• Bulletin of the Korean Chemical Society
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• 제21권7호
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• pp.720-726
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• 2000

The analytic gradient method for the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) energy has been extended to employ a reduced molecular orbital (MO) space. Not only the innermost core MOs but also some of the outermost virtua l MOs can be dropped in the reduced MO space, and a substantial amount of computation time can be reduced without deteriorating the results. In order to study the magnitudes and trends of the effects of the dropped MOs, the geometries and vibrational properties of the ground and excited states of BF, CO, CN, N2, AlCl, SiS, P2, BCl, AIF, CS, SiO, PN and GeSe are calculated with different sizes of molecular orbital space. The 6-31 G* and the aug-cc-pVTZ basis sets are employed for all molecules except GeSc for which the 6-311 G* and the TZV+f basis sets are used. It is shown that the magnitudes of the drop-MO effects are about $0.005\AA$ in bond lengths and about 1% on harmonic frequencies and IR intensities provided that the dropped MOs correspond to (1s), (1s,2s,2p), an (1s,2s,2p,3s,3p) atomic orbitals of the first, the second, and the third row atoms, respectively. The geometries and vibrational properties of the first and the second excited states of HCN and HNC are calculated by using a drastically reduced virtual MO space as well as with the well defined frozen core MO space. The results suggest the possibility of using a very smalI MO space for qualitative study of valence excited states.

• International Journal of Control, Automation, and Systems
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• 제2권3호
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• pp.384-389
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• 2004

Recently a method of model reduction for discrete systems has been proposed in the literature based on a new impulse response Gramian. In this method, the system matrix$A_r$ of a reduced model is computed by approximating the reduced-order impulse response Gramian. The remaining matrices $b_r$ and $c_r$ are obtained so that various initial Markov parameters and time-moments of the original system are preserved in the reduced model. In this paper a different approach is presented based on the recursive relationship among the impulse response Gramians.

• 한국음향학회지
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• 제37권5호
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• pp.263-270
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• 2018

외부 유동소음 문제를 다루는 대부분의 산업현장에서 FW-H(Ffowcs Williams and Hawkings) 방정식을 이용한 복합전산공력음향 기법이 수치적인 효율성으로 인하여 널리 사용되고 있다. 그러나 사중극자항을 무시한 면적분 형태의 FW-H 방정식을 사용할 때 경우에 따라 무시할 수 없는 비물리적인 소음이 발생한다고 알려져 있다. 특히, 수중 프로펠러와 같이 날개 끝 와류 공동(tip vortex cavitation)이 하류방향으로 길게 형성되는 유동에 대해서는 적절하게 모델링하지 않으면 소음 예측의 정확도가 떨어지게 된다. 따라서 본 연구에서는 사중극자 보정항을 추가하여 적분면에서 FW-H 방정식으로부터 발생하는 비물리적인 음향을 저감시키고자 하였다. 먼저 FW-H 방정식에 기초하여 개발한 내부 예측코드의 정확성을 확인하기 위하여 에어컨 실외기에 사용되는 축류팬을 대상으로 검증을 수행하였으며, ANSYS Fluent의 결과와 비교하여 잘 일치하는 것을 확인하였다. 사중극자 보정항의 효과를 확인하기 위하여 등엔트로피 와류 전파에 대한 소음 해석을 수행하였으며, 사중극자 보정항에 의한 오차의 저감 효과가 발생하는 것을 확인하였다. 마지막으로 Clark-Y 수중익형에서 발생하는 공동 유동장을 대상으로 소음 해석을 수행하였으며, 공동이적분면을 통과할 때 발생하는 오차를 사중극자 보정항을 이용하여 저감할 수 있다는 것을 확인하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.299-304
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• 2001

The current study describes experimental and computational work on the passive control of the steady and unsteady condensation shock waves, which are generated in a transonic nozzle. The bleed slots are installed on the contoured wall of the transonic nozzle in order to control the magnitude of the condensation shock wave and its oscillations. For computations, a droplet growth equation is incorporated into the two-dimensional Navier-Stokes equation systems. Computations are carried out using a third-order MUSCL type TVD finite-difference scheme with a second-order tractional time step. Baldwin-Lomax turbulence model is employed to close the governing equations. An experiment using an indraft transonic wind tunnel is made to validate the computational results. The current computations represented well the experimental flows. From both the experimental and computational results it is found that the magnitude of the condensation shock wave in the bleed slotted nozzle is significantly reduced, compared with no passive control of solid wall. The oscillations of the condensation shock wave are successfully suppressed by a bleed slot system.

• 대한기계학회논문집B
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• 제26권7호
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• pp.997-1004
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• 2002

The current study describes experimental and computational works on the passive control of the steady and unsteady condensation shock waves, which are generated in a transonic nozzle. The bleed slots are installed on the contoured wall of the transonic nozzle in order 10 control the magnitude of the condensation shock wave and its oscillations. For computations, a droplet growth equation is copuled with two-dimensional Navier-Stokes equation systems. Computations are carried out using a third-order MUSCL type TVD finite-difference scheme with a second-order fractional time step. Baldwin-Lomax turbulence model is employed to close the governing equations. An experiment using an indrafi transonic wind tunnel is made to validate the computational results. The current computations represented well the experimental flows. From both the experimental and computational results it is found that the magnitude of the condensation shock wave in the bleed slotted nozzle is signi ficantly reduced, compared with no passive control of solid wall. The oscillations of the condensation shock wave are successfully suppressed by a bleed slot system.

