• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.4
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• pp.329-335
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• 2014

This study aimed to understand the internal flow characteristics of a liquid droplet subject to periodic forced vibration. In order to predict the resonance frequency of a droplet, a high-speed camera and macro lens were used to capture internal flow characteristics of a droplet placed on a vibrating hydrophobic surface. Results showed that the droplet assumed a variety of shapes depending on the resonance mode of free droplet, particularly in modes 2, 4, 6, and 8. In addition, the induced internal vortex flow inside the droplet was also observed in each mode. Typically, the induced flow moved upwards along the axis of symmetry and downwards along the surface of the droplet, that is, from the apex to the contact line in modes 2 and 4, after which it broke into a smaller vortex. On the other hand, the large-scale vortex always remained steady in modes 6 and 8. The speed of the flow in mode 4 was always greater than that in mode 2, but those in modes 6 and 8 were similar.

• Korean Journal of Optics and Photonics
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• v.22 no.4
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• pp.179-183
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• 2011

An external cavity laser operating at near infrared wavelength is demonstrated by incorporating polymer waveguide Bragg reflectors. 3rd order Bragg grating and oversized rip waveguide structure were designed by using the effective index method and the transmission matrix method. The polymer waveguide was fabricated using polymer materials with refractive indices of 1.462 and 1.435 for the core and the cladding layers, respectively. The external feedback laser with 875-nm Bragg grating exhibits single mode lasing located at 850-nm wavelength with an output power of 0 dBm, a 20-dB bandwidth of 0.2 nm and a side mode suppression ratio of 40 dB.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.10
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• pp.720-727
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• 2019

Resonating thermal lags of solid fuel with heat transfer oscillations generated by boundary layer oscillation is the primary mechanism of the occurrence of the LFI (Low Frequency Combustion Instability) in hybrid rocket combustion. This study was experimentally attempted to confirm that how the boundary layer was perturbed and led to the LFI. Special attention was also made on oxidizer swirl injection to investigate the contribution to combustion stabilization. Also the overall behavior of fluctuating boundary layer flow and the occurrence of the LFI was monitored as swirl intensity increased. Fluctuating boundary layer was successfully monitored by the captured image and POD (Proper Orthogonal Decomposition) analysis. In the results, oscillating boundary layer became stabilized as the swirl intensity increases. And the coupling strength between high frequency p', q' diminished and periodical amplification of RI (Rayleigh Index) with similar frequency band of thermal lag was also decreased. Thus, results confirmed that oscillating axial boundary layer triggered by periodic coupling of high frequency p', q' is the primary mechanism to excite thermal resonance with thermal lag characteristics of solid fuel.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.4
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• pp.464-469
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• 2019

Tapered roller bearings are used widely in vans, trucks, and trains because they can support the vehicle in a stable manner even under a heavy load. The cage of a tapered roller bearing maintains the gap between the rollers, which prevents friction wear and suppresses heating. If the cage is severely deformed due to resonance, the roller may not be able to roll smoothly and even leave the cage. Consequently, it is very important to analyze the dynamic characteristics of the cage for reliable performance of a bearing. The cage essentially has geometrical tolerance in the manufacturing process. In this paper, the effects of those geometrical imperfections on the dynamic characteristics of the cage were investigated. As a result, natural frequency separation occurred near the natural frequency of the ideal cage due to geometrical imperfections. In addition, the interval was proportional to the magnitude of the geometric error, and the interval increased with increasing mode number.

• Journal of The Institute of Information and Telecommunication Facilities Engineering
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• v.2 no.1
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• pp.52-62
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• 2003

In this paper, photonic bandgap(PBG) bandstop filters which are composed of periodically etched circles in the ground plane show good microwave characteristics with the harmonic suppression on stopband. The PBG structures were analyzed using a finite-difference time-domain(FDTD) simulation and experimental measurement. The FDTD technique is used because it can simulate arbitrary 3-D structures and provide broadband frequency response. The analysis results are presented it is the same that only one row of etched circles and 2-dimension three rows of etched circles. And we show the PBG resonator characteristics between etched circles using field pattern and frequency characteristics as functions of etched circle number n, etched circle radius r and period a.

