| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 98 === Experiments were made to study the process of laminar-turbulent transition in a pulsating pipe flow. Particular interest was placed upon the initial growth of velocity fluctuations in the flow field. Measurements were made with a spanwise array of Self-made MEMS thermal film sensors flushed with the wall surface, and hot-wire velocity signals measured in the flow field. In general, I was found that initially the disturbances grew in the developing region, about 20 to 25 diameters downstream of the inlet of the pipe. By examining the hot-wire and MEMS signals measured, it was clearly seen that initially the disturbances grew in a thin layer very near the wall. The raw signals obtained allowed us to observed the time duration of the disturbances under different flow condition. The characteristics instantaneous frequency and amplitude of the disturbances were further analyzed with the Hilbert-Huang Transformation (HHT). Meanwhile, signals of the MEMS sensors measured at different circumferential angles were examined by correlation analysis, from which further physical insights were gained with respect to the two-dimensionality of the unstable disturbances developed initially on the wall surface.
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