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김건우 ( Geonwoo Kim ),김재윤 ( Jaeyun Kim ),김찬우 ( Chanwoo Kim ),윤창빈 ( Changbin Yoon ),최지호 ( Jiho Choi ),윤희철 ( Heecheol Yoon ),박완일 ( Wan-il Park ) 전북대학교 과학교육연구소 2019 과학과 과학교육 논문지 Vol.44 No.2
만유인력 법칙의 구조를 이론적으로 이해하고, 거리-역제곱법칙을 실험으로 검증해 보았다. 이론적으로는, 만유인력 법칙의 수학적 구조를 뉴턴의 제 2 운동법칙을 통해 추론해 보고 뉴턴이 했던 것과 같이 달의 공전을 이용하여 거리의 역제곱 법칙이 성립함을 확인해 보았다. 실험적으로는, 정밀저울을 이용하여 일정한 질량을 가진 분동의 높이 변화에 따른 무게변화를 측정함으로써 중력가속도의 크기 변화를 측정하고, 이로부터 만유인력의 법칙이 가진 거리-역제곱 법칙을 실험적으로 검증해 보았다. We inspected the theoretical structure of the law of universal gravity, and verified it experimentally. Theoretically, we deduced the mathematical structure of the law of universal gravity from Newton’s second law of motion, and confirmed the law of inverse square-distance via moon’s orbital motion as Newton did. Experimentally, in order to verify the law of inverse square-distance, a characteristic of the law of universal gravity, using a precision scale, we measured the variation of the magnitude of the gravitational acceleration as a function of the hight from the ground by measuring the weight of a weight having a given mass.
KR60 레일의 결함 탐상을 위한 위상배열 초음파 변환기 개발 (Ⅱ)
김건우(Geonwoo Kim),서무경(Mu-Kyung Seo),최남경(Namkyoung Choi),김기복(Ki-Bok Kim),권세곤(Segon Kwon),김영철(Young-cheol Kim) 한국비파괴검사학회 2017 한국비파괴검사학회지 Vol.37 No.6
본 연구에서는 KR60 레일 두부와 복부의 기하학적 형상 및 재질에 최적화된 위상배열 초음파 변환기를 CIVA 시뮬레이션 결과를 바탕으로 제작하였으며, 제작된 위상배열 초음파 변환기의 성능 평가를 위해 상용 위상배열 초음파 변환기와의 비교 실험을 진행하였다. 성능 평가는 KR60 레일에 표준인공결함을 제작하고 개발된 위상배열 초음파 변환기와 상용 위상배열 초음파 변환기의 감도와 상용 위상배열 초음파 변환기의 반사 신호 획득 제한 구역에서의 반사 신호 획득 가능 여부에 초점을 두었다. In this study, phased array ultrasonic transducers which are optimized in terms of the geometric and material features for detecting defects on head and body parts of the KR60 rail were fabricated based on CIVA simulation results. For the performance test of developed phased array ultrasonic transducers, these were compared with a commercial phased array ultrasonic transducer was compared. A performance test was conducted by comparing the sensitivities of the two types of phased array ultrasonic transducers, focusing on detecting defects that could not be inspected by commercial phased array ultrasonic transducers.
KR60 레일의 결함 탐상을 위한 위상배열 초음파 변환기 개발 (Ⅰ)
김건우(Geonwoo Kim),서무경(Mu-Kyung Seo),최남경(Namkyoung Choi),김기복(Ki-Bok Kim),권세곤(Segon Kwon),김영철(Young-cheol Kim) 한국비파괴검사학회 2017 한국비파괴검사학회지 Vol.37 No.6
현재 우리나라 철도 레일의 초음파 탐상은 복수의 단일 초음파 변환기를 이용한 탐상이다. 단일 초음파 탐상의 한계를 극복하기 위해, 본 연구에서는 CIVA 시뮬레이션을 이용하여 KR60 레일의 결함 탐상을 위한 최적 위상배열 초음파 변환기를 설계하고, 그 탐상 결과를 분석하였다. 상용 위상배열 초음파 변환기를 KR60 레일의 비파괴 탐상에 이용할 경우 레일의 기하학적 특성에 따라 검출 가능한 결함의 종류가 제한되었다. 따라서, KR60 레일의 구조적 특징과 결함의 위치와 형상에 따라 위상배열 초음파 변환기의 제원(중심주파수, 대역폭, 집속 깊이, 소자 길이, 너비, 높이 등)을 도출하였으며, 도출된 위상배열 초음파 변환기의 음장과 다양한 인공결함에 대한 S-scan 이미지를 비교·분석하였다. Ultrasonic testing for the KR60 railway in Korea has been conducted by single element ultrasonic testing. In this study, to overcome the limits of single element ultrasonic testing, phased array ultrasonic transducers for detecting defects on the KR60 rail were designed and analyzed based on the inspection results obtained by CIVA software simulation. It was difficult to detect various defects using commercial phased array ultrasonic transducers because of geometric features of KR60 railroad. Therefore, according to the geometric characteristics of the KR60 railroad, the specifications of phased array ultrasonic transducers for the KR60 rail were determined such as nominal frequency, bandwidth, focusing length, width, height, length of piezo-electric material, etc. The acoustic field of the phased array ultrasonic transducers and their S-scan images of artificial defects on the KR60 rail were compared and analyzed.
