• 대한조선학회논문집
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• 제45권5호
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• pp.487-494
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• 2008

An experimental assessment has been made of the drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins (2003). The drag reduction efficiency of the blades was reported to reach as much as 30%. The assessment of the drag reducing efficiency is mainly restricted to the downstream region of the blades. Indeed, sufficient care has not been taken to such adverse effects as the increase in the wetted surface area and the flow disturbances due to the presence of the blades. In the present study, a series of drag force measurements in towing tank and circulating water channel has been performed toward the assessments of the total drag reduction efficiency of the outer-layer vertical blades.

• 대한조선학회논문집
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• 제41권4호
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• pp.30-37
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• 2004

Flow characteristics of the inflow ahead of a rotating propeller attached to a container ship model were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases. The mean velocity fields show the acceleration of inflow due to the rotating propeller and the velocity deficit in the near-wake region. The axial velocity distribution of inflow in the upper plane of propeller is quite different from that in the lower plane due to the thick hull boundary layer. The propeller inflow also shows asymmetric axial velocity distribution in the port and starboard side. As the inflow moves toward the propeller, the effect of phase angle variation of propeller blade on the inflow becomes dominant. In the upper plane above the propeller axis the inflow has very low axial velocity and large turbulent kinetic energy, compared with the lower plane. The boundary layer developed along the bottom surface of stern hull forms a strong shear layer affecting vortex structure of the propeller near-wake.

• 대한기계학회논문집B
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• 제24권1호
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• pp.21-28
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• 2000

Although commercial PIV systems have been widely used for the non-intrusive velocity field measurement of fluid flows, they are still under development and have considerable room for improvement. In this study, a single-frame double-exposure PIV system using a high-resolution CCD camera was developed. A pulsed Nd:Yag laser and high-resolution CCD camera were synchronized by a home-made control circuit. In order to resolve the directional ambiguity problem encountered in the single-frame PIV technique, the second particle image was genuinely shifted in the CCD sensor array during the time interval dt. The velocity vector field was determined by calculating the displacement vector at each interrogation window using cross-correlation with 50% overlapping. In order to check the effect of spatial resolution of CCD camera on the accuracy of PIV velocity field measurement, the developed PIV system with three different resolution modes of the CCD camera (512 ${\times}$ 512, lK ${\times}$ IK, 2K ${\times}$ 2K) was applied to a turbulent flow which simulate the Zn plating process of a steel strip. The experimental model consists of a snout and a moving belt. Aluminum flakes about $1{\mu}m$ diameter were used as scattering particles for the liquid flow in the zinc pot and the gas flow above the zinc surface was seeded with atomized olive oil with an average diameter of 1-$3{\mu}m$. Velocity field measurements were carried out at the strip speed $V_s$=1.0 m/s. The 2K ${\times}$ 2K high-resolution PIV technique was significantly superior compared to the smaller pixel resolution PIV system. For the cases of 512 ${\times}$ 512 and 1K ${\times}$ 1K pixel resolution PIV system, it was difficult to get accurate flow structure of viscous flow near the wall and small vortex structure in the region of large velocity gradient.

• 대한기계학회논문집
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• 제13권5호
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• pp.1032-1043
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• 1989

본 연구에서는 해석적으로 열적 입구 길이를 규명하는데 필요한 와류 열확산 계수를 실험 결과를 이용하여 결정하고, 시험관 입구의 형상 변화가 열전달 특성에 미치는 영향을 실험적으로 결정하며, 열적 입구 길이 영역에서 국소 열전달 계수를 표시할 수 있는 실험식을 제시하고, 유체의 전단율에 따른 점성 계수의 실험 결과와 점탄성 유체의 특성시간을 이용한 새로운 무차원 수인 Weissenberg수를 결정하여 퇴화 현상을 분석하고저 한다.

