• Journal of Korean Society of Steel Construction
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• v.20 no.6
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• pp.761-771
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• 2008

When the crossing frequency of a train meets the natural frequency of a railway bridge, the bridge is bound to become resonant. There are few available time response samples involving a train that passes a bridge at high speed. Very effective modal-parameter extraction techniques for such special high-speed railway bridge conditions are introduced in this paper. Utilizing the cross-correlations of the free-vibration responses after the train passes, mode shapes and the temporal modal parameters (e.g., natural frequency and damping ratio) are extracted using the TDD and SI techniques, respectively. This approach has been applied to a two-span steel composite bridge in the Kyung-Bu high-speed railway system. The estimation results were compared with those obtained using the existing methods. The results fully coincide with those that were extracted using the existing aforementioned technique.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.04a
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• pp.7-12
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• 2007

Swirl injector with multi-stage tangential entry was analyzed to suppress high-frequency combustion instability in Liquid Rocket Engines. In order to analyze the effect of swirl injector as an acoustic absorber, swirl injector was regarded as a quarter-wave resonator and it's damping capacity is verified in atmospheric temperature. It has a finite mode of vibration and natural frequencies which can be tuned to the natural frequencies of a model combustion chamber. When the targeted injector for each modes is located at anti-node point, the amplitude of modes was decreased. And when the injector of large diameter is mounted, the split of mode which accompanies the decrease of amplitude appeared. From the experimental data, it is proved that if the location of injector mounted is located at an anti-node position of the targeted modes with proper volume, the amplitude of modes is decreased and the split of modes occurs at anti-node point.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.1
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• pp.24-33
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• 2020

In this study, model combustion tests were conducted to investigate the combustion instability characteristics of swirl coaxial injectors for a liquid rocket engine. To examine the effects of the combustion chamber resonant frequency and the injector mixing conditions, pressure fluctuations in the combustion chamber were measured by changing the combustion chamber length, injector recess length, and propellant mixture ratio. From the test results, the variation in the pressure fluctuations for each experimental condition was confirmed and the combustion stability was evaluated by stability mapping. It was found that the longitudinal mode and Kelvin-Helmholtz instabilities occurred due to the change in the combustion chamber and recess lengths.

• Korean Journal of Optics and Photonics
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• v.3 no.1
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• pp.27-36
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• 1992

Frequency modulation spectroscopy of the super-cavity, of which finesse is app. 40,000 has been demonstrated by using a sigle mode He-Ne laser. In-phase and quardrature components of frequency modulation signals (FM signal) were obtained by using the 1.5 MHz-driven-electrooptic phase modulator. The vector locus of the FM signa in the phase space, which is consisted of in-phase and quardrature components of the FM signal, was observed and analyzed for the dependence of FM signal upon the phase of the reference signal of a phase-sensitive-detector. According to rotating the phase of the reference signal, the vector locus was observed to rotate with the same phase angle as the reference signal. The in-phase component of the FM signals will be used to stabilize the frequency of the He-Ne laser to the resonant frequency of the super-cavity.

• Journal of the Korean Society of Propulsion Engineers
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• v.25 no.5
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• pp.18-26
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• 2021

In this work, we have developed a 1D network model aimed at predicting eigenvalues for resonance frequency analysis in a lab-scale industrial gas turbine single nozzle combustion system. Modern industrial gas turbines generally adopt combustors with very complex geometry and flow path to meet various design requirements simultaneously. The current study has developed a network model for combustion systems with backflow at the same axial location. The modeling results of resonance frequencies and mode distributions for a given system using the network model were validated from comparisons with prediction results using a 3D Helmholtz solver.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.20 no.2
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• pp.147-153
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• 2009

