• Wind and Structures
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• v.34 no.1
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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.

• Geophysics and Geophysical Exploration
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• v.11 no.2
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• pp.116-126
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• 2008

In gas hydrate survey, seismic amplitude and frequency characteristics play a very important role in determining whether gas hydrate exists. According to the variation of source frequency and scatterer size, we study seismic amplitude characteristics using elastic modeling applied at staggered grids. Generally speaking, scattering occurs in proportion to the square of source frequency and the scatterer volume, which has an effect on seismic amplitude. The higher source frequency is, the more scattering occurs in gas hydrate bearing zone. Therefore, BSR is hardly observed in high frequencies. On the other side, amplitude blanking zone and BSR is clearly observed in lower frequencies although the resolution is poor as a whole. Seismic reflections traveling through free-gas layer below gas hydrate bearing zone decay so severely a high frequency component that a low frequency term is dominant. Amplitude anomaly of BSR result from high acoustic impedance contrast due to free-gas, which is a very crucial factor to estimate gas hydrate bearing zone. Seismic frequency analysis is carried out using wavelet transform method that frequency component could be decomposed with time variation. In application of wavelet transform to the seismic physical experiments data, we can observe that reflections traveling through air layer, which corresponds to the free-gas layer, decay a high frequency component.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.54 no.5
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• pp.348-357
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• 2005

We have tried to analyze the SEMG interference pattern quantitatively and automatically using T/A variables ZC, TN, mSA, mSD, UCA, AIPEA, ACT and NSS. For the analysis, we have carried out experiments on 14 SEMG interference patterns recorded from the biceps brachii, first dorsal interosseus and abductor policis brevis muscles. Emphasis was placed on the following 3 points in the experiments. 1) Suitable amplitude threshold for the automatic detection of the T/A variables. 2) Variation of the T/A variables to varying $\%$MVC. 3) Variation of the T/A variables to the sustained contraction for 30 seconds. Results of the experiments showed that T/A analysis of the SEMG interference Pattern can be effective tools for diagnostic purposes instead of the conventional NEMG method.

• The Journal of the Korea institute of electronic communication sciences
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• v.11 no.2
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• pp.191-196
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• 2016

Van der Pol's oscillators is non-conservative oscillator that having nonlinear damping phenomena. The energy of its system is dissipative at a high amplitude whereas its system creates the energy at low amplitude. This paper deals with the Van der Pol oscillator model with a fractional order when the external force apply into Van der Pol oscillator. This paper confirms the status of variation for the limit cycle according to the parameter variation of fractional order in the Van der Pol oscillator that can be represented by fractional differential equation.

• Journal of the Optical Society of Korea
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• v.19 no.2
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• pp.123-129
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• 2015

A pulse is generated when the heart pumps blood into the arterial system. The heart pumps blood only when it contracts, not when it relaxes; therefore, blood enters the arterial system in a cyclical form. Artery beating is visible in some parts of the body surface, such as the radial artery of the wrist. This paper mainly uses the feature in which near-infrared spectroscopy penetrates skin to construct a non-invasive measurement system that can measure small vibration in the subcutaneous tissue of the human body, and then uses it for the pulse measurement. This measurement system uses the optical moir$\acute{e}$ principle, together with the fringe displacement made by small vibration in the subcutaneous tissue, and an image analysis program to calculate the height variation from small vibrations in the subcutaneous tissue. It completes a measurement system that records height variation with time, and that together with a fast Fourier transform (FFT) program, they can convert the pulse waveform generated by vibration (time-amplitude) to heartbeat frequency (frequency-amplitude). This is a new and non-invasive medical assistance system for measuring the pulse of the human body, with the advantages of being simple, fast, safe and objective.

• Journal of the Korean earth science society
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• v.26 no.5
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• pp.417-428
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• 2005

The seasonal variation and frequency of precipitation phenomenon of the Korean Peninsula in summer show strong local weather phenomena because of its topographical and geographical factors in the northeastern area of Asia. The characteristics of the prevailing weather patterns in summer precipitation in Korea have great influences on the variation patterns and the appearances over a ten-day period during the summer precipitation. The purpose of this paper is to induce variation patterns over a period 10 days during the summer precipitation, clarify the variations of their space scales, and study the subdivision of precipitation regions in Korea according to the combinations of precipitation amounts and variation pattern during the period, using the mean values during the years $1991\~2003$ at 78 stations in Korea. The classified precipitation of a period of 10 days of summer precipitation, and the principal component vector and the amplitude coefficient by the principal component analysis were used for this study. The characteristics of variation pattern over the ten-day period can be chiefly divided into two categories and the accumulated contributory rate of these is $64.3\%$. The variation patterns of summer precipitation during period of 10 days in Korea are classified into 9 types from A to K. In addition, regional divisions of summer precipitation in Korea can be classified into 17 types.

