• Wind and Structures
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• 제32권5호
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• pp.509-520
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• 2021

The streamlined box is a common type of girders for long-span suspension bridges. Spanning deep canyons, long-span bridges are frequently attacked by strong winds with large angles of attack. In this situation, the flow field around the streamlined box changes significantly, leading to reduction of the flutter performance. The wind fairings have different effects on the flutter performance. Therefore, this study examines the flutter performance of box girders with different wind fairings at large angles of attack. Computational fluid dynamics (CFD) simulations were carried out to extract the flutter derivatives, and the critical flutter state of a long-span bridge was determined. Further comparisons of the wind fairings were investigated by a rapid method which is related to the input energy by the aerodynamic force. The results show that a reasonable type of wind fairings could improve the flutter performance of long-span bridges at large angles of attack. For the torsional flutter instability, the wind fairings weaken the adverse effect of the vortex attaching to the girder, and a sharper one could achieve a better result. According to the input energies on the girder with different wind fairings, the symmetrical wind fairings are more beneficial to the flutter performance

• International Journal of Naval Architecture and Ocean Engineering
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• 제12권1호
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• pp.928-942
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• 2020

The pressure responsiveness property of a skirt-cushion system, which is closely related to the overall performance of Air Cushion Vehicles (ACVs), has always been the difficulty and challenging problem involving cushion aerodynamics and flexible skirt dynamics. Based on a widely used bag and finger skirt-cushion system, the pressure responsiveness properties are investigated numerically. The physical process and mechanism are analyzed and a numerical method for evaluating the pressure responsiveness property is proposed. A cushion-skirt information communication platform is also presented for interchanging the force and the skirt configuration between cushion aerodynamics and flexible skirt dynamics. The pressure responsiveness of a typical skirt-cushion system is calculated and the results demonstrate that the pressure responsiveness property helps alleviate the influence of the cushion height changing on the overall performance of ACVs. Finally, the influences of skirt geometrical and cushion aerodynamic parameters on the pressure responsiveness properties are discussed systematically, giving insight into the design of skirt-cushion systems.

• 한국가시화정보학회지
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• 제19권3호
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• pp.106-114
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• 2021

In the present study, we investigate the flow characteristics of a paraglider canopy with leading-edge tubercles by performing force measurement and surface flow visualizations. The experiment is conducted at Re = 3.3×10⁵ in a wind tunnel, where Re is the Reynolds number based on the mean chord length and the free-stream velocity. The canopy model with leading-edge tubercles has flow characteristics of a two-step stall, showing an earlier onset of the first stall than the canopy model without leading-edge tubercles. However, the main stall angle of the tubercled model is much larger than that of the canopy model without tubercles, resulting in a higher aerodynamic performance at high angles of attack. The delay in the main stall is ascribed to the suppression of separation bubble collapse around the wingtip at high angles of attack.

• Advances in aircraft and spacecraft science
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• 제9권3호
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• pp.263-278
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• 2022

The time-varying structural reliability of an aeroelastic launch vehicle subjected to stochastic parameters is investigated. The launch vehicle structure is under the combined action of several stochastic loads that include aerodynamics, thrust as well as internal combustion pressure. The launch vehicle's main body structural flexibility is modeled via the normal mode shapes of a free-free Euler beam, where the aerodynamic loadings on the vehicle are due to force on each incremental section of the vehicle. The rigid and elastic coupled nonlinear equations of motion are derived following the Lagrangian approach that results in a complete aeroelastic simulation for the prediction of the instantaneous launch vehicle rigid-body motion as well as the body elastic deformations. Reliability analysis has been performed based on two distinct limit state functions, defined as the maximum launch vehicle tip elastic deformation and also the maximum allowable stress occurring along the launch vehicle total length. In this fashion, the time-dependent reliability problem can be converted into an equivalent time-invariant reliability problem. Subsequently, the first-order reliability method, as well as the Monte Carlo simulation schemes, are employed to determine and verify the aeroelastic launch vehicle dynamic failure probability for a given flight time.

