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이정훈(Jeung Hoon Lee),김삼묘(Sam Myo Kim) 한국정보과학회 1996 한국정보과학회 학술발표논문집 Vol.23 No.2A
본 논문은 프랙탈 영상 압축 방식의 속도를 개선하기 위하여 새로운 도메인 분류 및 검색 방안을 제안한다. 도메인 분류는 DCT에 의하여 얻어진 한 개의 DC값과 최상위 두 개의 AC값을 바탕으로 이루어졌으며 도메인을 근접 빈도(nearest frequency neighborhood)에 따라 검색하는 방안을 택하였다. 실험결과 종전의 방법에 비하여 영상의 질을 유지하며 약 5배의 속도 향상을 얻을 수 있었다.
이정훈(Lee, Jeung-Hoon),김광준(Kim, Kwang-Joon) 한국소음진동공학회 2008 한국소음진동공학회 논문집 Vol.18 No.1
Pneumatic vibration isolator typically consisting of dual-chamber pneumatic springs and a rigid table are widely employed for proper operation of precision instruments such as optical devices or nano-scale equipments owing to their low stiffness- and high damping-characteristics. As environmental vibration regulations for precision instruments become more stringent, it is required to improve further the isolation performance. In order to facilitate their design optimization or active control, a more accurate mathematical model or complex stiffness is needed. Experimental results we obtained rigorously for a dual-chamber pneumatic spring exhibit significantly amplitude dependent behavior, which cannot be described by linear models in earlier researches. In this paper, an improvement for the complex stiffness model is presented by taking two major considerations. One is to consider the amplitude dependent complex stiffness of diaphragm necessarily employed for prevention of air leakage. The other is to employ a nonlinear model for the air flow in capillary tube connecting the two pneumatic chambers. The proposed amplitude-dependent complex stiffness model which reflects dependency on both frequency and excitation amplitude is shown to be very valid by comparison with the experimental measurements. Such an accurate nonlinear model for the dual-chamber pneumatic springs would contribute to more effective design or control of vibration isolation systems.
인체 머리부 6 자유도 운동 측정의 신뢰성 향상을 위한 가속도계 감도축의 옵셋(offset) 추정
이정훈(Lee, Jeung-Hoon),김광준(Kim, Kwang-Joon),장한기(Jang, Han-Kee) 한국소음진동공학회 2008 한국소음진동공학회 논문집 Vol.18 No.9
Notion sickness is well known to be caused by long time exposure to the very low frequency motion in the multiple axes of human body Since the vestibular system for the perception of low frequency motion is located in the head, accurate measurement of 6 degree of freedom head motion is of great importance. In this study, the measurement system consisting of a safety helmet and 9 translational accelerometers was constructed for the estimation of 3 translational and 3 rotational motions of human head. Since estimation errors of 3 rotational components can be significantly magnified even by small offset of the sensitivity axis from the geometric center of an accelerometer, accurate measurement of sensitivity axis must be preceded. The method for accurate estimation of the offset was proposed, and the effect of offset on the estimation of angular acceleration was investigated.
공기층 음향반사 현상을 이용한 프로펠러 캐비테이션 유발 가진력 저감
이정훈(Jeung-Hoon Lee),이경준(Kyung-Jun Lee),박형길(Hyoung-Gil Park) 한국소음진동공학회 2017 한국소음진동공학회 논문집 Vol.27 No.6
Single nozzle arrangement in the full-scale ship reveals that a significant reduction of propeller cavitation induced pressure amplitude is possible at the outside of air-bubble layer, where the isolation effect of the air is not involved. Resulting hull-vibration reduction is approximately 75 %. Instead of excessive consumption of air, that is, considerably small amount is necessary for a reduction of propeller excitation, which can make the constitution of relevant system simple. Hence the purpose of this study is to provide a physical proof for such a beneficial phenomenon. By approximating the solution of acoustic reflection from a bubble, we could find that phase-reversal reflection provoking a destructive interference is the main reasoning for a pressure reduction outside the layer.
풍선을 이용한 프로펠러 캐비테이션 유발 선체 가진력 저감
이정훈(Jeung-Hoon Lee),이경준(Kyung-Jun Lee) 한국소음진동공학회 2018 한국소음진동공학회 논문집 Vol.28 No.1
The purpose of this work is to give analytical evidence that air-balloon is able to replace the previous effort of air-injection for a mitigation of propeller cavity induced hull excitation. We exploit the acoustical nature of rubber-like material, which is similar to that of water; thus a rubber layer existing at water-to-air interface appears to be transparent in the propagation of acoustic wave. That is, the balloon could be anticipated to play only the role of air-packing without any deterrence of the desired acoustic phenomenon, i.e., the destructive interference. A simple design strategy that tunes the frequency of maximum destructive interference to an exciting frequency is also presented, which can be simply done by adjusting the size of balloon. Finally, the suggested scheme is verified by two experimental demonstrations.
