• Title/Summary/Keyword: 항력

Dynamic Position Control Method for the Buffer Unit of a Deepsea Mining System (해석심해자원개발용 버퍼의 동적위치제어기법)

• Kim, Ki-Hun;Choi, Hang-S.;Hong, Sup
• Journal of the Society of Naval Architects of Korea
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• v.39 no.3
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• pp.57-63
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• 2002
• This paper describes a control algorithm for the buffer of a deep-sea mining system, in which the buffer is connected to a long slender pipe and then to a surface ship on one end, and to a collector on sea floor through a flexible hose on the other end. A mathematical modeling is established for designing the controller for buffer thrusters, in which the dynamic response of the long pipe is taken into account based on the mode superposition method. The fluid loading acting on the pipe is estimated by using Morison's formula. For simplicity, the surface ship is assumed to be kept stationary, the reaction from the flexible hose is ignored and only the lateral motions are considered. In order to guide the buffer to react only to the low-frequency motion of the surface vessel, the FIR digital filter is introduced to a PID-based controller It can be shown numerically that the high frequency component of the ship's motion can be effectively filtered out by using the FIR low pass filter.

Dynamic Modeling and Simulation of a Towing Rope using Multiple Finite Element Method (다물체 요소이론을 이용한 예인줄 동역학의 모델링 및 시뮬레이션)

• Yoon, Hyeon-Kyu;Lee, Hong-Seok;Park, Jong-Kyu;Kim, Yeon-Gyu
• Journal of Navigation and Port Research
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• v.36 no.5
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• pp.339-347
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• 2012
• After towing rope connecting a barge to a tug was subdivided into multiple finite elements, then those dynamic models was established using Newton's second law and considering the external force and moment such as tension, drag, Coriolis force, gravity, buoyancy, and impact due to free surface acting on each element. While the previous research on the model of towing rope considered only translation, five-degree-of-freedom equations of motion except roll based on the body-fixed frame were established in this paper. All elements are connected by a spring and a damper, and the stiffness of the spring was set as the equivalent value of the real rope. In order to confirm the established multiple finite element model, various scenarios such as freely falling of towing rope in the air and above the free surface, accelerating of a tug which tows a barge connected by towing rope, and sinusoidal moving of a tug were set up and simulated. As the results, the trajectories of the tug, the barge, and the towing rope showed good tendencies to the ones of real expected situations.

Three-Phase Eulerian Computational Fluid Dynamics (CFD) of Air-Water-Oil Separator with Coalescer (유적 합체기가 포함된 공기-물-기름 분리 공정에 대한 3상 Eulerian 전산유체역학)

• Lim, Young-Il;Le, Thuy T.;Park, Chi-Kyun;Lee, Byung-Don;Kim, Byung-Gook;Lim, Dong-Ha
• Korean Chemical Engineering Research
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• v.55 no.2
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• pp.201-213
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• 2017
• Water is removed from crude oil containing water by using oil separator. This study aims to develop a three-dimensional (3D) Eulerian computational fluid dynamics (CFD) model to predict the separation efficiency of air-water-oil separator. In the incompressible, isothermal and unsteady-state CFD model, air is defined as continuous phase, and water and oil are given as dispersed phase. The momentum equation includes the drag force, lift force and resistance force of porous media. The standard k-${\varepsilon}$ model is used for turbulence flow. The exit pressures of water and oil play an important role in determining the liquid level of the oil separator. The exit pressures were identified to be 6.3 kPa and 5.1 kPa for water and oil, respectively, to keep a liquid level of 25 cm at a normal operating condition. The time evolution of volume fractions of air, water and oil was investigated. The settling velocities of water and oil along the longitudinal separator distance were analyzed, when the oil separator reached a steady-state. The oil separation efficiency obtained from the CFD model was 99.85%, which agreed well with experimental data. The relatively simple CFD model can be used for the modification of oil separator structure and finding optimal operating conditions.

