Jo May 20, 2024
Recently, aluminum has been widely used in various industries including aerospace, mechanical and wire cable industries, for its good mechanical properties and engineering properties.
Aluminum used in the wire cable industry has relatively low density, low melting point and good corrosion resistance compared to other metallic materials such as copper, steel, etc. However, it has worse mechanical properties.
It is also possible to increase the mechanical properties of aluminum by adding various alloying elements. However, in pure aluminum (more than 99.5%), mixed elements such as Fe, Si, Cu, etc. bring several negative effects. Therefore, it is important to investigate and introduce new ways to improve the mechanical properties of pure aluminum.
Carbon nanotubes have been widely used as reinforcement material for composites because of their mechanical, physicochemical and electrical properties. From the mechanical point of view, the combination of sp2 hybridization mode of carbon nanotubes is one of the very strong chemical bonds. Therefore, carbon nanotubes with ideal structure have very good mechanical properties.
Hence, addition of carbon nanotubes, an ideal reinforced material for composites, can be the most suitable method for achieving grain refinement of the matrix and enhancing the mechanical properties.
Jon Song Won, a researcher at the Faculty of Materials Science and Technology, has investigated the effect of carbon nanotubes on the properties of aluminum-carbon nanotube composites fabricated by the melting method.
He has found that the tensile strength, elongation, bending number and hardness are all high when the carbon nanotube content is 0.3%.
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Jo May 19, 2024
Rotor system, which plays an important role in modern industries, is used in many machines such as internal combustion and jet engines, various types of motors, generator turbines and so on. High speed and high power are the development direction of modern rotating machinery, and fatigue crack is apt to happen as one of the most serious damages in the heavy loading environment. Therefore, detecting the crack in a timely way is very important for safe operation.
Kim Yu Jong, a researcher at the Faculty of Mechanical Science and Technology, has proposed a finite element model of a hollow shaft with open cracks. She derived the time-varying stiffness matrix of finite element based on the transformation matrix by which node displacements are converted from the rotating coordinates into the fixed ones. She employed the harmonic balance method (HBM) to find the responses of a cracked rotor system.
She analyzed the nonlinear vibration of rotor system and compared the results with some published results, considering the eccentricity of disk, the dimensionless crack depth, the ratio between the inner radius and outer radius of the rotor and the angle between the crack and imbalance directions.
As a result, she drew the following conclusions.
The subcritical rotational speeds corresponding to the first critical forward whirling speeds are not excited in the open cracked hollow rotor system. For the deep crack, the first forward whirling rotational speeds decrease more rapidly than the first backward ones as the dimensionless crack depth increases.
The whirl orbits near the subcritical backward whirl speeds have the same shape and behavior for both breathing and open crack models. However, the variations of the whirl orbits shape with the change in the rotational speeds are slower at the deep crack than at the shallow crack.
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Jo May 17, 2024
Continuous casting is widely used for production of steel slab. The main problem is that if the stress and strain exceed a threshold value, steel slab will be cracked as the layer of steel slab depends on variable thermodynamic load in the solidification process.
The main cause of steel slab crack is the thermal stress that is generated by uneven temperature distribution during the solidification of molten steel.
Ri Yong Hwa, a researcher at the Faculty of Aerospace Engineering, proposed a method of determining the optimum heat transfer coefficient to prevent thermal crack of slab, on the basis of the identification of the cause of crack by analysis of unsteady thermal stress in the continuous casting of the slab whose width is relatively long for its height.
He analyzed unsteady temperature field and thermal stress of 800×150mm2 steel slab in the continuous casting process by using ANSYS to find out the reasons why steel slab is cracked. The analysis revealed that the reason is that the layer is not thick enough to withstand the thermal stress and the static pressure of the molten steel in the height direction of this band, and that all the thermal stresses on the surface exceed the yield limit of 1.9Mpa.
Then, based on the band analysis data and the optimization algorithm in the continuous casting process, he determined the reasonable coefficient of band heat transfer to prevent the crack of the steel slab by using ANSYS-workbench.
