• 대한조선학회논문집
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• 제52권1호
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• pp.25-33
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• 2015

Following the previous works on the natural frequency of heaving circular cylinder, i.e. Lee and Lee (2013) and Kim and Lee (2013), an investigation of the same spirit on the 2-dimensional cylinder of Lewis form has been conducted. As before, the natural frequency is defined as that corresponding to the local maximum of the MCFR (Modulus of Complex Frequency Response), which is given by the equation of motion in the frequency domain analysis. Hydrodynamic coefficients were found by using the Ursell-Tasai method, and numerical results for them were obtained up to much higher frequencies than before, for which the method was known as numerically unstable in the past. For a wide range of H, the beam-draft ratio, and ${\sigma}$, the sectional area coefficient, including their practical ranges for a ship, results for the natural frequency were computed and presented in this work. Two approximate values for the natural frequency, one proposed by Lee (2008) and another one by the damped harmonic oscillator, were also compared with the current results, and for most cases it was observed that the current result is between the two values. Our numerical results showed that the values of the local maximum of MCFR as well as the natural frequencye increase as ${\sigma}$ increases while H decreases. At present, extension of the present finding to the 3-dimensional ship via the approximate theory like the strip method looks promising.

• 한국강구조학회 논문집
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• 제29권3호
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• pp.205-215
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• 2017

본 연구에서는 10MW급 풍력하중을 받는 멀티기둥 타워시스템에 원형강관 부재의 구조안전성 및 경제성을 함께 검토하는 방식으로 부재 유용도에 근거한 개념설계의 예를 보였다. 단일 실린더형 타워를 대체할 수 있는 멀티기둥타워 구조의 구성에 관한 기본적인 가정을 정립하였고, 그에 따라 제안된 구조물을 모델링하고 해석하여 부재력을 확인하였다. 산정된 부재강도와 작용하중을 근간으로 제안된 멀티기둥타워의 각 부재별로 축력, 전단, 휨, 비틂에 대한 유용도가 산정되었고, 풍력타워로서의 적합성이 평가하였다. 멀티기둥 풍력타워의 개념설계에 채택될 수 있는 수준의 유용도 범위에서 강관 치수, 세장비 및 수평재 단수 등의 설계 매개변수를 제안하였다.

• 한국지능시스템학회논문지
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• 제20권6호
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• pp.748-754
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• 2010

깨어진 항아리 조각들을 가상 공간에서 조립하기 위하여 조각 표면을 이용한 빠른 3차원 회전축 추정 방법을 제안한다. 물체의 원형성과 표면의 국지적 평면성을 이용하여 대칭축을 찾는 방법을 사용한다. 항아리 조각 같은 회전축 대칭 물체는 반지름이 다른 여러 원통의 중첩으로 생각될 수 있다. 각 원통의 원형성을 회전축 계산에 이용한다. 먼저, 표면 위 임의의한 점을 지정하고 그 점을 통과하는 여러 개의 원통상의 궤도를 각각의 곡률의 변화를 측정 검사하여 조사한다. 올바른 원의 궤도는 곡률의 변화가 없을 것이므로 가장 작은 곡률의 변화가 원의 궤도로 선택된다. 또한 원의 중심점으로 축이 통과하는 경로가 되므로 원의 중심점이 축의 위치가 된다. 표면의 국지적 평면성과 프로파일 곡선 근사를 통한 축 위치 추정 방법 또한 연구 되었다. 제안된 방법은 기존의 방법에 비해 계산 속도가 향상되었고 조각의 부위에 영향을 받지 않는 강건성을 가짐을 실험적으로 입증하였다.

• 대한토목학회논문집
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• 제29권4D호
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• pp.555-561
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• 2009

굴착된 터널 형상 재현에서 단면의 종단 데이터는 터널의 유지를 위해서는 아주 중요하다. 터널이 완성되기전에 설계된 모델을 고려한 완성된 터널의 변형이 고려되어져야 한다. 그리고 변형은 터널 단면 전체를 따라 연속적으로 나타날 수 있다. 본 연구에서는 먼저 수학적 분석으로 접근하였고, 그것을 관측된 터널단면 데이터에 실험 하였다. 그 다음 선추적 방법, 유전자 알고리즘, 패턴 추적 방법 등으로 3D 터널 형상 재현을 비교하였다. 수학적 방법론은 철도 터널과 같은 간단한 원통형은 쉽게 해결이 되었으나, 도로터널과 같은 더욱 복잡한 모델(복심 곡선형과 비원통형)은 구속된 상태하에서 소프트 컴퓨팅 툴을 가지고 해결할 수 있었다. 유전자 알고리즘과 직접탐색법은 많은 계산 시간이 걸렸으나 복잡한 상태하에서 더욱 유연함을 보였으며, 선추적 방법은 초기값들이 제한된 범위 하에서 가장 빠르게 계산되어졌다.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2003년도 춘계학술대회 논문집
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• pp.214-214
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• 2003

