• Geophysics and Geophysical Exploration
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• v.20 no.1
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• pp.18-24
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• 2017

We performed seismic imaging based on relative S-wave travel times to examine S-wave velocity of upper mantle structure beneath East Asia. We used teleseismic events recorded at 129 broadband stations of the Korea Institute of Geoscience and Mineral Resources (KIGAM), Korea Meteorological Administration (KMA), and National Research Institute for Earth Science and Disaster Prevention (NIED). Relative travel time residuals were obtained by a multi-channel cross-correlation method designed to automatically determine accurate relative phase arrival times. The resulting images show high-velocity anomalies along plate boundaries around the Japanese islands region. These anomalies may indicate subducting Pacific and Philippine Sea plates. On the other hand, a low-velocity anomaly is revealed beneath east of the Korean peninsula down to around 300 km depth, which is thought to be related to the formation of the Ulleung basin and the Ulleung island. Low-velocity anomalies revealed beneath the Jeju island may imply that the formation and volcanism of the Jeju island have been caused by magmatic sources from the deep mantle.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.14 no.6
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• pp.1323-1330
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• 2010

The amount of acoustic data gathered from the acoustic data gathering system is increased dramatically as the acoustic data gathering system become various and highly effective. It is needed to transmit this acoustic data to analysis environment for precise analysis. In this gathering/analysis system, it is also needed the stable transmitting as well as highly perfect security. But the existing system using the conveying method needs a lot of time. In case of using networks, the low speed networking vulnerable in the stable transmitting and the information security is impossible to materialize. In this paper, I would like to propose a transmitting and management system sending a massive gathering acoustic data in the unsecurity and low speed networking environment. The implemented system is to transmit the acoustic data safely in low speed networking environment and secure the acoustic data from various threats.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.2
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• pp.99-108
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• 2020

In the field of aircraft icing prediction, surface roughness has been considered as critical factor because it enhances convective heat transfer and changes local collection efficiency. For this significance, experimental studies have been conducted to acquire the quantitative data of the formation process. Meanwhile, these experiments was conducted under low-speed condition due to the measurement difficulties. However, it has not been investigated that how the flow characteristic of low-speed will effects to the surface roughness. Therefore, the present study conducted experiment under low-speed icing condition, and analyzed the relation between surface roughness characteristics and icing condition. As an analysis method, the dominant parameters used in the previous high-speed experiments are employed, and roughness characteristics are compared. The size of roughness element was consistent with the previous known tendency, but not the smooth zone width.

• Composites Research
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• v.31 no.5
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• pp.215-220
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• 2018

In this study, finite element analysis of Carbon-Steel Laminates with different layup pattern was conducted to verify similarity to the results of previous studies and to develop the effective model for low-velocity impact analysis. As in the experiment, Finite element analysis of the Fiber metal laminates (FMLs) with five different lamination patterns was carried out, and the impact resistance of the FMLs was confirmed by comparing the energy absorption ratio. The FMLs showed the higher energy absorption ratio than the mild steel having the same thickness, and it was confirmed that all the FMLs had the high energy absorption ratio over than 96%. In addition, the low-velocity impact analysis model proposed in this study can be effectively used to study composite forms and automotive structures.

• Composites Research
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• v.13 no.6
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• pp.47-54
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• 2000

One promising method for impact detection of composite structures is based on the use of piezopolymer thin fim (PVDf) sensor. In this paper, the relationship between the contact force and the signals of the attached strain gage and PVDF sensor to the composite plate subjected to low-velocity impact were derived. The relation for the open circuit and short circuit voltage of PVDF sensor was derived based on the equivalent circuit model of the piezoelectric sensor. The work was then extended to include experimental investigation into the use of short circuit voltage of PVDF sensor without using charge amplifier to detect low-velocity impact. The natural frequencies and damping ratio of the composite plate obtained from the vibration test were used to modify the analytical model and therefore the differences between measured and simulated signal of the modified analytical model in both forward and backward problem were considerably reduced. The reconstructed contact force and simulated sensor signals agreed well with the measured contact force, strain gage signal, and PVDF sensor singanl.

