• 대한기계학회논문집
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• 제16권11호
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• pp.2090-2097
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• 1992

본 연구에서는 밸브판의 거동을 2차의 회전계로 간주하고, 양력 및 항력(drag force)을 구하는데 있어 Reif등이 구한것과 동일한 방법을 이용하되 정지핀에 의해 그 최대 열림각이 제한을 받는 물리적 사실을 고려하고, 밸브를 통과하는 유량도 단위 입 력이 아닌 정현파(sine wave)로 간주하여 밸브판의 동적거동을 해석하였다. 해석대 상의 밸브 모델로는 Bjork-Shiley 27mm 1엽 밸브를 선정하였다.

• 한국군사과학기술학회지
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• 제20권3호
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• pp.345-351
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• 2017

A wind tunnel test for the scaled parachute models was performed to verify aerodynamic characteristics for practical usage of Korean low cost low altitude aerial delivery system. The cruciform shaped cargo parachute models for heavy and light weight were ejected into wind tunnel test section; and the drag forces acting on the models in steady condition were measured in accordance with velocity. Also, the maximum opening forces during inflation were obtained and captured by a high speed camera to analyze the inflation characteristics and evaluate the design of the low cost aerial delivery system. The results showed a reliable stability and met the design requirement of delivery operation system for R.O.K Air Force.

• 한국입자에어로졸학회지
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• 제5권3호
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• pp.123-131
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• 2009

This paper presents simulation results of particle transport in low-pressure, low-temperature plasma environment. The size dependent transport of particles in the plasma is investigated with a two-dimensional simulation tool developed in-house for plasma chamber analysis and design. The plasma model consists of the first two and three moments of the Boltzmann equation for ion and electron fluids respectively, coupled to Poisson's equation for the self-consistent electric field. The particle transport model takes into account all important factors, such as gravitational, electrostatic, ion drag, neutral drag and Brownian forces, affecting the motion of particles in the plasma environment. The particle transport model coupled with both neutral fluid and plasma models is simulated through a Lagrangian approach tracking the individual trajectory of each particle by taking a force balance on the particle. The size dependant trap locations of particles ranging from a few nm to a few ${\mu}m$ are identified in both electropositive and electronegative plasmas. The simulation results show that particles are trapped at locations where the forces acting on them balance. While fine particles tend to be trapped in the bulk, large particles accumulate near bottom sheath boundaries and around material interfaces, such as wafer and electrode edges where a sudden change in electric field occurs. Overall, small particles form a "dome" shape around the center of the plasma reactor and are also trapped in a "ring" near the radial sheath boundaries, while larger particles accumulate only in the "ring". These simulation results are qualitatively in good agreement with experimental observation.

• 대한조선학회지
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• 제27권1호
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• pp.11-16
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• 1990

거친 해상에서 케이블이 형성된 수 있는 큰 동장력(large dynamic tensile forces)과 기하학적 비선형성(geometric nonlinearity)의 고려는 비선형 케이블 운동방정식(nonlinear cable dynamics)의 해에 상당한 영향을 끼치며 이 결과의 응용은 케이블의 극단장력(extreme tensions)과 slack-and-snapping 케이블의 연구에서 필수적인 부분이 될 것이다. 비선형 유체항력만을 포함한 경우와 기하학적 비선형성과 큰 동장력항을 함께 포함하는 경우의 케이블 운동방정식의 해를 비교하여, 케이블의 동적 거동에 대한 기하학적 비선형과 큰 동장력항의 복합적인 영향을 연구한다. 큰 동장력항과 기하학적 비선형성의 고려는, 최대 동장력의 증가를 가져오나 반면에 최소 동장력의 크기에서의 감소를 가져옴으로, 결국 동장력의 평균값의 상승과 그로인한 케이블의 피로수명 단축을 유발할 수 있다.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2002년도 추계학술대회 논문집
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• pp.273-276
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• 2002

Many artificial steel reefs are being built in Japan, however, in Korea, only few steel reefs were manufactured by POSCO, recently. In order to develop their novel types suitable for marine environments near the Korean Peninsula, it is very important to carry out model tests in the initial design stage. In the Ocean Engineering Wide Tank($L{\times}B{\times}D=30{\times}20{\times}2.5m$) and the Circulating Water Channel of the University of Ulsan, Korea, fluid drag forces acting on models of steel reefs with different sizes are measured in waves and currents. Also numerical predictions based on the Wavier-Stokes equation are made and compared with experimental results.

