Jo Mar 12, 2024
Electron density is a key parameter in designing and constructing a pumping system for high power transversely excited atmospheric (TEA) pressure CO2 laser. It is necessary to ensure uniform volumetric discharge in order to enhance the output power and the efficiency of gas laser. It is, however, very hard for TEA-CO2 laser to achieve electron density of more than 1014cm-3 necessary for volumetric discharge as the pressure of active gas of the laser is higher than atmospheric pressure.
Recent-developed external pre-ionization systems such as those based on high-energy field emission, ultraviolet or X-ray have enabled atmospheric pressure volumetric discharge in a short time of about 1㎲ and raised the output power of pulsed laser up to the level of GW. The main factor of getting higher power (over tens of GW) is to ensure that electron avalanche caused by inelastic collisions between CO2 (or N2 and He) molecule and cold electrons with energy of hundreds of keV is spatially uniform, and that non-self-sustained main discharge is conducted as a uniform volumetric one. It means that finding out the characteristics of change of electron density is of primary importance in developing high power TEA-CO2 laser.
The model of electron density for uniform volumetric discharge is a nonlinear model describing microphysical processes such as pre-ionization, avalanche ionization, recombination, attachment and so on. So far, there have been no general solutions of this model but only special solutions for some extreme cases.
Yun Tu Hon, a researcher at the Faculty of Physical Engineering, has proposed a method of general solutions for the nonlinear model and verified the accuracy and generality of his method by comparing the results with the data reported in the literature. He also reported some new special solutions unknown in the literature and the experimental results for them.
The proposed method will prove to be valuable for high power TEA-CO2 laser of hundreds of MW.
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Jo Mar 7, 2024
Submerged arc welding process with high productivity has been one of the widely-used surfacing methods for recycling worn components or fabricating bimetal. Surfacing with a lay-down plate electrode is a new variation of submerged arc surfacing, in which metal is deposited on the surface of a workpiece by arc heat between the workpiece and a plate laid down on it.
Both direct and alternate current can be used as welding power source. Surfacing with a lay-down plate shows the possibility of reducing the cost for restoring worn components like large-scale straight teeth gears with large module, and of improving the productivity and quality of surfacing.
Temperature distribution resulted by spontaneous movement of electric arc has a big effect on the stability and quality of surfacing with a lay-down plate. However, to the best of our knowledge, there have not been many discussions regarding the spontaneous movement of electric arc and the analysis of temperature field during surfacing with a lay-down plate electrode.
In order to make an optimal surfacing process condition, Pak Sang Yun, a researcher at the Faculty of Materials Science and Technology, has modeled the spontaneous movement orbit of arc during surfacing with a lay-down plate electrode and investigated its temperature distribution by ANSYS.
He has drawn the following conclusions.
First, the periodic and stable movement of electric arc, which is self-controlled in the channel during surfacing, results from periodic variation of forces acting on electric arc.
Second, the dimension and shape of the weld pool varies with time, but the boundary of solidus temperature is constant regardless of time. Thus, the temperature field remains in quasi-stationary state during surfacing with a lay-down plate electrode.
Third, the distribution of quasi-stationary temperature field depends on build-up technical conditions and the width of plate electrodes.
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Jo Mar 2, 2024
CMM, one of precision measuring instruments, is an important component of advanced manufacturing technology. Nowadays, CMMs are widely used in the fields of machine-building, automobile manufacturing, aerospace, flexible manufacturing system and computer integrated manufacturing system due to the advantages of strong versatility, high degree of automation and high precision.
However, CMMs have some weaknesses. They are large, expensive and not portable.
In comparison with traditional CMMs, AACMM offers many advantages such as large workspace and ease of use, high degree of freedom, portability and low cost. However, it also reveals some drawbacks due to its many articulated arms like bigger accumulated errors and lower measuring precision than CMMs.
In an attempt to improve the measurement accuracy of AACMM and reduce its manufacturing cost, Rim Chang Hyon, a researcher at the Faculty of Mechanical Science and Technology, has performed an error analysis and an optimum tolerance allocation.
Firstly, based on the analysis of error source of AACMM, he constructed a systematic error model.
Next, he calculated by the analytical and numerical methods the position error of measuring probe due to the errors of each structural parameter and that due to the systematic error of individual rotary encoders.
Lastly, from the perspective of processing and assembly, he established a mathematical model that converts the perpendicularity error of the AACMM into the arm’s length error, and performed a calculation about the optimal tolerance allocation of each structural parameter and then verified its accuracy.
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Jo Feb 29, 2024
The geometry of TICS has an influence on the output signal of sensors. This relation is determined by the cell constant given by the size of a sensor.
Several researchers demonstrated three methods to determine the cell constant of TICS― experimental method using standard solutions with known conductivity, single-loop calibration method and analytical method by rough approximation. They pointed out that in order to measure the lower ranges of conductivity which require a smaller cell constant, the diameter of sensor and measuring container must be enlarged.
