• 대한기계학회논문집B
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• 제27권8호
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• pp.1174-1181
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• 2003

The flow around a circular cylinder was controlled by attaching O-rings to reduce drag force acting on the cylinder. Four experimental models were tested in this study; one smooth cylinder of diameter D (D=60mm) and three cylinders fitted with O-rings of diameters d=0.0167 D, 0.05D and 0.067 D with pitches of PPD=2D, 1D, 0.5D and 0.25D. The drag force, mean velocity and turbulence Intensity profiles in the near wake behind the cylinders were measured for Reynolds numbers based on the cylinder diameter in the range of Re$_{D}$=7.8$\times$10$^3$~1.2$\times$10$^{5}$ . At Re$_{D}$=1.2$\times$10$^{5}$ , the cylinder fitted with O-rings of d=0.0167D in a pitch interval of 0.25D shows the maximum drag reduction of about 5.4%, compared that with the smooth cylinder. The drag reduction effect of O-rings of d=0.067D is not so high. For O-ring circulars, as the Reynolds number increases, the peak location of turbulence intensity shifts downstream and the peak magnitude is decreased. Flow field around the cylinders was visualized using a smoke-wire technique to see the flow structure qualitatively. The size of vortices and vortex formation region formed behind the O-ring cylinders are smaller, compared with the smooth cylinder.der.

• 한국항행학회논문지
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• 제20권2호
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• pp.134-140
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• 2016

본 연구는 항력을 고려한 탄도미사일의 비행궤적 특성에 대한 해석이다. 탄도미사일은 대부분 대기권 밖에서 초고속으로 비행하고 RCS가 작기 때문에 요격이 어렵다. 특히 부스트 및 종말단계에서는 급격한 항력의 변화에 따라 속도의 변화가 매우 커진다. 따라서 성공적인 탄도미사일 방어체계를 구축하기 위해서는 탄도미사일의 비행궤적 해석 시 항력 특성을 반드시 고려해야 한다. 이러한 관점에서 본 연구는 항력에 대한 특성을 분석하고, 이를 고려하여 스커드 B, C 및 노동미사일의 비행궤적 특성을 도출하였다. 탄도미사일의 비행궤적 모델은 지구 자전에 의한 코리올리힘과 원심력을 반영하였으며, 탄도미사일의 제원은 공개된 자료를 활용하였다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제36회 춘계학술대회논문집
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• pp.381-384
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• 2011

본 연구에서는 초음속 비행체의 조파저항을 감소시키기 위하여, 최대 주파수 80 kHz의 반복 레이저 펄스에 의해 야기된 에너지 부가법에 관한 실험적 연구가 수행된다. 기류 마하수 1.94의 흡입식 초음속 풍동의 바깥에 설치된 초점렌즈에 의하여 레이저 펄스가 실린더 모델 전단부에 집약된다. 시간변동 항력과 정체압력은 로드셀과 PCB 압력센서에 의해서 측정되며, 동시에 고속 카메라를 이용하여 가시화가 수행된다. 본 연구의 결과로부터, 레이저 펄스 에너지 부가에 의한 항력 저감량은 레이저 펄스 주파수가 증가할 때, 최대 21%까지 거의 선형적으로 증가하였다. 부가 에너지 효율은 레이저 펄스 에너지에만 의존하는 결과를 얻었으며, 최대 1000%까지 달성되었다.

• 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.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1997년도 춘계학술대회 논문집
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• pp.140-144
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• 1997

Although the cone type venturi flow meter is superior to another differential pressure type flow meter in precision, the venturi is installed at the pipe as an L beam, so that the dynamic load due to drag force and flow-induced force is generated in the flow meter. In this paper we propose a methodology to evaluate this dynamic load directly by using stain-gages attached on the venturi and we discuss about the dynamic characteristics on the basis of flow-induced vibration theory.

• 한국막학회:학술대회논문집
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• 한국막학회 2008년도 춘계 총회 및 학술발표회
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• pp.33-47
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• 2008

Non-equilibrium (irreversible) themodynamics is used to investigate colloidal back-diffusion during crossflow membrane filtration. The chemical potential is generalized as a superposition of equilibrium and irreversible contributions, originating from Brownian and shear-induced diffusion, respectively. As a result, an effective drag force is derived using the irreversible thermodynamics for a particle undergoing both Brownian and shear-induced diffusion in a sheared concentrated suspension. Using the drag force, a hydrodynamic force bias Monte Carlo method is developed for crossflow membrane filtration to determine the critical flux of hard sphere suspensions. Effects of shear rate and particle size on the critical flux are studied, and results show a good agreement with experimental observations reported in the literature.

