• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.2
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• pp.137-142
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• 2010

A plane wave expansion method (PWEM) was applied for photonic band structure calculation. We examined zero group velocity modes in photonic crystal. The zero group velocity modes were obtained at the second band along F-K direction. We expanded higher order Brillouin zone (BZ) to find the locations of zero group velocity modes and to investicate their properties. We found twelve locations, inside the first Brillouin zone, where the group velocities became zero. Also, we calculated band structure and group velocity in off-plane configuration.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.2
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• pp.128-132
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• 2010

Photonic crystal is dielectric materials or a set of different dielectric materials with periodic structure. Line defect is obtained by leaving out a row of rods along the $\Gamma$-X direction. We showed the change of group velocity in waveguide mode and found resultant small group velocity. Characteristics of the small group velocity were described by electric field distribution. Investigating the phase shift, it is confirmed if small group velocity is positive or negative.

• Transactions of the Society of Information Storage Systems
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• v.2 no.2
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• pp.150-155
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• 2006

Vertical L/UL (Load/Unload) velocity is very important parameter to determine the L/UL performance, and the vertical velocity is determined by the actuator velocity and the ramp profile. However, it is not easy to precisely control the actuator rotating velocity during the L/UL process. Especially in emergency parking, servo system doesn't operate, it is impossible to control an actuator velocity. Then, the vertical unloading velocity depends on only ramp profile. The ramp height and the sliding length for L/UL process in SFF (Small Form Factor) HDD are restricted due to slimness and small media. For these reasons, it is very difficult to design the ramp profile in SFF HDD. Therefore, this study analyzes the unloading dynamic characteristics for various ramp profiles and makes the thesis for ramp profile design.

• Journal of the Korean Wood Science and Technology
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• v.25 no.3
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• pp.8-15
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• 1997

The ultrasonic velocity and dynamic MOE of acetylated bamboos were investigated using PUNDIT, a transit time measuring device for longitudinal ultrasonic propagation. Bamboo specimens were boiled in acetic anhydride for 2, 4 and 6 hours, and the maximum average WPG (Weight Percentage Gain) of 19% was obtained at 6 hours. The volumes of acetylated bamboos increase with boiling time and WPG, while as WPG increases their oven-dry densities generally increase with a concave around 5% WPG. This oven-dry density pattern likely influences the trends of ultrasonic velocity and dynamic MOE. which generally decrease with a convex around 5% WPG. It is postulated that during boiling extractives in a bamboo move and aggregate at its surfaces transiently, resulting in the increase of ultrasonic velocity and dynamic MOE. To explain the fact that ultrasonic velocity varies with WPG a simple model was proposed and some ultrasonic properties of a transmitted wave were examined.

• 한국가시화정보학회:학술대회논문집
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• 2002.11a
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• pp.27-30
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• 2002

An experimental study is performed to investigate the characteristics of near wake behind a circular cylinder with serrated fins using the constant temperature anemometer and through flow visualization. Previous report(Boo at al., 2001) shows that there are three different modes in vortex shedding behavior. This paper is focused on the identification of the physical reasons why the difference is occured in vortex shedding. The through flow velocity crossing fins decreases as increasing fin height and decreasing fin pitch mainly due to the flow resistence. Vortex shedding is affected strongly by velocity distribution around fin tube, especially by the velocity gradient. The velocity distribution at X/d=0.0 has lower gradient with increasing freestream velocity and fin height and decreasing fin pitch. Those differences in velocity gradients generate different vortex shedding mechanism.

• Journal of the Korean Society for Precision Engineering
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• v.17 no.1
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• pp.209-215
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• 2000

Precision forging technology that can control flow velocity of workpiece have been developed to minimize the amounts of machining. To get the uniform rib length, flow velocity distribution is needed to be estimated and controlled. Computer-aided design is known for very effective to estimate the deformation behavior and design the die for controlling the flow velocity. In this study, die design to control the deformation velocity are investigated using the DEFORM-2D about rib-web shape parts. Also we can get uniform rib length by enforcing the back pressure at end section of rib. The applied load of back pressure farming is lower than that of conventional forging. These results are analysed and confirmed by the experiment.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.5
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• pp.511-516
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• 2004

The dynamic behavior of an automatic ball balancer (ABB) is studied considering the effects of gravity and angular velocity profiles. In this study, a physical model for an ABB installed on the Jeffcott rotor is adopted in order to investigate the effects of gravity and angular acceleration. The equations of motion for the rotor with ABB are derived by using Lagrange's equation. Based on derived equations, dynamic responses for the rotor are computed by using the generalized-o method. From the computed responses, the effects of gravity and angular velocity profiles on the dynamic behavior are investigated. It is found that the balancing of the rotor with ABB can be achieved regardless of gravity. It Is also shown that a smooth velocity profile yields relatively smaller vibration amplitude than a non-smooth velocity profile.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.586-591
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• 2004

A load/unload mechanism is applied to most small form factor hard disk drives. Loading velocity is ones of the most important factors to ensure the reliability of the load/unload operations. Especially, the lower the flying height of the head becomes, the more important the accuracy of the loading velocity control becomes. Because there is no sensor during the load/unload operations, BEMF voltage is used to sense the head velocity. To measure BEMF voltage, a simple circuit is added to the load/unload HDD control system. Overall acceptable results are obtained by using the simple PI controller but the errors from the target velocity occur when the head moves along the ramp and then reaches the disk. Feedforward profile and disturbance observer are used to remove those errors effectively.

• Wind and Structures
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• v.2 no.1
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• pp.51-68
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• 1999

Applying and extending some preceding researches, this paper proposes a map of Italian extreme winds assigning the reference velocity, i.e., the wind velocity averaged over 10 minutes, at 10 m height, in a flat open terrain, with 50 years mean return period, depending on the site and the altitude. Furthermore, an objective criterion is formulated by which the actual values of the local wind velocity are given as a function of the reference velocity. The study has been carried out in view of the revision of the Italian Standards dealing with safety and loads and the introduction of the aeolic Italian map into Eurocode 1.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.15 no.11
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• pp.910-917
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• 2003

Fluid flow in a rectangular duct for 90$^{\circ}$ bend elbow with the ratio of 1.5 between its curvature radius and width is measured by 5 W laser doppler velocity meter. The fluid flow is also computed by commercial software of STAR-CD for comparison between measured and computed velocity profiles in the duct. Reynolds numbers for the comparison are 11,643, 19,746 and 24,260. From the comparison, computation of principal velocity components in the duct predicts the experimental data somewhat satisfactorily even though those of minor velocity components and turbulent kinetic energy do not match with the experimental data quite well. K-factor for the bend elbow is computed to be average 0.086 while the equivalent ASHRAE data is 0.07.