• 국제초고층학회논문집
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• 제11권4호
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• pp.265-276
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• 2022

Double-Skin Facades (DSF) on tall buildings are becoming increasingly common in urban environments due to their ability to provide architectural merit, passive design, acoustic control and even improved structural efficiency. This study aims to understand the effects of porous DSF on the aerodynamic characteristics of tall buildings using wind tunnel tests. High Frequency Force Balance and pressure tests were performed on the CAARC standard tall building model with a variable porous DSF on the windward face. The introduction of a porous DSF did not adversely affect the overall mean forces and moments experienced by the building, with few differences compared to the standard tall building model. There was also minimal variation between the results for the three porosities tested: 50%, 65% and 80%. The presence of a full-height porous DSF was shown to effectively reduce the mean and fluctuating wind pressure on the side face of the building by about 10%, and a porous DSF over the lower half height of the building was almost as effective. This indicates that the porous DSF could be used to reduce the design load on cladding and fixtures on the side faces of tall buildings, where most damage to facades typically occurs.

• 터널과지하공간
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• 제11권4호
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• pp.319-327
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• 2001

장대도로터널 환기시스템의 최적 설계 및 운영은 새로운 설계도구의 개발을 요구하며 최근 환기 관련 변수들의 동적인 관계를 정량적으로 분석할 수 있는 도구로써 널리 이용되고 있다. 본 연구는 도로터널 환기시뮬레이션 모델인 NETVEN의 현장 검증연구를 국내 도로터널을 대상으로 수행함을 목적으로 하였다. 환기방식 및 차량통행방식이 다른 4개 터널을 대상으로 시뮬레이션 및 현장 측정연구를 수행하였으며 시뮬레이션 모델의 적용 결과와 측정 결과의 차이는 다음 4가지 요인에 기인한 것으로 분석된다. (1)실제 터널내 기류는 정상류 유동을 가정하는 시뮬레이션 모델과는 달리 천이-지연계의 특성을 가지고 있다. (2) 시뮬레이션 모델은 정상상태를 가정하나 측정값은 순간값을 대상으로 하므로 분산이 상대적으로 크게 나타난다. 환기력 관련변수들은 거의 정상상태가 아니며 교통 및 기상관련 변수들은 특히 순간적인 변화가 심한 편이다. (3) CO 농도의 경우에 출구부근에서의 차이가 상대적으로 크게 나타나는 현상이 뚜렷하며 특히 농도가 상대적으로 클수록 차이가 크다. 이는 난류확산에 의한 농도 저하가 원인으로 보인다. 난류확산 가능성은 최대 농도지점이 출구점이 아닌 약간 안쪽 지점에서 나타나는 현상에서도 나타나고 있다. (4) 교통량이 많을 경우 환기속도 오차가 크게 나타나므로 차량의 배기특성뿐만 아니라 공기역학적 특성에 대한 지속적인 연구가 필요하다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.671-679
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• 2008

The effect of Pre-cooled Turbojet Engine installation and nozzle exhaust jet on Hypersonic Turbojet EXperimental aircraft(HYTEX aircraft) were investigated by three-dimensional numerical analyses to obtain aerodynamic characteristics of the aircraft during its in-flight condition. First, simulations of wind tunnel experiment using small scale model of the aircraft with and without the rectangular duct reproducing engine was performed at M=5.1 condition in order to validate the calculation code. Here, good agreements with experimental data were obtained regarding centerline wall pressures on the aircraft and aerodynamic coefficients of forces and moments acting on the aircraft. Next, full scale integrated analysis of the aircraft and the engine were conducted for flight Mach numbers of M=5.0, 4.0, 3.5, 3.0, and 2.0. Increasing the angle of attack $\alpha$ of the aircraft in M=5.0 flight increased the mass flow rate of the air captured at the intake due to pre-compression effect of the nose shockwave, also increasing the thrust obtained at the engine plug nozzle. Sufficient thrust for acceleration were obtained at $\alpha=3$ and 5 degrees. Increase of flight Mach number at $\alpha=0$ degrees resulted in decrease of mass flow rate captured at the engine intake, and thus decrease in thrust at the nozzle. The thrust was sufficient for acceleration at M=3.5 and lower cases. Lift force on the aircraft was increased by the integration of engine on the aircraft for all varying angles of attack or flight Mach numbers. However, the slope of lift increase when increasing flight Mach number showed decrease as flight Mach number reach to M=5.0, due to the separation shockwave at the upper surface of the aircraft. Pitch moment of the aircraft was not affected by the installation of the engines for all angles of attack at M=5.0 condition. In low Mach number cases at $\alpha=0$ degrees, installation of the engines increased the pitch moment compared to no engine configuration. Installation of the engines increased the frictional drag on the aircraft, and its percentage to the total drag ranged between 30-50% for varying angle of attack in M=5.0 flight.

