• Wind and Structures
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• 제12권5호
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• pp.457-474
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• 2009

Techniques for stabilising slender bridges under wind loads are presented in this article. A mathematically consistent description of the acting aerodynamic forces is essential when investigating these ideas. Against this background, motion-induced aerodynamic forces are characterised using a linear time-invariant transfer element in terms of rational functions. With the help of these functions, the aeroelastic system can be described in the form of a linear, time-invariant state-space model. It is shown that the divergence wind speed constitutes an upper bound for the application of the selected mechanical actuators. Even active control with full state feedback cannot overcome this limitation. The results are derived and explained with methods of control theory.

• Wind and Structures
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• 제9권2호
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• pp.159-178
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• 2006

For the slender and flexible cable supported bridges, identification of all the flutter derivatives for the vertical, lateral and torsional motions is essential for its stability investigation. In all, eighteen flutter derivatives may have to be considered, the identification of which using a three degree-of-freedom elastic suspension system has been a challenging task. In this paper, a system identification technique, known as covariance-driven stochastic subspace identification (COV-SSI) technique, has been utilized to extract the flutter derivatives for a typical bridge deck. This method identifies the stochastic state-space model from the covariances of the output-only (stochastic) data. All the eighteen flutter derivatives have been simultaneously extracted from the output response data obtained from wind tunnel test on a 3-DOF elastically suspended bridge deck section-model. Simplicity in model suspension and measurements of only output responses are additional motivating factors for adopting COV-SSI technique. The identified discrete values of flutter derivatives have been approximated by rational functions.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1988년도 한국자동제어학술회의논문집(국제학술편); 한국전력공사연수원, 서울; 21-22 Oct. 1988
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• pp.934-938
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• 1988

In this paper we present a method of determining the unknown degree of any 2D discrete linear shift-invariant system which is characterized only by the coefficients of the double power series of a transfer function, i.e. a 2D impulse response array. Our method is based on a 2D extension of Berlekamp-Massey algorithm for synthesis of linear feedback shift registers, and it gives a novel approach to identification and approximation of 2D linear systems, which can be distinguished in its simplicity and potential of applicability from the other 2D Levinson-type algorithms. Furthermore, we can solve problems of 2D Pade approximation and 2D system reduction on a reasonable assumption in the context of 2D linear systems theory.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집A
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• pp.324-329
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• 2001

A novel method for non-matching interfaces on the boundaries of the finite elements in partitioned domains is presented by introducing interface elements in this paper. The interface element method (IEM) satisfies the continuity conditions exactly through interfaces without recourse to the Lagrange multiplier technique. The moving least square (MLS) approximation in the present study is implemented to construct the shape functions of the interface elements. Alignment of the boundaries of sub-domains in the MLS approximation and integration domains provides a consistent numerical integration due to one form of rational functions in an integration domain. The compatibility of displacements on the boundaries of the finite elements and the interface elements is always preserved in this method, and the completeness of the shape functions of the interface elements guarantees the convergence of numerical solutions. The numerical examples show that the interface element method is a useful tool for the analysis of a partitioned system and for a global-local analysis.

• 대한기계학회논문집A
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• 제30권12호
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• pp.1626-1633
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• 2006

In the present paper, in the context of the meshless interpolation of a moving least squares (MLS) type, a novel method which uses primary and secondary nodes in the domain and on the global boundary is introduced, in order to improve the accuracy of solution. The secondary nodes can be placed at any location where one needs to obtain a better resolution. The support domains for the shape functions in the MLS approximation are defined from the primary nodes, and the secondary nodes use the same support domains. The shape functions based on the MLS approximation, in an integration domain, have a single type of a rational function, which reduces the difficulty of numerical integration to evaluate the weak form. The present method is very useful in an adaptive calculation, because the secondary nodes can be easily added and moved without an additional mesh. Several numerical examples are presented to illustrate the effectiveness of the present method.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2000년도 봄 학술발표회 논문집
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• pp.663-667
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• 2000

The equivalent load method has been widely used in the design and analysis of prestressed concrete structures. The purpose of this paper is to explore several important method of obtaining equivalent loads and to clarify the advantages and limitations of each method. The methods devised in this study include the use of curvature of tendon, characteristics of primary moment, self-equilibrium conditions, and linear segments approximation of tendon. It is shown that equivalent lading system is not uniquely determined in some cases and careful engineering judgement is required from the view point of accuracy and practical convenience.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제16권1호
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• pp.65-84
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• 2012

We present an application of the variational multiscale methodology to the computation of concentric annular pipe flow. Isogeometric analysis is utilized for higher order approximation of the solution using Non-Uniform Rational B-Splines (NURBS) functions. The ability of NURBS to exactly represent curved geometries makes NURBS-based isogeometric analysis attractive for the application to the flow through the curved channel.

