Investigations of Unsteady Three-dimensional Flows of a Circular Cylinder in the Critical Regime

• Main Authors: Ying-Ju Lin, 林映如
• Other Authors: Jiun-Jih Miau
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/84226570560442813894

碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 95 === This study is intended to investigate the characteristics of unsteady three-dimensional flows around a circular cylinder at Reynolds numbers between 1.56×105 ~ 5.32×105. The experiment was carried out to measure pressure fluctuations along the spanwise direction on the circular cylinder. The purpose of this study is to quantify the characteristics of the three-dimensionality of the circular cylinder flow in the critical regime by means of correlation analysis and wavelet transformation. Correlation coefficients of pressure distributions along the spanwise direction on the circular cylinder were obtained in the regions of turbulent reattachment, the laminar and turbulent separations and separation bubble. The results obtained in the turbulent reattachment region show strongest three-dimensionality, compared to the findings obtained around turbulent and laminar separations. Upstream of laminar separation, flow near the circular cylinder behaves most two-dimensional like at any Reynolds number studied. In order to analyze the instantaneous frequency components contained in pressure fluctuations, the raw signals obtained by pressure transducers were firstly reduced by the empirical mode decomposition method (EMD). Subsequently, the reduced signals were analyzed by the wavelet transformation to obtain the instantaneous behavior of vortex shedding along the spanwise direction of the circular cylinder. Where the separation bubble occurred, the instantaneous vortex shedding frequency varies discontinuously and fluctuates violently. The instantaneous vortex shedding frequency obtained in the region downstream of the separation line shows a continuous distribution.