• Proceeding of EDISON Challenge
• /
• 2013.04a
• /
• pp.398-402
• /
• 2013

NACA 계열 익형의 경우 양력 계수와 양항비는 레이놀즈수가 낮아질수록 감소하는 경향을 보이기 때문에 초소형 항공기에 적용이 어렵다. 따라서 저레이놀즈수 영역에서 높은 양항비를 가지는 익형의 개발이 필요하다. 이에 본 연구에서는 원격 조종 비행기에서 사용되고 있는 KF Airfoil이 저레이놀즈수 영역에서 효율이 좋을 것이라 기대되어 연구를 수행하였다. EDISON CFD와 Fluent를 이용하여 $3{\times}10^3{\sim}3{\times}10^6$ 레이놀즈수 범위에서 KF Airfoil의 공력계수 해석을 수행하였다. 그 결과 $8{\times}10^3{\sim}1{\times}10^5$ 레이놀즈수 영역에서 양항비가 향상되었고 $1{\times}10^4$ 영역에서는 최대 37.3% 향상된 양항비를 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.12
• /
• pp.1161-1169
• /
• 2012

A numerical study has been conducted to investigate the aerodynamic characteristics and behavior of a wing-tip vortex around a three-dimensional symmetric wing (NACA0015) in the vicinity of the ground. The aerodynamic characteristics and the wing-tip vortex change as a wing approaches the ground as a result of two different phenomena: the ground effect and the Venturi effect. The ground effect increases lift and decreases drag whereas the Venturi effect generates negative lift and increases drag suddenly. A symmetric airfoil experiences both phenomena with respect to changes in the angle of attack. In the case of a NACA0015 airfoil, the Venturi effect is dominant at small angles of attack but the ground effect is dominant at large angles of attack. Interestingly, both phenomena can be observed at the 4 degree of angle of attack. The vortex core moves inside a wing when the wing experiences the Venturi effect, whereas the vortex core moves outward when the wing experiences the ground effect.

• Journal of Navigation and Port Research
• /
• v.37 no.3
• /
• pp.257-262
• /
• 2013

A ship operating in rough sea may suffer from an undesirable motion which may severely degrade the performance of equipment onboard and give a person an uncomfortable feeling. Hence, roll stabilization received a considerable attention and various devices including bilge keels, stabilizing fins, gyroscopic, anti-rolling tanks, rudders and flaps have been conceived and utilized for the purpose. The Coanda effect is evident when a jet stream is applied tangential to a curved surface of a hydrofoil since then the jet increases the circulation around the foil and consequently the lift. Model tests and numerical simulation have been conducted to examine the practicality of a fixed type fin stabilizer augmented by the Coanda jet. The results show that the lift coefficient of the modified Coanda fin at the zero angle of attack identically coincides with that of the original fin at ${\alpha}=\26^{\circ}$ when Coanda jet is supplied at the rate of $C_j$ = 0.25. It is also shown that fixed type fin stabilizers for active control of the motions of ships and the other mobile units without rotation can be put to practical use if the Coanda effect is applied.

• Journal of the Society of Naval Architects of Korea
• /
• v.28 no.2
• /
• pp.159-173
• /
• 1991

This paper describes a potential-based panel method formulated for the analysis of a super-cavitating two-dimensional hydrofoil. The method employs normal dipoles and sources distributed on the foil and cavity surfaces to represent the potential flow around the cavitating hydrofoil. The kinematic boundary condition on the wetted portion of the foil surface is satisfied by requiring that the total potential vanish in the fictitious inner flow region of the foil, and the dynamic boundary condition on the cavity surface is satisfied by requiring thats the potential vary linearly, i.e., the tangential velocity be constant. Green's theorem then results in a potential-based integral equation rather than the usual velocity-based formulation of Hess & Smith type. With the singularities distributed on the exact hydrofoil surface, the pressure distributions are predicted with improved accuracy compared to those of the linearized lilting surface theory, especially near the leading edge. The theory then predicts the cavity shape and cavitation number for an assumed cavity length. To improve the accuracy, the sources and dipoles on the cavity surface are moved to the newly computed cavity surface, where the boundary conditions are satisfied again. This iteration process is repeated until the results are converged. Characteristics of iteration and discretization of the present numerical method are much faster and more stable than the existing nonlinear theories. The theory shows good correlations with the existing theories and experimental results for the super-cavitating flow. In the region of small angles of attack, the present prediction shows and excellent comparison with the Geurst's linear theory. For the long cavity, the method recovers the trends of the Wu's nonlinear theory. In the intermediate regions of the short super-cavitation, the method compares very well with the experimental results of Parkin and also those of Silberman.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.26 no.7
• /
• pp.931-941
• /
• 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.