• The Transactions of the Korean Institute of Power Electronics
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• v.9 no.2
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• pp.112-118
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• 2004

Recently, development of rare earth permanent magnet with the high remanence, high coercivity allows the design of brushless motors with very high efficiency over a wide speed range. Cogging torque is produced in a permanent magnet by magnetic attraction between the rotor mounted permanent magnet and the stator teeth. It is an undesired effect that contributes to output ripple, vibration, and noise of machine. This cogging torque can be reduced by variation of magnet arc length, airgap length, magnet thickness, shifting the magnetic pole and varying the radial shoe depth and etc. In this paper, some airgap length and magnet arc that reduce cogging torque are found by finite element method(FEM) and Maxwell stress tensor method. The SPM(Surface Permanent Magnet) type of high-power Brushless DC (BLDC) motor is optimized as a sample model.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.7
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• pp.44-50
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• 2002

Low-velocity impact on composite sandwich panel has been investigated. Contact force is computed from a proposed modified Hertzian contact law. The Hertzian contact law is constructed by adjusting numerical value of the exponent and reducing the through-the- thickness elastic constant of honeycomb core. The equivalent transverse elastic constant is calculated from the rule of mixture. Nonlinear equation to calculate the contact force is solved by the Newton-Raphson method and time integration is done by the Newmark-beta method. A finite element program for the low-velocity impact analysis is coded by implementing these techniques and an 18-node assumed strain solid element. Behaviors of composite sandwich panels subjected to low-velocity impact are analyzed for various cases with different geometry and lay-ups. It has been found that the present code with the proposed contact law can predict measured contact forces and contact times for most cases within reasonable error bounds.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.6
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• pp.516-525
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• 2015

In this paper, the acoustic pressure of a helicopter rotor in hovering and low speed descent flight is predicted and compared with experimental data. Ffowcs Williams-Hawkings equation is used to predict the acoustic pressure. Two different wind tunnel test data are used to validate the predicted results. Boeing 360 model rotor test results are used for the low-frequency noise in hover, and HART II test results are employed for the mid-frequency noise, especially BVI noise, in low speed descent flight. A simple free-wake model as well as the state-of-the-art CFD/CSD coupling method are adopted to perform the analysis. Numerical results show good agreement against the measured data for both low-frequency and mid-frequency harmonic noise signal. The noise carpet results predicted using the FFT(Fast Fourier Transform) shows also reasonable correlation with the measured data.

• Aerospace Engineering and Technology
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• v.10 no.1
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• pp.183-192
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• 2011

The demand for weight saving and high performance of aircraft require the more uses of composite materials. However the complicate behaviors and various failure characteristics restrict usage of composite materials. Low-velocity impact damage is a major concern in the design of structures made of composite materials, because impact damage is hidden and cannot be detected by visual inspection. Especially, the reduction on compressive strength after impact is influenced by the ply delaminations introduced as damage by impact event. In this research, the numerical analysis was performed to investigate impact damage and compressive strength after impact. It was found that impact force history and compressive strength after impact calculated by the numerical analysis were compared and shown a good agreement with experimental results.

• Composites Research
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• v.18 no.3
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• pp.7-13
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• 2005

This paper presents a study on the low velocity impact response of the woven fabric laminates for the hybrid composite bodyshell of a tilting railway vehicle. In this study, the low velocity impact tests for the three laminates with size of $100mm\times100mm$ were conducted at three impact energy levels of 2.4J, 2.7J and 4.2J. Based on the tests, the impact force, the absorbed energy and the damaged area were investigated according to the different energy levels and the stacking sequences. The damage area was evaluated by the visual inspection and the C-scan device. The test results show that the absorbed energy of [fill]8 laminate is highest whereas (fill2/warp2)s is lowest. The [fill]8 laminate has the largest damage area because of the highest impact energy absorption.