• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.6
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• pp.1-9
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• 2006

본 연구의 목적은 여러 가지 비행 모드 상의 로터 성능을 효율적으로 예측하는 것이다. 헬리콥터의 공력 특성을 예측하기 위한 비정상 source-doublet 패널 기법 기반의 수치 기법을 개발하였다. 후류의 형상 예측에는 시간 전진 자유후류모델이 사용되었다. 점성에 의한 확산을 고려한 후류의 roll-up 모사를 위하여 후류의 doublet 패널은 같은 강도의 와류고리로 대체하여 계산하였다. 후류와 양력면의 충돌 문제는 표면격자 내부에 들어간 와류고리의 포텐셜값을 제거하여 해결하였다. 제자리비행의 해석 시에 나타나는 와류 불안정성의 해결에는 slow starting과 vortex core growth 모델을 사용하였다. 로터 공력 해석 프로그램은 제자리비행과 전진비행에 대한 실험 결과와 비교하여 검증하였으며, 실험치와 일치하는 결과를 얻을 수 있었다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.10
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• pp.761-769
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• 2013

Effects of helicopter wakes on helicopter aerodynamics are serious, but the wake configuration is very complicated and hard to predict. The purpose of this study is the detailed observation of wake using numerical methods. Vortex lattice method and freewake method are used to track the vortices in the wake. In this paper, the wake configuration is observed during hovering flight. In the case of hovering flight at the moderate thrust level, besides tip vortex, counter-rotating vortex can be observed at the inboard part of blade. When the vortices move downward, tip vortex and counter-rotating vortex get close and influence to each other. Therefore, vortices are highly distorted due to their own instability.

• Journal of the Korean Society of Marine Environment & Safety
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• v.25 no.6
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• pp.795-801
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• 2019

The aim of this study was to investigate the possibility of the skin-friction reduction by vortex control. A vortical system such as a horseshoe vortex, a hairpin vortex, and a wake region was induced around a hemisphere attached on a Perspex flat plate in the circulating water channel. Hairpin vortices were developed from the wake region and horseshoe vortices were formed by an adverse pressure gradient in front of the hemisphere. The horseshoe vortices located on the flank of the hemisphere induced a high momentum flow in the wake region by the direction of their vorticity. This process increased the frequency of the hairpin vortices as well as the frictional drag on the surface of the wake region. To reduce the skin-friction drag, suction control in front of the hemisphere was applied through a hole. Flow visualization was performed to optimize the free-stream velocity, size of the hemisphere, and size of the suction hole. Once the wall suction control mitigated the strength of the horseshoe vortex, the energy supplied to the wake region was reduced, causing the frequency of the hairpin vortex generation to decrease by 36.4 %. In addition, the change in the skin-friction drag, which was measured with a dynamometer connected to a plate in the wake region, also decreased by 2.3 %.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.6
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• pp.9-15
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• 2004

In this study the development of the two dimensional flowfield around an impulsively translated circular cylinder is numerically simulated using the diffusion vortex method. A detailed streamline pattern of fore wake and main wake of Re=1200, 9500 flowfields are investigated. The results of streamline pattern, the size of main wake and the axial velocities along the rear symmetry axis of the circular cylinder show good agreement with the reported experimental results. The long term wake delvelopment for Re=1200 flowfield was calculated up to ${\tau}=50$. The vortex shedding frequency shows the similar value as that of reported.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.11
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• pp.891-896
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• 2015

This experimental study investigated the wake flow around an elastically supported circular cylinder. In this study, the Reynolds numbers are varied in the region of $1.4{\times}10^4{\leq}Re{\leq}3.2{\times}10^4$. Under these conditions, we have captured the process of the wake mechanism and the moving path of the vortex by measuring the velocity at each position in the wake around the cylinder. Further, these facts from the wind tunnel test are proved by a flow visualization test through a water channel. From the result, we have arrived at the following conclusions : i) The process (formation${\rightarrow}$growth${\rightarrow}$collapse) of vortex is observed in the wake around the cylinder, ii) The vortex efflux angle is approximately $16^{\circ}{\sim}17^{\circ}$ under the experimental conditions. These angles have no relationship with the velocity change and the existence of flow-induced vibrations of the cylinder, and iii) The moving path of the vortex center is obtained by spectrum analysis of the fluctuating velocity behind the cylinder. These are confirmed by conducting visualization tests.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1264-1271
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• 1990

The vortex lattice method was adopted to predict the aerodynamic performance of a horizontal axis wind turbine. For this simulation. the rotor blade was divided into many panels both in chordwise and spanwise direction and then replaced by horseshoe vortices. The wake was divided into two parts of near wake and far wake : the near wake was assumed as helical vortex line elements and the far wake was modeled by semi-infinite circular vortex cylinder. The induced velocity components were calculated by the Biot-Savart law. By this way the power coefficient was obtained and represented as a function of the tip speed ratio. The numerical results obtained were compared with those of the other methods and experimental results and showed good agreement with experimental results.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.12 no.3
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• pp.109-115
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• 2000

Numerical model is proposed to estimate dynamic responses of riser with vortex excitation by inline current. Galerkin's finite decomposition method is implemented for the development of a numerical model and vortex excitation is modeled by coupled wake oscillator proposed by Blevins. The numerical results are inspected through the physical interpretation to give the verification and usefulness of the suggested numerical model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.7
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• pp.709-715
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• 2011

In order to observe the wake interaction between tip vortices generated by a two-bladed rotor with slightly different pitch angles, the velocity components of the tip vortices were measured by using a two-dimensional LDV system. It was observed that the swirl velocity components of the ensuing blade deviated from the Vatistas' n = 2 vortex model and the axial velocity components of the preceding blade deviated from the Gaussian profile. It was also found that in the wake-age range of $200^{\circ}$ to $240^{\circ}$, the filament of the ensuing blade tip vortex was stretched as result of the closing in of two vortices. The results from these observations suggest the possibility that a similar wake interaction is generated in actual rotor blades, especially, in the ones with articulated hubs.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.3
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• pp.57-63
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• 2002

지면효과를 받는 와류면의 2차원 비정상 전개를 이산와류법을 사용하여 연구하였다. 와류면이 말리는 정확한 형상을 얻기 위하여, 두 개의 평활기법을 비교하였다. 타원형 하중분포를 가지는 지면가까이에 있는 날개의 후류말림에 대하여 본 연구 방법을 적용하였다. 날개가 지면가까이 비행할 때, 지면효과로 인하여 와류가 날개길이방향으로 이동하였으며 날개익단부근에서 와류면의 늘림운동이 발생하였다.

• Proceeding of EDISON Challenge
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• 2013.04a
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• pp.382-386
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• 2013

본 연구에서는 Main Rotor Blade Tip 형상 변화에 따른 후류해석을 통해 와류 생성 및 주변 유동을 분석하여 블레이드 팁 형상의 변화가 와류 간섭을 감소시키는지의 여부를 확인하였다. EDISON CFD를 이용하여 블레이드 Blade Tip 형상에 따라 유동이 어떻게 나타나며, Blade 후류의 압력과 점성의 변화를 분석하여 와류의 양상을 해석하였다. 비교 Blade 형상은 2세대 긴 직사각형 모형, KUH 수리온의 Blade, 유로콥터사의 'Blue Edge'로 비교적 최근에 개발된 대표적인 Blade Tip 형상 3개로 정하였다. 결과를 토대로 블레이드 뒷전의 와류흐름 양상을 확인하여 블레이드 와류 간섭현상의 감소를 확인하였다.