• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1534-1539
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• 2004

The objective of this study is to simulate the etching characteristics with oscillation angle for the optimization of etching system. The etching characteristics were analyzed under different etching conditions. The spray characteristics were measured by Phase Doppler Anemometer (PDA). The correlation between the spray characteristics and the etching characteristics was investigated and used for fundamental data to simulate the etching characteristics with oscillation angle. The smaller coefficient of variation, the more uniform etching characteristic distribution became. It was found that numerical predictions of etching factor generally agreed well with the measured results with distance from nozzle tip. Oscillation leads to decrease of etching factor and increase of uniformity.

• Journal of Mechanical Science and Technology
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• v.17 no.10
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• pp.1533-1542
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• 2003

In the present study, the characteristics of pressure oscillation and heat transfer performance in an oscillating capillary tube heat pipe were experimentally investigated with respect to the heat flux, the charging ratio of working fluid, and the inclination angle to the horizontal orientation. The experimental results showed that the frequency of pressure oscillation was between 0.1 Hz and 1.5 Hz at the charging ratio of 40 vol.%. The saturation pressure of working fluid in the oscillating capillary tube heat pipe increased as the heat flux was increased. Also, as the charging ratio of working fluid was increased, the amplitude of pressure oscillation increased. When the pressure waves were symmetric sinusoidal waves at the charging ratios of 40 vol.% and 60 vol.%, the heat transfer performance was improved. At the charging ratios of 20 vol.% and 80 vol.%, the waveforms of pressure oscillation were more complicated, and the heat transfer performance reduced. At the charging ratio of 40. vol.%, the heat transfer performance of the OCHP was at the best when the inclination angle was 90$^{\circ}$ the pressure wave was a sinusoidal waveform, the pressure difference was at the least, the oscillation amplitude was at the least, and the frequency of pressure oscillation was the highest.

• Journal of Electrical Engineering and Technology
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• v.10 no.4
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• pp.1518-1526
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• 2015

For now, there are some low frequency oscillations in the power system which feature low frequency oscillation with positive damping and cannot be explained by traditional low frequency oscillation mechanisms. Concerning this issue, the dynamic damping effect is put forward on the basis of the power-angle curve and the study of damping torque in this article. That is, in the process of oscillation, damping will dynamically change and will be less than that of the stable operating point especially when the angle of the stable operating point and the oscillation amplitude are large. In a situation with weak damping, the damping may turn negative when the oscillation amplitude increases to a certain extent, which may result in an amplitude-increasing oscillation. Finally, the simulation of the two-machine two-area system verifies the arguments in this paper which may provide new ideas for the analysis and control of some unclear low frequency phenomena.

• 한국전산유체공학회:학술대회논문집
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• 2009.11a
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• pp.84-87
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• 2009

Electrowetting is a versatile tool to handle tiny droplets and forms a backbone of digital microfluidics. Numerical analysis is necessary to fully understand the dynamics of electrowetting, especially in designing electrowetting-based devices, such as liquid lenses and reflective displays. We developed a numerical method to analyze the general contact-line problems, incorporating dynamic contact angle models. The method is based on the conservative level set method to capture the interface of two fluids without loss of mass. We applied the method to the analysis of spreading process of a sessile droplet for step input voltages and oscillation of the droplet for alternating input voltages in electrowetting. The result was compared with experimental data. It is shown that contact line friction significantly affects the contact line motion and the oscillation amplitude. The pinning process of contact line was well represented by including the hysteresis effect in the contact angle models.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.2
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• pp.318-327
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• 2002

In this paper, heat transfer and pressure oscillation characteristics on oscillating capillary tube heat pipe(OCHP) according to input heat flux, mixture ratio of working fluid and inclination angle were investigated and were compared single working fluid(R-142b) with binary mixture working fluid(R-142b-Ethano1). OCHP was made to serpentine structure of loop type with 10 turns by drilling the channels of length 220mm, width 1.5mm, and depth 1.5mm on the surface of brass plate. In this study, R-l42b and R-l42b-Ethanol were used as working fluids, the charging ratio of working fluids was 40(vol.%), the input heat flux to evaporating section was changed from 0.3W/㎠ to 1.8W/㎠, and mixture ratio of working fluid was R(100%), R(95%)-E(5%), R(90%)-E(10%), and R(85%)-E(15%). From the experimental results, it was found that the effective thermal conductivity of single working fluid was better than that of binary mixture working fluid. But, in case of binary mixture working fluid, critical heat flux was higher than that of single working fluid. And, the higher the mixture ratios of working fluid, the lower heat transfer performance. In case of pressure oscillation, as the inclination angle was lower, pressure wave was more irregular. These phenomena were more serious when the working fluid was binary mixture. Besides, when mixture ratio was higher, saturated pressure was increased, more irregular wave was observed and the mean amplitude was increased. For the same input heat flux, inclination angle and charging ratio, when pressure oscillation has sinusoidal wave, mean amplitude was small, and saturated pressure was low value, the heat transfer was excellent.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.434-440
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• 2011