• 한국수자원학회논문집
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• 제31권6호
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• pp.845-854
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• 1998

Ebersole(1995)의 접근법을 사용하여 Massel(1993)의 확장형 완경사방정식에서 유도되는 eikonal 식과 파랑 에너지전송식과 또한 파수의 비회전성을 이용하여 파랑변형을 예측하였다. 완경사방정식에 무시되었으나 확장형 완경사방정식에 고려된 고차의 수심변화 효과, 즉 수심경사의 제곱 및 수심의 곡률이 고려되면 수심변화가 심한 경우에 더 정확한 해석이 될 것이라는 예측이 수치실험 결과 제대로 나타나지 않았다. 이는 수심변화가 심한 경우 eikonal 식에서 고려된 회절의 효과가 제대로 반영되지 않아서 해석결과에 오류가 발생하는 것이 아닌가 판단된다.

• Structural Engineering and Mechanics
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• 제57권2호
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• pp.327-355
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• 2016

A new analytical derivation of the elastodynamic point load solutions for an isotropic multi-layered half-space is presented by means of the precise integration method (PIM) and the approach of dual vector. The time-harmonic external load is prescribed either on the external boundary or in the interior of the solid medium. Starting with the axisymmetric governing motion equations in a cylindrical coordinate system, a second order ordinary differential matrix equation can be gained by making use of the Hankel integral transform. Employing the technique of dual vector, the second order ordinary differential matrix equation can be simplified into a first-order one. The approach of PIM is implemented to obtain the solutions of the ordinary differential matrix equation in the Hankel integral transform domain. The PIM is a highly accurate algorithm to solve sets of first-order ordinary differential equations and any desired accuracy of the dynamic point load solutions can be achieved. The numerical simulation is based on algebraic matrix operation. As a result, the computational effort is reduced to a great extent and the computation is unconditionally stable. Selected numerical trials are given to validate the accuracy and applicability of the proposed approach. More examples are discussed to portray the dependence of the load-displacement response on the isotropic parameters of the multi-layered media, the depth of external load and the frequency of excitation.

• 한국정밀공학회지
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• 제9권3호
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• pp.110-116
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• 1992

Electromgnetic joining of aluminum alloy tubes to high toughness polyurethane rubber cores is studied in order to estimate the joining strength and to analyze the effect of the process variables. The equation which can estimate the joining strength is proposed under considering the elastic recovery of the polyurethane core and the radial shrinkage of the core by pulling it axially. The obtained results are as follows : 1) The joining strength is mainly dependent on the magnitude of residual elastic strain of the polyurethane core. 2) The radial shrinkage (residual strain reduction) of the core during the axial pulling causes the joining strength to decrease severely. The equation for the reduced axial strength is proposed and it is found that the estimated values agree well with experimental results. 3) The magnitude of radial shrinkage could be reduced for the smaller value of ratio l/r. 4) The joining strength in metal/polymer joining increases as the friction coefficient increases. But its effect of friction coefficient is insignificant in comparison with the case of metal/metal joining.

• Steel and Composite Structures
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• 제28권6호
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• pp.655-670
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• 2018

A coupled method, that combines the Ritz method and the finite element (FE) method, is proposed to solve the vibration problem of rectangular thin and thick plates with general boundary conditions. The eigenvalue partial differential equation(s) of the plate is (are) first reduced to a set of eigenvalue ordinary differential equations by the application of the Ritz method. The resulting eigenvalue differential equations are then reduced to an eigenvalue algebraic equation system using the finite element method. The natural boundary conditions of the plate problem including the free edge and free corner boundary conditions are also implemented in a simple and accurate manner. Various boundary conditions including simply supported, clamped and free boundary conditions are considered. Comparisons with existing numerical and analytical solutions show that the proposed mixed method can produce highly accurate results for the problems considered using a small number of Ritz terms and finite elements. The proposed mixed Ritz-FE formulation is also compared with the mixed FE-Ritz formulation which has been recently proposed by the present author and his co-author. It is found that the proposed mixed Ritz-FE formulation is more efficient than the mixed FE-Ritz formulation for free vibration analysis of rectangular plates with Levy-type boundary conditions.

• 대한기계학회논문집
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• 제18권10호
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• pp.2640-2652
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• 1994

The stiffness of 6-node isotropic element is stiffer than that of 8-node isotropic element of same configuration. This phenomenon was called 'Relative Stiffness Stiffening Phenomenon'. In this paper, an equation of sampling point modification which correct this phenomenon was derived for the composite plate, as well as an equation for an isotropic plate. The relative stiffness stiffening phenomena of an isotropic plate element could be corrected by modifying Gauss sampling points in the numerical integration of stiffness matrix. This technique could also be successfully applied to the static analyses of composite plate modeled by the 3-dimensional 16-node elements. We predicted theoretical errors of stiffness versus the number of layers that result from the reduction of numerical integration order. These errors coincide very well with the actual errors of stiffness. Therefore, we can choose full integration of reduced integration based upon the permissible error criterion and the number of layers by using the thoretically predicted error.