• 한국수자원학회논문집
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• 제31권5호
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• pp.565-574
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• 1998

Colebrook(1938)이 수집한 상용관에 대한 관측자료에 의하면 상용관의 마찰계수 분포는 관의 종류와 크기에 따라 그 변이가 상이함을 알 수 있다. 본고는 그의 자료를 재분석하여 관로설게에 적용이 용이한 지수함수 형태의마찰계수 산정식을 도출하였다. 일반적으로 단일관로 설계에서 요구되는 사항들은 어떤 관로 조건이 주어져 있을 때 펌프동력 또는 관내 통과유량이나 적정관경의 산정이다. 균일조도관인 경우는 이미 유동훈(1995b)에 의해서 설계 기준식이 제시되었고, 유동훈과 강찬수(1996)에 의해서 더욱 일반적인 경우에 대한 해석으로 진전되었다. 또한 상용관인 경우도 유동훈과 강찬수(1997)에 의해서 보다 용이한 해석법이 이루어졌지만 그 연구결과는 보다 정확한 해를 구한다는 장점이 있는 반면에 산정식의 형태가 다소 복잡한 단점을 내포하고 있다. 이에 본고는 상용관에 대하여 두가지 유형의 지수형 관마찰계수 산정식을 개발하고 이들을 적용하여 보다 간단하게 관로 설계시 요구되는 사항들을 산출할 수 있는 양해법 산정식을 개발하여 제시하였다.

• 기술혁신학회지
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• 제15권1호
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• pp.28-46
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• 2012

본 연구는 최근 기술변화환경에 중요한 패러다임으로 대변되는 융합기술환경에서 바람직한 과학기술정책체계수립을 위한 정책적 시사점을 제시하고자 동태적 기술역량분포에 대한 실증 분석하였다. 본 연구결과를 간단하게 요약하면 다음과 같다. 먼저 비모수적 Hill추정치와 모수적 Rank-1/2추정치가 유사하게 멱법칙의 존재를 나타냈다. 둘째, 기술역량분포를 결정하는 멱법칙의 추정계수가 1990년부터 2008년 분석기간동안 지속적으로 증가하여 파레토분포에 접근하고 있다. 본 분석결과는 다음과 같은 과학기술정책수립에 시사점을 제공한다. 첫째, 최근 멱법칙 결정계수의 증가추이로 볼 때, 그 작동원리가 상대적으로 약화되고 있으며, 연구대상 국가간에 기술역량이 수렴화 되고 있다는 사실을 보여준다. 이러한 기술역량의 국제적 수렴화경향은 국가간 기술격차역량의 차이가 줄어들고 있음을 보여준다. 둘째, 본 연구에서 분석한 바와 같이 만일 기술역량분포의 구조결정에 지속적으로 멱법칙이 존재한다면, 국제적으로는 과학기술역량의 규모가 큰 국가의 기술개발변동이 전 세계의 과학기술변동에 매우 큰 영향을 미치게 된다. 국내적으로는 과학기술역량이 몇 기업에 국한된 경우에 그 기업의 성과가 우리나라 과학기술발전에 큰 영향을 미치게 된다는 것을 의미한다. 즉 이러한 멱법칙의 존재는 융합기술환경에서 한 국가의 기술역량이 소수의 기술잠재역량있는 기술개발기업에 너무 의존적이 될 수 있다는 기술정책적 시사점을 제공한다.

• Structural Engineering and Mechanics
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• 제52권4호
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• pp.663-686
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• 2014

This paper deals with free vibration analysis of bidirectional functionally graded annular plates resting on a two-parameter elastic foundation. The formulations are based on the three-dimensional elasticity theory. This study presents a novel 2-D six-parameter power-law distribution for ceramic volume fraction of 2-D functionally graded materials that gives designers a powerful tool for flexible designing of structures under multi-functional requirements. Various material profiles along the thickness and in the in-plane directions are illustrated by using the 2-D power-law distribution. The effective material properties at a point are determined in terms of the local volume fractions and the material properties by the Mori-Tanaka scheme. The 2-D differential quadrature method as an efficient and accurate numerical tool is used to discretize the governing equations and to implement the boundary conditions. The fast rate of convergence of the method is shown and the results are compared against existing results in literature. Some new results for natural frequencies of the plates are prepared, which include the effects of elastic coefficients of foundation, boundary conditions, material and geometrical parameters. The interesting results indicate that a graded ceramic volume fraction in two directions has a higher capability to reduce the natural frequency than conventional 1-D functionally graded materials.

• Steel and Composite Structures
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• 제33권6호
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• pp.865-875
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• 2019

The current article proposed to develop a geometrical model for the analysis and modelling of the uniaxial functionally graded structure using the higher-order displacement kinematics with and without the presence of porosity including the distribution. Additionally, the formulation is capable of modelling three different kinds of grading patterns i.e., Power-law, sigmoid and exponential distribution of the individual constituents through the thickness direction. Also, the model includes the distribution of porosity (even and uneven kind) through the panel thickness. The structural governing equation of the porous graded structure is obtained (Hamilton's principle) and solved mathematically by means of the isoparametric finite element technique. Initially, the linear frequency parameters are obtained for different geometrical configuration via own computer code. The comparison and the corresponding convergence studies are performed for the unidirectional FG structure for the validation purpose. Finally, the impact of different influencing parameters like aspect ratio (O), thickness ratio (S), curvature ratio (R/h), porosity index (λ), type of porosity (even or uneven), power-law exponent (n), boundary condition on the free vibration characteristics are obtained for the FG panel and discussed in details.