• Transactions of the Korean Society of Automotive Engineers
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• v.9 no.4
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• pp.148-156
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• 2001

Brake judder, one of low Sequency vibrations in brake system is determined by the excitation of Brake Torque Variation (BTV). The largest contributor to BTV is disc thickness variation. In this study, the static loads of brake torque at Suspension Mounting Points (SW) are obtained by the quasi-static analysis using DADS. The dynamic loads with frequency of BTV at SW are derived from correlation between forced vibration analysis with static loads and brake test results. And the accelerations at steering wheel were analyzed by forced vibration analysis with dynamic loads using commercial finite element program MSC/NASTRAN so that vibration characteristics of vehicle due to brake judder were investigated. Reliability of analysis results was verified through comparing the brake test results. Also, a parametric study with natural frequencies of frame, such as the 1st torsional mode and 1st bending mode, was conducted to reduce vibration amplitudes. As a result we could detect frame natural frequency conditions to improve vibration characteristics and obtained the frame model to reduce vibration amplitude.

• Journal of The Korean Astronomical Society
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• v.43 no.6
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• pp.201-211
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• 2010

We present an analysis of the measurements of mid-eclipse times of V839 Oph, collected from literature sources. Our analysis indicates a period increase of $3.2{\times}10^{-7}$ day/yr. This period increase of V839 Oph can be interpreted in terms of mass transfer of rate $1.76{\times}10^{-7}M_{\odot}/yr$, from the less to the more massive component. The O - C diagram shows a damping sine wave covering two different complete cycles of 36.73 yr and 19.93 yr with amplitudes approximately equal to 0.0080 and 0.0043 day, respectively. The third cycle has to be expected to cover about 13.5 years with lower amplitude than those of the former two cycles. These unequal duration cycles show a non periodicity which may be explained as resulting from either the presence of a tertiary component to the system or cyclic magnetic activity variations due to star spots. For the later mechanism, the obtained characteristics are consistent when applying Applegate (1992) mechanism.

• Economic and Environmental Geology
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• v.47 no.6
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• pp.625-633
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• 2014

The Aquistore project is the world's first commercial capture, transportation, utilization and storage project of post-combustion $CO_2$ from a coal-fired thermo electric power plant, and the proposed storage is a saline aquifer at a depth of about 3,500 m. Deep saline aquifer, compared to hydrocarbon reservoir, provides the great volumetric potential for storage of $CO_2$ anywhere in the world, therefore the research results from the project may be exported globally to other sites. Geological $CO_2$ storage characterization for saline aquifer instead of hydrocarbon reservoir needs to estimate the geophysical properties of subsurface geology. This study calculated the geophysical property of water-saturated formation by applying amplitude variation analysis developed from oil and gas exploration. We correlated horizon tops at the well logs to seismic traveltime of 1,815 and 1,857 ms as Winnipeg and Deadwood formations. Gradient analysis from seismic traces showed correlation coefficient of 45 - 81 % on amplitude variation with respect to incident angle. Crossplot of intercept and gradient shows the inverse proportional trend which represents typical water saturated sediments. Product attribute of intercept and gradient described the base of wet sediment. Poisson's ratio change attribute increased at the top of target area satisfying with wet sediment and decreased at the top of basement in a dry rock bed.

• Steel and Composite Structures
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• v.14 no.1
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• pp.73-83
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• 2013

In this paper Hamiltonian Approach (HA) have been used to analysis the nonlinear free vibration of Simply-Supported (S-S) and for the Clamped-Clamped (C-C) Euler-Bernoulli beams fixed at one end subjected to the axial loads. First we used Galerkin's method to obtain an ordinary differential equation from the governing nonlinear partial differential equation. The effect of different parameter such as variation of amplitude to the obtained on the non-linear frequency is considered. Comparison of HA with Runge-Kutta 4th leads to highly accurate solutions. It is predicted that Hamiltonian Approach can be applied easily for nonlinear problems in engineering.