• Journal of Korea Water Resources Association
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• v.42 no.8
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• pp.619-629
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• 2009

The objective of this study is to estimate bed-resistance in gravel-bed rivers using the equivalent roughness height($k_s$). We calculated the friction factor(f) with the measured data from 8 domestic gravel-bed rivers and investigated the size distributions of the bed materials. The averaged $k_s$ in each cross-section, which is determined under the hypothesis that the vertical velocity distribution follows the logarithmic law, is compared with the reach $k_s$ which is calculated with the cumulative grain diameter distribution curve of bed materials. Moreover, the applicability of existing formulae, such as Strickler type equations, is examined by comparing with Manning's n value converted from the $k_s$. According to the results, the reach $k_s$ proves to be a good indicator of representative characteristic of bed materials in a reach, and the Manning's n based on the reach $k_s$ is appropriate for practical estimation of the bed-resistance, for RMS errors between calculated and measured Manning's n is less than 0.003. The correlation between the $k_s$ and specified bed-material size($D_i$) is very low, so it is difficult to select a proper one among the existing empirical equations.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2010.04a
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• pp.774-777
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• 2010

본 논문에서는 MR 감쇠기의 비선형 특성을 등가의 선형감쇠로 치환하기 위한 방법으로 Newmark와 Hall이 제안한 감쇠계수를 이용하였다. 응답스펙트럼의 감쇠율을 나타내는 감쇠기계수는 구조물의 최대변위를 이용하여 등가감쇠비를 추출할 수 있다는 장점을 가지고 있다. 하지만 이러한 응답스펙트럼은 구조물에 작용하는 지진하중의 특성에 따라 경향이 달라진다. 본 논문에서는 기존의 등가감쇠비 산정식을 수정하여 제안하고, 지진하중 특성에 따른 등가감쇠비의 변화를 살펴보았다. MR 감쇠기의 등가감쇠비는 가진하중과 최대마찰력의 비율에 따라 결정되며, 장주기 구조물의 경우 등가감쇠비가 감소한다. MR 감쇠기의 점성은 마찰감쇠와의 상호작용에 의해 단순한 산술합 이상의 감쇠비 증가 효과를 가진다.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.4
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• pp.457-462
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• 2011

In this study, a simplified design procedure for friction dampers of a multi-story structure in order to reduce seismic response is proposed. To get insight for control effect of the structure with friction dampers is difficult, because of a nonlinear characteristic by a friction damper. Since a control force of a friction damper is influenced by coupling velocity between floors, adjoining modes are coupled. Thus structural response are derived by assuming steady-state response in resonance. As it is impossible that an exact solution is obtained for seismic load, first, a closed form solution can be achieved under harmonic vibration. Second, to convert a three-story building into a single-degree-of-freedom(SDOF) structure, modal analysis is performed. Third, an equivalent damping ratio is derived with utilizing closed form solution. And response reducing factor is proposed by it. Finally, friction force of a damper is designed for using response reducing factor, and then designed dampers are verified for seven seismic data. The nonlinear analysis results confirm the validity of the proposed procedure.

• Water for future
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• v.28 no.2
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• pp.155-168
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• 1995

The present paper discusses the traditional empirical equations of friction factor or cross-sectional mean velocity of open channel flow and suggests the new form of friction factor equation. Dimensional analysis is conducted for the possible forms of traditional empirical equations in order to satisfy the dimensional equality, and new forms of empirical equations are presented with introducing equivalent roughness height. Considering the distribution of friction factor against Reynolds number which has a similar characteristics to that of smooth turbulent flow in circular pipe, the friction factor equation of rectangular open channel flow is developed by modifying the friction factor equation of circular pipe flow for the region of smooth turbulent flow. The equations including the dimensionally-corrected empirical equations are tested against Bazin's laboratory experiments.

• Tunnel and Underground Space
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• v.25 no.3
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• pp.284-292
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• 2015

The generalized Hoek-Brown failure criterion, the strength parameters of which are determined by using the GSI index, is an empirical nonlinear failure criterion of rock mass and has been widely employed in various rock engineering practices. Many rock engineering practitioners, however, are still familiar with the description of the strength of rock mass in terms of friction angle and cohesion. In addition, almost all rock mechanics softwares incorporate the simple linear Mohr-Coulomb function. Therefore, it is necessary to provide a tool to implement the Hoek-Brown function in the framework of the Mohr-Coulomb criterion. In this study, the use of upper-bound solution of limit analysis for bearing capacity of a strip footing resting on the ground surface is proposed for the estimation of the equivalent friction angle and cohesion of rock mass incorporating the generalized Hoek-Brown failure criterion. The upper-bound bearing capacity is expressed in terms of friction angle by use of the relationship between tangential friction angle and tangential cohesion implied in the generalized Hoek-Brown function. The friction angle minimizing the upper-bound bearing capacity is taken as the equivalent friction angle. Through the illustrative implementations of the proposed method, the influences of GSI, $m_i$ and D on the equivalent friction angle and cohesion are investigated.

