• 한국전산유체공학회지
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• 제7권4호
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• pp.35-41
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• 2002

The projectile afterbodies for zero-lift drag reduction has been analyzed using the Navier-Stokes equations with the κ-εturbidence model. The numerical method of a second order upwind scheme has been used on an unstructured adaptive grid system. Base drag reduction methods that have been found effective on axisymmetric bodies are boattailing, base bleed, base combustion, locked vortex afterbodies and multistep afterbodies. In this paper, turbulence flow and pressure charateristics have been studied for geometries of multistep afterbodies. The important geometrical and flow parameters relevant to the design of such afterbodies have been identified by step number, length and height. The flow over multistep aftoerbodies or base have many kinds of compressible flow characteristics including expansion waves at the trailing edge, recompression waves, separation and recirculating flow in the base region, shear flow and wake flow. The numerical results have been compared and analyzed with the experimental data. The flow characteristics have been clearly shown.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2011년 춘계학술대회논문집
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• pp.343-349
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• 2011

A loosely coupling method is adopted to combine a computational fluid dynamics (CFD) solver and the comprehensive structural dynamics (CSD) code, CAMRAD II, in a systematic manner to correlate the airloads, vortex trajectories, blade motions, and structural loads of the HART I rotor in descending flight condition. A three-dimensional compressible Navier-Stokes solver, KFLOW, using chimera overlapped grids has been used to simulate unsteady flow phenomena over helicopter rotor blades. The number of grids used in the CFD computation is about 24 million for the isolated rotor and about 37.6 million for the rotor-fuselage configuration while keeping the background grid spacing identical as 10% blade chord length. The prediction of blade airloads is compared with the experimental data. The current method predicts reasonably well the BVI phenomena of blade airloads. The vortices generated from the fuselage have an influence on airloads in the 1st and 4th quadrants of rotor disk. It appeared that presence of the pylon cylinder resulted in complex turbulent flow field behind the hub center.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.804-809
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• 2003

In this study, 2D computations of the Aeolian tones for some obstacles (circular cylinder, square cylinder and NACA0012 airfoil) are simulated. First of all, we calculate the flow noise generated by a uniform flow around a two-dimensional circular cylinder at Re=150 are simulated by applying the finite difference lattice Boltzmann method (FDLBM). The third-order-accurate up-wind scheme (UTOPIA) is used for the spatial derivatives, and the second-order-accurate Runge-Kutta scheme is applied for the time marching. The results show that we successively capture very small acoustic pressure fluctuation with the same frequency of the Karman vortex street compared with the pressure fluctuation around a circular cylinder. The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream due to the Doppler effect in the uniform flow. For the downstream, on the other hand, it is faster. To investigate the effect of the lattice dependence, furthermore, simulations of the Aeolian tones at the low Reynolds number radiated by a square cylinder and a NACA0012 airfoil with a blunt trailing edge at high incidence are also investigated.

• 한국소음진동공학회논문집
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• 제24권12호
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• pp.962-968
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• 2014

The lattice Boltzmann method(LBM) has attracted attention as an alternative numerical algorithm for solving fluid mechanics since the end of the 90's. In these days, its intrinsic unsteadiness and rapid increase in computing power make the LBM be more applicable for computing flow-induced noise as well as fluid dynamics. The lattice Boltzmann method is a weakly compressible scheme, so we can get information about both aerodynamics and aeroacoustics from single simulation. In this paper, numerical analysis on Aeolian tone noise generated by tandem-twin square cylinders in duct is performed using the LBM. For simplicity, laminar two-dimensional fluid models are used. To verify the validity and accuracy of the current numerical techniques, numerical results for the laminar duct and the cylinder flows are compared with the analytical solution and the measurement, respectively. Then, aerodynamic noise of the twin tandem square cylinders is investigated. It is shown that the aerodynamic noise from the twin tandem square cylinders can be reduced by controlling the distance between the cylinders.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2002년도 추계학술대회 논문집
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• pp.234-239
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• 2002

Acoustic sounds generated by a uniform flow around a two-dimensional circular cylinder at Re=200 are simulated using finite difference lattice Boltzmann method. A third-order-accurate up-wind scheme is used for spartial derivatives, and a second-order-accurate Runge-Kutta scheme is used for time marching. The results show that in capturing very small acoustic pressure fluctuation with same frequency of Karman vortex street compared with the pressure fluctuation around a circular cylinder. The propagation velocity of acoustic sound is presented that acoustic which approaches tire upstream due to Doppler effect in the uniform flow slowly propagates, while that for the downstream quickly propagates. It is also apparent that the size of sound pressure is proportional for central distance $r^{-1/2}$ of the cylinder.

