and compared the results with experimental values to verify the validity of the presented method.
Her method can be applied to determination of design parameters and installing places of anti-vibrators.
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