• 한국화재소방학회논문지
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• 제11권1호
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• pp.21-35
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• 1997

The natural convection and combined heat transfer induced by fire in a rectangular enclosure is numerically studied. The model for this numerical analysis is partially opened right wall. The solution procedure includes the standard k-$\varepsilon$ model for turbulent flow and the discrete ordinates method (DOM) is used for the calculation of radiative heat transfer equation. In numerical study, SIMPLE algorithm is applied for fluid flow analysis, and the investigations of combustion gas induced by fire is performed by FAST model of HAZARD I program. In this study, numerical simulation on the combined naturnal convection and radiation is carried out in a partial enclosure filled with absorbed-emitted gray media, but is not considered scattering problem. The streamlines, isothermal lines, average radiation intensity and kinetic energy are compared the results of pure convection with those of the combined convection-radiation, the combined heat transfer. Comparing the results of pure convection with those of the combined convection-radiation, the combined heat transfer analysis shows the stronger circulation than those of the pure convection. Three different locations of heat source are considered to observe the effect of heat source location on the heat transfer phenomena. As the results, the circulation and the heat transfer in the left region from heating block are much more influenced than those in the right region. It is also founded that the radiation effect cannot be neglected in analyzing the building in fire. And as the results of combustion gas analysis from FAST model, it is found that O2 concentration is decreased according to time. While CO and CO2 concentration are rapidly increased in the beginning(about 100sec), but slowly decreased from that time on.

• 한국산업융합학회 논문집
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• 제25권1호
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• pp.61-70
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• 2022

Fire damage time in high-rise buildings and wildland fire increasing every year. The use of high-pressure fire pumps is required to effectively extinguish fires. Reflecting the curvature effect of the fire hose occurring at the actual fire fighting site, this study provides a database of pressure drop, discharge velocity and maximum discharge height through C FD numerical analysis and it can provide using standards for fire extinguishing. Two Reynolds numbers of 200000 and 400000 were numerically analyzed at 0° -180° bending with water of 25℃ as a working fluid in hoses with a diameter of 65mm, a length of 15m, and a radius of curvature of 130mm. Realizable k-ε turbulence model was used and standard wall function was used. The pressure drop increases as the bending angle increases, and the maximum value at 90° and then decreases. The increasing rate is greater than the decrease. The velocity of the secondary flow also decreases after having the maximum value at 90°. The decreasing rate is greater than the increase. The turbulent kinetic energy increases to 120° and decreases with the maximum value. Pressure drop, velocity of the secondary flow, and turbulence kinetic energy are measured larger in the second bending region than in the first bending region.

• 한국해안·해양공학회논문집
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• 제32권3호
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• pp.180-201
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• 2020

유한길이의 혼성방파제 선단에서 발생되는 회절파의 영향으로 방파제 길이를 따라 중복파고가 변동하고, 이로 인하여 케이슨에 작용하는 파압이 공간적으로 변동하며, 또한 케이슨의 활동거리가 상이한 사행피해가 발생한다는 것은 잘 알려져 있다. 제체에 작용파력의 공간적인 변동은 2차원적인 실험이나 수치해석으로서는 접근될 수 없는 문제이다. 본 연구는 olaFlow 모델을 적용하여 고천단의 사석마운드 상에 놓인 케이슨의 선단 주변에서 회절파의 발생과 배후역으로의 영향 및 제체에 작용하는 충격쇄파압을 포함한 파압의 공간적인 변동 등을 2차원 및 3차원수치기법으로 접근한다. 또한, 수치해석에서는 혼성방파제 주변에서 평균파고, 평균수평유속 및 평균난류운동에너지의 변동특성을 면밀히 분석·검토한다. 이로부터 동일한 입사파랑에 대해 케이슨에 작용하는 파압분포가 방파제의 길이에 따라 크게 변동하며, 2차원수치해석에서는 발생되지 않았든 충격쇄파압이 3차원수치해석에서는 발생되는 경우가 나타나고, 충격쇄파압의 발생 시 경우에 따라 기존의 설계조건보다 매우 큰 파압이 정수면 근방의 케이슨 전면 벽체에 작용되는 등의 중요한 결과를 확인할 수 있었다.

