• Coupled systems mechanics
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• 제1권1호
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• pp.19-37
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• 2012

In this work, the iterative coupling of finite element and boundary element methods for the investigation of coupled fluid-fluid, solid-solid and fluid-solid wave propagation models is reviewed. In order to perform the coupling of the two numerical methods, a successive renewal of the variables on the common interface between the two sub-domains is performed through an iterative procedure until convergence is achieved. In the case of local nonlinearities within the finite element sub-domain, it is straightforward to perform the iterative coupling together with the iterations needed to solve the nonlinear system. In particular, a more efficient and stable performance of the coupling procedure is achieved by a special formulation that allows to use different time steps in each sub-domain. Optimized relaxation parameters are also considered in the analyses, in order to speed up and/or to ensure the convergence of the iterative process.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.597-601
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• 2008

Three-dimensional structures of detonation wave propagating in circular tube were investigated. Inviscid fluid dynamics equations coupled with a conservation equation of reaction progress variable were analyzed by a MUSCL-type TVD scheme and four stage Runge-Kutta time integration. Variable-$\gamma$ formulation was used to account for the variable properties between unburned and burned states and the chemical reaction was modeled by using a simplified one-step irreversible kinetics model. The computational code was parallelized based on domain decomposition technique using MPI-II message passing library. The computations were carried out using a home made Windows based PC cluster having 160 AMD AthloxXP and Athlon64 processor. The computational domain consisted of through a roundshaped tube with wall conditions. As an initial condition, analytical ZND solution was distributed over the computational domain with disturbances. The disturbances has circumferential large gradient. The unsteady computational results in three-dimension show the detailed mechanisms of multi-cell mode of detonation wave instabilities resulting diamond shape in smoked-foil record.

• 한국소음진동공학회논문집
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• 제19권7호
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• pp.719-725
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• 2009

An efficient time-domain numerical method for the analysis of broadband noise generation and propagation due to turbulence-cascade interaction is developed. The core algorithm of the present method is based on the B-periodicity of the acoustic response function of the flat-airfoil cascade to the ingesting gust(B denotes the number of airfoils in the cascade). To confirm this periodicity, gust-cascade interaction problem are solved by using the time-domain method, which shows that the incident gust with the circumferential mode number having the same remainders when divided by the airfoil number excites the same acoustic response of the cascade. Using the proposed fast algorithm with this periodicity, we show that the total computation time for the model broadband problem using the total 525 incident gust modes can be reduced to about 1/4 of that taken in using the previous time-domain program.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2009년도 춘계학술대회 논문집
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• pp.477-482
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• 2009

An efficient time-domain numerical method for the analysis of broadband noise generation and propagation due to turbulence-cascade interaction is developed. The core algorithm of the present method is based on the B-periodicity of the acoustic response function of the flat-airfoil cascade to the ingesting gust (B denotes the number of airfoils in the cascade). To confirm this periodicity, gust-cascade interaction problem are solved by using the time-domain method, which shows that the incident gust with the circumferential mode number having the same remainders when divided by the airfoil number excites the same acoustic response of the cascade. Using the proposed fast algorithm with this periodicity, we show that the total computation time for the model broadband problem using the total 525 incident gust modes can be reduced to about 1/4 of that taken in using the previous time-domain program.

• 한국전산구조공학회논문집
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• 제23권4호
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• pp.369-378
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• 2010

포화된 다공질매체의 수치해석에서는 일반적으로 고체영역과 유체영역을 동시에 고려한 혼합유한요소해석(Mixed Finite Element Analysis)이 쓰인다. 여기서 고체영역과 유체영역에서의 변수를 계산하기 위해서는 직접법(Direct Method) 또는 반복법(Iterative method)을 사용할 수 있으나, 각 구성물질의 상이한 물리적 특성 때문에 수치안정성을 확보하기 위해서는 대부분 스태거드 방법(Staggered method)이 제안된다. 본 논문에서는 수치안정성을 높인 스태거드 방법에서 영역 분할기법 중 하나인 FETI(Finite Element Tearing and Interconnecting)기법을 고체영역에 접목시켜 수치효율성을 증대시키는 방법이 제안되었다. 고체영역에서 라그랑지 승수와 Conjugated Gradient Method를 이용해 영역 분할이 진행되고 MPI(Message Passing Interface) 라이브러리를 사용하여 수치 효율성을 검증하였다.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1995년도 춘계학술대회
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• pp.235-238
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• 1995

In this paper, we propose an effective coding method of the angiography by using the scalable structure in the frequency domain for MPACS(Medical Picture Archiving and Communication System). We employed the subband decomposition method and MPEG-2 system which is the international standard coding method of the general moving picture. After the subband decomposition is applied to split an input image into 4 bands in the spatial frequency domain, the motion compensated DPCM coding method of MPEG-2 is carried out for each subband. As a result, an easily controllable coding Structure is accomplished by composing the compound hit stream for each subband group. Follows are the simulation results of the proposed sheme for the angiography. A scalable structure which can be easily controlled for a loss of transmission or the band limit can be accomplisbed in the MPEG-2 stucture by the subband decomposition minimizing the side information. And by reducing the search area of the motion vector between -4 and 3, the processing speed of a codec is enhanced by more than two times without a loss of the picture quality compare with the conventional DCT coefficients decompositon method. And the processing speed is considerably improved in the case of the parallel construction of each subband in the hardware.

