Kyoung-Su Park

MODAL CHARACTERISTIC ANALYSIS OF THE APR1400 NUCLEAR REACTOR INTERNALS FOR SEISMIC ANALYSIS

Reactor internals are sensitive to dynamic loads such as earthquakes and flow induced vibration. Thus, it is essential to identify the dynamic characteristics to evaluate the seismic integrity of the structures. However, a full-sized system is too large to perform modal experiments, making it difficult to extract data on its modal characteristics. In this research, we constructed a finite element model of the APR1400 reactor internals to identify their modal characteristics. The commercial reactor was selected to reflect the actual boundary conditions. Our FE model was constructed based on scale-similarity analysis and fluid-structure interaction investigations using a fabricated scaled-down model.

Design Modifications to Improve the Antishock Performance of a Stamped Base for a 2.5-in HDD

This paper presents a modified design for a stamped base for a 2.5-in hard disk drive (HDD). A stamped base is a thinner thickness and lower stiffness than a die-casting base. To improve shock performance of the stamped base, the finite element (FE) model of the HDD is constructed. Then, the FE model was verified by modal analysis. Drop test was performed to confirm the shock simulation model. Parametric analysis was used to identify the key parameters for the shock performance which were the base thickness, rib shape, number of ribs, and reinforcement. The correlation between the relative displacement and natural frequency of the HDD was investigated for each parameter. The dominant parameter was identified and applied to the FE model of the stamped base. The modified stamped-base model was then evaluated under nonoperational shock conditions.

Analysis of the Loading Process of a Slider With Reflowed Lubricant in a Hard Disk Drive

As the recording density of hard disk drives has increased exponentially recently, the load/unload process can be affected severely by the nanolevel changes in the air-bearing surface (ABS) induced by the reflow behavior of perfluoropolyether molecules. In this paper, clean-up time and reflow time were first measured by using thermal flying controller measurements to check whether reflowed lubricant can affect loading performance. The measured clean-up time and reflow time were 120 s and 40 min, respectively. Because the clean-up time is so much longer than the loading process, almost all the reflowed lubricant remained on the bottom of the ABS during the loading process. Next, the lubricant behavior at rest was modeled and simulated theoretically by using a diffusion process. At rest, the time-dependent profiles of the reflowed lubricant were investigated. In addition, the ABS was modified with reflowed profiles at various time intervals and loading process was carried out with the modified ABS for various parameters such as reflow time and loading velocity. As a result, the slider–disk contact distinctly occurred during the loading process in the case with reflowed lubricant profiles. The simulation results were verified with the experimental results. Consequently, the effects of reflowed lubricant should be considered when designing the loading process of a slider to improve head–disk interface reliability.

Tension analysis of a 6-degree-of-freedom cable-driven parallel robot considering dynamic pulley bearing friction:

Recently, cable-driven parallel robots have been used in many industrial fields. Especially, cable-driven parallel robots have been used in specialized industrial fields where dynamical features such as high accuracy in systems requiring quick transitions. In this article, Dahl friction model was proposed to predict the tension during quick transitions in cable-driven parallel robots. And the Dahl friction equation for a combination of pulleys connected in series was derived. From their relationships, a modified kinematic equation was developed for redundant actuation cable-driven parallel robot systems. Because Dahl friction has non-linear characteristics, the actual tension set problem was solved using the Newton–Raphson method combined with the constrained non-linear multivariable optimal method. Finally, the tension profiles were simulated using both Dahl and Coulomb friction models with fast and slow velocities. In conclusion, according to simulation, Dahl friction model expressed dynamic characteristics such as pre-sliding in the transition range. Thus, it was in better agreement with the experiments than the Coulomb friction model. In particular, Dahl friction model was more accurate in the case of slow motion in the pre-sliding range.

Application of seismic analysis methodology to small modular integral reactor internals

The fluid–structure interaction (FSI) effect should be carefully considered in a seismic analysis of nuclear reactor internals to obtain the appropriate seismic responses because the dynamic characteristics of reactor internals change when they are submerged in the reactor coolant. This study suggests that a seismic analysis methodology considered the FSI effect in an integral reactor, and applies the methodology to the System-Integrated Modular Advanced Reactor (SMART) developed in Korea. In this methodology, we especially focus on constructing a numerical analysis model that can represent the dynamic behaviors considered in the FSI effect. The effect is included in the simplified seismic analysis model by adopting the fluid elements at the gap between the structures. The overall procedures of the seismic analysis model construction are verified by using dynamic characteristics extracted from a scaled-down model, and then the time history analysis is carried out using the constructed seismic analysis model, applying the El Centro earthquake input in order to obtain the major seismic responses. The results show that the seismic analysis model can clearly provide the seismic responses of the reactor internals. Moreover, the results emphasize the importance of the consideration of the FSI effect in the seismic analysis of the integral reactor.

Analysis of Thermal Flying-Height Control System About Humid Air Condition in Hard Disk Drive

This research investigates to analyze the effect of humid air on TFC system. Required parameters of humid air and flying attitude of slider are calculated. Condensation of air bearing at over the saturation vapor pressure is considered. Heat transfer coefficient for humid air condition is calculated. TFC simulation about humid air is conducted.Copyright

Operational Shock Analysis for 2.5-in Multi-Disk HDD Considering Ramp–Disk Gap Irregularity

With an increased mobility, hard disk drives (HDDs) now use ramp load/unload technology, and multiple disks are used to increase the data capacity. This can result in contact between the ramp and the disks, which negatively affects the slider dynamics under operational shock conditions. There are several types of ramp–disk contact, such as simultaneous, successive, and single contact, and these depend on the distance between the ramp and each disk in a multi-disk HDD. This paper examines the effect of ramp–disk contact on the slider dynamics using a transient shock analysis. The ramp–disk contact affects the head stack assembly bending mode and slider dynamics. The effect of irregular gaps between the disks and adjacent ramp on the slider dynamics is analyzed with the ramp–disk contact force. The simulation shows that the gap irregularity between the ramp and each disk should be as small as possible to improve the stability of the slider dynamics.

Message from the APMRC 2016 Publication Chairs

It was our great honor and pleasure to host the Asia-Pacific Magnetic Recording Conference (APMRC) at Yonsei University, Seoul, South Korea, from July 13–15, 2016. All members of the international scientific community interested in new developments in magnetic recording technologies were invited to attend. Among other topics the Conference addressed issues related to ultrahigh-density recording in the multi-terabits-per-square-inch regime.

Analysis of loading process of the slider with accumulated lubricant in HDD

In this paper, the lubricant dynamics is simulated from the disk to slider. Due to the change of accumulated lubricant during loading process, the dynamics of the slider is changed and investigated in the simulation. In the simulation result, the slider-disk contact occurs in case of w/ accumulated lubricant. As a result, it is very dominant to determine the loading performance in HDD. And based the simulation result, the accumulated lube is another dominant factor to evaluate the loading performance in the load/unload technology.

Workspace Analysis of Cable-Driven Parallel Robot With Coulomb Friction Under Ultra-High-Acceleration

Cable driven parallel robots (CDPRs) are a class of parallel robots in which the rigid links are replaced by cables. It consists of a moving end-effector and a number of active cables connected to the end-effector.Copyright