• Proceedings of the Korea Water Resources Association Conference
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• 2009.05a
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• pp.919-923
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• 2009

케이블 교량에서 발생하는 사장케이블의 진동현상에 대한 현상학적 특성을 명확히 이해해야, 사장케이블의 적합한 제진설계가 가능하다. 본 연구에서는 ADINA CFD 코드를 이용하여, 이중원형실린더에서 발생하는 와류방출현상과 실린더에 작용하는 외력을 해석함으로써, 사장케이블에서 발생하는 진동현상의 원인을 규명하고자 하였다. 이를 위하여 본 연구에서는 흐름의 레이놀즈수, 실린더간 간격과 이중원형실린더에 대한 바람의 입사각을 변화시키며 이중원형실린더에서 발생하는 와류방출현상과 이로 인해 실린더에 작용하는 항력, 양력과 같은 외력을 분석하였다. 분석 결과, 풍상측 케이블에는 일방향의 항력이 주로 작용하므로 케이블 진동이 제한적으로 발생하나, 풍하측 케이블에는 항력과 같은 규모의 진동하는 양력이 작용하는 것으로 나타났으며 이로 인해 풍하측 케이블에서 진동이 크게 발생하는 것으로 나타났다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.2132-2146
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• 1991

Extensive experiments were carried out to investigate the mass transfer and flow structures around a circular cylinder with annular fins in crossflow. The naphthalene sublimation method was employed to measure the circumferential and longitudinal variations of mass transfer from the circular cylinder between annular fins and H is the height of the fin from the cylinder surface. A remarkable enhancement of mass transfer due to the horseshoe vortices was observed near the corner junction between the annular fin and circular cylinder. The present results indicate that the local circumferential Sherwood number shows the higher values on the front stagnation point. The maximum augmentation of mass transfer rate at the center of cylinder is found near L/H-0.15 due to the separation bubble along the annular fins. The secondary flows, which are the corner vortices V2 and V3 near the end wall of the annular fin, are fairly predicted from the distributions of local Sherwood number in the spanwise direction. The average Sherwood number of overall surface at L/H=0.15 is larger 2.0 times than that of without annualr fins. The correlations of total average mass transfer rate with L/H and Re$_{L}$ can also be obtained.d.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.8
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• pp.16-23
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• 2006

The effects of strake planform shapes on the vortex formation, interaction, and breakdown characteristics of double-delta wings were investigated through pressure measurements of upper wing surface and off-surface flow visualization. Three different shapes of strakes were attached to a delta wing respectively to form double-delta wing configurations and tested in a medium-sized subsonic wind tunnel. The results of the pressure measurements indicated that the strake planform having a higher sweep angle generated more concentrated vortex systems at upstream locations, which, however, tended to diffuse and break down much faster at the downstream locations. It was also found from the off-surface visualization results that the cause for the vortex concentration was due to the acceleration of coiling and merging processes between the wing and strake vortices.

• Journal of the Korean Institute of Gas
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• v.23 no.3
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• pp.27-32
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• 2019

In this study, the vorticity distribution and the temperature distribution change around a circular cylinder were compared and analyzed with time for constant inlet velocity and periodic inlet velocity. Also, the frequency characteristics of the flow were analyzed by analyzing the time variation of lift and drag and their PSD(power spectral density). In the case of constant inlet velocity, the well known Karman vorticity distribution was shown, and vortices were alternately generated at the upper and lower sides of the circular cylinder. In case of periodic inlet velocity, it was observed that vortex occurred simultaneously in the upper and lower sides of the circular cylinder. In both cases, it was confirmed that the time dependent temperature distribution changes almost the same behavior as the vorticity distribution. For the constant inlet velocity, the vortex flow frequency is 31.15 Hz, and for the periodic inlet velocity, the vortex flow frequency is equal to the preriodic inlet velocity at 15.57 Hz. The mean surface Nusselt number was 99.6 for the constant inlet velocity and 110.7 for the periodic inlet velocity, which showed 11.1% increase in surface heat transfer.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.4
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• pp.446-451
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• 2020

Flow induced vibration in a heat exchanger may cause damage to piping. The purpose of this study was to compare the characteristics of vortex shedding behavior through the circular tube banks at several tube locations, No.1, No. 10, and No. 19, with respect to time when the flow velocity of the inlet is constantly and periodically fluctuating.(60) The time characteristics of lift and the PSD characteristics were also investigated. In the case of periodic inlet flow velocity, strong vortex occurred at some time and after that time, a weak vortex was generated through the tube banks simultaneously. In the case of constant inlet flow velocity, the lift fluctuating frequency was 37.25Hz and that at the No. 19 tube was 18.63Hz and near 50Hz. In the case of periodic inlet flow velocity, the lift fluctuating frequency was 37.25Hz and 18.63Hz. The lift fluctuating frequency at No. 19 tube was observed broadly from 20Hz and 50Hz.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.5
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• pp.359-367
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• 2018