Kim, Junseong,Lee, Woo Seung,Kim, Jinsub,Song, Seunghyun,Nam, Seokho,Jeon, Haeryong,Baek, Geonwoo,Ko, Tae Kuk The Korea Institute of Applied Superconductivity a 2016 한국초전도저온공학회논문지 Vol.18 No.3
Recently, production technique and property of the High-Temperature Superconductor (HTS) tape have been improved. Thus, the study on applying an HTS magnet to the high magnetic field application is rapidly increased. A Nuclear Magnetic Resonance (NMR) spectrometer requires high magnitude and homogeneous of central magnetic field. However, the HTS magnet has fabrication errors because shape of HTS is tape and HTS magnet is manufactured by winding HTS tape to the bobbin. The fabrication errors are winding error, bobbin diameter error, spacer thickness error and so on. The winding error occurs when HTS tape is departed from the arranged position on the bobbin. The bobbin diameter and spacer thickness error occur since the diameter of bobbin and spacer are inaccurate. These errors lead magnitude and homogeneity of central magnetic field to be different from its ideal design. The purpose of this paper is to investigate the effect of winding error, bobbin diameter error and spacer thickness error on the central field and field homogeneity of HTS magnet using the virtual NMR signals in MATLAB simulation.
Case Study on the Changes in the Physical Environment in Forest Healing Spaces
Kim, Geonwoo,Park, Bum-Jin,Yeon, Poung-Sik,Lee, Sanggoo,Joung, Dawou,Park, Choonghee,Koga, Shinya FACULTY OF AGRIC PUBLICATIONS-KYUSHU UNIV 2016 Journal of the Faculty of Agriculture, Kyushu Univ Vol. No.
<P>This study was conducted to investigate changes in the physical environment in forest healing spaces by examining each of two tree species in a space within the Kasuya Research Forest, Kyushu University. At the study site, culmination and sunset showed high TNVOCs, low concentrations of particulate matter, and low PPD. Of the NVOCs, camphor was detected with the highest concentrations, 0.359 ng/m(3) and 0.142 ng/m(3), in C. camphora and M. thunbergii, respectively. Coniferous trees are generally used when creating an environment for healing purposes, as they are known to emit a large amount of NVOCs. However, according to our results, deciduous trees also emit NVOCs of sufficient concentrations. Therefore, if forest healing spaces are created with the appropriate components, it can have effective results.</P>
A PROBABILISTIC APPROACH FOR VALUING EXCHANGE OPTION WITH DEFAULT RISK
Kim, Geonwoo The Youngnam Mathematical Society 2020 East Asian mathematical journal Vol.36 No.1
We study a probabilistic approach for valuing an exchange option with default risk. The structural model of Klein [6] is used for modeling default risk. Under the structural model, we derive the closed-form pricing formula of the exchange option with default risk. Specifically, we provide the pricing formula of the option with the bivariate normal cumulative function via a change of measure technique and a multidimensional Girsanov's theorem.
Geonwoo Kim,Young-In Hwang,서무경,Ki-Bok Kim 대한기계학회 2020 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.34 No.8
In this study, two simple electrical impedance tuning methods derived from Krimholtz, Leedom, and Matthaei (KLM) modeling were proposed for increasing the sensitivity of a piezo-electric material-based ultrasonic transducer. In the first method, the reactance components of the ultrasonic transducer were removed, and in the other method, the electrical impedance of an ultrasonic transducer was matched to that of the ultrasonic system. To these ends, electrical circuits composed of inductors and capacitors were de-signed and KLM simulations of the two methods were performed. To verify the simulation results, ultrasonic transducers were fabricated and operated under the same conditions as in the simulations, and the experimental results were compared with the simulated results. In conclusion, the electrical tuning method (elimination of the reactance compo-nents) was more effective and the transducer circuit structure was simpler. The experi-mental results showed good agreement with the KLM simulation results.
Application of KLM model for an ultrasonic through-transmission method
Geonwoo Kim,Mu-Kyung Seo,최남경,백광세,김기복 한국정밀공학회 2019 International Journal of Precision Engineering and Vol.20 No.3
KLM model has been widely used for designing PZT based ultrasonic transducers by using an ultrasonic pulse-echo method in the field of non-destructive testing. In this study, to apply the KLM model for an ultrasonic through-transmission method, transmitting and receiving ultrasonic transducers, which have different resonance frequency, were simulated and acquired the through-transmitted ultrasonic waveforms. To verify and analyze the KLM model based ultrasonic through-transmission method, an ultrasonic through-transmission system including PZT based ultrasonic transducers, pulser/receiver, a test specimen was constructed and compared with through-transmitted response signals of fabricated ultrasonic transducers based on the same conditions of the KLM simulation.
Kim, Young Gon,Baek, Geonwoo,Han, Seunghak,Choi, Yojong,Kim, Junseong,Jeon, Haeryong,Ko, Tae Kuk The Korea Institute of Applied Superconductivity a 2019 한국초전도저온공학회논문지 Vol.21 No.4
Recently, many studies have been reported on the magnetoresistance and Hall effect of REBCO thin films and bulk. The voltage interferes quench detection of high-temperature superconducting magnet and generates leakage current in no insulation high-temperature superconducting coil. Therefore, in this paper, experiments on magnetoresistance and Hall effect of commercial YBCO and GdBCO tapes have been carried out. As a result, anomalous voltages expected for the magnetoresistance and Hall effect of REBCO tapes were observed and analyzed. In addition, the voltage characteristics of REBCO have been identified, and the Hall coefficient are calculated for use in high magnetic field magnet applications.