Oscillating airfoil haw been challenged for the dynamic stalls of airfoil am wind turbines at high angle of attach. Especially, the pressure oscillation has a huge effect on noise generation, structure damage, aerodynamic performance am safety, because the flow has strong unsteadiness at high angle of attack. In this paper, the unsteady aerodynamics coefficients were analyzed for the oscillating airfoil at high angle of attack around two dimensional NACA0012 airfoil. The two dimensional unsteady compressible Navier-Stokes equation with a LES turbulent model was calculated by OHOC (Optimized High-Order Compact) scheme. The flow conditions are Mach number of 0.2 and Reynolds number of $1.2{\times}10^4$. The lift, drag, pressure distribution, etc. are analyzed according to the pitching oscillation. Unsteady velocity field, periodic vortex shedding, the unsteady pressure distribution, and the acoustic fields are analyzed. The effects of these unsteady characteristics in the aerodynamic coefficients are analyzed.

• Wind and Structures
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• v.26 no.2
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• pp.57-68
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• 2018

Wind tunnel tests are conducted to investigate wake-induced vibrations of two circular cylinders with a center-to-center spacing of 4 diameters and attack angle varying from $0^{\circ}$ to $20^{\circ}$ for Reynolds numbers between 18,000 and 168,800. Effects of structural damping, Reynolds number, attack angle and reduced velocity on dynamic responses are examined. Results show that wake-induced vortex vibrations of the downstream cylinder occur in a wider range of the reduced velocity and have higher amplitudes in comparison to the vortex-induced vibration of a single circular cylinder. Two types of wake-induced instability phenomena with distinct dynamic characteristics are observed, which may be due to different generation mechanisms. For small attack angles like $5^{\circ}$ and $10^{\circ}$, the instability of the downstream cylinder characterizes a one-degree-of-freedom (1-DOF) oscillation moving in the across-wind direction. For a large attack angle like $20^{\circ}$, the instability characterizes a two-degree-of-freedom (2-DOF) oscillation with elliptical trajectories. For an attack angle of $15^{\circ}$, the instability can transform from the 1-DOF pattern to the 2-DOF one with the increase of the Reynolds number. Furthermore, the two instabilities show different sensitivity to the structural damping. The 1-DOF instability can be either completely suppressed or reduced to an unsteady oscillation, while the 2-DOF one is relatively less sensitive to the damping level. Reynolds number has important effects on the wake-induced instabilities.

• Journal of the Korean Society of Visualization
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• v.12 no.2
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• pp.23-29
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• 2014

Although many studies have been done on an open-end capillary, the invasion into a closed end capillary is still novel in its investigation. In this research we have explored the fluid invasion in closed-end capillaries where the shape of the meniscus and the height of invasion were accompanied by gas compression inside the capillary. Theoretically, the one dimensional momentum balance equation shows the fluid oscillation. In the experiments, we have found the different phenomena, either the fluid oscillation with low frequency or no oscillation. This discrepancy is mostly caused by two factors. First, a continuous decrease of the advancing contact angle due to decreasing invasion velocity as increasing pressure inside the closed-end capillary reduces the invasion velocities. Second, the high shear stress within the entrance length region was generated by the plug like velocity profile.

• Journal of the Korean Society for Precision Engineering
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• v.32 no.7
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• pp.661-666
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• 2015

This paper presents control performance improvement by modifying center of gravity (COG) of an underwater robotic platform. To reduce the oscillation or to increase the positioning accuracy, it is important to accurately know the COG of an underwater robotic platform. The COG is determined by the three measured tilting angles of the platform in different postures. The tilting angle is measured while the platform is hanged by two strings. Using coordinate transformation, the plane of intersection is defined from the angle of the platform and the position of the string. The COG of the robotic platform is directly calculated by the intersected point in three defined planes. The measured COG is implemented to the control algorithm that is pre-designed in the previous research, and the empirical result on tilting gives 48.26% improved oscillation performance comparing to the oscillation result with the ideal COG position.

• Structural Engineering and Mechanics
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• v.82 no.4
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• pp.517-542
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• 2022

The effect of mean angle of wind attack on the flutter critical wind speed of two generic bridge deck cross-sections, viz, one closed box type streamlined section (deck-1) and closed box trapezoidal bluff type section with extended flanges/overhangs (deck-2) type of section have been studied using Computational Fluid Dynamics (CFD) based forced vibration simulation method. Owing to the importance of the effect of the amplitude of forcing oscillation on the flutter onset, its effect on the flutter derivatives and flutter onset have been studied, especially at non-zero mean angles of wind attack. The flutter derivatives obtained have been used to evaluate flutter critical wind speeds and flutter index of the deck sections at non-zero mean angles of wind attack studied and the same have been validated with those based on experimental results reported in literature. The value of amplitude of forcing oscillation in torsional degree of freedom for CFD based simulations is suggested to be in the range of 0.5° to 2°, especially for bluff bridge deck sections. Early onset of flutter from numerical simulations, thereby conservative estimate of occurrence of instability has been observed from numerical simulations in case of bluff bridge deck section. The study aids in gaining confidence and the extent of applicability of CFD during early stages of bridge design, especially towards carrying out studies on mean incident wind effects.