Experimental Investigation of Aerodynamic Force Coefficients and Flutter Derivatives of Bridge Girder Sections (교량단면의 공기력계수 및 플러터계수에 관한 실험적 연구)

• Cho, Jae-Young;Lee, Hak-Eun;Kim, Young-Min
• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.5A
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• pp.887-899
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• 2006
• The aim of this study is to investigate a correlation between fundamental data on aerodynamic characteristics of bridge girder cross-sections, such as aerodynamic force coefficients and flutter derivatives, and their aerodynamic behaviour. The section model tests were carried out in three stages. In the first stage, seven deck configurations were studied, namely; Six 2-edge girders and one box girder. In this stage, changes in aerodynamic force coefficients due to geometrical shape of girders, incidence angle of flow, wind directions and turbulence intensities were studied by static section model tests. In the second stage, the dynamic section model tests were carried out to investigate the relativity of static coefficients to dynamic responses. And finally, the two-dimensional (lift-torsion) aerodynamic derivatives of three bridge deck configurations were investigated by dynamic section model tests. The aerodynamic derivatives can be best described as a representation of the aerodynamic damping and the aerodynamic stiffness provided by the wind for a given deck geometry. The method employed here to extract these unsteady aerodynamic properties is known as the initial displacement technique. It involves the measurement of the decay in amplitude with time of an initial displacement of the deck in heave and torsion, for various wind speeds, in smooth flow. It is suggested that the proposed aerodynamic force coefficients and flutter derivatives of bridge girder sections will be potentially useful for the aeroelastic analysis and buffeting analysis.

Grid Tests for Large Eddy Simulation of Transitional Flows around Turbulence Stimulators (난류 촉진기 주위 천이 유동의 대형 와 모사를 위한 격자 테스트)

• Lee, Sang Bong;Park, Dong Woo;Paik, Kwang-Jun
• Journal of the Korean Society of Marine Environment & Safety
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• v.23 no.1
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• pp.112-121
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• 2017
• Large eddy simulations of transitional flows around a stud installed on a flat plate have been performed to investigate an influence of grid resolution on turbulence stimulation by the stud. Because streamwise vortical structures generated by the stud played an important role in turbulence stimulation of boundary layer, streamwise vorticity was compared in the wake region behind the stud when the number of grids increased or decreased by a ratio of ${\sqrt{2}}$ in streamwise, wall-normal and spanwise directions respectively. The streamwise vorticity was shown to be mainly affected by spanwise grid resolution (${\Delta}z^+$) rather than streamwise and wall-normal grid resolution. In a viewpoint of numerical efficiency as well as physical resolution, ${\Delta}x^+{_{min}}=7.6$, ${\Delta}x^+{_{max}}=41$, ${\Delta}y^+{_{wall}}=0.25$ and ${\Delta}z^+=7.6$ was found to be desirable. Once a grid resolution was determined in each direction, a grid verification was carried out by increasing or decreasing the grid resolution y a ratio of ${\sqrt{2}}$ in all directions. The grid uncertainties of pressure and drag coefficients were 21.6 % and 2.8 % while the corrected uncertainties were 2 % and 0.3 %, respectively.

Shape and Spacing Effects on Curvy Twin Sail for Autonomous Sailing Drone (무인 해상 드론용 트윈 세일의 형태와 간격에 관한 연구)

• Pham, Minh-Ngoc;Kim, Bu-Gi;Yang, Changjo
• Journal of the Korean Society of Marine Environment & Safety
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• v.26 no.7
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• pp.931-941
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• 2020
• There is a growing interest this paper for ocean sensing where autonomous vehicles can play an essential role in assisting engineers, researchers, and scientists with environmental monitoring and collecting oceanographic data. This study was conducted to develop a rigid sail for the autonomous sailing drone. Our study aims to numerically analyze the aerodynamic characteristics of curvy twin sail and compare it with wing sail. Because racing regulations limit the sail shape, only the two-dimensional geometry (2D) was open for an optimization. Therefore, the first objective was to identify the aerodynamic performance of such curvy twin sails. The secondary objective was to estimate the effect of the sail's spacing and shapes. A viscous Navier-Stokes flow solver was used for the numerical aerodynamic analysis. The 2D aerodynamic investigation is a preliminary evaluation. The results indicated that the curvy twin sail designs have improved lift, drag, and driving force coefficient compared to the wing sails. The spacing between the port and starboard sails of curvy twin sail was an important parameter. The spacing is 0.035 L, 0.07 L, and 0.14 L shows the lift coefficient reduction because of dramatically stall effect, while flow separation is improved with spacing is 0.21 L, 0.28 L, and 0.35 L. Significantly, the spacing 0.28 L shows the maximum high pressure at the lower area and the small low pressure area at leading edges. Therefore, the highest lift was generated.