The vortex ring state that occurs during the descending flight of a rotorcraft generates a circulating flow like a donut near the rotating surface, and it often causes a rotorcraft fall due to loss of thrust. In this paper, we have physically identified the flow field in the vortex ring state of the quadcopter, one of the types of unmanned aerial vehicles. The descending flight of the quadcopter was simulated in a 1m subsonic wind tunnel of the Korea Aerospace Research Institute(KARI) and the Particle Image Velocimetry(PIV) was used for the flow field measurement. The induced velocity in the hovering state is estimated by using the momentum theory and the test was carried out in the range of descent rate at which the vortex ring condition could be caused. The development and the direction of the vortex ring were confirmed by the measurement of the flow field according to not only the descent rate but also propeller separation distance. In addition, the results of the study show the vortex ring state can be predicted sufficiently by measuring the flow velocity around the quadcopter.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.4
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• pp.613-618
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• 2019

The flow-induced vibration in a heat exchanger may cause the damage to piping. Therefore, it is necessary to establish the flow induced vibration characteristics for the structural stability of a heat exchanger. The purpose of this study was to compare the generation, development, and separation characteristics of a vortex around a circular tube with respect to time when the flow velocity of the inlet was fluctuating constantly and periodically. The time characteristics of lift and drag and the PSD characteristics were also investigated. In the case of a constant inlet flow velocity, the well-known Kalman vorticity distribution was shown. The vortex generation, growth, and separation were also observed alternately at the upper and lower sides of the tube. In the case of periodic inlet flow velocity, the vortex occurred simultaneously in the upper and lower sides of the tube. In the case of constant inlet flow velocity, the magnitude of the lift PSD was 500 times larger than that of drag. The frequency was 31.15 Hz and that of drag was doubled at 62.3 Hz. In case of a periodic inlet flow velocity, the PSD of the drag was approximately 500 times larger than that of lift. The frequency was 15.57 Hz, which was the same as the inlet-flow velocity frequency. In addition, the frequency of lift was 31.15 Hz, which was the same Karman vortex frequency.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.1
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• pp.92-100
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• 2018

Vortical systems are considered a main feature to sustain turbulence in a boundary layer through interaction. Such turbulent structures result in frictional drag and erosion or vibration in engineering applications. Research for controlling turbulent flow has been actively carried out, but in order to show the effect of vortices in a turbulent boundary layer, it is necessary to clarify the mechanism by which turbulent energy is transferred. For this purpose, it is convenient to demonstrate and capture phenomena in a laminar boundary layer. Therefore, in this study, the interactions of disturbed flow around a hemisphere on a flat plate in laminar flow were analyzed. In other words, a street of hairpin vortices was generated following a wake region formed after flow separation occurred over a hemisphere. Necklace vortices surrounding the hemisphere also appeared due to a strong adverse pressure gradient that brought high momentum fluid into the wake region thereby leading to an increase in the frequency of hairpin vortices. To mitigate the effect of these necklace vortices, local suction control was applied through a hole in front of the hemisphere. Flow visualization was recorded to qualitatively determine flow modifications, and hot-film measurements quantitatively supported conclusions on how much the power of the hairpin vortices was reduced by local wall suction.

• Journal of Ocean Engineering and Technology
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• v.11 no.4
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• pp.130-140
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• 1997

원형주상체 주위의 유동을 규명하기 위해 회류수조에서 원주방향으로 24개의 위치에 대하여 압력을 계측하였으며, laser sheet을 이용하여 유동을 가시화 하였다. Reynolds수가 4800에서 40000인 범위에 대하여 실험을 수행하였다. 또한, 원형단면체 주위의 비정상 층류유동에 대한 Navier-Stokes방정식의 해를 구하는 수치해석기법을 개발하였다. 효과적인 격자배치를 위하여 H와 O-type의 중첩격자를 사용하였고, 이산화 방법으로는 정규격자시스템에서 유한차분법을 적용하였다. 실험과 수치해석결과에서 뚜렷한 와류박리현상을 볼 수 있었으며, 압력계수 (C$_{p}$ ), 항력계수(C$_{D}$), 스트로얼수(St)를 정량적으로 비교하였을 때, 비교적 잘 일치하는 것을 확인하였다.다.

• The Journal of the Acoustical Society of Korea
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• v.10 no.6
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• pp.82-100
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• 1991

본 연구에서는 회전톱날에서 발생하는 공기소음원 규명의 실현가능성을 검토하였다. 음향인텐시 티법은 3차원 선도, 인벤시티 벡터에너지선도, 동고선도 등의 표현에 유용한 장점을 갖고 있다. 근거리 음장 거동에 대한 추정, 주파수영역에서의 벡터 또는 스칼라 음향인텐시티는 소음원규명의목적으로 사 용되는 측정기법이다. 결과에 따르면 난류는 원형톱날의 이 부근에 나타나며, 톱날의 변동압력 측정에서 와류구조의 영향에 대한 근거는 측정된 음향인텐시티에 의해 도출된다. 도한 회전속도가 증가함에 딸k, 상호작용은 도플러현상을 일으키는 중요한 소음메타니즘이 될 수 있다.