| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 96 === The phenomenon of vortex shedding due to uniform flow over a circular cylinder with a normal slit was investigated in this study. Experiments were made for five cylinder models whose slit widths in terms of s/d, where s and d denote the width and the cylinder diameter, respectively, were 0, 0.05, 0.15, 0.20 and 0.30. For the experiments made in a water channel, the techniques of Particle Image Velocimetry (PIV) and flow visualization were employed. The PIV data obtained at the Reynolds numbers between 2,400 and 11,400 reveal a trend that the vortex shedding frequency reduced gets increased as s/d gets higher. More interestingly noted is that for the case of s/d= 0.15, the process of vortex shedding appears to be most periodic in time, which quantitatively can also be indicated by the signal quality defined. Results of flow visualization indicate that flow in the slit is oscillating in accordance with the frequency of vortex shedding, which in fact causes suction or blowing to flow over the circular cylinder. Thus, unsteady motions in the slit are realized very effective to modify the vortex shedding process.
Experiments made in a wind tunnel were aimed to examine the two-dimensionality of vortex shedding by means of spanwise correlation of hot-wire measurements at the Reynolds numbers between 30,000 and 50,000, for the five cylinder models studied. Results obtained indicate that the vortex shedding structures in the case of s/d=0.15 behave most two-dimensional. Hilbert-Huang Transformation (HHT) analysis of the velocity signals measured further show that among the five cases studied, the case of s/d=0.15 appears to be the one whose instantaneous vortex shedding frequency is most stabilized in time and its amplitude is the highest in value.
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