• 대한전자공학회논문지SP
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• 제37권3호
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• pp.17-30
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• 2000

본 논문에서는 NURBS (Non-Uniform Rational B-Splines) 곡면 근사에 기반하여 거리 데이터로부터 3차원 곡면 모델을 생성하는 기법을 제안한다 입력으로 이용되는 거리 데이터는 연결 정보가 알려지지 않고 정렬되지 않은 일반적인 3차원 점들의 집합으로 가정한다 제안하는 알고리듬은 초기 모델 추정, 계층적 모델 표현, NURBS 곡면 네트워크 생성의 3단계로 나뉘어진다 초기 모델 추정 단계에서는 K-평균 군집화 기법을 이용하여 다각형면과 삼각형으로 표현되는 근사 모델을 생성하고, 계층적 트리 구조를 이용하여 초기 모델을 표현한다. 계층적 트리 구조로 부터 생성된 사각형면 모델에 의하여 $G^1$ 연속인 NURBS 곡면 네트워크를 효율적으로 생성한다. 제안하는 알고리듬은 초기 모델의 계층적 그래프 해석을 통하여 곡면 네트워크 형성에 필요한 계산량을 감소시켰으며, 또한 정확한 NURBS 제어점 추정을 통하여 근사 오차를 감소시킨다. 모의 실험 결과 거리 데이터로 부터 초기 모델과 다양한 해상도의 NURBS 곡면 네트워크가 효과적으로 생성되었으며 생성된 NURBS 곡면 모델의 근사 오치는 무시할 수 있는 수준임이 관찰되었다.

• Steel and Composite Structures
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• 제37권6호
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• pp.663-678
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• 2020

This paper proposes a novel topology optimization method generating multiple materials for external linear plane crack structures based on the combination of IsoGeometric Analysis (IGA) and eXtended Finite Element Method (X-FEM). A so-called eXtended IsoGeometric Analysis (X-IGA) is derived for a mechanical description of a strong discontinuity state's continuous boundaries through the inherited special properties of X-FEM. In X-IGA, control points and patches play the same role with nodes and sub-domains in the finite element method. While being similar to X-FEM, enrichment functions are added to finite element approximation without any mesh generation. The geometry of structures based on basic functions of Non-Uniform Rational B-Splines (NURBS) provides accurate and reliable results. Moreover, the basis function to define the geometry becomes a systematic p-refinement to control the field approximation order without altering the geometry or its parameterization. The accuracy of analytical solutions of X-IGA for the crack problem, which is superior to a conventional X-FEM, guarantees the reliability of the optimal multi-material retrofitting against external cracks through using topology optimization. Topology optimization is applied to the minimal compliance design of two-dimensional plane linear cracked structures retrofitted by multiple distinct materials to prevent the propagation of the present crack pattern. The alternating active-phase algorithm with optimality criteria-based algorithms is employed to update design variables of element densities. Numerical results under different lengths, positions, and angles of given cracks verify the proposed method's efficiency and feasibility in using X-IGA compared to a conventional X-FEM.

• Journal of electromagnetic engineering and science
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• 제16권3호
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• pp.169-181
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• 2016

This work presents a new method for enhancing the performance of a dual band Planer Inverted-F Antenna (PIFA) and its lumped equivalent circuit formulation. The performance of a PIFA in terms of return loss, bandwidth, gain, and efficiency is improved with the addition of the proposed open stub in the radiating element of the PIFA without disturbing the operating resonance frequencies of the antenna. In specific cases, various simulated and fabricated PIFA models illustrate that the return loss, bandwidth, gain, and efficiency values of antennas with longer optimum open stub lengths can be enhanced up to 4.6 dB, 17%, 1.8 dBi, and 12.4% respectively, when compared with models that do not have open stubs. The proposed open stub is small and does not interfere with the surrounding active modules; therefore, this method is extremely attractive from a practical implementation point of view. The second presented work is a simple procedure for the development of a lumped equivalent circuit model of a dual band PIFA using the rational approximation of its frequency domain response. In this method, the PIFA's measured frequency response is approximated to a rational function using a vector fitting technique and then electrical circuit parameters are extracted from it. The measured results show good agreement with the electrical circuit results. A correlation study between circuit elements and physical open stub lengths in various antenna models is also discussed in detail; this information could be useful for the enhancement of the performance of a PIFA as well as for its systematic design. The computed radiated power obtained using the electrical model is in agreement with the radiated power results obtained through the full wave electromagnetic simulations of the antenna models. The presented approach offers the advantage of saving computation time for full wave EM simulations. In addition, the electrical circuit depicting almost perfect characteristics for return loss and radiated power can be shared with antenna users without sharing the actual antenna structure in cases involving confidentiality limitations.