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이차원 비압축성 유동 계산을 위한 Hermite 쌍 3차 유동 함수법
김진환(J.W. Kim) 한국전산유체공학회 2008 한국전산유체공학회 학술대회논문집 Vol.2008 No.-
This paper is an extension of previous study[9] on a development of a divergence-free element method using a hermite interpolated stream function. Divergence-free velocity bases defined on rectangles derived herein produce pointwise divergence-free flow fields. Hence the explicit imposition of continuity constraint is not necessary and the Galerkin finite element formulation for velocities does not involve the pressure. The divergence-free element of the previous study employed hermite serendipity cubic for interpolation of stream function, and it has been noted a possible discontinuity in variables along element interfaces. This deficiency can be removed by use of a hermite bicubic interpolated stream function, which requires at each element corners four degrees-of-freedom such as the unknown variable, its x- and y-derivatives and its cross derivative. Detailed derivations are presented for both solenoidal and irrotational bases from the hermite bicubic interpolated stream function. Numerical tests are performed on the lid-driven cavity flow, and results are compared with those from hermite serendipity cubics and a stabilized finite element method by Illinca et al[7].
이차원 비압축성 유동 계산을 위한 Hermite 겹 3차 유동 함수법
김진환(J.W. Kim) 한국전산유체공학회 2008 한국전산유체공학회지 Vol.13 No.4
This paper is an extension of previous study [1] on a development of a divergence-free element method using a hermite interpolated stream function. Divergence-free velocity bases defined on rectangles derived herein produce pointwise divergence-free flow fields. Hence the explicit imposition of continuity constraint is not necessary and the Galerkin finite element formulation for velocities does not involve the pressure. The divergence-free element of the previous study employed hermite (serendipity) cubic for interpolation of stream function, and it has been noted a possible discontinuity in variables along element interfaces. This deficiency can be removed by use of a hermite bicubic interpolated stream function, which requires four degrees-of-freedom at each element corners. Those degrees-of-freedom are the unknown variable, its x- and y-derivatives and its cross derivative. Detailed derivations are presented for both solenoidal and irrotational basis functions from the hermite bicubic interpolated stream function. Numerical tests are performed on the lid-driven cavity flow, and results are compared with those from hermite serendipity cubics and a stabilized finite element method by Illinca et al[2].
김진환(J.W. Kim) 한국전산유체공학회 2005 한국전산유체공학회지 Vol.10 No.2
Substructuring methods are often used in finite element structural analyses. In this study a multi-level substructuring(MLSS) algorithm is developed and proposed as a possible candidate for finite element fluid solvers. The present algorithm consists of four stages such as a gathering, a condensing, a solving and a scattering stage. At each level, a predetermined number of elements are gathered and condensed to form an element of higher level. At the highest level, each sub-domain consists of only one super-element. Thus, the inversion process of a stiffness matrix associated with internal degrees of freedom of each sub-domain has been replaced by a sequential static condensation of gathered element matrices. The global algebraic system arising from the assembly of each sub-domain matrices is solved using a well-known iterative solver such as the conjugare gradient(CG) or the conjugate gradient squared(CGS) method. A time comparison with CG has been performed on a 2-D Poisson problem. With one domain the computing time by MLSS is comparable with that by CG up to about 260,000 d.o.f. For 263,169 d.o.f. using 8 x 8 sub-domains, the time by MLSS is reduced to a value less than 30 % of that by CG. The lid-driven cavity problem has been solved for Re = 3200 using the element interpolation degree(Deg.) up to cubic. In this case, preconditioning techniques usually accompanied by iterative solvers are not needed. Finite element formulation for the incompressible flow has been stabilized by a modified residual procedure proposed by Ilinca et al.[9].
계층적 반복과 수정 잔여치법에 의한 비압축성 유동 계산
김진환(J. W. Kim) 한국전산유체공학회 2004 한국전산유체공학회지 Vol.9 No.3
The incompressible Navier-Stokes equations in two dimensions are stabilized by a modified residual method, and then discretized by hierarchical elements. The stabilization is necessary to escape from the Ladyzhenskaya-Babuska-Brezzi(LBB) constraint and hence to achieve an equal order formulation. To expedite a standard iterative method such as the conjugate gradient squared(CGS) method, a preconditioning technique called the Hierarchical Iterative Procedure(HIP) has been applied. In this paper, we increased the order of interpolation within an element up to cubic. The hierarchical elements have been used to achieve a higher order accuracy in fluid flow analyses, but a proper efficient iterative procedure for higher order finite element formulation has not been available so far. The numerical results by the present HIP for the lid driven cavity flow and others showed the present procedure to be stable, very efficient and useful in flow analyses in conjunction with hierarchical elements.
비파괴적 방법에 의한 ECAP 가공 알루미늄합금의 결정립 미세화 평가
김진환(J.H. Kim),안석환(S.H. Ahn),남기우(K.W. Nam),강석봉(S.B. Kang) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.4
The grain size of aluminum alloy was refined to the submicrometer level by using equal-channel angular pressing(ECAP). The effect of grain size refinement was evaluated by the tensile test, micro-hardness test, microstructure observations, ultrasonic test and acoustic emission test. The strength and the Vickers hardness were increased significantly according to grain size refinement after equal-channel angular pressed. The ultrasonic velocity was faster after equal-channel angular pressed, and the high frequency range appeared. The results of the ultrasonic velocity and the frequency range are expected to be basic data that can prove the grain size refinement.
김진환(J.H. Kim),안석환(S.H. Ahn),김현수(H.S. Kim),남기우(K.W. Nam),김선진(S.J. Kim) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.3
Fracture behaviors of pipes with local wall thinning are very important for the integrity of nuclear power plant. However, effects of local wall thinning on strength and fracture behaviors of piping system were not well studied. Acoustic emission(AE) has been widely used in various fields, because of its extreme sensitivity, dynamic detection ability and location of growing defects. In this study, we investigated failure modes of local wall thinning pipes and AE signals by bending test. As results of test, we could divide four types of failure modes of ovalization, ovalization+cracking, local buckling and buckling+cracking. And fracture behaviors of Elastic range, yield range, plastic range and cracking progress could be evaluated in bending test by AE counts, accumulation counts and time frequency analysis. It is expected to be basic data that can protect a risk according to local wall thinning of pipes, as a real time test of AE.
김진환(J. H. KIM),남기우(K. W. Nam),안석환(S. H. Ahn),강창룡(C. R. Kang),박인덕(I. D. Park) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.3
Even if duplex steel made improvement the strength and corrosion resistance properties, the necessity of steel which has long life in severe environment increases with industrial development. Newly developed duplex stainless steel is the super duple stainless steel which contain 25% of chromium. We investigated the fatigue crack propagation of heat affected zone(HAZ) in super duplex stainless steel. The HAZ of super duplex stainless steel propagated more faster base metal of super duplex stainless steel. And we analysed acoustic emission signals during the fatigue test by time-frequency analysis method. And as a result of time-frequency was 200-400 ㎑ was obtained by fatigue crack propagation and 500 ㎑ was obtained by dimple and separate of inclusion.