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고진환,변도영,Ko, Jin-Hwan,Byun, Do-Young 한국전산구조공학회 2007 한국전산구조공학회논문집 Vol.20 No.6
대수학 부구조법은 대형 문제들의 고유치 계산에 최고 성능을 지닌 방법이지만 근본적으로 최소 고유치만을 계산하기 위해 설계되었다. 본 논문에서는 이동값을 이용하여 특정범위 안의 내부 고유치를 계산하기 위해 대수학 부구조법의 갱신된 버전을 제안하고, 이를 이동 대수학 부구조법이라 명명한다. 그리고 구조문제의 유한요소모델에 대한 수치실험을 통해 제안된 방법이 다수의 내부고유치를 계산하는데 란쵸스방법보다 월등한 효율성을 가지고 있음을 보였다. Algebraic substructuring (AS) is a state-of-the-art method in eigenvalue computations, especially for large size problems, but, originally, it was designed to calculate only the smallest eigenvalues. In this paper, an updated version of AS is proposed to calculate the interior eigenvalues over a specified range by using a shift value, which is referred to as the shifted AS. Numerical experiments demonstrate that the proposed method has better efficiency to compute numerous interior eigenvalues for the finite element models of structural problems than a Lanczos-type method.
효과적인 열응력 해석을 위한 사각형 추가 변형률 요소의 개발
고진환,이병채,Ko, Jin-Hwan,Lee, Byung-Chai 대한기계학회 1999 大韓機械學會論文集A Vol.23 No.7
A new quadrilateral plane stress element is developed for efficient and accurate analysis of thermal stress problems. It is convenient to use the same mesh and the same shape functions for thermal analysis and stress analysis. But, because of the inconsistency between deformation related strain field and thermal strain field, oscillatory responses and considerable errors in stresses are resulted in. To avoid undesired oscillations, strain approximation is enhanced by supplementing several assumed strain terms based on the variational principle. Thermal deformation is incorporated into the generalized mixed variational principle for displacement, strain and stress fields, and basic equations for the modified enhanced assumed strain method are derived. For the stress approximation of bilinear elements, the $5{\beta}$ version of Pian and Sumihara is adopted. The numerical results for several problems show that the present element behaves well and reduces oscillatory responses. it also results in almost the same magnitude of error as compared with the quadratic element.
다중스케일 모델을 통한 나노외팔보 공진기의 파라메트릭 해석
고진환(Jin Hwan Ko),정준호(Junho Jung),조맹효(Maenghyo Cho) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
Nano-cantilever vibratory resonators operated in an ultra high frequency or a very high frequency have been used in applications including force detection, mass sensor, and biomolecular sensor. Molecular dynamics (MD) has been a typical simulation tool for nano-structure analysis, but it is known as very expensive. Alternatively, a continuum-based multi-scale model provides a good approximate solution with higher efficiency over MD simulation as well as directly interfaces to continuum-based models. In this paper, we model nano-cantilever and its support by the multi-scale model and predict its quality factor by implementing perfectly matched layer concept. This multi-scale model is verified by an analytic solution for the quality factor prediction. Parametric analyses on hole effects, taped shapes, clamping conditions are followed and give a hint about an optimal design of nano-cantilevers with high quality factors.
Algebraic sub structuring with frequency sweep algorithm for mid-frequency response analysis
고진환(Jin-Hwan Ko),Zhaojun Bai 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
Most of the studies use algebraic sub structuring (AS) based frequency response solvers for low frequency (LF) response analysis due to their high computational efficiency. In LF response analysis, AS retains all LF modes when using the modal superposition. However, in mid-frequency (MF) response analysis, the LF modes make very little contribution and also increase the number of retained modes, which leads to loss of computational efficiency. Therefore, MF response analysis should consider low truncated modes to increase the computational efficiency of the solver. The current work is focused on improving the computational efficiency using a variant AS algorithm which adopts shift strategy and a frequency sweep algorithm.
고진환(Jin-Hwan Ko),김지웅(Jee Woong Kim),변도영(Doyoung Byun),박수형(Soo Hyung Park) 한국항공우주학회 2008 韓國航空宇宙學會誌 Vol.36 No.11
최근 다분야 전산유체 역학에서는 설계 최적화, 공탄성, 강제 경계 운동 등에서 요구되어지는 이동경계문제를 다루게 된다. 이동경계의 변위가 클 경우 강건하고 효율적인 격자변형 알고리즘의 개발이 필요하다. 본 연구에서는 유한 대형요소와 초월유한보간에 근거한 체적격자 변형 코드를 개발하였고, 정렬격자 다중 블록 Navier-Stokes 코드와 연계하였다. 개발된 코드의 검증을 위해 주기적으로 진동 운동을 하는 에어포일 문제에 대해 계산을 수행 하였고 양력, 항력, 모멘트 계수의 이력 계산 결과가 실험 결과와 잘 일치하였다. Modern multidisciplinary computational fluid dynamics often incorporates moving boundaries, as would be required in the applications such as design optimization, aeroelasticity, or forced boundary motion. It is challenging to develop robust, efficient grid deformation algorithms when large displacement of the moving boundaries is required. In this paper, a volume grid deformation code is developed based on the finite macro-element and the transfinite Interpolation, and then interfaces to a structured multi-block Navier-Stokes in-house code. As demonstrated by an airfoil with pitching motion, the hysteresis loops of lift, drag and moment coefficients of the developed method are shown to be in good agreement with those of experimental data.
MEMS 공진기의 고주파 응답해석을 위한 고효율 해석기
고진환(Jin Hwan Ko),Zhaojun Bai 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
A modern MEMS resonator is a micro-scale structure operated over a high frequency range. In order to predict its resonant behavior in a design process, High-frequency response analysis (Hi-FRA) is demanded. Algebraic substructuring (AS) is known as a fast numerical technique to construct an eigenspace for FR and frequency sweep (FS) algorithm efficiently solves the frequency response system projected on the eigenspace. However, the existing FS algorithm using AS is developed for low-FRA, say over the range 1㎐-2㎑. In this work, we extend the FS algorithm using AS for FRA over an arbitrary frequency range. Therefore, it can be efficiently applied to systems operated at a high frequency, say over the range 230㎒-250㎒. The success of the proposed method is demonstrated by Hi-FRA of a checkerboard resonator.
고진환(Jin Hwan Ko),김지웅(Jee Woong Kim),박수형(Soo Hyung Park),투엔르광(Tuyen Quang Le),변도영(Doyoung Byun) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
As a novel bio-mimetic technology, flapping foils have attracted much attention, owing to their capacity of producing thrust with high efficiency. Flapping foils may fine applications on autonomous underwater vehicle, flapping micro-air vehicle and etc. In design stage of the new type vehicles, computational fluid dynamics (CFD) can be used to explore performance of the flapping foils. One of challenging problems in CFD is known to be the development of robust, efficient grid deformation algorithms for the large deformation of the foils. In this paper, a volume grid deformation is developed based on Finite Macro-Element and Transfinite Interpolation and then interfaces a structured multi-block Navier-Stokes code. As demonstrated by airfoils with pitching or heaving motion, the force results of presented method is in good agreement with those of a conventional method and is also close to experimental results.