• The Journal of the Acoustical Society of Korea
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• v.39 no.2
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• pp.77-86
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• 2020

In this study, the Eulerian/Lagrangian one-way coupling method is proposed to predict flow noise due to Blade-Tip Vortex Cavitation (BTVC). The proposed method consists of four sequential steps: flow field simulation using Computational Fluid Dynamics (CFD) techniques, reconstruction of wing-tip vortex using vortex model, generation of BTVC using bubble dynamics model and acoustic wave prediction using the acoustic analogy. Because the CFD prediction of tip vortex structure generally suffers from severe under-prediction of its strength along the steamwise direction due to the intrinsic numerical damping of CFD schemes and excessive turbulence intensity, the wing-tip vortex along the freestream direction is regenerated by using the vortex modeling. Then, the bubble dynamics model based on the Rayleigh-Plesset equation was employed to simulate the generation and variation of BTVC. Finally, the flow noise due to BTVC is predicted by modeling each of spherical bubbles as a monople source whose strength is proportional to the rate of time-variation of bubble volume. The validity of the proposed numerical methods is confirmed by comparing the predicted results with the measured data.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1997.04a
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• pp.63-72
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• 1997

본 논문은 분사제트 주위에 형성되는 와류를 조절하여 제트를 제어하기 위하여 유동가시화, 속도분포 및 난류성분을 측정하는 실험을 수행하였다. 와류를 조절하기 위한 방법으로 제트노즐 주위에 환형관을 설치하여 환형관으로부터 2차제트를 분사 또는 흡입함으로써 제트주위에 형성되는 전단류를 변화시켰다. 2차제트 분사시 주제트 주위에 형성되는 와류의 발달을 억제함으로써 제트 포텐셜코어의 길이가 아주 길어지는 제트유동을 얻을 수 있었다. 환형관으로부터 주제트주위의 유체를 흡입하는 경우 제트주위의 전단류가 흡입비 R=1.3∼l.65에서 대류불안정성에서 절대불안정성으로 바뀜으로써 형성된 와류가 하류에서 제트중심부까지 발전, 결합되는 것을 방지하여 더 긴포텐셜코어와 중심에서 낮은 난류강도를 얻었다. 위의 결과는 환형관 주위에 부착한 깃의 높이 변화에 따라서 변화하였는데, 이것은 깃이 환형관을 통한 흡입유동의 유로역할을 함으로써 제트밖으로부터 흡입되는 것을 방지할 수 있었다. 분사제트 벡터링을 위하여 제트노즐 주위의 환형관을 이등분하여 한쪽으로만 제트주위의 유동을 흡입함으로써 제트주위에 다른 전단류를 형성함과 동시에 Coanda효과를 이용하여 분사제트를 편향시켰다. 편향되는 정도 및 난류성분은 홉입속도 비에 따라서 크게 바뀌었다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2012.05a
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• pp.244-248
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• 2012

A series of hybrid rocket combustion experiments were carried out with PMMA/GOx changing diameter and length of the disk installed at pre-chamber. The disk can generate vortex shedding flow and change flow conditions prior to entering the fuel grain which could also alter the combustion characteristics and pressure oscillations. The interaction of vortex shedding in the pre-chamber and small-scale vortices adjacent to burning surfaces by using combustion test.

• The Journal of the Acoustical Society of Korea
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• v.43 no.1
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• pp.103-111
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• 2024

Axial-flow fans are used to transport fluids in relatively low-pressure flow regimes, and a variety of design variables are employed. The tip geometry of an axial fan plays a dominant role in its flow and noise performance, and two of the most prominent flow phenomena are the tip vortex and the tip leakage vortex that occur at the tip of the blade. Various studies have been conducted to control these three-dimensional flow structures, and winglet geometries have been developed in the aircraft field to suppress wingtip vortices and increase efficiency. In this study, a numerical and experimental study was conducted to analyze the effect of winglet geometry applied to an axial fan blade for an air conditioner outdoor unit. The unsteady Reynolds-Averaged Navier-Stokes (RANS) equation and the FfocwsWilliams and Hawkings (FW-H) equation were numerically solved based on computational fluid dynamics techniques to analyze the three-dimensional flow structure and flow noise numerically, and the validity of the numerical method was verified by comparison with experimental results. The differences in the formation of tip vortex and tip leakage vortex depending on the winglet geometry were compared through a three-dimensional flow field, and the resulting aerodynamic performance was quantitatively compared. In addition, the effect of winglet geometry on flow noise was evaluated by numerically simulating noise based on the predicted flow field. A prototype of the target fan model was built, and flow and noise experiments were conducted to evaluate the actual performance quantitatively.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.9
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• pp.745-751
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• 2012