But until now it has not been suggested to deal with the influence of the geometric parameter of a sensor on the cell constant and the regulation band of the cell constant with the size of a sensor. So, it is worthwhile to do some research on the relation between the cell constant and the size of a sensor.
Jang Hyon, a researcher at the Faculty of Electronics, has analyzed the relation between the geometry of a TICS (inner radius r1, outer radius r2 and height t of the sensor toroid) and its cell constant through simulations by the finite element method and experiments.
Taking account of actual conditions, he considered the sizes (r1, r2 and t) of a sensor within several tens of millimeters.
A key factor which influences the cell constant is the hole diameter (2r1) of the sensor toroid. The effective superior limit of inner radius of the sensor toroid that can give the smallest cell constant is about 30 mm.
The theoretical results on the cell constants of sensors in different sizes coincided with the experiment results with less than about 5% relative error.
The conductivity below limit value that can be adjusted by cell constant is about 0.36 times the lowest bound value of conductance measurement range.
You can find the details in his paper “Optimization of the cell constant for application of the transformer type inductive conductivity sensor (TICS) in low-conductive solution” in “IEEE Sensors Letters” (SCI).
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Jo Feb 28, 2024
Satellite-based free space optical quantum key distribution system is being considered as a very promising technique which is capable of overcoming the transmission limits of fiber-based QKD system and constructing a global quantum communication network.
It was demonstrated that laser beam has orbital angular momentum generated from spiral topological structure of itself in addition to spin angular momentum polarization property. In this case, unlike polarized quantum state, a photon may have a number of quantum states. Using optical orbital angular momentum as information carriers, information can be modulated in the infinite dimensional space. Furthermore, there is a possibility of increasing data throughput efficiently and enhancing the stability of system. Consequently, applying the orbital angular momentum to quantum key distribution, it is possible to increase the quantum key generation rate remarkably and this mode is suitable for free space optical quantum communication.
Some researchers considered polarization coding in FSO QKD system which is one of the good ways to realize quantum communication in free space. For convenience of analysis, they assumed that the air is in a steady state when QKD is performed between two parties in free space. However, the air is in a dynamic state of continuous flow rather than in a fixed state. It is known that there is a possibility of upgrading the performance of FSO QKD system when orbital angular momentum is applied to QKD.
O Ju Hyok, a researcher at the Faculty of Communications, has investigated more suitable QKD protocols for FSO quantum communication and analyzed the impact of atmospheric turbulence on quantum bit error rate of QKD system. He has also developed two QKD protocols suitable for FSO QKD system (one formed by combining random phase modulation with BB84 protocol and the other by combining entanglement modulation using photon orbital angular momentum with BB84 protocol) and analyzed the effects of atmospheric turbulence on the performance of FSO QKD system.
The simulation results demonstrate two facts. First, QBER of FSO QKD system increases with increase in air refraction index structural constant, that is, the intensity of atmospheric turbulence. Second, for FSO QKD, entanglement-based OAM QKD is more suitable than random phase modulation method.
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Jo Feb 26, 2024
Since the ocean covers over 70% of the earth’s surface and marine transportation has many advantages, it is continuously increasing and shipbuilding is still considered as an important industry. This is constantly increasing the demands for ships as well as for the efficiency improvement of shipbuilding. Therefore, shipbuilding field is widely introducing advanced techniques including computer programs for ship design and shipbuilding to raise its modernization level.
Some processes such as seam landing of hull surface are carried out on computers, but they are mainly based on the skills and experiences of designers.
Seam or butt landing (simply seam landing) of shell plates in the hull lofting process which is the first step of shipbuilding plays an important role in raising the speed and quality of shipbuilding and lowering its cost. However, this work has not been modernized until now because of its complexity and it has been done manually on paper and on mold lofts by the skills and experiences of designers.
Some computer programs for ship design and shipbuilding such as AVEVA, Ship Constructor, Maxsurf, etc. give help with seam landing of shell plate. These programs contain some graphics processing functions for seam landing in ship surfaces. However, seam landing of hull surface by computers still depends on the skills and experiences of designers. Therefore, the result of seam landing of hull surface is dependent on designer’s ability and this work lacks scientific accuracy. The main reason for low scientific accuracy of this work is its complexity.
Pae Ho Rim, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has proposed an optimization model for seam landing and a solving method by GA. On this basis, he has confirmed the validity of the proposed method.
First, for the optimum seam landing, he built an optimization model with the sum of cutting, shearing, forming and welding cost as an objective function. As obvious in the objective function, costs of cutting, shearing, forming and welding depend on seam landing.
Second, he proved that the optimization problem for seam landing of hull surface is a topological optimization one when the number and form of seams are design variables and it is a form optimization one when only the form is a design variable.
Finally, he demonstrated that optimization method using GA gives a better solution than the seam landing of an existing ship and that by Taguchi’s method.
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