• Structural Engineering and Mechanics
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• 제48권4호
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• pp.501-518
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• 2013

Apart from thinning of cortical layers, the local bone curvature, varying along bone periphery, modulates ultrasound waves as well, which is however often underestimated or overlooked in clinical quantitative ultrasound (QUS). A dedicated three-dimensional finite element modelling technique for cortical bones was established, for quantitatively exploring and calibrating the effect of local curvature of cortical bone on ultrasound. Using a correlation-based mode extraction technique, high-velocity group (HVG) and low-velocity group (LVG) wave modes in a human radius were examined. Experimental verification using acrylic cylinders and in vitro testing using a porcine femur were accomplished. Results coherently unravelled the cortical curvature exerts evident influence on bone-guided ultrasound when RoC/${\lambda}$ <1 for HVG mode and RoC/${\lambda}$ <2 for LVG mode (RoC/${\lambda}$: the ratio of local bone curvature radius to wavelength); the sensitivity of LVG mode to bone curvature is higher than HVG mode. It has also been demonstrated the local group velocity of an HVG or LVG mode at a particular skeletal site is equivalent to the velocity when propagating in a uniform cylinder having an outer radius identical to the radius of curvature at that site. This study provides a rule of thumb to compensate for the effect of bone curvature in QUS.

• Structural Engineering and Mechanics
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• 제75권4호
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• pp.435-444
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• 2020

Ultrasonic guided wave testing is a very promising non-destructive testing method for rails, which is of great significance for ensuring the safe operation of railways. On the basis of the semi-analytical finite element (SAFE) method, a analytical model of 59R2 grooved rail was proposed, which is commonly used in the ballastless track of modern tram. The dispersion curves of ultrasonic guided waves in free rail and supported rail were obtained. Sensitivity analysis was then undertaken to evaluate the effect of rail elastic modulus on the phase velocity and group velocity dispersion curves of ultrasonic guided waves. The optimal guided wave mode, optimal excitation point and excitation direction suitable for detecting rail integrity were identified by analyzing the frequency, number of modes, and mode shapes. A sinusoidal signal modulated by a Hanning window with a center frequency of 25 kHz was used as the excitation source, and the propagation characteristics of high-frequency ultrasonic guided waves in the rail were obtained. The results show that the rail pad has a relatively little influence on the dispersion curves of ultrasonic guided waves in the high frequency band, and has a relatively large influence on the dispersion curves of ultrasonic guided waves in the low frequency band below 4 kHz. The rail elastic modulus has significant influence on the phase velocity in the high frequency band, while the group velocity is greatly affected by the rail elastic modulus in the low frequency band.

• 지구물리와물리탐사
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• 제14권2호
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• pp.140-145
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• 2011

데이터에 포함된 잡음이 소규모 저속도 이상대 탐지를 위한 시추공 주시토모그래피에 미치는 영향을 분석하기 위하여, 터널모델에 대하여 유한차분 파선추적 법으로 기록의 초동주시를 산출하고 산출된 초동주시에 세 종류의 잡음과 잡음수준이 다른 네 종류의 백색 잡음을 첨가한 후 SIRT 반복역산 법으로 속도영상을 도출하였다. 첨가된 잡음의 세기가 터널로 인한 기록의 최대 예상 주시지연의 10% 정도인 속도영상에서 저속도 이상대가 정확하게 터널위치에 형성되었다. 그러나 터널의 위치에 산출된 속도는 상정된 속도모델의 속도와는 현저하게 다르며 배경 속도와 비슷하였다. 잡음이 결과영상에 미치는 영향은 무작위 잡음이 송신기나 수신기 종속적인 잡음보다 현저하게 적은 것으로 나타났다.

• 지구물리와물리탐사
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• 제6권3호
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• pp.153-159
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• 2003

본 연구에서는 상반성 원리에 의해 수진기 배열을 변경하여 영상화 범위를 확장한 탄성파 기록을 얻었다. 이를 역시간 구조보정에 적용하여 구조보정 영상의 개선을 시도하였다. 송신원과 수진기의 위치가 바뀌어도 물리적 반응이 변화 없다는 상반성의 원리를 이용하면 탄성파 탐사기록이 취득 되지 않은 수진기 위치에서도 탄성파 기록을 얻을 수 있다. 변경된 수진기 배열에 의한 탐사 기록의 효율성을 검증하기 위하여 Marmousi 속도 모형을 적용한 역시간 구조보정을 수행하였다. 현장 자료의 경우 정확한 지하 속도 모형을 얻기 어려우므로 이를 고려하여 선형적으로 속도가 증가하는 모형을 적용하였다. 구조보정 결과 탐사기록의 변경이 구조보정 영상의 해상도 개선에 효과가 있음을 확인하였다. 따라서 기존의 탐사기록을 걸다 유용하게 이용할 수 있을 것으로 보인다.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2010년도 한국우주과학회보 제19권1호
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• pp.40.4-41
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• 2010

Using magnetic field and plasma moment data obtained by THEMIS satellites(A, D, and E), we selected 203 fast flow events accompanied by dipolarization in the near-Earth region( X(GSM) = -7 ~ -12 RE) and statistically examined their properties. It was found that most of the fast flows show the maximum velocity between 1 minute before dipolarization onset and 2 minutes after onset and proceed earthward and duskward. We also found that only the flows with low velocity of less than 400 km/s are observed at X > -8 RE, while the high velocity flows(as well as low velocity flows) are observed at the further tailward region(X < -8 RE). And most of the tailward flows are slow regardless of distance at X(GSM) = -7 ~ -12 RE. On the other hand, if we consider the fast flow as a bubble (Pontius and Wolf, 1990), the entropy parameter, PV5/3 is an important factor to describe the plasma sheet dynamics. Thus we investigated the relationship between the flow velocity and the amount of change in PV5/3 before and after dipolarization onset and found out that the dipolarizations with more depleted entropy parameter tend to show higher flow velocity. Also we examined how the magnetic field at geosynchronous orbit responds to the fast flow accompanied by dipolarization in the near-earth plasma sheet, using the measurements from GOES 11 and 12 statellites. We found that most of the fast flows do not reach geosynchronous orbit as suggested by Ohtani et al. (2006).