• Journal of computational fluids engineering
• /
• v.6 no.2
• /
• pp.9-21
• /
• 2001

초음속 여객기의 천음속 순항 성능을 개선하기 위하여 날개의 플랩 꺽임각을 최적화하였다. 이를 위하여 3차원 Euler 코드와 adjoint 코드를 이용한 최적설계기법을 적용하였다. 설계변수로서, 앞전플랩 5개, 뒷전 플랩 5개 등 총 10개의 플랩의 꺽임각이 사용되었다. 설계과정중에 격자계 내부격자점의 수정을 위해 타원형방정식법을 이용하였다. 계산 시간의 단축을 위해 내부격자의 민감도는 무시하였다. 또한 본 설계문제에 근사구배기법의 적용가능성 여부를 조사하였다. 충격파가 없는 경우 앞전 플렙에 한하여 근사구배기법을 적용할 수 있음을 알았다. 최적설계기법으로 BFGS기법을 적용하여 항력을 최소화하였으며, 양력 및 날개 표면 마하수에 대한 제약조건을 적용하였다. 앞전 플랩의 최적화 및 앞전과 뒷전 플랩의 최적화 등 두 개의 설계 문제를 고려하였다. 성공적인 결과를 얻음으로써 본 설계방법의 타당성 및 효율성을 확인하였다.

• Journal of Ocean Engineering and Technology
• /
• v.25 no.4
• /
• pp.48-53
• /
• 2011

Current trends in propeller design have led to the need for extremely complex blade shapes, which place great demands on the accuracy of design and analysis methods. This paper presents a new proposal for improving the prediction of propeller performance with a vortex lattice method using the lifting surface theory. The paper presents a review of the theory and a description of the numerical methods employed. For 8 different propellers, the open water characteristics are calculated and compared with experimental data. The results are in good agreement in the region of a high advanced velocity, but there are differences in the other case. We have corrected the parameters for the trailing wake modeling in this paper, and repeated the calculation. The new calculation results are more in agreement with the experimental data.

• Electronics and Telecommunications Trends
• /
• v.24 no.5
• /
• pp.74-86
• /
• 2009

기존 전력망에 정보통신기술을 접목하여 전력망의 신뢰성, 효율성, 안전성 향상을 꾀하고자 하는 스마트 그리드의 비전은 태양력 및 풍력과 같은 신재생 에너지, 전기자동차(EV), 배터리와 같은 저장 장치, 수요반응 등으로 대표된다. 스마트 그리드에서는 에너지원, 전력, 통신, 소프트웨어, 컴퓨팅, 가전기기, 반도체 등 다양한 기술이 복합적으로 얽혀 있으며, 사업자 및 정책도 기술요소만큼 복합적이다. 본 고에서는 전력 정보를 양방향 실시간으로 유통함으로써 에너지 자원을 거래하는 모습까지 포함하는 차세대 전력망 기술인 스마트 그리드의 개념 및 특성, 관련 기술 파악을 통하여 중요성을 살펴보고, 관련된 국내외 정책 및 시장동향, 국내외 기술개발 동향에 대해서 기술하고자 한다.

• Proceedings of the Korean Society of Computer Information Conference
• /
• 2015.01a
• /
• pp.183-184
• /
• 2015

본 논문에서는 HTML5 기반의 대화형 스키점프 시각화 도구를 소개한다. 스키점프의 비거리는 다양한 요소에 영향을 받지만 받음각에 따른 양력과 항력의 비인 양항비에 의해 크게 결정된다. 본 논문에서는 스키의 벌림각, 구부린 각 등을 면적 계수로 두고 물리적인 2차원 좌표를 계산하여 시각화한다. 사용자의 상호작용성을 위하여 각 계수나 영향 요소를 조정할 수 있게 하여 실시간으로 스키점퍼의 자세와 비거리를 확인할 수 있게 하였다. 또한 구현 결과는 웹기반에서 동작하여 사용자의 접근성과 모델 변경에 따른 용이성을 제공한다. 본 논문의 결과물은 스키점프 경기력 향상을 위한 분석 도구로 활용될 수 있을 것이다.

• Proceeding of EDISON Challenge
• /
• 2012.04a
• /
• pp.13-16
• /
• 2012

본 논문에서는 항공기의 제어 안정성(Control Stability)과 조종성(Controllability), 그리고 기계 하중을 줄여 구조와 공기역학적인 안정성을 향상시키기 위해 고양력장치(High-lift Device)인 플랩에 탭이 추가된 익형을 설계하였다. 이 과정에서 한정된 해석자원 때문에 많은 설계조건을 해석하는데 어려움이 있었다. 이를 해결 하기위해 얇은 익형 해석(Thin Airfoil Theory)을 이용하여 지정된 설계구속조건을 통해 시위선을 지정하고 이를 바탕으로 두께를 부여하여 최종적인 익형을 설계하였다. EDISON CFD 2.0 Solver를 이용해 최종 설계한 익형의 성능을 해석하였다. 이를 바탕으로 플랩만 존재하는 익형에 비해 양력손실률이 15%이하로 감소하고, 힌지 모멘트가 최소인 공기역학적 익형을 산출하였다.