• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2003년도 춘계학술대회 논문집
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• pp.438-442
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• 2003

For an efficient sliding mode control system stability and chattering avoidance should be guaranteed. A continuation method using boundary layer is well known as one solution for this. However since not only model uncertainties and disturbances but also control task itself is variable. it is practically impossible to set controller parameters - control discontinuity, control bandwidth, boundary layer thickness - in advance. In this paper first an adaptation law of control discontinuity is introduced to assure system stability and then fuzzy logic based tuning algorithm of design parameters is applied based on monitored performance indices of tracking error, control chattering, and model precision. In the end maximum control bandwidth not exciting unmodeled dynamics and minimum control discontinuity, boundary layer thickness making system stable and free of chattering are found respectively. This eliminates control chattering and enhances control accuracy as much as possible under given control situation. In order to demonstrate the validity of the proposed algorithm safe headway maintenance control for autonomous transportation system is simulated. The control results show that the proposed algorithm guarantees system stability all the time and tunes control parameters consistently and in consequence implements an efficient control in terms of both accuracy and actuator chattering.

• 터널과지하공간
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• 제17권3호
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• pp.175-185
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• 2007

불연속면을 따라 점토 등의 충전물이 포함된 경우에는 불연속면의 방향이 유리하게 작용하더라도 사면안정에 불안정성을 초래한다. 연구지역의 사면활동은 점토충전물과 풍화토의 경계를 따라 발생하였다. 이것은 물성의 차이를 보이는 경계부분이 존재했기 때문이며, 암석에서 엽리 등이 구조적 운동시 연약대로 작용하여 층간의 미끄러짐을 유도하는 현상과 매우 유사하다. 대부분의 해석에서 점토충전물이 존재할 경우에는 하나의 불연속면으로 설정하여 해석한다. 이때의 불연속면은 점토층내에서의 활동면으로 해석되므로 점토와 원지반과의 경계부의 특성이 해석에 고려되지 못하여 불연속면의 거동은 결국 점토의 물리적 성질에 많이 지배된다. 점토충전물로 인한 활동사면의 안정성 평가 결과는 충전물에 의한 불연속면의 파괴유형에 따른 강도정수에 의해 안전율에 상당한 차이를 보인다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1086-1091
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• 2004

The boundary layer is a very important characteristic of a liquid-vapor interface since it governs the heat and mass transfer phenomena across an interface. However, the thickness of a boundary layer is generally micro- or nano-sized, which requires highly accurate measurement devices and, consequently, costs the related experiments very high and time-consuming. Due to these size dependent limitations, the experiments related with a nano-scaled size have suffered from the errors and the reliability of the obtained data. This study is performed to grasp the characteristics of a liquid-vapor interface, by using a molecular dynamics method. The simulation results were compared with other studies if possible. Although other studies reported that there existed a temperature discontinuity over an interface when the system was reduced to micro- or nano-sized, we confirmed that there was no such a temperature discontinuity.

• 한국전산유체공학회지
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• 제17권1호
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• pp.8-15
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• 2012

A code developed using the flux-difference splitting scheme on the hybrid Cartesian/immersed boundary method is applied to simulate three-dimensional internal waves. The material interface is regarded as a moving contact discontinuity and is captured on the basis of mass conservation without any additional treatment across the interface. Inviscid fluxes are estimated using the flux-difference splitting scheme for incompressible fluids of different density. The hybrid Cartesian/immersed boundary method is used to enforce the boundary condition for a moving three-dimensional body. Immersed boundary nodes are identified within an instantaneous fluid domain on the basis of edges crossing a boundary. The dependent variables are reconstructed at the immersed boundary nodes along local normal lines to provide the boundary condition for a discretized flow problem. The internal waves are simulated, which are generated by an pitching ellipsoid near an material interface. The effects of density ratio and location of the ellipsoid on internal waves are compared.