• 한국군사과학기술학회지
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• 제23권4호
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• pp.311-318
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• 2020

This study investigates the performance and blade airloads for a rigid coaxial rotor of high-speed compound unmanned rotorcrafts. The present compound unmanned rotorcraft uses not only a rigid coaxial rotor, but also wings and propellers for high-speed flights. For the rigid coaxial rotor in this work, CAMRAD II, a rotorcraft comprehensive analysis code, is used to study the performance at a flight speed of up to 250 knots and blade section lift forces at 230 knots. As the flight speed increases, the rotor power decreases; however, the power of propellers increases to overcome the drag force of a rotorcraft in high-speed flight. The effective lift-to-drag ratio of a rotor has the maximum value of about 11.6 which is much higher than the value of the conventional helicopter. The blade section lift forces of the upper and lower rotors at 230 knots show the similar variation trends for one rotor revolution, and the impulses because of the aerodynamic interaction between both rotors are observed.

• Geomechanics and Engineering
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• 제13권6호
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• pp.947-961
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• 2017

An innovative spiral variable-section capillary model is established for piping critical hydraulic gradient of cohesion-less soils causing water/mud inrush in tunnels. The relationship between the actual winding seepage channel and grain-size distribution, porosity, and permeability is established in the model. Soils are classified into coarse particles and fine particles according to the grain-size distribution. The piping critical hydraulic gradient is obtained by analyzing starting modes of fine particles and solving corresponding moment equilibrium equations. Gravities, drag forces, uplift forces and frictions are analyzed in moment equilibrium equations. The influence of drag force and uplift force on incipient motion is generally expounded based on the mechanical analysis. Two cases are studied with the innovative capillary model. The critical hydraulic gradient of each kind of sandy gravels with a bimodal grain-size-distribution is obtained in case one, and results have a good agreement with previous experimental observations. The relationships between the content of fine particles and the critical hydraulic gradient of seepage failure are analyzed in case two, and the changing tendency of the critical hydraulic gradient is accordant with results of experiments.

• 대한토목학회논문집
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• 제38권5호
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• pp.627-634
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• 2018

본 연구에서는 바람의 영향을 받는 박스형 콘크리트 거더교에 대한 풍력계수를 산정하기 위하여 전산유체해석(CFD)를 수행하였다. 방음벽이 없는 교량 단면에 대한 항력계수, 양력계수 및 비틀림모멘트계수를 산정하였고, 이 풍력계수 값들을 다양한 높이의 방음벽을 갖는 교량 단면에 대한 풍력계수 값들과 비교하였다. 전산유체해석에서 풍력계수들을 산정할 때 전단응력수송(SST) $k-{\omega}$ 난류 모델을 적용하였고, 마찰 항력계수가 전체 항력계수에 미치는 기여도를 조사하였다. 연구 결과, 바람이 수평으로 불 때 항력계수는 방음벽의 높이가 커질수록 증가하였고, 마찰 항력의 기여도는 교량 단면에 방음벽이 없을 때 가장 높았다. 따라서 교량설계에서 풍력을 산정할 때 방음벽의 높이의 영향을 고려할 필요가 있으며, 벽면 마찰력은 교량에 작용하는 풍력을 산정할 때 중요한 역할을 하였다.

• 한국항공우주학회지
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• 제33권6호
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• pp.15-20
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• 2005

타원형 실린더 주위를 흐르는 비정상 점성 유동을 해석하기 위해서 SIMPLER 방법을 사용한 이차원 비압축성 나비어-스??스 유동 해석 프로그램을 개발하였다. 실린더 직경에 대한 두께의 비가 0.6, 0.8, 1.0, 1.2인 실린더 주위의 점성 흐름을 레이놀즈 수 200, 400, 1000인 조건에서 해석하였다. 본 논문은 실린더 두께와 레이놀즈 수가 실린더에 작용하는 양력 및 항력에 미치는 영향을 파악하는데 주 관심을 두었다. 개발된 해석 프로그램의 검증을 위해서 가용한 실험 자료와 수치 해석 자료가 개발된 해석 프로그램의 수치 결과와 비교되었으며, 잘 일치하는 경향을 보여주었다. 본 연구를 통해서 실린더 두께와 레이놀즈수가 항력과 양력의 진동 주기뿐만 아니라 크기나 진폭에 큰 영향을 미치게 됨을 알 수 있었다.

• 대한조선학회논문집
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• 제43권4호
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• pp.414-421
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• 2006

Yacht is operated by wind-driven thrust on the saii, but also experiencing the side force. Thus the keel attached on the bottom of main hull prevents it from flowing sideway. Since the keel affects the stability and thrust of yacht, its selection is one of the most important factor in design. In the present paper the correlation between yacht hull and keel was investigated. through comparison of forces measured at various combinations of heeling and leeway angles with and without keel. Keel-only test was also performed to find out the drag and lift characteristics of keel itself. finally three different types of keel, i.e. fin keel, bulb keel and winglet keel were tested to compare their advantages and drawbacks.