• Bulletin of the Korean Space Science Society
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• 2010.04a
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• pp.28.1-28.1
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• 2010

이 연구에서는 기존 선형 상대운동방정식에 차등중력, 주위성의 이심율, J2 섭동 등의 비선형항을 추가하여 보다 정확한 상대운동방정식을 만든 후 섭동이론을 적용하여 위성편대 연료최적화 재배치 문제에 대한 근사 해석해를 구하고자 한다. 먼저, 비선형 섭동항을 테일러 급수를 이용하여 2차항까지 전개한 후, 이를 기존 선형상대운동방정식에 추가하여 새로운 비선형 상대운동방정식을 만든다. 이 때 사용된 선형상대운동방정식은 힐스 방정식으로 주위성의 궤도가 일반적인 타원이고 위성 간 상대거리가 충분히 가깝다고 가정한다. 최적화 조건으로부터 상태벡터와 라그랑지 곱수로 이루어진 연립 미분방정식이 만들어 지는데, 이 식은 힐스 방정식에 기인한 선형부분과 2차 비선형항에 기인한 섭동부분으로 나뉜다. 이 때, 이 연립미분방정식의 해는 선형부분의 해와 섭동으로 인한 변화량의 합으로 근사할 수 있으며 그 변화량은 섭동이론을 적용하여 얻을 수 있다. 이와 같이 얻어진 해는 여러 섭동의 비선형항을 2차까지 포함한 상대운동방정식을 사용했기 때문에, 기존 선형상대운동방정식을 사용하여 구한 최적해 보다 더 정확한 결과를 얻을 것이라 예상한다.

• The KIPS Transactions:PartB
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• v.9B no.3
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• pp.277-286
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• 2002

Genetic programming (GP) has been combined with quantum mechanical perturbation theory to make a new algorithm to construct mathematical models and perform predictions for chaotic time series from real world. Procedural similarities between time series modeling and perturbation theory to solve quantum mechanical wave equations are discussed, and the exemplary GP approach for implementing them is proposed. The approach is based on multiple populations and uses orthogonal functions for GP function set. GP is applied to original time series to get the first mathematical model. Numerical values of the model are subtracted from the original time series data to form a residual time series which is again subject to GP modeling procedure. The process is repeated until predetermined terminating conditions are met. The algorithm has been successfully applied to construct highly effective mathematical models for many real world chaotic time series. Comparisons with other methodologies and topics for further study are also introduced.

• 전기의세계
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• v.25 no.2
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• pp.58-61
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• 1976

최근 동기발전기에 각종 brushless여자방식이 고안되고 있으나 공통된 개발 목표란 신뢰도와 경제성에 있다. 이것은 구성재료의 발달과 반도체의 개발이 크게 작용하였으며 종래의 직류여자기에 비하여 전기적인 섭동부의 완전 제거와 동시에 보다 소형경량화된 동기발전기의 개혁이 필요하다고 본다. 실제 동기발전기에서 전기자와 계자사이에 energy전달은 불가피한 기본원리로서 전기적인 방식과 자기적인 방법으로 구분할 수 있다. 전자는 도체의 섭동 즉 commutator, seipring brush의 마모와 기계적 손실이 있어 신뢰도가 낮은 반면 적은 여자전력으로 가능하나 자기적인 방식은 공극의 reluctance로 인하여 여자전력이 증가한다. 이러한 문제점을 해결할 수단으로서 commutatorless, static excitation, exciterless, harmonics excitation 방식 등이 고안되고 있으나 섭동부의 완전제거란 불가능하여 본고에서는 동기발전기를 중심으로 Brushless여자방식의 종류와 원리에 대하여 이론적으로 해설하고자 한다.

