• Title/Summary/Keyword: 양항력비

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Cable Dynamics for Marine Applications -Nonlinearities- (해양 응용을 위한 케이블 동역학)

  • Shin, H. K.
    • Journal of Ocean Engineering and Technology
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    • v.4 no.2
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    • pp.35-40
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    • 1990
  • 해양 산업에서 심해로의 이동은 해양 구조물의 계류 장치의 중요성을 부각시켰고 그에 따른 기본적인 연구로서의 케이블 동력학에 대한 흥미를 일깨웠다. 거친 해상에서 케이블에 형성될 수 있는 큰 동장력과 기하학적 비선형성의 고려는 케이블의 비선형적 거동 해석에 주요 인자가 될 것이다. 또한 매우 큰 동장력 증폭에 의한 음의 큰 동장력은 케이블의 양의 정장력을 초과할 수 있고, 따라서 전체장력은 영 또는 음이 될 수 있다. 비선형 유체 항력을 포함한 모든 비선형성을 갖는 케이블의 이론적 해석 모델을 개발하고, 수치 결과와 기존의 실험 결과를 비교한다.

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Flowfield Experiments for a Circular Cylinder Having a Front Triangular Prism (정면에 정삼각주를 가진 원주의 유동장 특성실험)

  • Ro, Ki Deok;Han, Sang Yun;Ju, Hyung Gwan;Kang, Ja Un;Bae, Tae Beom;Noh, Woo Kyun
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.39 no.9
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    • pp.751-759
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    • 2015
  • In this study, we investigate the Characteristics of the flowfields of a circular cylinder having a small triangular prism at the upstream side. We measure the fluid force on the circular cylinder and obtain a visualization of the flow fields using particle image velocimetry (PIV). The experimental parameters employed were the width ratios (H/B = 0.2~0.6) of the triangular prisms to the circular cylinder's diameter, and the gap ratios (G/B = 0~3) between the circular cylinder and the triangular prism. We observed that the drag reduction rate and Strouhal number of the circular cylinder increased and then decreased with G/B in the case of the same H/B. The drag reduction rate increased with H/B in the case of the same G/B. In the case where the circular cylinder had a small triangular prism, the stagnation regions were represented in the upstream and downstream sides of the circular cylinder.

Characteristics of Flowfield of a Circular Cylinder Having a Detached Splitter Plate with High Reynolds Number (고 레이놀즈 수에서 분리된 분할판을 가진 원주의 유동장 특성)

  • Ro, Ki Deok;Lee, Han Gyun;Lee, Jong Ho;Lee, Jeong Min;Shin, Jin Ho;Cheon, Kang Bin
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.40 no.6
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    • pp.373-381
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    • 2016
  • In this study, we investigate the characteristics of the drag reduction of a circular cylinder having a detached splitter plate at the wake side. We measure the fluid force on a circular cylinder and visualize the field using particle image velocimetry (PIV) with a high Reynolds number, Re = 10,000. The experimental paraeters used were the width ratios (H/B = 0.5~1.5) of splitters to the prism width and the gap ratios (G/B = 0~2) between the prism and the splitter plate. The drag-reduction rate of the circular cylinder increased with H/B in the case of the same G/B, and it increased and then decreased with G/B in the case of the same H/B. The vortices of the opposite direction on the upper and lower sides of the detached splitter plate were generated by installing the plate. Reverse flow was caused by the vortices at the wake region of the circular cylinder, and the drag of the circular cylinder was decreased by the reverse flow.

A STUDY ON THE IONOSPHERE AND THERMOSPHERE INTERACTION BASED ON NCAR-TIEGCM: DEPENDENCE OF THE INTERPLANETARY MAGNETIC FIELD (IMF) ON THE MOMENTUM FORCING IN THE HIGH-LATITUDE LOWER THERMOSPHERE (NCAR-TIEGCM을 이용한 이온권과 열권의 상호작용 연구: 행성간 자기장(IMF)에 따른 고위도 하부 열권의 운동량 강제에 대한 연구)

