• 한국산학기술학회논문지
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• 제20권4호
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• pp.613-618
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• 2019

열교환기에서 유동 유발 진동은 배관의 파손을 유발할 수 있어서 열교환기의 구조 안정성을 위해 유동 유발 진동 특성을 규명할 필요가 있다. 본 연구는 단일 원관에서 입구의 유속이 일정한 경우와 주기적인 변동이 있는 경우에 대하여 시간에 따라 원관 주위에서 와류의 생성, 발달 및 분리 특성을 비교 분석하고 양력과 항력의 시간 변화 특성과 PSD 특성을 살펴보아 단일 원관에서 전방류의 주기적인 속도 변동에 따른 유동 특성을 규명하였다. 일정 입구 유속의 경우는 잘 알려진 칼만 와류 분포를 보여 주었으며 원관의 상하에서 교대로 와류가 발생, 성장, 분리하는 것을 관찰할 수 있었다. 주기적인 입구 유속의 경우는 원관의 상하에서 동시에 와류가 발생하는 것을 관찰할 수 있었다. 일정 입구 유속의 경우는 양력의 PSD 크기가 항력에 비해 약 500배 컸으며 주파수는 31.15 Hz이며 항력은 2배인 62.3 Hz로 나타났다. 주기적인 입구 유속의 경우는 항력의 PSD가 양력보다 약 500배 컸으며 주파수는 입구 유속의 주파수와 동일하게 15.57 Hz이며 양력의 주파수는 일정 입구 유속과 같은 칼만 와류 주파수인 31.15 Hz로 나타났다.

• 대한전기학회논문지:전기물성ㆍ응용부문C
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• 제48권8호
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• pp.577-579
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• 1999

DLC film remaining on device surface could be removed by eliminating AI sacrificial layer as a final step of lift-off process in the fabrication of DLC-coated Si-tip FEA. The field emission properties(I-V curves, hysteresis, and current fluctuation etc.) of the processed device were analyzed and the process was employed to 1.76 inch-sized FEA panel fabrication in order to evaluate its FED applicability.

• 한국전산유체공학회지
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• 제20권3호
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• pp.47-53
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• 2015

This study performed numerical simulation to elucidate the characteristics of flow past a rectangular cylinder with various values of the aspect ratio(AR) of the cylinder. We calculated the flow field, force coefficients and Strouhal number of vortex shedding depending on the Reynolds number(Re) and the aspect ratio. The $AR{\approx}1$ is preferred for drag reduction, and 0.375$AR{\approx}0$ is recommended if suppression of the lift-coefficient fluctuation and the shedding frequency is desirable. Furthermore the criticality of the Hopf bifurcation is also reported for each AR.

• 해양환경안전학회지
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• 제5권2호
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• pp.35-44
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• 1999

High negative pressure coefficient is formed in the corner of the bluff body structures. For many curtain wall designers this phenomena is of interest because this high negative pressure coefficient is adopted in structural calculation. The present study is aimed to investigate shedding vortex characteristics of two-dimensional rectangular prism flow. Unsteady calculation by finite difference method based upon SOLA is carried out for three aspect ratios(1:1, 1:2, 1:3) of Re=10$^4$ in viscous incompressible flow within infinite domain. Fluctuation of velocity components at various pick-up points and time variation of drag and lift coefficients are analysed by FFT method to reveal shedding vortex frequency patterns. At aspect ratio 1:1, one primary Strouhal number appears for about all pick-up points. At aspect ratio 1:2, two representative Strouhal numbers are classified by pick-up positions and their flows show two different reattachment patterns. For aspect ratio 1:3, frequency spectrum maintains multiple peaks.

• 농촌계획
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• 제10권3호
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• pp.53-58
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• 2004

In this study the effect caused by limited storage lift of agricultural products for determining shipping amount can be analyzed by $l^{st}$ order autoregressive model based on cobweb theorem. Carrying capacity and auction price of upland-grown cabbage and garlic from 2000 to 2003 in wholesale markets were used for analysis. In result regression models of cabbage can not be used in verification periods although those of garlic approximately predicted shipping amounts in verification periods. It can be inferred that it is hard to control shipping amounts depending on price fluctuation for agricultural products which have limited storage life so cultivated areas and meteorological risk should be managed for stable price.

• 인문언어
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• 제5권
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• pp.293-310
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• 2003

This study starts with a remark on the frequent usage of the term 'generation culture' in the explanations of cultural change. The term 'generation culture' itself does have neither a clear meaning nor any academic consensus for its operational definition yet. Depending on intuitions or common senses, in most cases, of everyday lift experience, the tends to designate the co-existence of different cultural layers as composing subsystems of a little broader Korean cultural trend. I tried here to analyze different positions and perspectives in the employment of this term, sometimes with strategic intentions of each social groups. Economic or political positions, for example, are intertwined with the quest of identification concerning 'we' and 'other' grouping dynamics in the Age of Globalization, which accelerates the speed of cultural re-territorialization. And the role of consumption activity as a kind of cultural indication has gained more weight in that process. This analysis will remind us, also, of the post-modem society's assumption about the space-and-time in transition, and its fluctuation.

