• 대한기계학회논문집
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• 제17권12호
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• pp.3135-3147
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• 1993

A numerical method is presented which can solve the steady flow and heat transfer from a rotating and heated circular cylinder in a uniform flow for a range of Reynolds number form 5 to 100. The steady response of the flow and heat transfer is simulated for various spin parameter. The effects on the flow field and heat transfer characteristics known as lift, drag and heat transfer coefficient are analyzed and the streamlines, velocity vectors, vorticity, temperature distributions around it were scrutinized numerically. As spin parameter increases the region of separation vortex becomes smaller than upper one and the lower region will vanish. The lift force, a large part is due to the pressure force, increases as the Reynolds number and it increases linearly as spin parameter increases. The pressure coefficient changes rapidly with spin parameter on the lower surface of the cylinder and the vorticity is sensitive to the spin parameter near separation region. As spin parameter increases the maximum heat coefficient and the thin thermal layer on front region are moved to direction of rotation. However, with balance between the local increase and decrease, the overal heat transfer coefficient is almost unaffected by rotation.

• Geomechanics and Engineering
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• 제22권6호
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• pp.509-518
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• 2020

Multi-layered primary linings have been proved to be highly effective for tunneling in severe squeezing grounds. But there still has not existed well-established design method for it. Basically, there are two main critical problems in this method, including determinations of allowable deformation and distribution of support stiffness. In order to address such problems, an attempt to investigate the mechanical response of a circular tunnel with double primary linings is performed in this paper. Analytical solutions in closed form for stresses and displacements around tunnels are derived. In addition, the effectiveness and reliability of theoretical formulas provided are well validated by using the numerical method. Finally, based on the analytical solutions, a parametric investigation on the effects of allowable deformation and distribution of support stiffness on tunnel performance is conducted. Results show that the rock pressure and displacement are significantly affected by these two design parameters. It can be found that rock pressure decreases as either allowable deformation increases or stiffness of the first primary lining decreases, but rock displacement shows an opposite trend. This paper can provide a useful guidance for the design of multi-layered primary linings.

• 한국항공우주학회지
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• 제38권7호
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• pp.664-672
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• 2010

본 논문은 표적의 방향 정보만을 측정할 경우 $t_{go}$-다항식 유도법칙의 표적 가관측성 분석을 다룬다. 본 연구에서는 직접적인 방법으로 표적의 가관측성을 분석한다. 등속도 표적에 대해 가관측성을 갖기 위한 조건들이 시선각의 함수로만 주어지기 때문에, $t_{go}$-다항식 유도법칙을 적용 했을 때 시선각의 닫힌 해를 도출하고, 이 닫힌 해를 가관측성 조건에 직접 대입하므로, $t_{go}$-다항식 유도법칙의 표적 가관측성 특성을 해석한다. 표적의 가관측성은 유도법칙의 유도이득, 초기 유도 조건과 유도명령의 형상에 따라 달라진다. 최종적으로 시뮬레이션을 통해 $t_{go}$-다항식 유도법칙의 표적 가관측성을 검증한다.

• Structural Engineering and Mechanics
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• 제24권5호
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• pp.543-560
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• 2006

In this paper, first, the equations of motion for a rectangular isotropic plate have been derived. This derivation is based on the Von Karmann theory and the effects of shear deformation have been considered. Introducing an Airy stress function, the equations of motion have been transformed to a nonlinear coupled equation. Using Galerkin method, this equation has been separated into position and time functions. By means of the dimensional analysis, it is shown that the orders of magnitude for nonlinear terms are small with respect to linear terms. The Multiple Scales Method has been applied to the equation of motion in the forced vibration and free vibration cases and closed-form relations for the nonlinear natural frequencies, displacement and frequency response of the plate have been derived. The obtained results in comparison with numerical methods are in good agreements. Using the obtained relation, the effects of initial displacement, thickness and dimensions of the plate on the nonlinear natural frequencies and displacements have been investigated. These results are valid for a special range of the ratio of thickness to dimensions of the plate, which is a characteristic of the Multiple Scales Method. In the forced vibration case, the frequency response equation for the primary resonance condition is calculated and the effects of various parameters on the frequency response of system have been studied.

• 대한수학회보
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• 제48권4호
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• pp.791-812
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• 2011

This is a survey on American options. An American option allows its owner the privilege of early exercise, whereas a European option can be exercised only at expiration. Because of this early exercise privilege American option pricing involves an optimal stopping problem; the price of an American option is given as a free boundary value problem associated with a Black-Scholes type partial differential equation. Up until now there is no simple closed-form solution to the problem, but there have been a variety of approaches which contribute to the understanding of the properties of the price and the early exercise boundary. These approaches typically provide numerical or approximate analytic methods to find the price and the boundary. Topics included in this survey are early approaches(trees, finite difference schemes, and quasi-analytic methods), an analytic method of lines and randomization, a homotopy method, analytic approximation of early exercise boundaries, Monte Carlo methods, and relatively recent topics such as model uncertainty, backward stochastic differential equations, and real options. We also provide open problems whose answers are expected to contribute to American option pricing.