Hybrid rocket shows interesting characteristics of complicated mixing layers developed by interactions between turbulent oxidizer flow and mass flow from surface due to fuel vaporization. In this study, compressible LES with a ring structure attached at the entrance of the combustor are performed. According to one recent report, adding a ring structure in the middle of the combustor helps increasing regression rate. From the numerical results, it is seen that vortex structures near the wall becomes stronger due to the interaction with surface mass injection, and the local heat flux increases due to the vortices. This phenomenon is obviously related to the generation of dimple structures which are seen in the number of experiments. Also, the ring structure at the entrance induces strong vortex flow which enhances heat transfer to the wall surface and mixing between fuel and oxidizer as well as reaction efficiency.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05a
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• pp.514-519
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• 1998

범용 전산유체해석(computational fluid dynamics) 코드인 CFX를 이용하여 지지격자 형상에 따른 봉다발 부수로에서의 난류유동 수치해석을 수행하였다 ABB와 SIMENS가 각각 개발한 split vane이 부착된 지지격자와 원자력연구소가 개발중인 회전유동 발생장치가 부착된 지지격자를 포함하는 부수로 난류유동을 분석하였다. 각각의 지지격자 형상에 대해 부수로에서의 축방향 속도, 횡방향 속도, 난류 운동에너지, 와류크기와 압력강하 둥을 비교-분석하였다. 세가지 경우 모두 유사한 경향을 나타냈으나 SIMENS split vane의 유동 전향날개가 크기때문에 와류와 압력강하가 다소 크게 예측되었다. 난류 운동에너지와 와류크기는 지지격자 근처에서 현저히 증가한 후 급격히 감소하는 측정결과를 CFX예측결과에서도 확인할 수 있었다. CFX 예측결과는 지지격자 근처에서 실험 결과와 다소 큰 차이를 보였으나 비교적 부수로 유동특성을 잘 나타낸다.

• Proceeding of EDISON Challenge
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• 2016.03a
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• pp.571-575
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• 2016

두꺼운 에어포일은 받음각이 클 때 역압력 구배가 일어날 수 있기 때문에 날개의 중앙부 후방에서 유동박리(Flow separation)와 와류 진동(Vortex shedding)이 쉽게 발생할 수 있다고 알려져 있다. 항공기가 이 착륙할 때 받음각이 커짐에 따라 유동박리에 의한 실속이 발생할 수 있는데 이를 지연 시켜 실속각을 크게 하면 안전성이나 효율 면에서 유리하다. 이를 위해 날개에 Hole을 만들어 와류를 잡아 유동의 박리를 지연시키고자 하였다. 본 연구에서는 EDISON_CFD 시스템의 2D_Incomp_P 솔버를 사용하여 NACA0018 에어포일의 윗면에 다른 위치의 Hole이 있을 때와 크기가 다른 Hole이 있을 때의 실속각이 가장 커지는 경우를 찾아보았다. Hole의 위치와 반지름 크기를 변화시켰을 때 각각의 최대 양력 계수를 비교하여 실속각의 증가와 Streamline을 그려 유동박리가 지연됨을 확인하였다.

• Aerospace Engineering and Technology
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• v.8 no.1
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• pp.197-206
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• 2009

To improve the aerodynamic efficiency of Smart Unmanned Aerial Vehicle (SUAV), vortex generators and flow fence are applied on the surface and the tip of wing. The initially applied vortex generator increased maximum lift coefficient and delayed the stall angle while it produced excessive increase in drag coefficient. It turns out reduction of the airplane's the lift/drag ratio. The new vortex generators with L-shape and two different height, 3mm and 5mm, were used to TR-S4 configuration to maintain the desired level of maximum lift coefficient and drag coefficient. Flow fence was also applied at the end of both wing tip to reduce the interaction between nacelle and wing when nacelle tilting angles are large enough and produce flow separation. To examine the effect of flow fence, flow visualization and force and moment measurements were done. The variation of the aerodynamic characteristics of SUAV after applying flow control devices are summarized.

• 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
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• v.12 no.5
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• pp.1175-1188
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• 1988

Linear estimation in isotropic turbulence is examined to approximate conditional averages in the form of fluctuating velocity fields conditioned on local velocity. The conditional flow fields and their associated vorticity field are computer using experimental data [Van Atta and Chen] and energy spectrum model [Driscoll and Kennedy]. It appears that ring vorticies could be the dominant structure. Due to the extremely large vorticity in the viscous region of a conditional ring vortex, the energy spectrum model can be used appropriately by changing the Reynolds number. The hairpin vortex could be detected by combining vorticies in isotropic field with an anisotropic orientation imbedded in uniform mean shear flow and this is consistent with other studies [Kim and Moin].