During a model test of Hutton TLP, a "ringing" response was first observed about 20 years ago. This phenomenon is a resonant build up over the time of wave period and this burst-like motion can cause the extreme load on the TLP tether. It is often detected in the large and steep irregular waves but the generation mechanism leading to the "ringing" is not yet well understood. According to the research since then, the higher order harmonic components may account for the "ringing" on the floating offshore structures. The main purpose of the present research is, thus, to measure the higher harmonic forces exerted on a vertical truncated circular column and to compare them with available data. A vertical truncated cylinder with a diameter of 3.5inch and a draft of 10.5inch is used as a test structure, which is a scaled model of ISSC TLP column. The cylinder is installed at a distance of 45ft from the wave maker in order to avoid parasitic waves created in the wave flap. Attached to the upper part of the cylinder are two force gages to measure the horizontal (surge) and vertical (heave) forces on the cylinder. The incoming waves are Stokes waves with a slope ranging from 0.06 to 0.24. The forces and waves are measured for 60 seconds with a sampling rate of 50 Hz. Among the recorded data, the first 10 waves are excluded because of transient behavior of the waves and the next The horizontal and vertical forces are analyzed up to 5th order harmonics. The horizontal forces are then compared to the values from the theoretical model called "FNV model". In addition, force transfer functions are also investigated. Major findings in this research are below. 1) The first order forces measured are slightly larger than the theoretical values of "FNV model" 2) The "FNV model" considerably overpredicts the second order forces. 3) The larger the amplitude and more extreme the wave slope, the smaller the predictions are compared to the experimental. 4) The higher harmonic forces are significantly smaller than the first harmonic force for all wave parameters. 5) The normalized forces vs. waves slopes are almost constant in the lower harmonics but vary a lot in the higher harmonics. 6) The trend of forces is more nonlinear in the horizontal forces than in the vertical forces as the wave slope increases. 7) The part of the results above is also observed by other researchers and confirmed again through the present work.

• Journal of Mechanical Science and Technology
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• 제17권3호
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• pp.409-421
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• 2003

The hierarchical recursive local-correlation PIV algorithm with CBC (correlation based correction) method was employed to increase the spatial resolution of PIV results and to reduce error vectors. The performance of this new PIV algorithm was tested using synthetic images, PIV standard images of Visualization Society of Japan, real flows including ventilation flow inside a vehicle passenger compartment and wake behind a circular cylinder with riblet surface. As a result, most spurious vectors were suppressed by employing the CBC method, the hierarchical recursive correlation algorithm improved the sub-pixel accuracy of PIV results by decreasing the interrogation window size and Increased spatial resolution significantly. However, with recursively decreasing of interrogation window size, the SNR (signal-to-noise ratio) in the correlation plane was decreased and number of spurious vectors was increased. Therefore, compromised determination of optimal interrogation window size is required for given flow images, the performance of recursive algorithm is also discussed from a viewpoint of recovery ratio and error ratio in the paper.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.2929-2934
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• 2007

Prediction methods for cavitation noise are presented. At first, direct numerical simulation of cavitating flow noise has been performed, and acoustic analogy equation based on the cavitation noise modeling is derived. For the direct numerical simulation, a density based homogenous equilibrium model is employed to simulate cavitating two-phase flow and the governing equations are solved with high-order numerical schemes to resolve cavitation noise. The compressible Navier-Stokes equations for mixture fluids are discretized with a sixth-order central compact scheme, and the steep gradient of flow variables and supersonic regions are treated with the selective spatial filtering technique. The direct simulation of cavitating flow noise is performed for a 2D circular cylinder at cavitation number 0.7 and 1. The far-field noise is also predicted with the derived analogy equation. Noise spectrum predicted with the equation is well compared with the result of direct numerical simulation and also agree well with the theory.

• Structural Engineering and Mechanics
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• 제6권7호
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• pp.817-826
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• 1998

A finite element model based on the incremental endochronic theory for flexible pavement materials was developed in this study. Three grid systems with eight-node cubic isoparametric elements, and different loading steps were used to perform the calculations for a specimen of circular cylinder. The uniaxial stress experimental results on an asphalt mixture at $60^{\circ}C$ in SHRP conducted by University of California at Berkeley were used to check the ability of the derived numerical model. Then, the numerical results showed isotropic response and deviatoric response on the specimen in a three dimensional manner, which provided a better understanding for a deformed flexible material under the specified loading conditions.

• 한국전산유체공학회지
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• 제12권1호
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• pp.1-8
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• 2007

A code is developed using the hybrid Cartesian/immersed boundary method and it is applied to simulate flows around a three-dimensional deforming body. A new criterion is suggested to distribute the immersed boundary nodes based on edges crossing a body boundary. Velocities are reconstructed at the immersed boundary nodes using the interpolation along a local normal line to the boundary. Reconstruction of the pressure at the immersed boundary node is avoided using the hybrid staggered/non-staggered grid method. The developed code is validated through comparisons with other experimental and numerical results for the velocity profiles around a circular cylinder under the forced in-line oscillation and the pressure coefficient distribution on a sphere. The code is applied to simulate the flow fields around a plate whose tail is periodically flapping under a translation. The effects of the velocity and acceleration due to the deformation on the periodic shedding of pairs of tip vortices are investigated.

• International Journal of Naval Architecture and Ocean Engineering
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• 제9권4호
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• pp.429-438
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• 2017

Cranke-Nicolson scheme in native OpenFOAM source libraries was not able to provide 2nd order temporal accuracy of velocity and pressure since the volume flux of convective nonlinear terms was 1st accurate in time. In the present study the simplest way of getting the volume flux with 2nd order accuracy was proposed by using old fluxes. A possible numerical instability originated from an explicit estimation of volume fluxes could be handled by introducing a weighting factor which was determined by observing the ratio of the finally corrected volume flux to the intermediate volume flux at the previous step. The new calculation of volume fluxes was able to provide temporally accurate velocity and pressure with 2nd order. The improvement of temporal accuracy was validated by performing numerical simulations of 2D Taylor-Green vortex of which an exact solution was known and 2D vortex shedding from a circular cylinder.