• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.256-256
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• 2022

레이놀즈 분해법은 유속을 비롯한 변수를 평균 성분과 변동 성분으로 분해하는 분석 방법으로, 난류 분석의 기본이 되는 방법이다. 그러나 유체 내에 장애물이 존재할 경우, 흐름에 큰 와류가 존재하여 난류 변동 성분과 구분되는 고유 구조가 형성되는데, 이러한 경우에 레이놀즈 분해법을 적용하면 고유 구조의 변동 성분이 난류로 처리되어 난류 강도가 과다하게 책정될 수 있다는 한계점이 있다. 이에 대한 대안으로 제안된 것이, 변수를 평균 성분, 파동 성분, 변동 성분으로 분해하는 삼중 분해법이다. 삼중 분해법은 흐름 내의 고유 구조를 추출하는 것을 가능하게 하여 다양한 연구에서 사용되어왔다. 삼중 분해법을 구현하기 위해 이용되는 방법론 중 하나로, 공분산 행렬을 이용하여 유속장을 분해하는 방법인 적합 직교 분해법이 많이 사용된다. 본 연구에서는 원기둥 후류에 적합 직교 분해법을 사용하여 삼중 분해법을 시행하고, 후류의 흐름 구조를 분석하는 것을 목표로 하였다. 영상 유속계를 사용하여 실험을 통해 원기둥 후류의 수평 유속장을 측정하였고, 측정 자료에 적합 직교 분해법을 적용한 결과, 첫 두 모드에서 큰 규모의 와류가 파동 형태로 전파되는 것이 관찰되어 고유 구조의 존재를 확인할 수 있었다. 해당 성분을 삼중 분해법의 파동 성분으로 상정하였고, 푸리에 분석을 적용한 결과에서도 원기둥 후류의 고유 진동수가 뚜렷하게 나타나는 것을 확인하였다. 또한, 원기둥 후류의 에너지 전달 구조를 확인하기 위하여 에너지 방정식에 삼중 분해법을 적용하여 식을 유도하고, 실험 자료로부터 각 항을 계산하여 비교해보았다.

• 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 B
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• v.38 no.3
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• pp.211-217
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• 2014

This study analyzes the nonlinear interaction between two different vortex shedding frequencies from a cylinder with two diameters. In particular, two different vortex shedding frequencies are generated by preparing a cylinder having two diameters artificially. Flow velocity fluctuations behind the cylinder are measured three-dimensionally. Additionally, we fabricated a hole and placed a pressure transducer for measuring the pressure on the cylinder surface. The pressure signal from the pressure transducer is used as basic signal. A TSC(Trans Spectrum Coherence) is used for checking the strength of the nonlinear interaction between two different vortex shedding frequencies. As a result, the following are clarified: i) frequency distribution behind the cylinder, ii) three-dimensional flow state behind the cylinder through calculation of ensemble average, and iii) close relationship between the vertical vortex and change of low frequency by nonlinear interaction between two different vortex shedding frequencies from the cylinder with two diameters.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.10
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• pp.859-864
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• 2014

This experimental study investigates the effects of a downstream cylinder in the wake change on upstream object's diameter. A V-shaped object is placed in the upstream of the test section and a circular cylinder containing a load-cell is placed in the downstream. The velocity distribution of the wake generated from the upstream object with a change in its diameter is investigated. Further, the fluctuation in the lift coefficient and Karman-vortex emission frequency with a change in the position of the downstream cylinder is examined. The study results reveal the following. i) The flow velocity in the wake is smaller than that in the main stream. ii) The lock-in phenomenon occurs when the diameter of the upstream object is larger than that of the downstream cylinder. iii) To generate maximum fluctuating lift force of the downstream cylinder in the wake, the position of the downstream cylinder must be moved with changing diameter of the upstream object together.