• The Journal of Asian Finance, Economics and Business
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• 제8권4호
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• pp.613-624
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• 2021

This paper analyzes the relationship between income distribution and energy consumption from a Pareto optimal approach. For this purpose, the causality relationship between electricity consumption per capita (kWh) with respect to country groups and energy consumption per capita (kg of oil equivalent) along with gross domestic product per capita was analyzed. In addition to this purpose, a Pareto analysis was conducted to determine the countries with the highest per capita national income, how much of the world total energy they consume, and whether the law of power in the energy and electricity markets exists. Finally, the impact of official development assistance provided to low-income countries by high-income countries on the low-income countries' electricity and energy consumption was analyzed. In other words, it was questioned whether pareto redistribution policies serve the purpose or not. The Engle-Granger causality approach was used in the analysis of the causality relationship between variables. Our analysis indicated that, first, the energy data of the country groups may be inadequate in revealing income inequalities. Second, the existence of Pareto law of power and global income inequality can be explained based on energy data. Finally, Pareto optimal redistribution policies to eliminate income inequality remain inadequate in practice.

• Advances in aircraft and spacecraft science
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• 제10권1호
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• pp.19-49
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• 2023

The present study proposes a theoretical and numerical investigation on the dynamic response behaviour of a functional graded (FG) ceramic-metal tapered rotor shaft system, by the differential quadrature finite elements method (DQFEM) to identify the natural frequencies for modelling and analysis of the structure with suitable validations. The purpose of this paper is to explore the influence of heat gradients on the natural frequency of rotation of FG shafts via three-dimensional solid elements, as well as a theoretical examination using the Timoshenko beam mode, which took into account the gyroscopic effect and rotational inertia. The functionally graded material's distribution is described by two distribution laws: the power law and the exponential law. To simulate varied thermal conditions, radial temperature distributions are obtained using the nonlinear temperature distribution (NLTD) and exponential temperature distribution (ETD) approaches. This work deals with the results of the effect on the fundamental frequencies of different material's laws gradation and temperature gradients distributions. Attempts are conducted to identify adequate explanations for the behaviours based on material characteristics. The effect of taper angle and material distribution on the dynamic behaviour of the FG conical rotor system is discussed.

• 대한안전경영과학회지
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• 제2권2호
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• pp.71-83
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• 2000

This paper presents accelerated life tests for Type I censoring data under probabilistic stresses. Probabilistic stress, $S_j$, is the random variable for stress influenced by test environments, test equipments, sampling devices and use conditions. The hazard rate, ,$theta_j$, is the random variable of environments and the function of probabilistic stress. Also it is assumed that the general stress distribution is uniform, the life distribution for the given hazard rate, $\theta$, is exponential and inverse power law model holds. In this paper, we obtained maximum likelihood estimators of model parameters and the mean life in use stress condition.

• 대한기계학회논문집A
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• 제21권12호
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• pp.2165-2171
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• 1997

The analysis model is the infinite power law creep material containing the rigid inclusion with crack shape. The present analysis is performed using the complex pseudo-stress function method. The strain rate intensity factor is developed as new fracture mechanics parameter which represents the stress and strain rate distribution near a crack tip in power law creep material. The strain rate intensity factor is developed in terms of Kolosoff stress functions.

• KEPCO Journal on Electric Power and Energy
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• 제7권2호
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• pp.241-248
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• 2021

High cycle fatigue life for the cables with two different types of clamps is estimated comparatively through acceleration testing. The high cycle fatigue fracture of overhead lines is caused mainly by the aeolian vibration which is induced by vortex shedding. It is necessary to manage the integrity of cables continuedly considering that the aeolian vibration is unavoidable since it occurs in steady and relatively low wind velocity. Two types of clamps which are largely used for overhead lines of the distribution grids are selected and failure data are obtained by step stress testing with a electrodynamic shaker with them. The inverse power law is assumed to describe the stress-life relationship and the fatigue limit at any specified life is supposed to follow Weibull distribution. The life of the cable is defined as the number of cycles to the time that one of strands is completely broken. Finally, the fatigue limits of the cables with two clamp types are estimated at the reference life of 500 Mcycles and compared each other based on a bending vibration amplitude.

• 한국멀티미디어학회논문지
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• 제9권3호
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• pp.333-341
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• 2006

최근의 웜은 CPU자원, 네트워크 대역폭등 주어진 자원을 최대한 소모하여 네트워크 전체 가용성을 심각히 저해하는 랜덤확산형(Random Constant Spreading) 웜이 점차 늘어나고 추세이다. 본 논문에서는 이러한 웜의 화산을 동적으로 억제하기 위하여 선호적 성장 특성을 가지는 멱함수 네트워크를 분석한다. 그리고 이러한 네트워크에서 공통적으로 나타나는 전달노드의 깊이분포 특성을 이용하여 랜덤확산형 웜을 동적으로 제어하는 모델을 제안하고 시뮬레이션을 통하여 각 노드의 부하가 최소화되면서 월 확산이 효과적으로 제어됨을 검증한다.