  • Kwak, Young-Sil;Richmond, Arthur D.;Ahn, Byung-Ho;Won, Young-In
    • Journal of Astronomy and Space Sciences
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    • v.22 no.2
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    • pp.147-174
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    • 2005
  • To understand the physical processes that control the high-latitude lower thermospheric dynamics, we quantify the forces that are mainly responsible for maintaining the high-latitude lower thermospheric wind system with the aid of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). Momentum forcing is statistically analyzed in magnetic coordinates, and its behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) is further examined. By subtracting the values with zero IMF from those with non-zero IMF, we obtained the difference winds and forces in the high-latitude 1ower thermosphere(<180 km). They show a simple structure over the polar cap and auroral regions for positive($B_y$ > 0.8|$\overline{B}_z$ |) or negative($B_y$ < -0.8|$\overline{B}_z$|) IMF-$\overline{B}_y$ conditions, with maximum values appearing around -80$^{\circ}$ magnetic latitude. Difference winds and difference forces for negative and positive $\overline{B}_y$ have an opposite sign and similar strength each other. For positive($B_z$ > 0.3125|$\overline{B}_y$|) or negative($B_z$ < -0.3125|$\overline{B}_y$|) IMF-$\overline{B}_z$ conditions the difference winds and difference forces are noted to subauroral latitudes. Difference winds and difference forces for negative $\overline{B}_z$ have an opposite sign to positive $\overline{B}_z$ condition. Those for negative $\overline{B}_z$ are stronger than those for positive indicating that negative $\overline{B}_z$ has a stronger effect on the winds and momentum forces than does positive $\overline{B}_z$ At higher altitudes(>125 km) the primary forces that determine the variations of tile neutral winds are the pressure gradient, Coriolis and rotational Pedersen ion drag forces; however, at various locations and times significant contributions can be made by the horizontal advection force. On the other hand, at lower altitudes(108-125 km) the pressure gradient, Coriolis and non-rotational Hall ion drag forces determine the variations of the neutral winds. At lower altitudes(<108 km) it tends to generate a geostrophic motion with the balance between the pressure gradient and Coriolis forces. The northward component of IMF By-dependent average momentum forces act more significantly on the neutral motion except for the ion drag. At lower altitudes(108-425 km) for negative IMF-$\overline{B}_y$ condition the ion drag force tends to generate a warm clockwise circulation with downward vertical motion associated with the adiabatic compress heating in the polar cap region. For positive IMF-$\overline{B}_y$ condition it tends to generate a cold anticlockwise circulation with upward vertical motion associated with the adiabatic expansion cooling in the polar cap region. For negative IMF-$\overline{B}_z$ the ion drag force tends to generate a cold anticlockwise circulation with upward vertical motion in the dawn sector. For positive IMF-$\overline{B}_z$ it tends to generate a warm clockwise circulation with downward vertical motion in the dawn sector.

3차원 표면효과익의 자유표면 효과에 관한 수치연구

  • Gwak, Seung-Hyeon
    • Journal of Ocean Engineering and Technology
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    • v.12 no.2 s.28
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    • pp.79-86
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    • 1998
  • A three-dimensional WIG (Wing In Ground effect) moving above free surface is numerically studied by means of finite difference techniques. The air flow field around the WIG is analyzed by MAC (Marker & Cell) method, and interactions between WIG and the free surface are appeared as the variation of pressure distribution acting on the free surface. To analyze the wavemaking phenomena by those pressure distributions, the NS (Navier-Stokes) solver is employed in which nonlinearities of the free surface conditions can be included. Through the numerical simulation, Cp values and lift/drag ratio are carefully reviewed by changing the height/chord ratio. The section shape of model is NACA0012 with the span/chord ratio of 3.0. Through computational results, it is confirmed that the effect of free surface is small enough to treat it as a rigid wavy wall.