• 한국추진공학회지
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• 제13권3호
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• pp.41-49
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• 2009

본 논문은 공기흡입식 추진기관의 고체 로켓 부스터 분리에 관한 수학적 모델링과 시뮬레이션 기법을 기술하였다. 비행체 및 부스터는 하나의 다물체(multi-body)로 고려하였고 부스터는 단지 비행체의 축 방향으로 움직이는 것으로 가정하였다. 비행체 및 부스터의 동적 운동은 Kane 방법에 의해 모델링 되었다. 다양한 부스터 위치에 따라 전체 시스템에 작용하는 공력은 DATCOM 소프트웨어를 사용하여 산출되었으며 부스터 분리 유효 작용면에 작용하는 내부 분리 압력은 일반적인 기체역학 및 Taylor-MacColl 관계식에 의해 산출되었다. 수치적 해석은 Mathworks사의 Matlab이 사용되었다. 해석 결과에 의하면 부스터 분리 동안 마하수 및 받음각 변화 등은 크지 않는 것으로 나타났으며, 실제 시험 장치를 이용한 부스터 분리 시험이 진행될 경우 자세 각 변화, 흡입 유동 특성 등은 무시할 만한 수치임을 확인할 수 있었다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2009년 추계학술대회논문집
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• pp.151-158
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• 2009

To implement the insects' flapping flight for developing flapping MAVs(micro air vehicles), the unsteady flow characteristics of the insects' forward flight is investigated. In this paper, two-dimensional FSI(Fluid-Structure Interaction) simulations are conducted to examine realistic flow features of insects' flapping flight and to examine the flexibility effects of the insect's wing. The unsteady incompressible Navier-Stokes equations with an artificial compressibility method are implemented as the fluid module while the dynamic finite element equations using a direct integration method are employed as the solid module. In order to exchange physical information to each module, the common refinement method is employed as the data transfer method. Also, a simple and efficient dynamic grid deformation technique based on Delaunay graph mapping is used to deform computational grids. Compared to the earlier researches of two-dimensional rigid wing simulations, key physical phenomena and flow patterns such as vortex pairing and vortex staying can still be observed. For example, lift is mainly generated during downstroke motion by high effective angle of attack caused by translation and lagging motion. A large amount of thrust is generated abruptly at the end of upstroke motion. However, the quantitative aspect of flow field is somewhat different. A flexible wing generates more thrust but less lift than a rigid wing. This is because the net force acting on wing surface is split into two directions due to structural flexibility. As a consequence, thrust and propulsive efficiency was enhanced considerably compared to a rigid wing. From these numerical simulations, it is seen that the wing flexibility yields a significant impact on aerodynamic characteristics.

• 한국항공우주학회지
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• 제32권3호
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• pp.10-16
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• 2004

본 논문에서는 축소형실험을 통하여 제자리 비행하는 헬리콥터 주로터에서 발생하는 소음의 특성을 측정하는 실험을 수행하였다. 이를 위하여 구동장치에서의 발생소음이 적은 저소음 로터시험장치를 설계/제작하였으며, 아울러 로터의 공력특성을 측정하기 위하여 작동조건에서의 회전수, 추력 및 토크를 함께 계측하였고 이에 대한 문제점을 검토하였다. 사용된 하중 측정장치의 교정을 통하여 이의 정확도를 확인하였으며, 두개의 블레이드로 구성된 직경 1.2m의 축소형 로터를 이용한 실험에서 측정된 추력값이 이론적 계산값과 비교하여 비교적 잘 일치함을 알 수 있었으나 토크 측정값은 잘 일치하지 않음을 확인하였다. 소음 측정실험을 통하여 로터 구동장치로 인한 배경소음이 로터 소음에 비하여 현저히 작음을 확인하였고, 각 로터 발생소음원의 특성에 따라 날개끝 속도에 의하여 달리 무차원화 됨을 확인하였고 방향성 특성도 변화함을 확인하였다.