Jo Mar 22, 2024
Numerical modeling and simulations of plasma is an indispensable means to overcome the technical limit in measuring plasma parameters, which has been performed by many researchers. Unlike conventional numerical modeling, lattice Boltzmann method (LBM) is a meso-scale scheme based on Boltzmann kinetic theory, which has some advantages such as relatively simple computational structure, easy handling on complex boundaries and possible parallel computation. Recently, the LBM has been widely used to simulate not only single phase flow but also multi-component and/or multi-phase thermal fluid transfer, electric and magnetic fields, acoustic fields and radiative transport. However, the previous models for plasma were limited to 2D schemes, so they were not enough to analyze 3D mechanism of arc dynamics.
Therefore, Kim Ju Song, a lecturer at the Faculty of Physical Engineering, has suggested a 3D lattice Boltzmann model to numerically simulate thermal fluid flow in non-transferred arc torch with swirl and validated its accuracy by comparing it with results in literature.
The Boltzmann model was built for local thermodynamic equilibrium. The boundary conditions were considered for both macro- and micro-parameters, and the curved boundaries were handled for the cathode and the anode.
The simulation results demonstrate that the maximal temperatures appear at positions downstream from cathode and the axial velocity profiles are remarkably different according to the swirl number and that the position in the maximal axial velocity shifts toward the cathode with increase in the swirl number.
For more information about it, please refer to his paper “Numerical Analysis for Argon Arc Plasma Jet Flow by Three-Dimensional Thermal Lattice Boltzmann Model” in “Bulletin of the Lebedev Physics Institute” (SCI).
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Jo Mar 20, 2024
Conventional approaches such as physical and chemical methods have been proposed for synthesis of metal nanoparticles. However, these are associated with the use of heavy equipment, huge amount of energy input, highly toxic and dangerous chemical compounds that cause biological hazards.
Apart from these, plant-mediated synthesis of metal nanoparticles seems to be very rapid, simple, dependable, non-toxic and ecofriendly.
Recently, great efforts have been made to use environmentally-friendly methods for the synthesis of silver (Ag) nanoparticles which have become a promising material in the recognition of ultrasensitive chemical and biological molecules owing to its low cost, high catalytic activity and noteworthy optical properties. Most plants used in green synthesis of Ag nanoparticles are medical herbs. The extracts of such medical herbs include several kinds of useful ingredients including polyphenol, protein and polysaccharide for reduction and stabilization of Ag nanoparticles.
Ri Yun Sik, a researcher at the Faculty of Materials Science and Technology, has investigated an eco-friendly route to the synthesis of Ag nanoparticles by extracts derived from Epimedium Koreanum Nakai leaves, which have been traditionally used as aphrodisiac and for low blood pressure, neurasthenia, infertility, impotence and amnesia, and their antibacterial properties and colorimetric sensing property on hydrogen peroxide.
As a result, he has found that the Ag nanoparticles he synthesized were small, nearly monodispersed and highly crystalline with face centered cubic structure and that these Ag nanoparticles show high antibacterial activity against bacteria such as E. coli and S. aureus. He has also proved that the degradation of Ag nanoparticles induced by catalytic decomposition of hydrogen peroxide causes a considerable change in the surface plasmon resonance absorbance with hydrogen peroxide concentration and that this sensor reaction has a very good sensitivity and a linear response over wide hydrogen peroxide concentration range of 10−6 to 10−1M hydrogen peroxide.
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Jo Mar 18, 2024
Nowadays, pearlite ductile cast irons are widely used for producing shafts, wheels and gears due to its good mechanical properties and excellent castability. Compared with carbon steels, ductile irons have relatively low melting temperatures, good fluidity and castability, and moderate shrinkage during solidification and cooling.
There are several heat treatment methods to improve the mechanical properties of ductile cast iron such as annealing, normalizing, quenching and tempering. These heat treatments could be partially replaced by adequate balances of alloying elements. It is an easy and economical way of obtaining different microstructures and good mechanical properties in ductile cast irons.
As the demand for pearlite ductile irons increases day by day, nickel, copper, and molybdenum are common alloying elements in as-cast pearlite ductile irons. In view of the international trend in the rising cost of nickel and molybdenum, there is a renewed interest in exploiting the strengthening effect of manganese and copper in ductile irons. Both manganese and copper are FCC metals and they are highly soluble in austenite. The use of alloying elements stabilizing the pearlite such as Cu and Mn can be considered as the most appropriate method to control the amount of pearlite in the matrix and enhance the mechanical properties.
