Jo May 25, 2025
Transversely excited atmospheric pressure (TEA) CO2 lasers have been widely developed and used due to their high peak power and short pulse width, and research into them is being further intensified. TEA CO2 lasers have been used in the photochemical fields such as light detection and ranging of atmospheric pollutants (LIDAR) and laser isotope separation. TEA CO2 lasers have also been used for laser particle acceleration, synthesis of nanoparticles, and rapid large paint stripping required in the aerospace industry.
In order to study the dynamics of TEA CO2 laser, multi-temperature models such as four-, five- and six-temperature models have been developed and different simulation methods have been introduced. Here, the six-temperature model is accepted as the most suitable model to describe the dynamics of TEA CO2 laser, taking into account all possible vibrational-rotational transitions of gas mixture. The six-temperature model considers the given gas mixture ratio, total pressure, laser cavity geometry, pumping mechanism and discharge gap geometry, etc. to determine simulation parameters including excitation rate and relaxation time, and to simulate TEA CO2 laser dynamics.
Although some preceding experimental results have reported that addition of a small amount of hydrogen to the gas mixture of TEA CO2 laser leads to higher power and efficiency, unfortunately, little has been investigated about the theoretical modeling and dynamics of hydrogen-doped TEA CO2 laser.
Pak Kwang Il, a researcher at the Faculty of Physical Engineering, has developed an improved six-temperature model, taking into account the effect of hydrogen on the vibrational levels of gas mixture. Through the simulation of the dynamic processes of the hydrogen-doped TEA CO2 lasers, he determined the optimized gas mixture ratio to increase the output power and the pulse energy.
You can find the details in his paper “Improved six-temperature model and simulation for dynamics of high-power TEA CO2 lasers considering effect of hydrogen” in “Optics and Laser Technology” (SCI).
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Jo May 23, 2025
Pulsed electric field (PEF) processing is a nonthermal treatment that uses high voltage electric pulses (HVEP) for many operations in food and bioengineering, including improvement of protein digestibility, potato processing, microalgae processing, exposure of biological cells, fruit processing, and meat preservation. The processing medium, placed between or passing through two electrodes, are affected by the HVEP of a few microseconds. There have been proposed several designs of PEF treatment chamber such as parallel, co-axial and co-linear configuration. Of all the configurations, the co-linear design is regarded as the most effective. Although this technology has already been introduced to industrial application such as food preservation, the detailed mechanisms of HVEP action on different processing media still remain imperfect.
To clarify the beneficial effect of HVEP and find the proper design of treatment chamber for different treatment objects, more reliable and optimized modeling on the electric field of PEF treatment chamber should be conducted because its measurement is difficult or impossible to perform. Many new data on the electric field of PEF treatment chamber have been presented but the simulation results are not identical with each other. The origin of different results in the literature is likely related to the difference in the simulation conditions, which might cause ambiguity in clarifying the mechanism of PEF treatment and designing the device.
Ham Kum Hae, a post-graduate student at the Faculty of Physical Engineering, has constructed a model to simulate the electric field in a PEF treatment chamber and verify the advantage of a new type of treatment chamber by lattice Boltzmann method.
First, she developed a lattice Boltzmann model (LBM) to describe the electric field distribution in co-linear PEF processing. Based on the assumption that PEF does not cause a time varying magnetic field, she carried out a simulation by using the charge conservation equation. For a two-dimensional LBM, she specified a macroscopic boundary condition for electric potential at high voltage and ground electrodes, and a bounce-back boundary condition for electric potential at the insulator. Next, she suggested a new type of treatment chamber with “holo-elliptical” geometry where the uniformity of electric field was remarkably improved.
For more details, you can refer to her paper “Lattice Boltzmann Simulation of Electric Field in Co-Linear Pulsed Electric Field (PEF) Treatment Chamber” in “Bulletin of the Lebedev Physics Institute” (SCI).
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Jo May 22, 2025
Magnetic resonance imaging (MRI) finds wide application in various studies and clinical practice related to the quantitative and intuitive assessment of cerebral nerve system because of its good contrast and high resolution for brain structures. Most of them require an image processing step called brain extraction by which only the brain part is segmented from cranial magnetic resonance (MR) images.
Though a number of brain extraction algorithms have been presented, brain extraction tool (BET) is still regarded as a favorable tool in the neuroimaging community, and most of the brain extraction algorithms proposed up to date have used the BET as an important competing method to compare their performance.
BET based on the deformable surface initializes the surface as a spherical mesh, and then evolves the surface toward the brain border with small movements applying iteratively a set of forces depending on local parameters to the vertices on the surface. Adopting the local parameters, in general, doesn’t guarantee the balanced evolution all over the surface to generate self-intersections because some vertices may move more quickly while others move slowly depending on the local conditions. This is why BET should have small movements for evolution. Because BET adopts local parameters and small movements, the evolution of deformable surface may not only require more iteration but also tend to easily fall into local optimum resulting in falsely negative regions. Though the computing efficiency of BET is acceptable for clinical applications at present, the computation time is still an important issue when taking the increasing resolution of MRI or large-scale studies into account.
Son Chang Il, a researcher at the Faculty of Biology and Medicine Engineering, has proposed a modified BET (BETWP) consisting of two steps of surface evolution for fast and accurate brain extraction.
He introduced a new fast model using a global parameter, the global mean inter-vertex distance of evolution surface. This fast model is adopted in the preprocessing step and then the original BET model completes the evolution of surface in the second step.