• 한국운동역학회지
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• 제16권1호
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• pp.129-138
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• 2006

The motion of a spinning table-tennis ball is investigated in both theory and experiment. The equation of motion of spinning table-tennis ball is made using aerodynamics and calculated by C++ program In theoretical part, gravity, drag force and lift force are regarded as main force. Velocity, angular velocity, mass and Drag and lift coefficients are considered as a independent variable. Experiments are made by a digital stroboscope, a digital camera and a mirror, and snap multi-exposed images were took as a dependent result In experimental part, both magnitude and direction of velocity and angular velocity are changed in each situation. The predicted three-dimensional trajectories of spinning balls are compared with experimental trajectories. As a result the theoretical trajectories were predicted within 10% of experimental trajectories.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2018년도 학술발표회
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• pp.149-149
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• 2018

Prediction on initial motion of sediment is crucial to evaluate sediment transport and channel stability. The condition of incipient movement of sediment is characterized by bed shear stress, which is generated from force of moving water against the bed of the channel, and by critical shear stress, which depends on force resisting motion of sediment due to the submerged weight of the grains. When the bed shear stress exceeds the critical shear stress, sediment particles begin rolling and sliding at isolated and random locations. In Mountain River, debris flow frequently occurs due to heavy rainfall and can lead some natural stones from mountain slope into the bed river. This phenomenon could add additional forces to sediment transport system in the bed of river and also affect or change direction and magnitude of sediment movement. In this paper, evaluations on incipient motion of uniform coarse gravel under falling spheres impacts using small scale flume channel were conducted. The drag force of falling spheres due to water flow and length movement of falling spheres were investigated. The experiments were carried out in flume channel made by glass wall and steel floor with 12 m long, 0.6 m wide, and 0.6 m deep. The bed slopes were selected with the range from 0.7% to 1.5%. The thickness of granular layer was at least 3 times of diameter of granular particle to meet grain placement condition. The sphere diameters were chosen to be 4cm, 6 cm, 8 cm, 10 cm. The spheres were fallen in to the bed channel for critical condition and under critical condition of motion particle. Based on the experimental results, the Shields curve of particles Reynold number and dimensionless critical shear stress were plotted. The relationship between with drag force and the length movement of spheres were plotted. The pathways of the bed material Under the impact of spheres falling were analyzed.

• 한국항공우주학회지
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• 제33권12호
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• pp.1-8
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• 2005

타원형 실린더의 두께와 받음각 및 레이놀즈수가 실린더에 작용하는 항력과 양력에 어떤 영향을 미치는가를 고찰하기 위해서 수치적 연구를 수행하였다. 타원형 실린더 주위를 흐르는 비정상 점성 유동을 해석하기 위하여 SIMPLER 기법을 이용한 2차원 비압축성 Navier-Stokes 유동 해석 프로그램을 개발하였으며, 두께-시위길이 비가 0.2, 0.4, 0.6인 타원형 실린더 형상에 대해서 레이놀즈수가 400, 600인 조건, 그리고 받음각이 10도, 20도, 30도인 조건하에서 유동을 해석하였다. 본 연구를 통해서 실린더 두께 비와 받음각 및 레이놀즈수가 항력과 양력 계수의 시간 평균값 및 진폭의 크기, 그리고 진동 주기에 크게 영향을 미침을 확인할 수 있었다.

• 생물환경조절학회지
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• 제1권2호
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• pp.142-147
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• 1992

The wind pressure distributions were analyzed to provide fundamental criteria for the structural design on the two-span arched house according to the wind directions through the wind tunnel experiment. In order to investigate the wind force distributions, the variation of the wind force coefficients, the mean wind force coefficients, the drag force coefficients and the lift force coefficients were estimated using the experimental data. The results obtained are as follows : 1. The variation of the wind force with wind directions on the side walls was the greatest at the upwind edge of the walls. 2. The maximum negative wind force along the length of the roof appeared at the upwind edge at the wind direction of 60$^{\circ}$. 3. The maximum negative wind force along the width of the roof appeared at the width ratio and wind direction of 0$^{\circ}$ and 0.4 in the first house and 0.6 and 30$^{\circ}$ in the second house, respectively. 4. The mean negative wind force on the side walls of the first house at the wind direction of 0$^{\circ}$ was far greater than that of the second house, and the maximum negative wind force on the roof occurred at the wind direction of 30$^{\circ}$. 5. The maximum lift force appeared on the second house at the wind direction of 30$^{\circ}$, but the lift force on the first house was far greater than that on the second house at the wind direction of 0$^{\circ}$. 6. The parts to be considered for the local wind forces were the edges of the walls, and the edges of the x-direction and the width ratio, 0.4 of the y-direction in the roofs.