• International Journal of Naval Architecture and Ocean Engineering
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• 제12권1호
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• pp.799-809
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• 2020

In this study, the supercavitating flow of a high-velocity moving body near air-water surface is calculated and analyzed based on a commercial CFD software ANSYS Fluent. The effect of regular wave parameters including both wave height and wavelength on the cavitating flow and force characteristics of a body at different velocities is investigated. It is found that the cavity shape, lift coefficient and drag coefficient of the body vary periodically with wave fluctuation, and the variation period is basically consistent with wave period. When the wavelength is much greater than the cavity length, the effect of wave on supercavitation is the alternating effect of axial compression and radial compression. However, when the wavelength varies around the cavity length, the cavity often crosses two adjacent troughs and is compressed periodically by the two wave troughs. With the variation of wavelength, the average area of cavity shows a different trend with the change of wave height.

• 한국산업융합학회 논문집
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• 제20권4호
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• pp.275-284
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• 2017

As the high-level of the industrial and information age, the electricity become the indispensable element in the daily life including OA, FA, and computer, electric home appliances, and etc. In particular, The continuous use of the high capacity power supply system by applying a Switching Mode Power Supply(SMPS) according to the increase of the secondary side output terminal of the power load of the refrigerator of the home appliance or automation of the plant is pressed. The purpose using the way with this kind of high-capacity altogether is to supply the output voltage and output current regardless of the input voltage or to the external environmental conditions of the secondary-side load fluctuation. In this paper, a combination of a Buck Converter with Boost Converter by making a constant current source to control the inductor current and maintain stable power supply side operating characteristics, when load variations. While maintaining the same characteristics as conventional Buck Converter, and offer a DC-DC Converter system with the new switch pattern having a wide output range capable of operating in Buck-Boost Converter. In addition, after theoretical analysis, we carry out simulations and experiments to verify the validity and performance comparing with a conventional DC-to-DC converter.

• Journal of Multimedia Information System
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• 제7권2호
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• pp.189-196
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• 2020

We have developed a device for recording biological data by inserting three electrodes and a needle with an angular velocity sensor into the moth for the purpose of measuring the electromyogram of the flapping and the corresponding lift force. With this measurement, it is possible to evaluate the moth-physiological function of moths, and the amount of pesticides that insects are exposed to (currently LD50-based standards), especially the amount of chronic low-concentration exposure, can be reduced the dose. We measured and recorded 2-channel electromyography (EMG) and angular velocity corresponding to pitch angle (pitch-like angle) associated with wing flapping for 100 sweet potato hawkmoths (50 females and 50 males) with the animals suspended and constrained in air. Overall, the angular velocity and amplitude of EMG signals demonstrated high correlation, with a correlation coefficient of R = 0.792. In contrast, the results of analysis performed on the peak-to-peak (PP) EMG intervals, which correspond to the RR intervals of ECG signals, indicated a correlation between ΔF fluctuation and angular velocity of R = 0.379. Thus, the accuracy of the regression curve was relatively poor. Using a DC amplification circuit without capacitive coupling as the EMG amplification circuit, we confirmed that the baseline changes at the gear change point of wing flapping. The following formula gives the lift provided by the wing: angular velocity × thoracic weight - air resistance - (eddy resistance due to turbulence). In future studies, we plan to attach a micro radio transmitter to the moths to gather data on potential energy, kinetic energy, and displacement during free flight for analysis. Such physiological functional evaluations of moths may alleviate damage to insect health due to repeated exposure to multiple agrochemicals and may lead to significant changes in the toxicity standards, which are currently based on LD50 values.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2292-2314
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• 2006

Efficient numerical method is developed for the prediction of aerodynamic noise generation and propagation in low Mach number flows such as aeolian tone noise. The proposed numerical method is based on acoustic/viscous splitting techniques of which acoustic solvers use simplified linearised Euler equations, full linearised Euler equations and nonlinear perturbation equations as acoustic governing equations. All of acoustic equations are forced with immersed surface dipole model which is developed for the efficient computation of aerodynamic noise generation and propagation in low Mach number flows in which dipole source, originating from unsteady pressure fluctuation on a solid surface, is known to be more efficient than quadrupole sources. Multi-scale overset grid technique is also utilized to resolve the complex geometries. Initially, aeolian tone from single cylinder is considered to examine the effects that the immersed surface dipole models combined with the different acoustic governing equations have on the overall accuracy of the method. Then, the current numerical method is applied to the simulation of the aeolian tones from twin cylinders aligned perpendicularly to the mean flow and separated 3 diameters between their centers. In this configuration, symmetric vortices are shed from twin cylinders, which leads to the anti-phase of the lift dipoles and the in-phase of the drag dipoles. Due to these phase differences, the directivity of the fluctuating pressure from the lift dipoles shows the comparable magnitude with that from the drag dipoles at 10 diameters apart from the origin. However, the directivity at 100 diameters shows that the lift-dipole originated noise has larger magnitude than, but still comparable to, that of the drag-dipole one. Comparison of the numerical results with and without mean flow effects on the acoustic wave emphasizes the effects of the sheared background flows around the cylinders on the propagating acoustic waves, which is not generally considered by the classic acoustic analogy methods. Through the comparison of the results using the immersed surface dipole models with those using point sources, it is demonstrated that the current methods can allow for the complex interactions between the acoustic wave and the solid wall and the effects of the mean flow on the acoustic waves.