• Tunnel and Underground Space
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• v.13 no.5
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• pp.389-396
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• 2003

It is shown that the cubic law can be modified regarding the steady-state Navier-Stokes equations by using perturbation approximation method for a sinusoidal aperture variation. In order to adopt the perturbation theory, the sinusoidal function needs to be non-dimensionalized for the amplitude and wavelength. Then, the steady-state Navier-Stokes equations can be solved by expanding the non-dimensionalized stream function with respect to the small value of the parameter (the ratio of the mean aperture to the wavelength), together with the continuity equation. From the approximate solution of the Navier-Stokes equations, the basic cubic law is successfully modified for the steady-state condition and a sinusoidal aperture variation. A finite difference method is adopted to calculate the pressure within a fracture model, and the results of numerical experiments show the accuracy and applicability of the modified cubic law. As a result, it is noted that the modified cubic law, suggested in this study, will be used for the analysis of fluid flow through aperture geometry of sinusoidal distributions.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.04a
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• pp.152-156
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• 2007

It is very important to understand about mass flow rate variations of propellants generated by pressure fluctuation in the combustion chamber. Therefore, we have studied quantifying the variation of mass flow rate generated by pressure fluctuation. The flow velocity in orifice is acquired through theoretical approach after measuring the pressure in orifice and the flow area in orifice is measured by film thickness measuring device. Our results agreed with it in the very small error range comparing our results with velocity and mass flow rate in steady state. Thus, our result based on theoretical approach will help about measuring mass flow rate in non-steady state.

• The Journal of the Acoustical Society of Korea
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• v.28 no.3
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• pp.174-186
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• 2009

When we apply a propagation model to the ocean with boundaries, we can calculate reflected wave using reflection coefficient suggested by Rayleigh assuming the boundaries are flat. But boundaries in ocean such as sea surface and sea bottom have an irregular rough surface. To calculate the reflection loss for an irregular boundary, it is needed to compute the coherent reflection coefficient based on an experimental formula or scattering theory. In this article, we derive the coherent reflection coefficients for a fluid-fluid interface using perturbation theory, Kirchhoff approximation and small-slope approximation respectively. Based on each formula, we can calculate coherent reflection coefficients for a rough sea surface or sea bottom, and then compare them to the Rayleigh reflection coefficient to analyze the reflection loss for a random rough surface. In general, the coherent reflection coefficient based on small-slope approximation has a wide valid region. Comparing it with the coherent reflection coefficients derived from the Kirchhoff approximation and perturbation theory, we discuss a valid region of them.

• Nuclear Engineering and Technology
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• v.27 no.3
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• pp.374-384
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• 1995

The mathematical adjoint solution of the Analytic Function Expansion (AFEN) method is found by solving the transposed matrix equation of AFEN nodal equation with only minor modification to the forward solution code AFEN. The perturbation calculations are then performed to estimate the change of reactivity by using the mathematical adjoint The adjoint calculational scheme in this study does not require the knowledge of the physical adjoint or the eigenvalue of the forward equation. Using the adjoint solutions, the exact and first-order perturbation calculations are peformed for the well-known benchmark problems (i.e., IAEA-2D benchmark problem and EPRI-9R benchmark problem). The results show that the mathematical adjoint flux calculated in the code is the correct adjoint solution of the AFEN method.

• Journal of Navigation and Port Research
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• v.31 no.6
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• pp.523-530
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• 2007

Crane systems are very important in industrial fields to carry heavy objects such that many investigations about control of the systems are actively conducted for enhancing its control performance. This paper presents an adaptive control approach using the model matching for a complex 3-DOF nonlinear crane system. First, the system model is linearized through feedback linearization method and then PD control is applied in the approximated model. This linear model is considered as nominal to derive corrective control law for a perturbed crane model using Lyapunov theory. This corrective control is primitively aimed to compensate real-time control deviation due to partially known perturbation. We additionally study stability analysis of the crane control system using Lyapunov perturbation theory. Evaluation of our control approach is numerically carried out through computer simulation and its superiority is demonstrated comparing with the classical control.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.4
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• pp.381-387
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• 2016

In this paper, the methodology applied to the improvement of stress analyses is extended to free vibration and buckling analyses. The essence of the methodology is the Saint-Venant's principle that is applicable to beam and plate models. The principle allows one to dimensionally reduce three-dimensional elasticity problems. Thus the methodology can be employed to vibration and buckling as well as stress analysis. First, the principle is briefly revisited, and then the formations of classical beam theories are presented. To improve the predictions, the perturbed terms (unknowns) are introduced together with the warping functions that are calculated by stress equilibrium equations. The unknowns are then calculated by applying the equivalence of stress resultants (i.e., Saint-Venant's principle). As numerical examples, cantilever and simply supported beams are analytically solved. The results obtained are compared with those of the classical beam theories. It is shown that the methodology can be used to improve the predictions without introducing shear correction factors.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.10a
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• pp.51-51
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• 1998