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Unsteady Force Characteristics on Foils Undergoing Pitching Motion (피칭 운동익에 작용하는 비정상 유체력)

  • Yang Chang-Jo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.30 no.2 s.245
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    • pp.117-125
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    • 2006
  • In the present study the unsteady forces acting on the pitching foils such as a flat plate, NACA0010, NACA0020, NACA65-0910 and BTE have been measured by using a six-axis sensor in a circulating water tunnel at a low Reynolds number region. The unsteady characteristics of the dynamic drag and lift have been compared to the quasi-steady ones which are measured under the stationary condition. The pitching motion is available for keeping the lift higher after the separation occurs. Especially, the characteristics of the dynamic lift are quite different from the quasi-steady one at high pitching frequency regions. As the pitching frequency deceases, the amplitude of the dynamic lift becomes closer to the quasi-steady one. However, the phase remains different between the steady and unsteady conditions even at low pitching frequencies. On the other hand, the dynamic drag is governed strongly by the angle of attack.

Cavitating Flow Characteristics around a 2-Dimensional Hydrofoil Section (2차원 날개 단면 주위의 캐비테이팅 유동 특성 연구)

  • Choi, Jung-Eun;Chung, Seok-Ho;Lee, Dong-Hyun
    • Journal of the Society of Naval Architects of Korea
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    • v.44 no.2 s.152
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    • pp.74-82
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    • 2007
  • Recently, the erosion due to cavitation frequently occurs on a horn-type rudder of a high-speed large container carrier. It is necessary to understand the flow characteristics around a rudder in fully wetted and cavitating flow condition, and the process of generation and collapse of cavitation for a rudder design to minimize the cavity-induced erosion. The flow characteristics around a two-dimensional hydrofoil(NACA66) are investigated through the computational method utilizing a viscous flow theory applied to a cavitation model. The computational results from the viscous flow theory are verified by the comparison with the experimental results, and are compared with those from the potential flow theory. The effects of angle of attack, Reynolds number, cavitation number, and thickness ratio on the cavitating flow are also investigated.

A Study of the Force Majeure as Immunity by 3rd Party Liability of the Aircraft-Operator -With respect to the German Aviation Act- (항공기운항자의 제3자 책임에 관한 면책사유로서의 불가항력 조항에 관한 고찰 - 독일 항공법상의 해석을 중심으로 -)

  • Kim, Sung-Mi
    • The Korean Journal of Air & Space Law and Policy
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    • v.31 no.1
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    • pp.37-62
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    • 2016
  • Two controversial issues exist in interpretation of "Force Majeure" set forth in the Article 931 (4) of the Korean Commercial Code. Firstly, its scope of application is ambiguous. Secondly, there is a concern that the "immunity" under paragraph 1 and "Force Majeure" may overlap each other. "Force Majeure" refers an event resulted from either natural disaster or 3rd-party. Meanwhile, the latter implies relatively extensive and comprehensive meaning and its interpretation may vary depends on law enforcement. In general, the aircraft accident hardly results in damage or loss to the 3rd-party. Additionally, it is worth to review newly enacted clause and to define its applicability. When the 3rd party is suffered from damage or loss incurred by any external act, it is necessary to explicit the concept of the non-contractual liabilities with respect to 3rd party. From the perspective of protecting aviation industries, the commercial aviation operator may be entitled to immunity in respect of claim for damage incurred by the event of Force Majeure. However, this approach is directly opposite to the victim's benefit and protection by the law. Therefore, the priority of the legal protection should be considered. Although the interest of the commercial aviation operator is not negligible, the protection of the law should be favorable to the 3rd party. Otherwise, the innocent party has no right to claim for damage incurred by aviation accident. Another issue is about the possibility of overlapping of the provision set forth in the paragraph 1 and 4. The former states that the liabilities shall be exempted on account of either the unsettled political or economic situation but this clause is inconsistent with the interpretation on Force Majeure under the latter. As argued above, this may include any event resulted from either political or economic account by the external influence of the 3rd party, thus these two provisions are overlapped. Consequently, in order to develop ordinances and guidelines and to ensure an equal protection to both parties, above two issues must remain open for further discussions.