프로펠러 캐비테이션 유기 가진력 저감을 위한 풍선의 형상 의존성
이정훈(Jeung-Hoon Lee),신윤호(Yun-Ho Shin) 한국소음진동공학회 2021 한국소음진동공학회 논문집 Vol.31 No.4
The air-balloon is a simple, yet effective, device that can suppress propeller cavitation induced hull excitations using the principle of acoustic destructive interference. It is vital to have an accurate balloon destructive fre-quency for a design. The present study assumed the balloon as an oblate spheroid rather than as an ideal sphere. Beginning with the exact modal-series solution, we attempted an approximation for the concerned low frequency ranges whose corresponding acoustic wavelengths were significantly greater than the size of the balloon. The destructive frequency could then be derived in simple terms of the resonance frequency and a spheroidal variable. Based on this, it was emphasized that the asphericity stimulated a severe shift of the destructive frequency from the ideal spherical balloon with the same volume.
이정훈(Lee, Jeung-Hoon),김광준(Kim, Kwang-Joon) 한국소음진동공학회 2008 한국소음진동공학회 논문집 Vol.18 No.4
Pneumatic vibration isolator has a wide application for ground-vibration isolation of vibration-sensitive equipments. Recent advances In precision machine tools and instruments such as nano-technology or medical devices require a better isolation performance, which can be efficiently done by precise modeling- and design- of the isolation system. This paper will discuss an efficient transmissibility design method for pneumatic vibration isolator by employing the complex stiffness model of dual-chamber pneumatic spring developed in our previous research. Three design parameters of volume ratio between the two pneumatic chambers, the geometry of capillary tube connecting the two pneumatic chambers and finally the stiffness of diaphragm necessarily employed for prevention of air leakage were found to be important factors in transmissibility design. Based on design technique that maximizes damping of dual-chamber pneumatic spring, trade-off among the resonance frequency of transmissibility, peak transmissibility and transmissibility in high frequency range was found, which was not ever stated in previous researches. Furthermore this paper will discuss about negative role of diaphragm in transmissibility design. Then the design method proposed in this paper will be illustrated through experiment at measurements.
Spectral Kurtosis를 이용한 프로펠러 날개끝 보텍스 소음의 발생위치추정
이정훈(Jeung-Hoon Lee),김동호(Dong-Ho Kim),신윤호(Yun-Ho Shin) 한국소음진동공학회 2018 한국소음진동공학회 논문집 Vol.28 No.3
One primary concern for marine propeller design is the incipient tip vortex cavitation. The present study elaborates its hyperbolic localization, which is subjected to an estimation of time difference of arrival between measured acoustic signals. The generalized cross-correlation with phase transform, a conventional algorithm for time delay estimation, tends to be susceptible to the noise outside the frequency band of signal. In order to suppress these effects, we propose a weighting function, resembling the rectangular window in frequency domain. By further noting that the signal arisen by the tip vortex cavitation can be fall into the cyclostationary type, the spectral kurtosis is employed to design the optimum band of the frequency weights. Finally, the method is verified by experimental measurements in the water tunnel.
상용 유한요소해석 프로그램을 이용한 공압 스프링 내 다이아프램의 복소강성 산출
이정훈(Lee, Jeung-Hoon),김광준(Kim, Kwang-Joon) 한국소음진동공학회 2006 한국소음진동공학회 논문집 Vol.16 No.9
An accurate mathematical model for complex stiffness of the pneumatic spring would be necessary for an efficient design of a pneumatic spring used in vibration isolation tables for precision instruments such as optical devices or nano-scale equipments. A diaphragm, often employed for prevention of air leakage, plays a significant role of complex stiffness element as well as the pressurized air itself Therefore, effects of the diaphragm need to be included in the dynamic model for a more faithful description of dynamic behavior of pneumatic spring. But the complex stiffness of diaphragm is difficult to predict In an analytical way, since it is a rubber membrane of complicated shape in itself. Moreover, the diaphragm should be expandable in response to pressurization inside a chamber, which makes direct measurement of complex stiffness of diaphragm extremely difficult. In our earlier research, the complex stiffness of diaphragm was indirectly measured, which was just to eliminate the theoretical stiffness of pressurized air from the measured complex stiffness of the pneumatic spring. In order to reflect complex stiffness of inflated diaphragm on the total stiffness at the initial design or design improvement stage, however. it is required to be able to predict beforehand. In this paper, how to predict the complex stiffness of inflated rubber diaphragm by commercial FE codes (e.g. ABAQUS) will be discussed and the results will be compared with the indirectly measured values.