• Ocean and Polar Research
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• 제44권4호
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• pp.287-296
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• 2022

High-frequency (HF) radar measures sea surface currents from the radio waves transmitted and received by antenna on land. Since the data quality of HF radar measurements sensitively depend on the radio wave environment around antenna, Antenna Pattern Measurements (APM) plays an important role in evaluating the accuracy of measured surface currents. In this study, APM was performed by selecting the times when the background noise level around antenna was high and low, and radial data were generated by applying the ideal pattern and measured pattern. The measured antenna pattern for each case was verified with the current velocity data collected by drifters. The radial velocity to which the ideal pattern was applied was not affected by the background noise level around antenna. However, the radial velocity obtained with APM in the period of high background noise was significantly lower in quality than the radial velocity in a low noise environment. It is recomended that APM be carried out in consideration of the radio wave environment around antenna, and that the applied result be compared and verified with the current velocity measurements by drifters. If it is difficult to re-measure APM, we suggest using radial velocity in generating total vector with the ideal pattern through comparative verification, rather than poorly measured patterns, for better data quality.

• 한국지반공학회논문집
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• 제25권11호
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• pp.53-60
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• 2009

터널의 시공은 항상 주변 암반의 손상을 초래한다. 발생하는 손상의 정도는 주변암반의 역학적 및 수리학적 거동에 영향을 미친다. 본 논문에서는 크로스홀 시험을 통하여 P 파 속도를 측정하여 터널주변 암반의 손상을 계측하였다. 발진공에서의 탄성파 신호발생을 위하여 기계적인 충격을 가하였으며 그 결과로 발생된 P파 신호는 잡음이 적으며 파의 초동 도달시간 판별이 용이하였다. 실험결과 암반의 손상이 예상되는 구간에서 P 파의 속도가 낮게 검측 되었다. 크로스홀 공 내 다중의 지점에서 P 파 계측을 수행하여 이차원 P 파 토모그래피를 생성하였는데, 생성된 토모그래피는 터널 배면의 암반 손상이 발생한 구간에 대한 가시적인 결과를 나타내었다. 측정된 P파의 속도로부터 간극율 또는 Q 값과의 상관관계를 통해 암반 특성의 정량적인 손상 평가가 가능하였다.

• 한국정밀공학회지
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• 제34권3호
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• pp.179-184
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• 2017

Recently, traffic accidents and damage on the highway have increased because of overloaded vehicles. The existing overload-detecting system has a low accuracy rate. An overload-detecting system using a weigh-in-motion (WIM) system has been developed to solve this problem. The WIM system can be used to detect overloaded vehicles by measuring the weight of the vehicles. The WIM system is divided into high-speed and low-speed types. The inaccuracy rate in the low-speed WIM system results mainly from the low response rate of the sensor when the velocity is moving at more than 20 km/h. In this study, a low-speed overload-detecting pad with a hydraulic structure using a WIM system was developed to make the system more accurate. The structural and formal analysis was carried out by using a finite element method (FEM) in order to analyze the structural stability and the extrusion velocity of the system. In addition, a static load test was performed to confirm the linearity and accuracy of the pad.

• 대한물리의학회지
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• 제7권1호
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• pp.1-9
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• 2012

Purpose : This study aimed to compare the effect of two modes (proprioceptive neuromuscular facilitation combination patterns and ball exercise) of low back stability for chronic low back patients. Methods : The subjects were recruited fourty patients who had low back pain. All subjects were randomly assigned to PNF combination patterns group, ball exercise group. Measurements were performed fourtimes: pre test, 2weeks, 4weeks, and 6weeks. Main outcome measures comprised the visual analogue scale(VAS), balance performance monitor(BPM). Results : The results were as follows. In the comparison of VAS score, sway area, sway path, and sway velocity according treatment period, score was significantly reduced in both PNF combination pattern group and ball exercise group. In the comparison of the both VAS and sway area between groups, there were significant. PNF combination pattern group significantly more decrease than ball exercise group at 6 weeks. However, both sway path and sway max velocity between group, there were not significantly. Conclusion : These results of this study indicated that PNF combination pattern which performed for six weeks had a significant influence than ball exercise group on low back pain.

• Wind and Structures
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• 제20권3호
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• pp.469-488
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• 2015

A full-scale instrumented low-rise building with gable roof was built at a coastal site with a high incidence of tropical cyclones for monitoring of wind effects on the building during windstorms. This paper presents the field measurements of the wind velocity field around and the wind-induced pressures on the low-rise building during the passage of severe tropical storm Soudelor. Near-ground wind characteristics such as wind speed, wind direction, turbulence intensity, gust factor, turbulence integral length scale and wind velocity spectra were investigated. The wind-induced pressures on the roof of the building were analyzed and discussed. The results revealed that the eave and ridge edges on the roof were subjected to the most severe suction pressures under quartering winds. These suction pressures showed obvious non-Gaussian behavior. The measured results were compared with the provisions of ASCE 7-10 to assess the suitability of the code of practice for the wind-resistant design of low-rise buildings under tropical cyclones. The field study aims to provide useful information that can enhance our understanding of the extreme wind effects on low-rise buildings in an effort to reduce tropical cyclone wind damages to residential buildings.