• The Journal of Korea Institute of Information, Electronics, and Communication Technology
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• v.7 no.3
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• pp.142-146
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• 2014

In this paper, an antenna which has quad band in GSM/DCS/PCS/Bluetooth is proposed. This structure is designed with miniaturization for wide band characteristic based on monopole antenna and log-periodic toothed trapezoid patch antenna which has slots. To achieve multi-bandwidth is used the microstrip line on the substrate. An antenna size is $35mm{\times}20mm$ on FR-4(${\varepsilon}r=4.4$) ground substrate of $35mm{\times}75mm{\times}1mm$ size. And proposed antenna is satisfied with impedance bandwidth(VSWR ${\leq}$ 3). The simulated maximum radiation gain is 1.92 dBi, 3.26 dBi, 3.97 dBi at the center frequency of 0.92 GHz, 1.97 GHz, 2.45 GHz, respectively.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.7
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• pp.711-717
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• 2014

An accelerated ultrasonic fatigue test (UFT) was used for analyzing very high cycle fatigue (VHCF, $N_f$ > $10^7$) behaviors of a specimen with a test resonance of 20 kHz. Using the finite element method (FEM), the dynamic behaviors of the specimen was studied by calculating the stresses along its gauge portion, with displacement. The shape of gauge portion profile was assumed to be a hyperbolic according to the stress equation of the UFT. However, as the specimen used in the test had a circular arc profile, the FEM was used for studying the local stresses for two cases of the gauge profile. The results were compared with those obtain from the stress equation of the UFT. The dynamic behavior of the gauge portion could be understood for further comparison with the actual results.

• Journal of the Korean Society for Marine Environment & Energy
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• v.18 no.4
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• pp.291-297
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• 2015

The wave spectrum was generated from wave data measured at the Chagwi-do site in Jeju, where a 10MW class floating wave-offshore wind hybrid power generation system will be installed. The latching control technology (Sheng et al.[2015]) was applied in order to improve the extracted power from WEC (Wave Energy Converter), which is heaving in corresponding irregular waves. The peak period as a representative value of irregular waves was used when we determined the latching duration. From the numerical results in the time-domain analysis, the latching control technology can significantly improve the extracted power about 50%.

• Computational Structural Engineering
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• v.8 no.2
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• pp.153-161
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• 1995

The dynamic instability for snapping phenomena has been studied by many researchers. There is few paper which deal with the dynamic buckling under the load with periodic characteristics, and the behavior under periodic excitation is expected the different behavior against step excitation. In this study, the dynamic direct snapping of shallow elliptic paraboloidal shells is investigated under not only step excitation but also sinusoidal and seismic excitations, applied in the up-and-down direction. The dynamic nonlinear responses are obtained by the numerical integration of the geometrically nonlinear equations of motion, and examined by the Fourier spectral analysis in order to get the frequency-dependent characteristics of the dynamic instability for various load levels. The results show that the dynamic instability phenomenon carried out from stable to unstable region reveals considerably different mechanism depending on the characteristics of excitations.

• Journal of the Korean Institute of Telematics and Electronics D
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• v.36D no.5
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• pp.88-98
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• 1999

A refractive index modulated (RIM) DFB laser, in which the refractive index of a center region of the upper cladding layer comprising the grating region is different from that of side regions, is proposed to obtain and effective ${\lambda}$/4 phase shift in the center region. Since the coupling coefficient of a center region in a RIM-DFB laser is larger than that of side regions, a RIM-DFB laser has the effect of a distributed coupling coefficient. Simulation results show that RIM-DFB lasers have better operation characteristics - more uniform photon density profile, less SHB effect, and better single mode operation at high injection currents - compared to those of ${\lambda}$/4 phase-shifted DFB lasers and CPM-DFB lasers. In addition, the effect of the center region on the above threshold characteristics of a RIM-DFB laser is investigated.