A Propeller Design Method with a New Blade Section : Applied to Container Ships (새로운 날개단면을 이용한 프로펠러 설계법 - 콘테이너선에 응용 -)

• J.T. Lee;M.C. Kim;J.W. Ahn;S.H. Van;H.C. Kim
• Journal of the Society of Naval Architects of Korea
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• v.28 no.2
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• pp.40-51
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• 1991

Development of Generating Technique for Triangular Mesh by using Distinct Element Method (개별요소법을 이용한 삼각망 생성기법 개발)

• Kim, Nam-Hyeong;Yun, Hyeon-Cheol;Hur, Young-Teck
• Journal of Navigation and Port Research
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• v.34 no.5
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• pp.367-373
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• 2010
• When the numerical analysis is carried out, it is necessary to set proper elements as a feature of analysis domains for more accurate simulations. In this study, Distinct Element Method(DEM) is applied, only considering repulsive force and tensile force except for frictional force and resisting force of particle. When the filled particles with initial Quad-tree type is relocated by DEM, a blank space existing among the particles can be minimized because the shape of particle is circular. Finally, it is the effective feature that the centroidal disposion of the particles is similar to an equilateral triangle. Triangular mesh are formed by using the Delaunay triangular technique on these relocated particles, the quality of triangular mesh is more improved by carrying out Laplace interpolations. The compared result of Aspect Ratio before and after the Laplace interpolation is shown that although the quality of triangular mesh made by DEM is good, the later triangular mesh are higher quality than the formers. In this study, although the developed technique takes a longer calculational time than the previous technique to generate triangular mesh, it is considered that the applicable possibility is very high in the generation of finite element mesh about wave analysis and various numerical simulation to need a complex or reappearance of exact topography.

Flow and Heat Transfer Characteristics of Heat Exchanger Tube Bank with the Sinusoidal Inlet Velocity (정현파 입구 속도 변동에 따른 열교환기 관군의 유동 및 열전달 특성)

• Ha, Ji Soo
• Journal of the Korean Institute of Gas
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• v.25 no.1
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• pp.14-19
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• 2021
• The change of the vorticity and the temperature distribution in heat exchanger tube bank were analyzed for the flows with the constant inlet velocity and the sinusoidal inlet velocity. The flow frequency characteristics were examined by analyzing power spectral density of lift and drag at a typical circular tube in the tube bank. Karman vortex street could be seen at the upstream region of tube bank for the case of constant inlet velocity. It could be seen that the Karman vortex street was affected by the change of inlet velocity near the circular tubes for the case with the sinusoidal inlet velocity. It was observed that the unsteady temperature distributions for both inlet velocity conditions had almost the same motion as the flow vorticity behavior. The flow frequency for the case with the constant inlet velocity is 37.25Hz, and that with the sinusoidal inlet velocity, the flow frequency is 18.63Hz, which is equal to the sinusoidal inlet velocity. The mean surface Nusselt number(Nu) for overall heat exchanger tube bank was 1051 for the case with the constant inlet velocity and 1117 for the case with the sinusoidal inlet velocity. From the result of heat transfer analysis, it could be seen that Nu with the sinusoidal inlet velocity showed 6.3% increase than that with the constant inlet velocity.

Wind Tunnel Test Study on the Characteristics of Wind-Induced Responses of Tall Buildings with Openings (중공부(中空部)를 가진 고층건축물(高層建築物)의 풍응답(風應答) 특성(特性)에 관한 풍동실험(風洞實驗) 연구(硏究))

• Kim, Dong Woo;Kil, Yong Sik;Ha, Young Cheol
• Journal of Korean Society of Steel Construction
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• v.17 no.4 s.77
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• pp.499-509
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• 2005
• The excessive wind-induced motion of tall buildings most frequently result from vortex-shedding-induced across-wind oscillations. This form of excitation is most pronounced for relatively flexible, lightweight, and lightly damped high-rise buildings with constant cross-sections. This paper discusses the aerodynamic means ofmitigating the across-wind vortex shedding induced in such situations. Openings are added in both the drag and lift directions in the buildings to provide pressure equalization. Theytend to reduce the effectiveness of across-wind forces by reducing their magnitudes and disrupting their spatial correlation. The effects of buildings with several geometries of openings on aerodynamic excitations and displacement responses have been studied for high-rise buildings with square cross-sections and an aspect ratio of 8:1 in a wind tunnel. High-frequency force balance testshave been carried out at the Kumoh National University of Technology using rigid models with 24 kinds of opening shapes. The measured model's aerodynamic excitations and displacement were compared withthose of a square cylinder with no openings to estimate the effectiveness of openings for wind-induced oscillations. From these results, theopening shape, size, and location of buildings to reduce wind-induced vortex shedding and responses were pointed out.