• 한국항공우주학회지
• /
• 제42권4호
• /
• pp.298-304
• /
• 2014

Weissinger의 방법을 기반으로 양력선 방법을 지면효과를 고려한 해석이 가능하도록 확장하였다. 이 방법을 이용하여 가로세로비 10의 타원형 날개와 인간동력항공기 날개에 대해 지면효과에 의한 공력성능 변화를 예측하였다. 날개가 지면에 가까워질수록 날개의 와류강도는 약간 증가하지만 내리흐름은 크게 감소한다. 인간동력항공기 날개의 경우에 날개가 지면에 가까울수록 양력은 증가하여 높이가 2m에서 양력계수는 지면효과를 받지 않을 때보다 5% 증가한다. 유도항력계수는 날개가 지면에서 스팬길이 높이에 있을 때에 지면효과가 없을 때보다 10% 감소하고 높이가 2m이면 55% 감소한다.

• 한국항공우주학회지
• /
• 제41권12호
• /
• pp.931-939
• /
• 2013

본 논문에서는 항공기 날개의 개념 설계에서 적용하기에 적합한 양력선 방법을 찾고 정확성과 적용범위를 분석한다. Prandtl의 양력선 이론에서 발전된 두 가지 양력선 방법으로서 얇은 익형의 가정을 갖고 3/4 시위의 제어점에서 속도경계조건을 부여하는 Weissinger방법과 3차원 와류 양력법칙을 적용한 Phillips의 방법을 택하였다. 계산 대상은 타원형 날개, 후퇴각이 있는 날개, 그리고 상반각과 비틀림이 있고 후퇴각 없는 테이퍼 날개이다. 계산을 통해 포텐셜 유동의 공력 데이터로 날개의 순환분포, 내리흐름 분포, 양력과 유도항력을 추출하여 이론식 결과 및 풍동시험 데이터와 비교하였다. Weissinger 방법은 날개의 형상에 상관없이 정확도와 신뢰성 있는 결과를 보여주지만 Phillips 방법은 후퇴각이 있는 날개에서는 부정확한 결과를 나타내었다.

• 한국항공우주학회지
• /
• 제49권4호
• /
• pp.273-279
• /
• 2021

윙렛은 유도항력을 줄이기 위한 장비로, 1980년대부터 보잉사에서 제작한 항공기에 장착되기 시작했고, 에어버스에서는 2009년부터 'Sharklet'이라는 이름의 윙렛을 개발하여 A320 Family에 장착 옵션으로 제공하기 시작했다. 윙렛은 날개 끝단에서 발생하는 유도항력을 감소시켜 이륙성능 향상, 연료소모량 감소, 유상탑재량 증대 및 운항거리 증가의 효과를 내고 있다. 본 연구는 Sharklet이 장착된 A321 항공기와 미장착된 A321 항공기의 실제 운항 데이터를 분석하여 윙렛 장착에 따른 연료 효율성 향상을 검증하고 이에 따른 경제성 분석을 목적으로 한다. 이를 통해 항공기 도입 시 윙렛 장착 사양 결정 혹은 기존 항공기의 업그레이드를 위한 의사 결정에 활용할 수 있다. 이를 위해 윙렛 장착에 따른 공력 특성 연구 사례조사와 효과 확인 연구를 수행하고 경제성을 검증하였다.

• Wind and Structures
• /
• 제34권5호
• /
• pp.407-419
• /
• 2022

Many factors should be considered by architects and designers for designing a tall building. Wind load is one of these important factors that govern the design of tall building structures and can become a serious challenge when buildings tend to be built very tall and slender. On the other hand, through the initial stages of a design process, choosing the design geometry greatly affects the wind-induced forces on a tall building. With this respect, geometric shapes with 3-fold rotational symmetry are one of the applied plan shapes in tall buildings. This study, therefore, aims to investigate the aerodynamic characteristics of 8 different geometrical shapes using Computational Fluid Dynamics (CFD) by measuring the drag and lift forces. A case study approach was conducted in which different building shape models have the same total gross area and the same height of 300 meters. The simulation was an incompressible transient flow that ran 1700 timesteps (85 seconds on the real-time scale). The results show a great difference between wind-induced force performance of buildings with different plan shapes. Generally, it is stated that the shapes with the same area, but with smaller perimeters, are better choices for reducing the drag force on buildings. Applying the lift force, the results show that the buildings with plan shapes that have rounded corners act better in crosswind flow while, those with sharp corners induce larger forces in the same direction. This study delivers more analytical understanding of building shapes and their behavior against the wind force through the parametric modelling.

• Wind and Structures
• /
• 제4권1호
• /
• pp.63-72
• /
• 2001

This paper presents results of a study on wind loads and wind induced dynamic response of bridge jointed twin-towered buildings. Utilizing the high-frequency force balance technique, the drag and moment coefficients measured in wind tunnel tests, and the maximum acceleration rms values on the top floor of towers, are analyzed to examine the influence of building's plan shapes and of intervals between towers. The alongwind, acrosswind and torsional modal force spectra are investigated for generic bridge jointed twin-towered building models which cover twin squares, twin rhombuses, twin triangles, twin triangles with sharp corners cut off, twin rectangles and individual rectangle with the same outline aspect ratio as the twin rectangles. The analysis of the statistical correlation among three components of the aerodynamic force corroborated that the correlation between acrosswind and torsional forces is significant for bridge jointed twin-towered buildings.