• Tunnel and Underground Space
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• v.14 no.5
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• pp.373-380
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• 2004

In this study, the emission rate of pollutant was modified according to the published standards, and the distribution of pollutant concentration was analyzed for each vehicle velocity. This modified emission rate was applied to a model tunnel and it was proved that the required air quantity was reduced to 49%, compared to the PIARC method. From the simulation result, it was proved by using statistics that the most sensitive factor among them is the friction coefficient and it was modified to the value in the range of 0.018 to 0.021. It is also expected that the required air quantity can be decreased form 14.4% to 19.2% according as the coefficient is applied to the domestic model tunnels. In conclusion, it is proposed that the number of jet fans can be reduced and the annual operating cost can be curtailed as well.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.1
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• pp.116-122
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• 1988

Interfacial shear stresses have been determined for countercurrent stratified flow of air and water in a nearly-horizontal rectangular channel, based upon measurements of pressure drop, gas velocity profiles and mean film thickness. A dimensionless correlation for the interfacial friction factor has been developed as a function of the gas and liquid Reynolds numbers. Equivalent surface roughnesses for the interfacial friction factor have been calculated using the Nikuradse correlation and have been compared with the intensity of the wave height fluctuation on the interface. The results show that the interfacial shear stress is mainly affected by turbulent mixing near the interface due to the wave motion rather than by the roughened surface.

• Proceedings of the KIPE Conference
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• 2006.06a
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• pp.342-344
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• 2006

전동기 구동시스템의 사양 결정과 설계를 위해서는 부하의 부하토크, 관성계수, 마찰계수 등의 정보에 의한 부하 모델링이 반드시 필요하다. 전동지게차 시스템의 모델링을 위해서는 차체의 중량과, 바퀴의 제원 등 제한적인 정보를 기초로 하여 번거로운 계산과정을 거쳐야 하지만 정확한 정보를 얻기에는 어려움이 따른다. RecurDyn은 기계시스템의 모델링과 시뮬레이션을 하는 3차원 해석이 가능한 Functionbay사에서 개발한 시뮬레이션 프로그램으로서 3D의 기계적인 형상을 작성하고, 물성을 입력하면 부하토크, 등가 관성계수, 마찰계수를 출력할 수 있어 전동기구동시스템의 시뮬레이션 프로그램인 Matlab/Simulink와 결합하여 시뮬레이션을 수행 할 경우 부하 모델링 없이 전체 구동시스템의 특성 시뮬레이션이 가능하다. 본 논문에서는 영구자석 동기전동기로 구동하는 전동지게차 시스템을 대상으로 RecurDyn 프로그램의 활용을 소개하고 Matlab/Simulink를 이용한 결함 시뮬레이션의 가능성을 보고자 한다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.299-307
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• 1991

A unified correlation of the interfacial friction factor for air-water and steam-water flows in inclined rectangular channels has been developed. The correlation was expressed in the form of a power law of the liquid and the gas Reynolds number, and the liquid-to-gas viscosity ratio. In addition, a relation between the equivalent roughness and the intensity of wave height fluctuation of the interface has been investigated. A new dimensionless intensity of fluctuation including a liquid film Reynolds number is proposed. It has been shown that the dimensionless equivalent roughness, which is calculated from the Nikuradse equation, can be uniquely related to this dimensionless intensity of fluctuation for both air-water and steam-water flows.

• Journal of Korea Water Resources Association
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• v.31 no.3
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• pp.223-234
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• 1998

Recently, the interests of the construction of the permeable submerged breakwaters have been increased to preserve and to improve the coastal environment, and to control the incident waves and littoral transport. It is very important to predict the wave transformation precisely over the permeable submerged breakwaters. This study discusses nonlinear wave transformation and characteristics by using BEM based on the frequency domain method of the 3rd-order Stokes waves. The Dupuit-Forchheimer formula is applied to the analysis of the fluid resistance of rubble stones, and the equation about equivalent linear frictional coefficient is newly modified based on the Lorentz's condition for the equivalent work. The numerical results are compared with the experimental ones for verification. These two results give a close agreement each other. It is confirmed that the present method of the 3rd-order Stokes waves estimates more precisely than that of the 2nd-order Stokes waves.