In this paper, FSS(Frequency Selective Surface) using defect mode characteristic is proposed. The unit cell using defect mode characteristic of the proposed FSS is offered lower resonant frequency in the same cell size. The number of suitable array is optimized 13 by 13. Also, the patch antennas operated in WCDMA(Wideband Code Division Multiple Access) Tx band and Rx band are designed for the comparison. The gain value of proposed FSS-1 complex structure (the patch antenna of Tx band and FSS) is improved 3.3 dB from 9.98 dBi to 13.28 dBi in Tx band. The gain value of proposed FSS-2 complex structure(the patch antenna of Rx band and FSS) is improved 5.53 dB from 9.81 dBi to 15.34 dBi in Rx band. Also the measured impedance bandwidth($VSWR{\leq}2$) of manufactured $13{\times}13$ array antenna is from 337 MHz(1.87 to 2.21 GHz). The measured radiation gain is 11.39 dBi(1.94 GHz), 13.11 dBi(2.05 GHz), 11.09 dBi(2.14 GHz). The measured radiation efficiency is 81 %. Because the proposed FSS structure has more higher gain, it will be applied to antenna of WCDMA repeater system.

• Journal of the Optical Society of Korea
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• v.1 no.2
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• pp.110-115
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• 1997

We report effcient cascaded Raman generation and signal amplification at 1.3.mu.m achieved in a ring resonator constructed solely from fiber components, i.e. fusion WDM couplers. Low-loss single-mode fiber with moderate $GeO_2$ content (18 mole %) is used as an active medium and pumped by a Nd:YAG laser at 1.064.mu.m. In a resonant cascaded geometry, this generates the third Stokes line at 1.24.mu.m, which acts as a pump for signal wavelength around 1.3.mu.m. A DFB laser operating at 1.315.mu.m is used to provide an input signal. An output signal powers up to 20 dBm (100 mW) with a 28 dB Raman gain are attained, where the Nd:YAG pump power is 3.4 W. It is also shown experimentally that it is important to use optical filters to suppress feedback from the resonator, permitting high Raman gain and good signal quality.

• Journal of the Microelectronics and Packaging Society
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• v.18 no.3
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• pp.39-43
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• 2011

Effects of cavity material on the Q-factor measurement of microwave dielectric materials were studied by HFSS simulation and the measurements using metal cavity. $TE_{01\delta}$ mode resonant frequency was determined from the electric and magnetic field patterns and the loaded Q-factor was calculated from 3dB bandwidth of $S_{21}$ spectrum. When the cavity metal materials were Cu, SUS and Au cavity, the level of Q-factor was similar. However, Q-factor was significantly decreased when the cavity metal material was CuO. The Q-factor measurements of dielectric resonator by network analyzer using various metal cavity exhibits consistent behavior.

• Journal of Power Electronics
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• v.17 no.2
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• pp.570-578
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• 2017

This paper proposes a method to stabilize the output voltage of the secondary side of an Inductive Power Transfer (IPT) system through tuning/detuning via a serial tuned DC Voltage-controlled Variable Capacitor (DVVC). The equivalent capacitance of the DVVC changes with the conduction period of a diode in the DVVC controlled by DC voltage. The output voltage of an IPT system can be made constant when this DVVC is used as a variable resonant capacitor combined with a PI controller generating DC control voltage according to the fluctuations of the output voltage. Since a passive diode instead of an active switch is used in the DVVC, there are no active switch driving problems such as a separate voltage source or gate drivers, which makes the DVVC especially advantageous when used at the secondary side of an IPT system. Moreover, since the equivalent capacitance of the DVVC can be controlled smoothly with a DC voltage and the passive diode generates less EMI than active switches, the DVVC has the potential to be used at much higher frequencies than traditional switch mode capacitors.

• Proceedings of the KIEE Conference
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• 1996.07a
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• pp.395-399
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• 1996

The Clamp Mode(CM) Forward Zero Voltage Switching Multi Resonant Converter(ZVS-MRC) with self-driven synchronous rectifier in studied. The loss at the synchronous rectification stage of the converter is analyzed using MOSFET linear model and is compared with the loss at the conventional schottky diode rectification stage of the converter. From the results of the analysis, it is known that the use of MOSFETs as a synchronous rectifier reduces the loss at the rectification stage over the whole load range comparing the use of schottky diodes as a conventional rectifier in the converter. In order to verify the validity of the analysis, we have built a 33W(3.3V/10A) CM Forward ZVS-MRC with self-driven synchronous rectifier, in which switching frequency is 1MHz, and tested. From the experimental results, it is known that the synchronous rectification achieved about 1W improvement in the loss at the rectification stage and about 3% in the efficiency at the converter as compared with the conventional schottky diode rectification.