• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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• pp.209-212
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• 2006

Insect and birds in nature flap their wings to generate fluid dynamic forces that are required for the locomotion. Most of the previous published papers discussed mainly on the effect of flapping parameters such as flapping frequency and amplitude on the thrust at a fixed Reynolds number. However, it is not much known on the values of the flapping parameters that the flapping wing requires to generate the thrust at the low Reynolds number flow. In this paper, the onset of the thrust generation is investigated using the lattice Boltzmann method. The wake patterns and velocity profiles behind a flat plate in heaving oscillation are investigated for the heaving amplitude of 0.5C. The time-averaged thrust coefficient value is investigated by changing the reduced frequency from 0.125 to 3.0 for three values of heaving amplitude (h/C=0.25, 0.325, 0.50). It is also found that the critical Strouhal number over which the flat plate starts to produce the thrust is around 0.1 and the thrust is an exponential function of the Strouhal number.

• Wind and Structures
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• 제12권6호
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• pp.541-551
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• 2009

This paper explains how to correctly measure the drag coefficient of a circular cylinder in wind tunnels with large blockage ratios and for the sub-critical to the super-critical flow regimes. When dealing with large blockage ratios, the drag has to be corrected for wall constraints. Different formulations for correcting blockage effect are compared for each flow regime based on drag measurements of smooth circular cylinders performed in a wind tunnel for three different blockage ratios. None of the correction model known in the literature is valid for all the flow regimes. To optimize the correction and reduce the scatter of the results, different correction models should be combined depending on the flow regime. In the sub-critical regime, the best results are obtained using Allen and Vincenti's formula or Maskell's theory with ${\varepsilon}$=0.96. In the super-critical regime, one should prefer using Glauert's formula with G=0.6 or the model of Modi and El-Sherbiny. The change in the formulations appears at the flow transition with a variation of the wake pattern when passing from sub-critical to super-critical flow regimes. This parameter being not considered in the known blockage corrections, these theories are not valid for all the flow regimes.

• 한국전산구조공학회논문집
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• 제15권1호
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• pp.1-8
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• 2002

물체가 수중에 입수할 때, 원래의 운동에너지는 물체와 그 주위의 물에 부가질량 형태로 분배된다. 이러한 에너지 혹은 운동량의 전달에 기인하여 물체는 유체동력학적 충격력과 가속도를 받는다. 이러한 충격거동은 수중운동체의 공중 발사에 중요한 고려 요인이 된다. 본 논문에서는 구명정 모델을 바탕으로 원통형 물체의 입수에 대한 충격거동을 해석하는 근사기법을 제안하였다. 충격력은 von Karman의 운동량 이론으로 계산하고, 운동, 특히 가속도는 유체동력학적 힘의 평형에 의하여 유도된 운동방정식의 수치 적분으로 계산하였다. 제안된 방법은 입수충격을 받는 물체의 초기설계나 운동 해석을 위한 단순하면서도 효과적인 방안이 될 수 있을 것으로 기대된다.

• 한국소음진동공학회논문집
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• 제26권3호
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• pp.281-289
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• 2016

In this paper, a new dynamic model for modal analysis of a rotating cantilever beam with a tip-mass is developed. The nonlinear strain such as von Karman type and the corresponding linearized stress are used to consider the geometric nonlinearity, and Euler-Bernoulli beam theory is applied in the present model. The nonlinear equations of motion and the associated boundary conditions which include the inertia of the tip-mass are derived through Hamilton's principle. In order to investigate modal characteristics of the present model, the linearized equations of motion in the neighborhood of the equilibrium position are obtained by using perturbation technique to the nonlinear equations. Since the effect of the tip-mass is considered to the boundary condition of the flexible beam, weak forms are used to discretize the linearized equations. Compared with equations related to stiffening effect due to centrifugal force of the present and the previous model, the present model predicts the dynamic characteristic more precisely than the another model. As a result, the difference of natural frequencies loci between two models become larger as the rotating speed increases. In addition, we observed that the mode veering phenomenon occurs at the certain rotating speed.

• 한국구조물진단유지관리공학회 논문집
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• 제6권3호
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• pp.139-149
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• 2002

Explicit 직접적분법을 사용하여 충격하중을 받는 박판의 후좌굴거동을 해석할 수 있는 알고리즘을 제안하였다. von Karman의 대변위 판 이론과 Marquerre의 쉘 이론을 이용하여 유도한 직사각형 평판 유한요소는 박판의 초기처짐과 기하학적 비선형 거동을 고려할 수 있다. 중앙차분법을 바탕으로 해석 알고리즘을 개발하였고 이를 프로그램화 시켜, 하중형상과 재하시간이 다른 충격하중에 대하여 박판의 동적 좌굴거동을 해석 하였다. 수치해석 예제를 통하여 Explicit 직접적분법의 특성을 평가하였다.

