Flow Structure in the Vortex-Formation Region behind a Circular Cylinder. LDV Measurement and Flow Visualization by an FFT-Spectrum Method.

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  • 円柱渦形成域の流れ構造　ＬＤＶ計測とＦＦＴスペクトル法による可視化
  • エンチュウ ウズ ケイセイイキ ノ ナガレ コウゾウ LDV ケイソク ト F

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Abstract

The simple mechanism of the vortex formation behind a circular cylinder is visualized using two LDVs and FFT-spectrum method. The present method can more easily eliminate the FM-AM modulations of the fluctuating velocities than the phase-average method. These distribution based on the mean and fluctuating velocity components clearly indicate a simple mechanism of vortex formation in a cycle. Namely, in the mean flow, the stationary symmetric twin vortices exist in the dead water region as well as in the stationary thin vortex layers along the upper and the lower separated shear layers. In the periodically fluctuating flow, however, an unsteady vortex whose center is on the x-axis is generated just behind the cylinder and moves within the wake. Therefore, a large vortex forms asymmetrically in the dead water region, as the stationary thin vortex layers and the unsteady vortex interact, affecting one another's vorticity and kinetic energy.

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