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Jo May 15, 2024
The vortex street flow meter is an instrument that measures a flow rate in the pipe line by measuring the frequency of vortex streets in the wake of a vortex generator located perpendicular to the flow direction in the pipe line. When Reynolds number reaches a specific value in the pipe line through which fluid runs, alternate vortex street occurs in the wake of the vortex generator and the generating frequency of this vortex street is in proportion to flow velocity. Therefore, the generating frequency is used for prediction of flow rate. The frequency depends on the type of generator.
The vortex street flowmeter has good accuracy and stability in flow measurement, but they are not widely used for low flow rate due to the small vortex frequency signal when the diameter of a pipe line is small or flow rate is low. In industrial pipe lines, flows with Re below 2×104 are often found, so in such cases, the vortex street flowmeter cannot guarantee measurement accuracy.
Compared to other types of vortex generators, a triangular prism-shaped vortex generator is now used in most vortex street flowmeters because it is easier to make and has better stability in vortex separation.
Jong Ji Song, a researcher at the Faculty of Mechanical Science and Technology, has solved and optimized hydrodynamic problems arising in the design of a triangular prism-shaped vortex generator available in the low discharge domain with minimum pressure loss.
First, he analyzed the influence of the structural parameters of the vortex generator such as thickness and width on the vortex generation and fluid flow by CFD simulation of the vortex flow around the triangular prism-shaped vortex generator. As a result, he found that with great thickness, the vortex generator easily generates vortex but has large flow resistance, and its great width is not good for generation of vortex and its narrow width cannot guarantee the stability of vortex. Next, on the basis of this analysis of the vortex generation and the flow loss in the pipe line, he determined the structural parameters of the triangular prism-shaped vortex generator available in the low discharge domain by optimization with BP neural network and genetic algorithm.
The proposed CFD simulation method and the optimization of structural parameters can also be applied in the design of vortex generators of any shape.
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Jo May 14, 2024
As the rotary angular sensor is important for measuring accurate positions and directions, it plays an important role in various applications such as robotic controller, camera and industrial machines.
At present, lots of angular position sensors such as simple resistive potentiometers, capacitive potentiometers, optical sensors and magnetic sensors are in use, and optical sensors which operate in visible light or infrared light offer the advantages of contactless measurement and insensitivity to electric and magnetic fields. So, an optical angular sensor has become recognized as an indispensable displacement/position sensor due to its high resolution, light weight and excellent immunity to electromagnetic interface.
Jo Myong Jin, a researcher at the Semiconductor Institute, has proposed an absolute rotary angular sensor with a nonlinear transparent disc. This rotary angular sensor is of transparent type and it has a nonlinear transparent disc between light source and LDRs. This sensor consists of five elements, that is, light source, a shaft coupled nonlinear transparent disc, a lens, a pair of light dependent resistors (LDR) and a signal processing circuit. In this sensor, absolute rotary angle is measured by output resistance of double LDRs which has a linear change by nonlinear transparency of the disc in the whole measurement range (from 0° to 360°) according to the characteristics of LDR’s resistance via irradiance. Its operation principle is similar to that of a resistive potentiometer.
The advantage of the sensor is that it is immune to shock and vibration as it has a nonlinear transparent disc instead of a binary coded disc and a large gap distance between the disc and the optical sensor. Another advantage is that as the output signal of LDRs is analogue, its resolution is determined by the A/D converter in the signal processing circuit. Therefore, this rotary angular sensor provides a high resolution in the range of 0~360°.
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Jo May 11, 2024
Every country and every nation in the world have their own manners of salutation.
However, hardly any of them are like ours, which satisfactorily and clearly expresses politeness to people in different social standings and age ranges.
From olden times, our people have observed the proprieties by either making a deep bow or bowing their heads when they see or part from one another.
According to our greeting etiquette, a deep bow is supposed to be made to the elders and seniors respectfully with some polite words for their health, and a slight bow to the people of the same age. Lowering one’s heads is appropriate for greeting younger ones.
Josonjol, the morally superior, hygienically impeccable and cultured greeting manners of our own style, is the best of all.
How our country could be widely known to the world as a country of courteous people in the East is also attributable, to some extent, to the fact that the people had created and constantly developed such admirable greeting etiquette.
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