• 정보처리학회논문지C
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• 제13C권6호
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• pp.749-756
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• 2006

단일 안테나를 사용하는 사용자들 사이의 협력통신은 무선 매체의 전송 특성이나 경로 손실을 이용하여 안테나의 숫자를 증가하지 않고도 다중 안테나 시스템의 강력한 장점을 얻을 수 있다. 본 논문에서는 ML(Maximum Likelihood) 검파기의 구조를 간단하게 할 뿐만 아니라 에러확률을 최소화 할 수 있는 전송신호 증폭인자의 최적화를 통해 전파환경의 변화에 순응하는 협력통신의 방법에 대해 제안한다. 제안하는 협력통신에 대해 수학적인 해석에 의한 BER(Bit Error Ratio) 표현을 유도할 뿐만 아니라 모의 실험 결과를 통해 그 성능을 비교한다. 다양한 수학적 계산의 결과를 통해 주파수 비선택적 Rayleigh 페이딩 채널에 AWGN(Additive White Gaussian Noise)이 합쳐진 채널 환경에서 실험한 결과 협력통신이 비 협력통신 보다 현저히 성능이 좋음을 알 수 있다.

• 한국강구조학회 논문집
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• 제14권3호
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• pp.423-432
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• 2002

비대칭 박벽단면을 갖는 곡선보의 자유진동해석을 수행할 수 있는 엄밀해 및 유한요소 이론을 제시한다. 단순지지된 일축대칭 박벽단면을 갖는 곡선보의 면내 자유진동 모드에 대응하는 엄밀해를 산정하였으며, 곡섬보를 유한요소로 분할하여 요소의 변위장을 요소의 변위벡터의 대하여 축방향 신장조건에서는 3차 그리고 비신장조건에서는 5차의 Hermitian 다항식으로 나타내고, 이를 운동방정식에 대입함으로써 탄성강성행렬과 질량행렬을 유도한다. 또한 본 연구에서 얻어진 엄밀해와 곡선보요소를 이용한 유한요소 해석결과를 ABAQUS 쉘요소를 이용하여 얻어진 결과와 비교 검토함으로써 본 연구의 타당성을 입증한다. 특히 곡선보의 축방향 비신장조건과 두께-곡률효과가 동적거동에 미치는 영향을 조사한다.

• Steel and Composite Structures
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• 제13권1호
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• pp.91-107
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• 2012

In this research, mechanical buckling of hybrid functionally graded plates is considered using a new four variable refined plate theory. Unlike any other theory, the number of unknown functions involved is only four, as against five in case of other shear deformation theories. The theory presented is variationally consistent, does not require shear correction factor, and gives rise to transverse shear stress variation such that the transverse shear stresses vary parabolically across the thickness satisfying shear stress free surface conditions. The plate properties are assumed to be varied through the thickness following a simple power law distribution in terms of volume fraction of material constituents. Governing equations are derived from the principle of minimum total potential energy. The closed-form solution of a simply supported rectangular plate subjected to in-plane loading has been obtained by using the Navier method. The effectiveness of the theories is brought out through illustrative examples.

• Structural Engineering and Mechanics
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• 제20권5호
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• pp.589-608
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• 2005

There is a growing need for the development and implementation of new methods for the rapid and cost-effective rehabilitation of deteriorating steel structural components to offset the drawbacks related to welding and/or bolting in the field. Carbon fiber reinforced polymer (CFRP) composites provide a potential alternative as externally bonded patches for strengthening and repair of metallic structural members for building and bridge systems. This paper describes results of an investigation of tensile and fatigue response of steel/CFRP joints simulating scenarios of strengthening and crack-patching. It is shown that appropriately designed schemes, even when fabricated with levels of inaccuracy as could be expected in the field, can provide significant strain relief and load transfer capability. A simplified elasto-plastic closed form solution for stress analysis is presented, and validated experimentally. It is shown that the bond development length remains constant in the linear range, whereas it increases as the adhesive is deformed plastically. Fatigue resistance is shown to be at least comparable with the requirements for welded cover plates without attendant decreases in stiffness and strength.

• 천문학회지
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• 제53권5호
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• pp.99-102
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• 2020

I show that when the observables (π_E, t_E, θ_E, π_s, µ_s) are well measured up to a discrete degeneracy in the microlensing parallax vector π_E, the relative likelihood of the different solutions can be written in closed form P_i = KH_iB_i, where H_i is the number of stars (potential lenses) having the mass and kinematics of the inferred parameters of solution i and B_i is an additional factor that is formally derived from the Jacobian of the transformation from Galactic to microlensing parameters. Here t_E is the Einstein timescale, θ_E is the angular Einstein radius, and (π_s, µ_s) are the (parallax, proper motion) of the microlensed source. The Jacobian term B_i constitutes an explicit evaluation of the "Rich Argument", i.e., that there is an extra geometric factor disfavoring large-parallax solutions in addition to the reduced frequency of lenses given by H_i. I also discuss how this analytic expression degrades in the presence of finite errors in the measured observables.