• Wind and Structures
• /
• 제20권6호
• /
• pp.793-810
• /
• 2015

Tests were conducted at the Florida International University (FIU) Wall of Wind (WOW) to investigate the susceptibility of Variable Message Signs (VMS) to wind induced vibrations due to vortex shedding and galloping instability. Large scale VMS models were tested in turbulence representative of the high frequency end of the spectrum in a simulated suburban atmospheric boundary layer. Data was measured for the $0^{\circ}$ and $45^{\circ}$ horizontal wind approach directions and vertical attack angles ranging from $-4.5^{\circ}$ to $+4.5^{\circ}$. Analysis of the power spectrum of the fluctuating lift indicated that vertical vortex oscillations could be significant for VMS with a large depth ratio attached to a structure with a low natural frequency. Analysis of the galloping test data indicated that VMS with large depth ratios, greater than about 0.5, and low natural frequency could also be subject to galloping instability.

• Wind and Structures
• /
• 제1권4호
• /
• pp.351-364
• /
• 1998

In wind tunnel experiments, the blockage effect is a very important factor which affects the test results significantly. A number of investigations into this problem, especially on the blockage correction of drag coefficient, have been carried out in the past. However, only a limited number of works have been reported on the wind tunnel blockage effect on wind-induced vibration although it is considered to be fairly important. This paper discusses the aerodynamic characteristics of the square model and square model with corner cut based on a series of the wind tunnel tests with various blockage ratios and angles of attack. From the test results, the aerodynamic behavior of square models with up to 10% blockage ratio are almost the same and square models with up to 10% blockage ratio can be tested as a group which behaves similarly.

• Geomechanics and Engineering
• /
• 제5권3호
• /
• pp.195-221
• /
• 2013

A centrifuge model study is carried out to investigate the behavior of pile subject to negative skin friction induced by pile installation, ground water drawdown and surcharge loading. A single end-bearing pile is examined as the induced negative skin friction would induce the most severe stress on the pile structural material as compared to friction piles. In addition, the behavior of the pile under simultaneous negative skin friction and dead/live loads is examined. To facilitate detailed interpretations of the test results, the model setup is extensively instrumented and involves elaborate test control schemes. To further examine the phenomenon of negative skin friction on an end-bearing pile, finite element analyses were conducted. The numerical analysis is first validated against the centrifuge test data and subsequently extended to examine the effects of pile slenderness ratio, surcharge intensity and pile-soil stiffness ratio on the degree of mobilization of negative skin friction induced on the pile. Finally experimental and numerical studies are conducted to examine the effect of applied transient live load on pile subject to negative skin friction.

• Wind and Structures
• /
• 제30권6호
• /
• pp.633-648
• /
• 2020

The objective of this work was to investigate the interference effects of two-parallel bridge decks on aerodynamic coefficients, vortex-induced vibration, flutter instability and flutter derivatives. The two bridges have significant difference in cross-sections, dynamic properties, and flutter speeds of each isolate bridge. The aerodynamic static tests and aeroelastic tests were performed in TU-AIT boundary layer wind tunnel in Thammasat University (Thailand) with sectional models in a 1:90 scale. Three configuration cases, including the new bridge stand-alone (case 1), the upstream new bridge and downstream existing bridge (case 2), and the downstream new bridge and the upstream existing bridge (case 3), were selected in this study. The covariance-driven stochastic subspace identification technique (SSI-COV) was applied to identify aerodynamic parameters (i.e., natural frequency, structural damping and state space matrix) of the decks. The results showed that, interference effects of two bridges decks on aerodynamic coefficients result in the slightly reduction of the drag coefficient of case 2 and 3 when compared with case 1. The two parallel configurations of the bridge result in vortex-induced vibrations (VIV) and significantly lower the flutter speed compared with the new bridge alone. The huge torsional motion from upstream new bridge (case 2) generated turbulent wakes flow and resulted in vertical aerodynamic damping H1^* of existing bridge becomes zero at wind speed of 72.01 m/s. In this case, the downstream existing bridge was subjected to galloping oscillation induced by the turbulent wake of upstream new bridge. The new bridge also results in significant reduction of the flutter speed of existing bridge from the 128.29 m/s flutter speed of the isolated existing bridge to the 75.35 m/s flutter speed of downstream existing bridge.

• Journal of Biosystems Engineering
• /
• 제33권5호
• /
• pp.289-295
• /
• 2008

The task of chemical spraying has been seriously considered as an irritating and annoying job for Korean rice farmers. An agricultural unmanned helicopter was suggested to solve this problem so as the farmers to have more decent farming condition. The objectives of this study were to analyze the adoptability of an experimental rotor blade (SW05) using the CFD simulation and also to compare the simulation results with experimental results. The simulation results showed that the induced power of this rotor reached to $57{\sim}63%$ of total power and the profile power was about $37{\sim}43%$ of total power. Therefore it can be concluded that this rotor's performance characteristics were not so efficient for the size of unmanned helicopter due to the low induced power and high profile power relatively compared with ones of conventional rotors. The comparison with experimental results showed that the tested lifts were less than 70% of simulated ones at the grip pitch of $12^{\circ}$ and decreased to 40% at the $18^{\circ}$ grip pitch. Therefore, it can be concluded that the rotor was too oversized to be used for a 15.4 kW (21 PS) engine.