Kim Yu Chol, a researcher at the Faculty of Materials Science and Technology, studied the effect of cheaper alloying elements like copper and manganese on the mechanical properties and microstructure of ductile cast iron, thus obtaining as-cast pearlite ductile cast iron with high tensile strength, good elongation and high hardness.
There was, practically, no free ferrite in the ductile cast iron alloyed with Mn and Cu. When the content of Mn was 0.53% and the additive amount of Cu was 0.6%, the pearlite content was 86%. At that time, the as-cast pearlite ductile cast iron had tensile strength of 720 MPa, elongation of 4.6% and hardness of HB 268.
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Jo Mar 16, 2024
A charge sensitive amplifier (CSA) is widely used as the preamplifier of an X-ray or gamma-ray detector. It is based on a low-noise high frequency field-effect transistor and a current feedback operational amplifier allowing the readout of fast charge pulses.
The pile-up of output pulses in a CSA is attributable to its long discharge time constant. A CR differentiator which shortens pulse width is used to remove pile-up. Due to the exponential decay of a CSA feedback circuit in combination with a differentiator circuit, undershoot occurs at the output of a shaping amplifier with the same time constant as the CSA. PZC circuit is inserted between the CSA and the shaping amplifier to eliminate undershoot. But this makes it difficult to decide the real differential time constant of the CR differentiator after elimination of undershoot and to process the signals like peak detection, time analysis, etc. An analog PZC circuit always has some uncertainties that result in pulse tail pile-up. This drawback can be overcome with a digital pole-zero, zero-pole compensation unit.
A digital PZC method also has some uncertainties. Although digital real time processing of pulses could, in principle, eliminate almost all pulse pile-up distortion in spectroscopy, it is not yet widespread.
Pae Won Sik, a researcher at the General Assay Office, has performed an analysis to search for the cause of undershoot after differentiation of input signals and designed a new PZC circuit with unchanged differential time constant by using a pot resistor. Then, he has conducted some simulations and experiments to verify its successful performance.
The results show that the proposed circuit effectively eliminates undershoot and significantly reduces pulse width, which leads to the improvement of pile-up cancelling, peak detection and time analysis.
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Jo Mar 15, 2024
Welding deformation is one of the most complex problems arising in manufacturing of welded structures and it is impossible to remove or correct it. Welding deformation reduces accuracy in the size and shape of structures and increases the amount of correction work which leads to rise in manufacturing cost. Therefore, predicting and controlling welding deformation is of key importance in manufacturing welded structures. Predicting and controlling of welding deformation can reduce the amount of correction work, thus saving labor and cost and raising productivity.
At present, analysis of welding deformation is conducted by thermal-elastic-plastic (TEP) FEM and inherent strain method. In the past, there were a large number of experiments and numerical models to obtain inherent deformation. Although constraint is, in general, a simple practical method to reduce angle distortion, past researches were focused on numerical analysis of welding deformation, and the control of welding deformation considering the effect of constraint on angle distortion of large structures have not been sufficiently investigated.
Kim Chang Son, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has predicted the welding distortion of a ship panel structure with butt and zigzag joints and determined the reasonable constraint position to reduce fabrication cost by considering the effect of constraint on angle distortion.
First, he investigated the inherent strains of the welded structure by TEP FEM analysis under both constraint and free conditions. Next, through the elastic FEM based on inherent strain, he estimated the welding deformation of ship panel structure under consideration. After that, in order to quantitatively estimate the effect of constraint on angle distortion of welded structure, he performed a numerical analysis in a non-constraint free condition and various constraint positions, and determined the reasonable constraint position. Finally, he compared the angle distortion of ship panel structure under the constraint condition with that under the non-constraint free condition.
The simulation results show that angular distortion can be effectively reduced under a constraint condition.
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Jo Mar 13, 2024
Laser cutting, which is one of the laser machining technologies, has been widely used in industrial production due to its good processing flexibility, high quality, high cutting speed, high precision and high production rate.
In order to ensure high quality in laser cutting, not only the appropriate process parameters (laser power, focal position, cutting speed, type and pressure of assist gas, etc.) must be set, but the stand-off distance between a work piece and a cutting head has to be kept at a fixed distance.
An Chol Min, a section head at the Faculty of Physical Engineering, has built a precise mathematical model of a truncated cone-shaped capacitive sensor based on the theory related to the electrostatic field and the properties of conductors and dielectrics in the field. He has also proposed limitations and suppositions to get a unique solution of the capacitance while analyzing the model by MATLAB, and verified the correctness of the model through experiments.
The theoretical analysis and the experiments show that the truncated cone sensor has much higher sensitivity than a parallel-plate sensor in laser cutting.
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