The experiments for evaluating the computation efficiency and segmentation quality have shown that the proposed scheme has a couple of advantages over BET. First, it can improve the evolution speed at least twice for brain extraction without any failure due to self-intersection. Second, it can significantly improve the segmentation quality on JC, TE and NE including the false negative ratio for both MRI modalities (T1-weighted image and DW image).
For more information, please refer to his paper “Fast BET Based on Pre-Processing Evolution Using Global Mean Inter-Vertex Distance of Deformable Surface for MRI Brain Extraction” in “International Journal of Image and Graphics” (SCI).
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Jo May 21, 2025
Banach’s contraction principle, which holds an important position in the fixed point theory, has been extended in two directions; one is the generalization of metric spaces (MS), and the other is the change of contraction conditions.
Recently, a new generalization of MS called DCMS has been introduced. The concept of F-contraction, first proposed by a researcher in 2012, has become an important generalization of Banach’s contraction mapping. Since then, many authors have further investigated his result, suggesting various extensions and modifications. They first presented results that underwent change or omission of some of the three conditions for the mapping F.
Kil Chol Jin, a researcher at the Faculty of Applied Mathematics, has investigated the generalization of DCMS using a previous researcher’s idea.
First, he defined a new space, namely, triplecomposed metric space (TCMS), by combining the properties of RMS and DCMS. He formed TCMS by using the composition of three functions in a quadrilateral inequality. Second, he presented fixed point results for nonlinear FT-contractions using only (F1) in TCMS. Finally, he led the obtained results to the application of nonlinear integral equations.
You can find details in his paper “Triple-Composed Metric Spaces and Related Fixed Point Results With Application” in “Journal of Function Spaces” (SCI).
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Jo May 20, 2025
Weightlifting is one of the Olympic sports, which is divided into two lifts: snatch and jerk. In the clean and jerk, the barbell is first lifted above the chest and then jerked overhead. Barbell’s trajectory and kinematic characteristics such as velocity are important factors for evaluating weightlifting technique.
Previous studies revealed some drawbacks in the research into the jerk in clean and jerk. They paid little attention to the elasticity of the weightlifting bar, which is not a rigid body. Therefore, effective use of the elasticity enables lifting more weight.
Jon Min Hyok, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has investigated the kinematic analysis and optimization of jerk considering the elasticity of weightlifting bar and the damping effect of the body, to help weightlifters to achieve good technical skills for clean and jerk.
First of all, in order to derive the barbell’s velocity and acceleration over time, he tracked the barbell’s trajectory from the video clip of lifter’s performance by using image processing technologies such as pattern recognition. Since the pulling action of jerk is similar to that of snatch, he set focus only on jerking. To evaluate jerk technique, he newly introduced five kinematic characteristics “Jerk-Preparing-Time, ” “Jerk-Drive-Velocity, ” “Jerk-Time, ” “Jerk-Offset, ” and “Jerk-Drop”. Then, he proposed a kinematic model comprised of springs and dashpots in order to simulate and optimize the jerk using the elasticity of weightlifting bar. He estimated the spring constant and damping coefficient of the model by analyzing the barbell’s oscillation on the shoulders. He also calculated the spring constant by the theory of bending of beam. Finally, he formulated an optimal control problem based on the proposed kinematic model to optimize jerk.
The advantages of the proposed method are its simplicity, flexibility and no need for any equipment except a camera. It will help coaches or weightlifters improve their jerk technique based on correct numerical evaluation.
For the details, please refer to his paper “Kinematic Analysis and Optimization of Jerk Considering Elasticity of Weightlifting Bar and Damping Effect of Body in Clean and Jerk” in “Engineering Reports” (EI).
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Jo May 19, 2025
Impellers with two-dimensional blades are mainly employed in a centrifugal fan. The blade profile can be defined according to the distribution of flow parameters such as specified loading (velocity) and circulation. Here, velocity distribution is closely related with the increase of the boundary layers on the blade, flow separation, secondary flows and pressure pulsation, etc. So, it directly determines and controls the internal flow through the impeller. Thus, what is the most important in the blade design is to determine proper velocity distribution.
According to the literature, it is impossible to avoid secondary flow because of turning and curvature inside blade passage. However, there are several ways of decreasing the influence of secondary flow. Those are increasing the curvature radius of inlet vane and meridian surface, prevention of overlapping of the position of maximum vane curvature and the position of maximum meridian curvature, and controlling the enlargement of boundary layers inside flow passage.
A number of researches on the relative velocity distribution law of a centrifugal impeller have been conducted, but practical applications in the impeller design have not been reported. Instead, most impeller design methods considered average velocity distribution only. Velocity distribution on the lower surface, however, has an important influence on the flow pattern of impeller passage.
Kim Won Il, a section head at the Faculty of Aerospace Engineering, has proposed a calculation method for improving the impeller performance with flow control parameters such as reduction ratio of inlet area and maximum reduction ratio of mean relative flow velocity, and inlet gradient of rear relative flow velocity, and verified its effectiveness through CFD simulations.
The simulation result shows the following.
First, it is better to reduce the decrease in average flow relative velocity of gas flow, since it decreases the attack loss in the blade inlet. On the other hand, it is better to slightly increase the average relative velocity, since it suppresses the flow separation in the blade outlet. In addition, appropriate inlet gradient on the lower surface and position of maximum loading guarantees small change in the relative velocity of a real gas throughout the whole flow passage from the blade inlet to the outlet, and minimization of the thickness of the boundary layers.
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