• Nuclear Engineering and Technology
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• 제45권6호
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• pp.707-720
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• 2013

We analyze piecewise homogenization with flux-weighted cross sections and preservation of averaged currents at the boundary of the homogenized domain. Introduction of a set of flux discontinuity ratios (FDR) that preserve reference interface currents leads to preservation of averaged region reaction rates and fluxes. We consider the class of numerical discretizations with one degree of freedom per volume and per surface and prove that when the homogenization and computing meshes are equal there is a unique solution for the FDRs which exactly preserve interface currents. For diffusion submeshing we introduce a Jacobian-Free Newton-Krylov method and for all cases considered obtain an 'exact' numerical solution (eight digits for the interface currents). The homogenization is completed by extending the familiar full assembly homogenization via flux discontinuity factors to the sides of regions laying on the boundary of the piecewise homogenized domain. Finally, for the familiar nodal discretization we numerically find that the FDRs obtained with no submesh (nearly at no cost) can be effectively used for whole-core diffusion calculations with submesh. This is not the case, however, for cell-centered finite differences.

• Steel and Composite Structures
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• 제25권3호
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• pp.337-346
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• 2017

Sandwich shells made of composite materials are the main focus on recent literature parallel to the requirements of industry. They are commonly chosen for the modern engineering applications which require moderate strength to weight ratio without dependence on conventional manufacturing techniques. The investigations on hyperbolic paraboloidal formed sandwich composite shells are limited in the literature contrary to shells that have a number of studies, consisting of doubly curved surfaces, arbitrary boundaries and laminations. Because of the lack of contributive data in the literature, the aim of this study is to present the effects of curvature on hyperbolic paraboloidal formed, layered sandwich composite surfaces that have arbitrary boundary conditions. Analytical solution methodology for the analyses of stresses and deformations is based on Third Order Shear Deformation Theory (TSDT). Double Fourier series, which are specialized for boundary discontinuity, are used to solve highly coupled linear partial differential equations. Numerical solutions showing the effects of shell geometry are presented to provide benchmark results.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2003년도 추계 학술대회논문집
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• pp.5-10
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• 2003

In order to reduce the excessive numerical dissipation, the new spatial discretization scheme is introduced. The present method in this paper has the formula that has an additional procedure of defining transferred properties at a cell-interface, based on AUSMPW+. The newly defined transferred property could eliminate numerical dissipation effectively in non-flow aligned grid system. In addition, the present method guarantees the monotonic characteristic in capturing a discontinuity. Through a stationary or moving contact discontinuity and a stationary or moving shock discontinuity, a vortex discontinuity and shock wave/ boundary layer interaction, it is verified that the accuracy of the present method is improved.

• 한국해안해양공학회지
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• 제13권3호
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• pp.254-261
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• 2001

장주기파가 깊은 바다에서 수심이 급격히 감소하는 대륙사면을 지나 대륙붕 위로 전파해 들어오는 경우 육지에서 반사된 파가 대륙사면에서 외해로 전파해 나가지 못하고 재반사되어 들어오기 때문에 육지 쪽에서 커다란 수위진동을 일으키는 것은 잘 알려진 사실이다. Liu(1986)는 반원형 외해 경계를 따라 수심의 불연속이 존재하는 지형을 가정하여 이러한 현상을 해석하였으나 그의 해석해에는 오류가 있었다. 본 논문에서는 직선해안 및 직사각형 항만에 대한 Liu(1986)의 해석해를 수정하고, 이를 정 등(1998)의 확장형 완경사방정식에 기초한 Galerkin 유한요소 모형에 의한 결과와 비교하여 확인하였다.

• 대한기계학회논문집
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• 제17권9호
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• pp.2138-2144
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• 1993

An integral equation representation of cracks was presented which differs from well-known "dislocation layer" representation. In this new representation, the equations are written in terms of the displacement discontinuity across the crack surfaces rather than derivatives of the displacement-discontinuity. It was shown in that the new technique is well-suited to the treatment of kinked cracks. In the present paper, this integral equation representation is coupled to the direct boundary-element method for the treatment of finite bodies containing kinked cracks. The method is demonstrated for two-dimensional finite domains but extension to three-dimensional problems would appear to be possible. The resulting approach is shown to be simple, yet very accurate. accurate.

• 대한기계학회논문집B
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• 제29권1호
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• pp.159-168
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• 2005

In recent studies, it was reported that there existed the temperature discontinuity at a liquid-vapor interface in an equilibrium state. However, from the viewpoint of the classical thermodynamics, it is highly questionable result although considering that the experiments related with a boundary layer is very difficult due to the extremely thin thickness of it. To clarify whether the temperature discontinuity over a liquid-vapor interface really exists, the computer simulations were performed. From the simulation results, it could be concluded that the misconception in a temperature calculation might result in non-uniform temperature distributions over an interface under an equilibrium state.