• 한국해양환경ㆍ에너지학회지
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• 제13권2호
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• pp.112-120
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• 2010

이 논문에서는 Yura 해역에서 폭풍 중 24시간 연속으로 측정된 freak wave가 포함된 파랑자료를 분석하여 비선형 파도의 통계적 특성을 살펴보았다. 파고의 초과확률분포는 Edgeworth-Rayleigh(ER) 분포와, Rayleigh 분포를 이용하여 계측된 자료와 비교하였다. 파형이 stationary 상태를 유지하는 10시간 동안 계측된 파랑자료(파랑의 개수가 4600 내외)에서는, 파형의 분포는 Gram-Chalier분포가 Gaussian분포보다 파랑자료와 일치를 한다. 전반적인 파고의 확률분포는 Rayleigh($H_{rms}$)분포를 잘 따르는 것을 볼 수 있다. Freak wave 발생확률은 Rayleigh($m_{o}{^{1/2}}$) 분포로 잘 표시가 되며, ER 분포는 과대하게 예측하는 것을 볼 수 있다. 30분간 계측된 파랑자료(파랑 개수가 250개 내외)에서 freak wave가 발생된 경우는 ER 분포가 freak wave 발생확률과 잘 일치한다. 그러나 전반적인 파고의 발생확률이 과대평가된다. Freak wave가 발생하지 않은 경우는 거의 모든 파고에 걸쳐 Rayleigh($H_{rms}$)분포가 계측된 파고분포와 잘 일치를 한다. 파고가 10 m 이하의 freak wave의 파고확률분포는 10 m 이상의 파고를 가지는 freak wave와 비슷한 경향을 보인다. $H_{max}/H_{1/3}$은 파형의 kurtosis와 연관이 있는 것을 보여주었다. Freak wave의 발생은 높은 kurtosis 값과 관계가 있으며, freak wave가 발생하는 임계 kurtosis 값이 존재할 가능성이 있을 것으로 추정된다.

• 한국소음진동공학회논문집
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• 제16권10호
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• pp.1057-1066
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• 2006

Acoustic streaming Induced by longitudinal vibration at 30 kHz is visualized for a test fluid flow between the stationary glass plate and ultrasonic vibrating surface with particle imaging velocimetry (PIV) To measure an increase in the velocity of air flow due to acoustic streaming, the velocity of air flow in a gap between the heat source and ultrasonic vibrator is obtained quantitatively using PIV. The ultrasonic wave propagating into air in the gap generates steady-state secondary vortex called acoustic streaming which enhances convective cooling of the stationary heat source. Heat transfer through air in the gap is represented by experimental convective heat transfer coefficient with respect to the gap. Theoretical analysis shows that gaps for maximum heat transfer enhancement are the multiple of half wavelength. Optimal gaps for the actual design are experimentally found to be half wavelength and one wavelength. A drastic temperature variation exists for the local axial direction of the vibrator according to the measurement of the temperature distribution in the gap. The acoustic streaming velocity of the test fluid in the gap is at maximum when the gap agrees with the multiples of half wavelength of the ultrasonic wave, which are specifically 6 mm and 12 mm.

• 한국가스학회지
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• 제12권4호
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• pp.58-62
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• 2008

본 연구에서는 직렬 6기통 압축천연가스 엔진의 피스톤에 대한 3차원 모델링을 수행하여 정상상태에서의 온도분포 및 그에 따른 열응력과 변형을 예측하고, 이를 기존의 해석결과와 비교 검토를 통하여 피스톤의 유한요소해석의 기준을 구축하고자 한다. 또한 냉각시스템의 성능이 피스톤의 열부하에 미치는 영향을 평가하기 위하여 냉각수 온도의 변화에 따른 피스톤의 온도분포 및 열응력 분포 그리고 그에 따른 변형을 분석하였다. 분석결과 피스톤의 최고 온도는 크라운부의 중앙에서 나타났고, 피스톤의 크라운 하부에서 최대 열응력이 발생하였다.

• 한국추진공학회지
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• 제16권4호
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• pp.33-41
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• 2012

KM6 단기추진제의 저장수명을 감마과정 이론을 이용한 확률론적 방법으로 추산하였다. 장기 저장에 따른 안정제 함량의 저하량이 0.8%일 때를 상태고장으로 보았으며 정상감마과정으로 가정하였을 때 형상함수의 상수와 척도모수를 모멘트법으로 추정하였다. 저장기간별 확률밀도함수로부터 각 저장기간에서의 상태분포를 확인할 수 있으며 누적고장분포함수 곡선에서 누적고장확률이 10%인 $B_{10}$수명은 25년이며 $B_{50}$수명은 36년으로 추산되었다. 실용적 관점에서 볼 때 $B_{50}$수명을 평균저장수명으로 볼 수 있으며 확률과정론을 이용하면 저장수명을 분포곡선으로 표현할 수 있다.

• Wind and Structures
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• 제37권5호
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• pp.361-373
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• 2023

Suspended monorail trains (SMTs) are sensitive to crosswinds, and instantaneous aerodynamic characteristics of two SMTs passing each other under crosswinds are particularly complicated. In this study, a pressure measurement test is carried out on stationary train-bridge models arranged in several critical positions. In addition, a validated moving CFD model is developed with the dynamic and sliding mesh method to explore the realistic train movement effects. The time-varying aerodynamic forces and surface pressure distribution on, as well as the flow field around running trains and bridges during trains passing each other, are computed in detail to illustrate the shielding effect of the upstream train. The results reveal that when two trains begin to pass each other, the side force coefficient of the downstream train reduces significantly to negative values due to the wind shielding effect of the upstream train. The moving model successfully captures that airflow is separated on the middle line of the head car for the suspended monorail train, and the surrounding bluff double-beams can significantly affect the flow structures around the train. The wind shielding effect of the upstream train on the downstream train will weaken as the relative yaw angle decreases.

