• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.2
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• pp.443-450
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• 1995

An analysis of the dynamics of a mechanical monoleaflet heart valve prosthesis in the closing phase is presented. Employing the moment equilibrium principles on the occluder motion and the squeeze film dynamics of the fluid between the occluder and the guiding strut at the instant of impact, the velocity of the occluder tip and the impact force were computed. The dynamics of fluid being squeezed between the occluder and the guiding struts is accounted for by Reynold's equation. The effect of the fluid being squeezed between the occluder and the guiding strut was to reduce the velocity of the occluder tip at the instant of valve closure as well as dampen the fluttering of the occluder before coming to rest in the fully closed position. The squeeze film fluid pressure changed rapidly from a high positive value to a relatively large negative value in less than 1 msec. The results of this study may be extended for the analysis of cavitation inception, mechanical stresses on the formed elements and valve components as well as to estimate the endurance limits of the prosthetic valves.

• Journal of Korea Water Resources Association
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• v.44 no.6
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• pp.449-459
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• 2011

By examining the experimental results, the critical mean velocity which initiates the movement of riprap is increased with the riprap size in mean diameter, the mean diameter over water depth (d/h), Froude number (Fr), and turbulent shear velocity over critical mean velocity (u*/${\nu}$) which have great correlations among them so these parameters are adopted governing hydraulic stability for riprap. The hydraulic stability equation for riprap is developed by regression analysis. The developed equation is expanded from 0.36~0.73 m/s of experimental range to 0~5.0 m/s for the application in engineering discipline. So many useful relations among those parameters including critical mean velocity are derived by expanding to high Reynolds regions. Mean diameter calculation results by expanding to high Reynolds regions coincide with the calculations of ASCE and USBR at the range of 0~3.0 m/s and the calculation result of ASCE at the range of 3.0~5.0 m/s. The results by developed formulae coincide well with the formulae of ASCE in general and also the results by recently developed existing formulae of hydraulic stability for riprap. Thus, the developed equation has the high applicability in engineering discipline to evaluate the hydraulic stability for riprap.

• Journal of Korea Water Resources Association
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• v.33 no.2
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• pp.181-193
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• 2000

This paper presents the flow and the suspended sediment movement over ripples for oscillatory flows. A new numerical model system is developed, and applied to a laboratory experimental condition of regular waves and a fictitious condition of irregular waves. The flow field is obtained from a programme proposed by Kim et. al.(1994), which is a modified version of SOLA based on SMAC scheme. The sub-model solves the continuity and Reynolds momentum equations in the x-z plane. The wave orbital velocities, shear stresses, and pressure are all reasonably reproduced by the model. The model results on the vertical velocity component show good agreement with the measurements. The suspended sediment transport sub-model is newly set up to solve the advection-diffusion equation of suspended sediment using a split method, and involving a special shear entrainment from the whole ripple surface. The calculated suspended sediment concentrations for regular waves show reasonable agreement with measurements at Deltaflume. The model results for random waves show that the suspended sediment concentration is higher than those for regular waves and that the sediment diffuses higher than for regular waves with the significant wave height and the peak wave period of the irregular waves.

• Journal of the Korean Society of School Health
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• v.25 no.2
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• pp.214-221
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• 2012

Purpose: This study was to assess the association of internet addiction and depression, stress and suicidal ideation among adolescents. Methods: The authors have surveyed 2 middle schools and 2 high schools located in Seoul and analyzed 525 questionnaires. The questionnaire included the Internet Addiction Self Diagnosis Index (K-index), Child Depression Index (CDI), Stress Index and Reynold's Suicidal Ideation Index. Structural equational model analysis was done to explore the association between internet addiction and suicidal ideation. Results: The adolescent internet addiction groups were classified as the high risk group 12 (2.3%), the potential risk group 64 (12.2%) and the normal group 449 (85.5%). When classified according to sex, the high risk group is consisted of 8 (3.9%) boys, 4 (1.3%) girls and the potential risk group is consisted of 41 (64.1%) boys and 23 (35.9%) girls which leads to the conclusion that boys are relatively more addicted compared with girls. The group with more addiction showed higher depression scores, stress scores and suicidal ideation scores. The correlation analysis showed the significant relations between internet addiction and depression (r=0.263), stress (r=0.265) and suicidal ideation (r=0.284). According to the structural equation model analysis, internet addiction had direct effects on the depression and indirect effects on suicidal ideation. Stress had the significant direct effects on both internet addiction and depression. Depression had a significant direct effect on suicidal ideation. Conclusion: Internet addiction affects directly and indirectly an depression and suicidal ideation among adolescent.