| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 91 === The characteristic behaviors of low-frequency modulations embedded in the vortex shedding process were studied experimentally with a normal plate situated in a low-speed wind tunnel at Re = 1.8×103 to 2.7×104. Hot-wire signals and smoke-wire visualization images were acquired simultaneously to examine the correlation between low-frequency modulations and the vortex formation length. Wavelet analysis was performed to extract the instantaneous properties from the raw hot-wire signals measured in the region upstream of the normal plate model. Results show that the variations of instantaneous vortex shedding frequency appear to be correlated in a negative manner with the low-frequency modulations, that the cross-correlation coefficient can reach —0.7 in value. This substantiates that the low-frequency modulations observed are linked with the vortex shedding process.
Further experiments using three hot-wires situated at different spanwise locations at the edge of separated shear layer were performed to investigate the three-dimensionality of the vortex shedding. Meanwhile, the three-dimensionality was evident by the streamwise vortices and the spanwise incoherence in the separated shear layer as visualized using the smoke-wire technique. It is noted that the phase difference of vortex shedding between the two signals with spanwise separation of two characteristic length may reach as high as 35°, at which the amplitude of vortex shedding sensed by either of the hot-wires appeared to be minimal. More specifically, the correlation coefficient of the spanwise phase differences of vortex shedding and the amplitude of vortex shedding reduced from the hot-wire signals measured amounts to —0.4, signifying that the linkage between the low-frequency modulations and three-dimensionality is noticeable. As a result, it is suggested that the low-frequency unsteadiness and the three-dimensionality of vortex shedding are can be described into two modes; namely, the long formation region mode, called mode L, and the short formation region mode, called mode S. In mode S, the instantaneous vortex shedding frequency appears to be higher, the instantaneous vortex shedding amplitude detected at a point outside the separated shear layer is weaker, and the three-dimensionality appears to be more pronounced. The mode S, corresponding to the events of vortex shedding with the spanwise phase difference larger than 20°, occupies less than 5% of the total time measured. In this study, the mode S is referred as the burst mode in vortex shedding.
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