• 대한기계학회논문집B
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• 제22권2호
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• pp.240-252
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• 1998

The purpose of this study is to develop a method for predicting the aerodynamic performance of the low speed airfoils in the 2-dimensional, steady and viscous flow. For this study, the airfoil geometry is specified by adopting the longest chord line system and by considering local surface curvature. In case of the inviscid incompressible flow, the analysis is accomplished by the linearly varying strength vortex panel method and the Karman-Tsien correction law is applied for the inviscid compressible flow analysis. The Goradia integral method is adopted for the boundary layer analysis of the laminar and turbulent flows. Viscous and inviscid solutions are converged by the Lockheed iterative calculating method using the equivalent airfoil geometry. The analysis of the separated flow is performed using the Dvorak and Maskew's method as the basic method. The wake effect is also considered by expressing its geometry using the formula of Summey and Smith when no separation occurs. The computational efficiency is verified by comparing the computational results with experimental data and by the shorter execution time.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.612-621
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• 2008

For the purpose of improving accuracy in jet noise prediction and investigating its generation mechanism, high subsonic jets were computed by using compressible Large Eddy Simulation(LES), wherein the inflow forcing or disturbance added in the inflow shear layer was incorporated. The far-field Sound Pressure Levels(SPL) as well as the flow field resulted in good agreement with available experimental data by applying only the high azimuthal modes among the inflow forcing parameters. We found that this result was due to an important role of the inflow forcing upon breaking down the axiymmetric vortices that caused high amplitude velocity and pressure fluctuations. In order to examine generation mechanism of the dominant noise component, wavelet transformation was introduced to reveal the presence of a well-organized structure of pressure fluctuations that originated mainly from vortex motions near the end of the jet potential core. This structure took a train of alternately positive and negative wavelet-transformed pressure regions along the jet distance, spreading towards the downstream with advection and propagation. It was concluded that this structure and its dynamic motion are the reason why a high subsonic jet produces the dominant noise with a particular downstream directivity.

• 한국항공우주학회지
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• 제33권7호
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• pp.1-8
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• 2005

풍동 시험부 비정상 벽면효과에 대한 연구를 위해 폐쇄형 시험부내의 원형 실린더 주위 유동장에 대한 수치적 연구를 수행하였다. 수치기법은 Roe의 flux-difference-splitting을 사용한 격자점 중심 유한체적법과 이중시간 전진 기법을 사용하는 내재적 시간적분법을 사용하였다. 계산 결과 폐쇄형 시험부에는 실린더 주위 비정상 유동장에 압력구배를 강화시켜 실린더의 양력 및 항력의 진폭을 크게 하고, 실린더 뒷전에서의 기저압력을 작게 하여 항력을 증가시키는 벽면효과가 있음을 확인하였다. 또한, 이러한 시험부 벽면은 실린더 와류 shedding 주파수를 커지게 하는 효과가 있다. 시험부 벽면에서의 압력은 벽면효과가 포함된 shedding 주파수를 기본으로 하는 고조파 현상을 보인다.

• 대한기계학회논문집
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• 제12권1호
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• pp.106-115
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• 1988

본 연구에서는 경계층 유동을 충류, 천이, 난류 영역을 포함하는 압축성 유동으로 가정하였고, Morgan 등이 제시한 새로운 질점분할 방법을 사용하여 속도분포를 계산하고, 점성 압축성 효과를 고려하기 위하여 viscous--inviscid interaction 법을 사용하였고 이 계산 결과를 기존의 실험값과 비교하여,타당성을 확인하였다.그리고 최적 양력의 익형 설계는 Augmented Lagrange multiplier 법을 사용하였고 비구속 조건을 갖는 목적함수 augmented lagrangian의 최소화는 Davidan-Fletcher-Powell 방법 중 self-scaling quasi-Newton algorithm을 사용하였다. 그리고 NACA 23012를 기본 익형으로 하고 NACA 64-2-415, NACA 64-2-A215, NACA 65-3-218를 보상 익형으로 하여 최대 양력익형을 설계 하였다.

• 대한기계학회논문집B
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• 제39권6호
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• pp.477-483
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• 2015

공동유동에 관한 연구는 공동시스템에서 발생하는 소음/진동 문제로 인하여 많은 연구가 이루어졌으며, 현제 항공우주 산업의 급속한 발전과 더불어 다양한 공학적 장치에 적용되고 있다. 하지만, 실제 공학적 응용에서 많이 적용되는 곡면 벽상에 설치한 공동유동에 관한 연구에 대해서는 거의 수행되지 않았다. 본 연구에서는 곡면벽상에 설치한 공동유동의 특성을 조사하기 위해 수치계산을 수행하였으며, 곡면의 곡률 반경의 비(L/R) 및 입구 유동의 마하수를 변화시켜, 천음속/초음속 공동유동에서 발생하는 압력진동을 조사하였다. 그 결과 곡면에 부착된 공동유동의 경우 와류의 상호작용으로 인한 압력 교란을 완화시켰으나, 압력 진동의 진폭을 증가시켰으며, 또한 마하수가 증가함에 따라 압력 진동의 진폭이 증가하였다. 주파수 분석 결과, 곡관의 무차원 진동수는 직관에 비해 낮은 값이 측정되었으며, 또한 Rossiter의 실험값에 비해 낮은 값을 가졌다.