• Composites Research
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• v.33 no.3
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• pp.133-139
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• 2020

In this research, the vibration characteristics of composite quadcopter are analyzed, and avoidance design and analysis are performed to avoid resonance. The full platform of a commercial quadcopter with composite rotor arm is analyzed to see the vibration characteristics using FEM program. The manufactured stacking sequences of rotor arm is used for analysis, and the natural frequencies are compared with experimental results and simple analytic model results. It is also confirmed that the natural frequency of the particular mode is included within the operation range of the motor. The resonance avoidance design is carried out by selecting three variables from the existing model: stacking sequence, rotor-arm pipe length, and pipe thickness.

• Journal of Veterinary Clinics
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• v.33 no.5
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• pp.290-294
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• 2016

A 13-month-old intact female poodle dog presented with an acute history of circling and seizure episodes. On the basis of the results of neurologic examination combined with magnetic resonance imaging and cerebrospinal fluid analysis (CSF), meningoencephalitis of unknown etiology (MUE) was suspected. Therapy with mycophenolate mofetil plus prednisolone was initiated, following which the clinical signs showed improvement for only one month before gradually worsening again. Acute progression of the clinical disease was observed, and the patient was euthanized 91 days after initial presentation. This case was definitively diagnosed as necrotizing meningoencephalitis (NME) according to the results of post-mortem histopathological examination. This report describes the clinical findings, serial magnetic resonance imaging (MRI) characteristics, and histopathological changes in a case of acute NME.

• Bulletin of the Korean Chemical Society
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• v.25 no.4
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• pp.452-459
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• 2004

The geometrical structures of various isomers of hexafluoro-1,3-butadiene (HFBD), tetrafluoro-1,3-butadiene (TFBD), and difluoro-1,3-butadiene (DFBD) have been studied theoretically. Natural steric and natural resonance theory (NRT) analyses indicate that the lower energy of skew s-cis conformer of hexafluoro-1,3-butadiene than that of the s-trans conformer is originated from the strong steric repulsions between fluorine atoms particularly in the s-trans conformer. The resonance structures generated by NRT also show that the lone electron pairs of fluorine atoms effectively extend the conjugation, and the large differences in energy among the structural isomers of tetrafluoro-1,3-butadiene and difluoro-1,3-butadiene are in part attributed to the differences in the delocalization energies, in addition to the steric repulsion between fluorine atoms. Other interatomic interactions, such as hydrogen bonding, also play important roles in determination of the structures of isomers of tetrafluoro-1,3-butadiene and difluoro-1,3-butadiene.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.11b
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• pp.588-592
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• 2002

There was the transient vibration on $H_2$ piping system for cooling the generator in a power plant. We found it was resulted from resonance between the natural vibration of the piping system and exciting force from the turbine rotor by measurement and simulation test. We verified it would be changed the mode shape of the piping system by several simulation test for the structural modification of the piping system. Therefore we concluded that the change of natural vibration mode depends on deeply changing effective length of pipe and reducing supports.

• Bulletin of the Korean Chemical Society
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• v.28 no.12
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• pp.2477-2480
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• 2007

MgAl2O4:Eu3+ red-light emitting powder phosphor was prepared at temperature as low as 500 oC within a few minutes by using the combustion route. The prepared powder was characterized by X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectrometry. The luminescence of Eu3+-activated MgAl2O4 shows a strong red emission dominant peak around 611 nm, which can be attributed to the 5D0-7F2 transition of Eu3+ ions from the synthesized phosphor particles under excitation (394 nm). Electron paramagnetic resonance (EPR) measurements at the X-band showed that no signal could be attributed to Eu2+ ions in MgAl2O4.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.552-558
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• 2001

When operating lathe gear box which is equipped with geared transmission, it sometimes generates loud noise and excessive vibrations. In order to identify their causes, in this study, torsional and lateral vibration characteristics including critical speeds of the gear transmission system are firstly analyzed using lumped parameter models. Natural frequencies and mode shapes of the gear box structure are also analyzed by using the modal test. Furthermore, measured vibration and noise signals during operations are analyzed and compared with theoretical analysis results. After all, it is concluded that the primary cause of the excessive noise and vibrations is the resonance between gear meshing frequency including its side bands, the frequencies of shaft bending and torsional vibrations, and the natural frequencies of the gear box structure. Consequently the noise and vibration levels are greatly reduced by avoiding resonance between the natural frequencies and gear meshing frequencies through the rearrangement of the gears on the transmission shaft without any gear ratio change.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.642-647
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• 2008

The boiler structure is determined by combustion characteristics and construction costs in the combustion chamber of a large commercial boiler. The heat transfer in boiler is composed of the radiation and the convection. The convective heat transfer has happened to back-pass heating surface. The combustion gas sequentially passes through the reheater tube, 1st economizer tube, and 2nd economizer tube. In case of being lowered in boiler height, we have to install additional tube bundle in back-pass heating surface for increasing the heat transfer of boiler, which causes the noise and vibration from combustion gas. When the combustion gas passes through the back-pass tube bundle in specified load of commercial boiler, this paper analyzes the acoustic characteristics between vortex-shedding frequency and natural frequency in tube bundle cavity. The case study reduce the resonance by changing natural frequency characteristics of tube-bundle cavity using a way to install ant-noise baffle in the direction of combustion gas flow.

• Journal of the Korean Magnetic Resonance Society
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• v.20 no.2
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• pp.41-45
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• 2016

Chemical investigation of a marine-derived fungus, Penicillium sp. 108YD020, resulted in the discovery of six bile acid derivatives, glycocholic acid (1), glycocholic acid methyl ester (2), cholic acid (3), glycochenodeoxycholic acid (4), glycodeoxycholic acid methyl ester (5), and cholic acid methyl ester (6). The structures of six bile acid derivatives 1-6 were determined by the detailed analysis of 1D, 2D NMR and LC-MS data, along with chemical methods and literature data analysis.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.12 s.255
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• pp.1588-1595
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• 2006

The stabilities of a pinned-pinned straight pipe conveying fluid are investigated by complexification-averaging method. The flow is assumed to vary harmonically about a constant mean velocity. Instability conditions of a governing equation are analytically obtained about parametric primary, secondary and combination resonances. The resulted stability conditions show that instabilities exist when the frequency of flow fluctuation is close to one and two times the natural frequency or to the sum of any two natural frequencies. In case that the fluctuated flow frequency is close to the difference of two natural frequencies, instabilities does not exist.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.902-905
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• 2004

Recently, the composite materials are widely used for manufacturing the helicopter rotor blades. furthermore, composites show great potential on the design of rotor blades due to the advantages of strength, durability and weight of the materials. To keep with this advantages, it is necessary to calculate natural frequencies of a rotating blades for avoiding resonance. In this paper, the structural design process of airfoil cross section is introduced, and natural frequencies of composite rotor blades with variable rpm we investigated.