• Structural Engineering and Mechanics
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• 제68권5호
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• pp.603-619
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• 2018

In this research, an effective computational technique is carried out for nonlinear and post-buckling analyses of cracked imperfect composite plates. The laminated plates are assumed to be moderately thick so that the analysis can be carried out based on the first-order shear deformation theory. Geometric non-linearity is introduced in the way of von-Karman assumptions for the strain-displacement equations. The Ritz technique is applied using Legendre polynomials for the primary variable approximations. The crack is modeled by partitioning the entire domain of the plates into several sub-plates and therefore the plate decomposition technique is implemented in this research. The penalty technique is used for imposing the interface continuity between the sub-plates. Different out-of-plane essential boundary conditions such as clamp, simply support or free conditions will be assumed in this research by defining the relevant displacement functions. For in-plane boundary conditions, lateral expansions of the unloaded edges are completely free while the loaded edges are assumed to move straight but restricted to move laterally. With the formulation presented here, the plates can be subjected to biaxial compressive loads, therefore a sensitivity analysis is performed with respect to the applied load direction, along the parallel or perpendicular to the crack axis. The integrals of potential energy are numerically computed using Gauss-Lobatto quadrature formulas to get adequate accuracy. Then, the obtained non-linear system of equations is solved by the Newton-Raphson method. Finally, the results are presented to show the influence of crack length, various locations of crack, load direction, boundary conditions and different values of initial imperfection on nonlinear and post-buckling behavior of laminates.

• Nuclear Engineering and Technology
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• 제51권6호
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• pp.1487-1503
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• 2019

The methods and performance of a pin-level nuclear reactor core thermal-hydraulics (T/H) code ESCOT employing the drift-flux model are presented. This code aims at providing an accurate yet fast core thermal-hydraulics solution capability to high-fidelity multiphysics core analysis systems targeting massively parallel computing platforms. The four equation drift-flux model is adopted for two-phase calculations, and numerical solutions are obtained by applying the Finite Volume Method (FVM) and the Semi-Implicit Method for Pressure-Linked Equation (SIMPLE)-like algorithm in a staggered grid system. Constitutive models involving turbulent mixing, pressure drop, and vapor generation are employed to simulate key phenomena in subchannel-scale analyses. ESCOT is parallelized by a domain decomposition scheme that involves both radial and axial decomposition to enable highly parallelized execution. The ESCOT solutions are validated through the applications to various experiments which include CNEN $4{\times}4$, Weiss et al. two assemblies, PNNL $2{\times}6$, RPI $2{\times}2$ air-water, and PSBT covering single/two-phase and unheated/heated conditions. The parameters of interest for validation include various flow characteristics such as turbulent mixing, spacer grid pressure drop, cross-flow, reverse flow, buoyancy effect, void drift, and bubble generation. For all the validation tests, ESCOT shows good agreements with measured data in the extent comparable to those of other subchannel-scale codes: COBRA-TF, MATRA and/or CUPID. The execution performance is examined with a mini-sized whole core consisting of 89 fuel assemblies and for an OPR1000 core. It turns out that it is about 1.5 times faster than a subchannel code based on the two-fluid three field model and the axial domain decomposition scheme works as well as the radial one yielding a steady-state solution for the OPR1000 core within 30 s with 104 processors.

• Wind and Structures
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• 제36권4호
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• pp.221-236
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• 2023

The lateral component of turbulence and the vortices shed in the wake of a structure result in introducing dynamic wind load in the acrosswind direction and the resulting level of motion is typically larger than the corresponding alongwind motion for a dynamically sensitive structure. The underlying source mechanisms of the acrosswind load may be classified into motion-induced, buffeting, and Strouhal components. This study proposes a frequency domain framework to decompose the overall load into these components based on output-only measurements from wind tunnel experiments or full-scale measurements. First, the total acrosswind load is identified based on measured acceleration response by solving the inverse problem using the Kalman filter technique. The decomposition of the combined load is then performed by modeling each load component in terms of a Bayesian filtering scheme. More specifically, the decomposition and the estimation of the model parameters are accomplished using the unscented Kalman filter in the frequency domain. An aeroelastic wind tunnel experiment involving a tall circular cylinder was carried out for the validation of the proposed framework. The contribution of each load component to the acrosswind response is assessed by re-analyzing the system with the decomposed components. Through comparison of the measured and the re-analyzed response, it is demonstrated that the proposed framework effectively decomposes the total acrosswind load into components and sheds light on the overall underlying mechanism of the acrosswind load and attendant structural response. The delineation of these load components and their subsequent modeling and control may become increasingly important as tall slender buildings of the prismatic cross-section that are highly sensitive to the acrosswind load effects are increasingly being built in major metropolises.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2009년도 정기 학술대회
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• pp.139-145
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• 2009

축소시스템은 반복적인 계산이 요구되는 문제에서 매우 유용하게 적용될 수 있는 해석 기법이다. 최근에는 영역분할 기법과의 연동을 통해 축소시스템의 효율성이 향상되었다. 그러나, 전체 도메인이 몇 개의 영역으로 분할될 때 구속조건이 부과되지않는 영역이 만들어지게 된다. 각 부영역의 축소시스템을 구축하기 위해서는 리츠벡터를 추출해야 하는데, 구속조건이 부과된 부영역에서는 일반적인 정적해석을 통해 가능하다. 그러나, 경계조건이 부과되지 않은 부영역에서는 리츠벡터 추출을 위해 의사역행렬을 이용해야 한다. 일반적으로, 의사역행렬의 사용은 상당한 계산시간과 전산자원을 필요로 하는 문제점이 있다. 본 연구에서는 이 문제점을 개선하기 위해 축소 의사역행렬 도입을 제안한다. 이 방법은 정적 축소방법을 기초로 축소 의사역행렬을 구축하여 축소된 리츠벡터 정보를 추출한 후, 변환관계를 이용하여 전체 리츠벡터 정보를 구하게 된다. 수치예제에서는 고유치 해석을 통해 제안방법의 신뢰성을 검증하고, 전체시스템 계산시간과 비교하여 그 효율성을 검증한다.