Aerodynamic Characteristics of Basic Airfoils for Agricultural Helicopter using Wind-tunnel Test and CFD Simulation (풍동실험과 CFD 시뮬레이션을 이용한 농용 헬리콥터 기본 익형의 공력특성 고찰)

  • Won, Yong Sik;Koo, Young Mo;Haider, B.A.;Sohn, Chang Hyun
    • Proceedings of the Korean Society for Agricultural Machinery Conference
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    • 2017.04a
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    • pp.54-54
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    • 2017
  • 본 연구는 무인 헬리콥터의 양력을 개선하기 위한 기초 단계로서 V1505A 및 V2008B 기본 두 익형의 400 mm 블레이드 섹션에 해당하는 모델에 대한 풍동실험을 실시하여 양력, 항력 및 동력특성을 분석하고 CFD 시뮬레이션의 결과와 비교하고 검증하였다. 시뮬레이션은 풍동 실험과 유사하게 설정하기 위하여 400 mm 블레이드 섹션의 양 끝을 벽으로 제한하여 3차원 와류현상을 억제하여 모델을 구성하였고, 시뮬레이션의 결과와 비교하여 모델을 검증하였다. 사용된 모델은 로터로부터 $Re=0.32{\times}10^6$ 영역까지는 aminar 모델을 사용하였으며, 그 이후 영역역(>$Re=0.32{\times}10^6$)은 양력 및 저항의 급격한 변화를 올바로 포착할 수 있다는 S-A 모델을 적용하여 확장하였다. 시뮬레이션의 격자는 유동 현상에 있어 박리로 야기된 와류 현상을 관찰하기 위하여 익형 주변에 접하는 부분에 격자를 집중시켰다. 시뮬레이션 방법은 유속은 36~141 m/s 까지 5 수준으로 하였으며, 받음각은 $0{\sim}16^{\circ}$로 7 수준으로 변화 시키면서 공력계수 및 동력을 분석하였다. 양력분석에 있어 익형 V1505A에 비해 익형 V2008B의 특성이 우수하였으나, 익형 V1505A는 실속 이후 양력이 급격히 떨어지지 않고 유지되는 특성을 보였다. 익형 V2008B는 낮은 받음각에서 높은 공력과 낮은 항력을 나타냈다. 동력 분석 결과로 익형 V1505A의 유도동력은 총 동력의 56~72%를 차지하고, 형상동력은 총 동력의 27~43%를 차지하였다. 익형 V2008B는 유도동력은 총 동력의 66~81%를 차지하고, 형상동력은 총 동력의 18~33%를 차지하였다. 익형 V2008B이 익형 V1505A보다 유도동력은 크며, 형상동력은 적게 나와 상대적으로 효율적이라 할 수 있다. 헬리콥터 동력원의 규모는 법률적인 총중량에 의하여 제한되므로 일반적인 농용 소형 무인 헬리콥터 엔진의 사양인 24.5 kW (32PS)를 적용한다면, 익형 V1505A은 받음각 $8{\sim}10^{\circ}$에서 그리고 익형 V2008B은 $7{\sim}9^{\circ}$정도에서 받음각이 제한되며 이때 총 양력은 1200~1300 N 정도로 예상된다.

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Effects of Wing Twist on Longitudinal Stability of BWB UCAV (날개의 비틀림이 동체-날개 융합익형 무인전투기의 종안정성에 미치는 영향에 대한 연구)

  • Ban, Seokhyun;Lee, Jihyeong;Kim, Sangwook;Cho, Jinsoo
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.46 no.1
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    • pp.1-9
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    • 2018
  • Lambda wing type Unmanned Combat Aerial Vehicle(UCAV) which adopts Blended Wing Body(BWB) has relatively less drag and more stealth performance than conventional aircraft. However, Pitching moment is rapidly increased at a specific angle of attack affected by leading edge vortex due to leading edge sweep angle. Wind tunnel testing and numerical analysis were carried out with UCAV 1303 configuration on condition of 50 m/s of flow velocity, $-4^{\circ}{\sim}28^{\circ}$ of the range of angle-of-attack. The effect of wing twist for longitudinal stability at the various angles of attack was verified in this study. When negative twist is applied on the wing, Pitch-break was onset at higher angle of attack due to delayed flow separation on outboard of the wing. On the other hand, pitch-break was onset at lower angle of attack and lift-to-drag ratio was increased when positive twist is applied on the wing.