• 한국해안·해양공학회논문집
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• 제32권6호
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• pp.531-552
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• 2020

선행연구에서는 3차원규칙파에 대해 olaFlow 수치모델을 적용하여 혼성방파제의 선단 주변에서 파랑특성 및 케이슨에 작용하는 파압특성을 검토하였다. 본 연구에서는 동일한 수치모델과 혼성방파제의 배치 및 형상을 적용하여 일방향불규칙파의 작용 하에 고천단의 마운드 상에 놓인 케이슨의 선단 주변에서 회절파의 발생과 배후역으로의 영향 및 케이슨에 작용하는 충격쇄파압을 포함한 파압의 공간적인 변동 등을 2차원 및 3차원적으로 검토한다. 또한, 수치해석으로부터 혼성방파제 주변에서 주파수스펙트럼, 평균유의파고, 평균수평유속 및 평균난류운동에너지의 변동특성도 면밀히 분석·검토한다. 이로부터 2차원수치해석에서는 발생되지 않았던 충격쇄파압이 3차원수치해석에서는 발생되는 경우가 나타나고, 충격쇄파압의 발생 시 경우에 따라 기존의 설계조건보다 매우 큰 파압이 정수면 근방의 케이슨의 전면 벽체에 작용되는 등의 중요한 결과를 얻을 수 있었다. 또한, 동일한 입사유의파랑에 대해 케이슨에 작용하는 파압분포가 방파제의 길이에 따라 변동하는 것을 확인할 수 있었으며, 이러한 변동은 크기에서 차이를 나타내지만, 3차원규칙파에 대한 선행연구의 경우와 유사한 특성을 갖는다.

• 에너지공학
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• 제2권3호
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• pp.268-275
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• 1993

수치해석 방법에 의해 큐백시의 스토커 소각로 유동장을 분석하였다. 수치모사의 변수는 큐백시의 스토커 소각로를 중심으로 한 5가지 내부 형상, 1차공기 속도, 2차공기 속도 및 주입각, 출구면적을 고려하였다. 검사체적에 기초한 Patankar의 유한차분 방법을 사용한 본 논문에서는 power-law scheme과 SIMPLEC 알고리즘을 사용했으며 난류 유동은 표준 k-e 모델을 이용했다. 소각로 유동장 분석을 위해서 재순환 영역의 크기, 난류 점성계수 및 이차공기의 질량분율 분포, 압력강하를 계산했다. 계산 결과는 물리적 의미에 잘 맞게 나타났으며, 큐백시의 스토커 소각로가 다른 내부 형상의 소각로에 비해 상부에 강한 난류를 가진 재순환 영역을 형성하였다.

• 한국유체기계학회 논문집
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• 제15권5호
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• pp.60-66
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• 2012

One of commonly physical phenomena encountered in pump sump systems in which its significant influence to the hydraulic performance of pump system plays an important role in the field of fluid engineering, is the appearance of free surface and submerged vortices. In this paper, a study of the vortices behavior and their formative mechanism of asymmetry is considered in this paper by using numerical approach. The Reynolds-Averaged Navier-Stokes (RANS) equations and k-omega Shear Stress Transport turbulence model used to describe the properties of turbulent flows, in company with VOF multiphase model, are implemented by Fluent code with multi-block structured grid system. In the numerical simulation, the calculated elevation of air-water interface and vortex core contours are used to classify visually surface vortices as well as submerged vortices. It is shown that the free surface vortex is identified by the concavity of liquid region from the free surface and swirling flow at that own plane. To investigate the distinctive behavior of these vortices corresponding to each given flow rate at the same water level, some numerical testing of them are considered here in such a manner that the flow pattern of surface vortex are obtained similarly to the obtained results from experiment. Furthermore, the influence due to the change of grid refinement and the variation of depth of the concavity are also considered in this paper. From that, these influential factors will be implemented to design a good pump sump with higher performance in the future.