• Wind and Structures
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• 제34권1호
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• pp.81-90
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• 2022

To better understand the vortex-induced vibration (VIV) characteristics of a 5:1 rectangular cylinder, the distribution of aerodynamic force and the non-dimensional power spectral density (PSD) of fluctuating pressure on the side surface were studied in different VIV development stages, and their differences in the stationary state and vibration stages were analyzed. The spanwise and streamwise correlations of surface pressures were studied, and the flow field structure partitions on the side surface were defined based on the streamwise correlation analysis. The results show that the variation tendencies of mean and root mean square (RMS) pressure coefficients are similar in different VIV development stages. The RMS values during amplitude growth are larger than those at peak amplitude, and the smallest RMS values are observed in the stationary state. The spanwise correlation coefficients of aerodynamic lifts increase with increase of the peak amplitude. However, for the lock-in region, the maximum spanwise correlation coefficient for aerodynamic lifts occurs in the VIV rising stage rather than in the peak amplitude stage, probably due to the interaction of vortex shedding force (VSF) and self-excited force (SEF). The streamwise correlation results show that the demarcation point positions between the recirculation region and the main vortex region remain almost constant in different VIV development stages, and the reattachment points gradually move to the tailing edge with increasing amplitude. This study provides a reference to estimate the demarcation point and reattachment point positions through streamwise correlation and phase angle analysis from wind tunnel tests.

• Journal of the Korean Data and Information Science Society
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• 제20권6호
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• pp.973-982
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• 2009

파론도 게임은 두 개의 지는 게임을 주기적으로 반복하거나 혹은 임의적으로 선택하면 궁극적으로 이기게 되는 역설적인 게임을 말한다. 여러 명의 게임자들이 파론도 게임을 구성하는 두 게임 중에서 하나를 집단적으로 선택해서 진행하는 게임을 고려하자. 본 논문에서는 이 집단 파론도 게임의 모든 모수의 범위에서 장기적으로 기대상금을 최대화하는 최적의 게임 선택 기준이 무엇인지를 찾고 그 기대상금을 구한다.

• Wind and Structures
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• 제26권3호
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• pp.179-190
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• 2018

Translation of tornadoes is an important feature in replicating the near-ground tornado flow field which has been simulated in previous studies based on Ward-type tornado simulators using relative motion of the ground plane. In this laboratory investigation, effects of translation on the near-ground tornado flow field were studied using the ISU Tornado Simulator that can physically translate over a ground plane. Two translation speeds, 0.15 m/s and 0.50 m/s, that scale up to those corresponding to slowly-moving tornadoes in the field were selected for this study. Compared with the flow field of a stationary tornado, the simulated tornado with translation had an influence on the spatial distribution and magnitude of the horizontal velocities, early reversal of the radial inflow, and expansion of the core radius. Maximum horizontal velocities were observed to occur behind the center of the translating tornado and on the right side of its mean path. An increase in translation speed, resulted in reduction of maximum horizontal velocities at all heights. Comparison of the results with previous studies that used relative motion of the ground plane for simulating translating tornadoes, showed that translation has similar effects on the flow field at smaller radial distances (~2 core radius), but different effects at larger radial distances (~4 core radius). Further, it showed that the effect of translation on velocity profiles is noticeable at and above an elevation of ~0.6 core radius, unlike those in studies based on the relative motion of the ground plane.

• Journal of Welding and Joining
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• 제25권1호
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• pp.24-29
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• 2007

Finite element analysis for induction heating process including magnetic and thermal situations is presented. Because magnetic and thermal material properties vary with the temperature, material properties depending on temperature are considered. As the inductor moves, the solution domains corresponding to the inductor change into those of the air and the solution domains corresponding to the air change into those of the inductor. For these reasons, modeling of induction heating process is very difficult with a general purpose commercial programs. In this paper, three dimensional analysis of induction heating process for moving inductor is analyzed using moving coordinate. The skin effect is confirmed inside the steel plate in the electro-magnetic analysis. The distribution of heat generation at the initial state is different from that at the quasi-stationary state. Therefore, material properties depending on temperature must be considered. The calculated results of finite element analysis agree well with the experimental temperature results. This approach is suitable to solve magneto-thermal coupled problems.

• 대한산업공학회지
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• 제21권4호
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• pp.609-628
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• 1995

This paper presents a reliability model and a data-analytic procedure for a repairable unit subject to failures due to multiple non-identifiable causes. We regard a failure cause as a state and assume the life distribution for each cause to be exponential. Then we represent the dependency among the causes by a Markov switching model(MSM) and estimate the transition probabilities and failure rates by maximum likelihood(ML) method. The failure data are incomplete due to masked causes of failures. We propose a specific version of EM(expectation and maximization) algorithm for finding maximum likelihood estimator(MLE) under this situation. We also develop statistical procedures for determining the number of significant states and for testing independency between state transitions. Our model requires only the successive failure times of a unit to perform the statistical analysis. It works well even when the causes of failures are fully masked, which overcomes the major deficiency of competing risk models. It does not require the assumption of stationarity or independency which is essential in mixture models. The stationary probabilities of states can be easily calculated from the transition probabilities estimated in our model, so it covers mixture models in general. The results of simulations show the consistency of estimation and accuracy gradually increasing according to the difference of failure rates and the frequency of transitions among the states.