| Summary: | 碩士 === 國立中興大學 === 機械工程學系所 === 98 === This study investigates the flow characteristics behind two staggered circular cylinders of diameter ratio two by experiment. The Reynolds number was kept 1000; and the streamwise and the transverse distances between the cylinder centers are located within the range of L/D=-2.0~1.0, T/D=1.0~2.0, respectively. All the experiments are performed in a low-speed recirculation water channel. The qualitative flow structures were observed by the dye flow visualization technique, and the quantitative velocity measurements were performed by the FLDV system. Some of the data are analyzed by the cross Wavelet transformation. Further, using the PIV system acquire the instantaneous vorticity and circulation of the related flow structures between these two staggered cylinders. Some important results are summarized as follows: When the distances (either streamwise or transverse) between the centers of staggered circular cylinders are large, the gap flow becomes nearly parallel; and the flow structure forms two parallel vortex streets shed alternatively in the downstream regions. In these cases, the characteristic frequencies are very close to those behind a single large and small circular cylinders. While the center spacing decreases, two modes of the flow structures are defined, mode 1 and 2, depending upon the phases of the gap vortices. At smaller gap, the flow of the narrow wake shows the characteristic with much higher frequency. However, the two outer shear layers of the wide wake sometimes show the pairing of the vortices, forming the 1/2 subharmonic component of the frequency of the narrow wake during the evolution process. In the further downstream region, the wake character may be formed with a low characteristic frequency. Further decrease of the distances (L/D and T/D) between two cylinders much shorter vortex formation length is found behind the narrow wake. On the other hand, the outer shear layers of the wide wake can only form the vortices of much low frequency due to instability. The widely spread shear layer interacts strongly with the vortex street of the narrow wake and will form a much large scale vortex in the farther downstream region. The characteristic frequencies of the two shear layers are close, and may form a vortex street shed alternatively with much lower frequency.
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