• 한국항공우주학회지
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• 제30권1호
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• pp.36-43
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• 2002

압전적층판의 비선형 열압전탄성 거동에서의 스냅-스루 현상을 뉴튼-랩슨기법에 호길이법을 적용하여 수치적으로 규명하였다. 층별변위장이론과 von Karman 변형률-변위 관계식을 적용하여 열압전탄성 복합적층 평판에 대한 비선형 유한요소정식화를 수행하였다. 다양한 압전 작동모드에 따라 대칭 및 편심된 구조모델에 대하여 정적 및 동적 관점에서 비선형 열압전탄성 거동과 진동특성을 연구하였다. 본 연구에서는 압전 작동기를 사용하여 유연한 열적 구조물들의 성능을 향상시킬 수 있는 가능성과 새로운 현상학적인 발견인 열압전탄성 스냅핑 거동이 좌굴된 압전탄성 복합적층 평판에서 과도한 압전작동력이 작용하는 경우에 발생할 수 있음을 제시하였다.

• Wind and Structures
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• 제9권5호
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• pp.387-396
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• 2006

This paper presents the wind characteristics of Typhoon Dujuan as measured at a 50 m guyed mast in Hong Kong. The basic wind speed, wind direction and turbulent intensity are studied at two measurement levels of the structure. The power spectral density of the typhoon is compared with the von Karman prediction, and the coherence between wind speeds at the two measurement levels is found to This paper presents the wind characteristics of Typhoon Dujuan as measured at a 50 m guyed mast in Hong Kong. The basic wind speed, wind direction and turbulent intensity are studied at two measurement levels of the structure. The power spectral density of the typhoon is compared with the von Karman prediction, and the coherence between wind speeds at the two measurement levels is found to compare with Davenport's prediction. The effect of typhoon Dujuan on the response of the structure will be discussed in a companion paper (Law, et al. 2006).with Davenport's prediction. The effect of typhoon Dujuan on the response of the structure will be discussed in a companion paper (Law, et al. 2006).

• Smart Structures and Systems
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• 제18권1호
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• pp.155-181
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• 2016

In the present paper we discuss the stability and the post-buckling behaviour of thin piezoelastic plates. The first part of the paper is concerned with the modelling of such plates. We discuss the constitutive modelling, starting with the three-dimensional constitutive relations within Voigt's linearized theory of piezoelasticity. Assuming a plane state of stress and a linear distribution of the strains with respect to the thickness of the thin plate, two-dimensional constitutive relations are obtained. The specific form of the linear thickness distribution of the strain is first derived within a fully geometrically nonlinear formulation, for which a Finite Element implementation is introduced. Then, a simplified theory based on the von Karman and Tsien kinematic assumption and the Berger approximation is introduced for simply supported plates with polygonal planform. The governing equations of this theory are solved using a Galerkin procedure and cast into a non-dimensional formulation. In the second part of the paper we discuss the stability and the post-buckling behaviour for single term and multi term solutions of the non-dimensional equations. Finally, numerical results are presented using the Finite Element implementation for the fully geometrically nonlinear theory. The results from the simplified von Karman and Tsien theory are then verified by a comparison with the numerical solutions.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2001년도 춘계학술대회 논문집
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• pp.40-45
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• 2001

Recently, the use of numerical simulation has been increased rapidly because of the development of high performance computer systems. The present study is aimed to investigate flow characteristics of a two-dimensional sharp plane. Unsteady calculation by FDM(Finite Difference Method) based upon SOLA scheme which was performed at $Re=2{\times}10^4$in viscous incompressible flow within a finite domain on the irregular grid formation. Total numbers of irregular grids are $8{\times}10^4$. The minimum grid size is 1/100 of the plane length L which is the representative length. The inclined angles of every objects are $15^{\circ}, \;30^{\circ}\;and\; 45^{\circ}.$ And, the edge angle of the plane is $30^{\circ}.$ This study discussed the flow characteristics in term of the turbulent intensity, vorticity and frequency analysis. Developed flows show that the periodic Karman vortices occur at the back of the plane.

• 설비공학논문집
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• 제12권8호
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• pp.771-781
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• 2000

The Karman boundary-layer, has been numerically investigated for the disturbance wave number, wave velocity, azimuth angle and radius (Reynolds number, Re). The disturbed flow over rotating disk can lead to transition at a much lower Re than that of the well-known Type 1 mode of instability. This early transition is due to the excitation of the Type II mode. Presented are the neutral stability results concerning these modes by solving new formulated vorticity equations with consideration of whole convective terms. When the present numerical results are compared with the previously known results, the value of critical Re corresponding to Type I is moved from Rec,! =285.3 to 270.2 and the value corresponding to Type II is from $Re_{c,2}$=69.4 to 36.9, respectively. Also, the corresponding wave number is moved from $k_1$ =0.378 to $k_1$ =0.389 for Type I; from $k_2$ =0.279 to $k_2$=0.385 for Type II. For Type II, the upper limit of wave number and azimuth angle is $k_U$=0.5872,$varepsilon_U=-18^{\circ}$ , while its lower limit is$k_L$ =0.05, $varepsilon_L=-27^{\circ}$ This implies that the disturbances will be relatively fast amplified at small Re and within narrow bands of wave number compared with the previous results.