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Application of Wavelet Transform to the Intermittent Switching Characteristics of Gap Flow between Two Bluff Bodies Arranged Side by Side
Yun, Yang Ting
|關鍵字:||gap flow;間隙流;switching;vortex shedding frequency;full-field velocity;phase analysis;不穩定搖擺;渦流脫離頻率;全域速度場;相位分析||出版社:||土木工程學系||摘要:||
根據實驗量測與小波分析結果顯示，可由其瞬時頻率變化特性將並列鈍形體尾流場間隙流之搖擺過程分為三種型態： 間隙流瞬間擺動至另一側； 間隙流擺動至中間側，持續不偏向後再轉回原偏向側，無完整之上下擺動發生； 間隙流擺動至中間側，持續一段時間後再偏向另一側。此外，本文亦對其不穩定搖擺過程中，上、下柱渦流脫離相位及頻率變化等相關性分佈特性進行詳細之探討，並比較傅立葉轉換與小波轉換法分析結果之相異處。
The flow characteristics of gap-flow downstream of two side-by-side bluff bodies are provided with temporal variation of unstable phenomenon in the uniform fluid field. The distinct identities of vortex shedding obtained by the single point measurement method and the analysis of Fast Fourier Transform (FFT) are insufficient. The primary objective of the present study is to develop a unique measurement technique that combines the use of two one-component fiber laser Doppler velocimetries (FLDV) and particle image velocimetry (PIV), aiming to perform simultaneous velocity measurement to integrate both time and flow domains of the switching gap flow and the wide and narrow wakes corresponding to these two stationary cylinders including square, circular, and flat forms. Moreover, by using the Wavelet transform (WT), the energy-history of each component shedding frequency can be described as time-frequency domain.
First, this study aims to validate the feasibility of Wavelet transform which is then applied to the analysis of velocity time signal obtained by FLDV. Meanwhile, the temporal variation of the full-field velocity maps in the flow domain of interest can be obtained by employing PIV. To synchronize the PIV and two FLDV data during measurement, the two FLDV velocity signals and PIV camera triggering signal were transferred into the data acquisition board in the same time, so the corresponding time (or phase) of each PIV velocity map can be precisely determined. Therefore, the instantaneous frequency of the wide and narrow wakes (analyzed by wavelet transform) as well as the corresponding instantaneous full-field velocity map (including the deflected gap flow) can be identified.
Based upon the experimental results, it is found that the switching characteristics of gap-flow patterns can be classified into three categories. These categories are defined as : (1) gap-flow switching from one side to the other side instantaneously; (2) gap-flow switching spontaneously from one side to the center direction, in which the gap flow was oriented parallel to the free-stream flow for short duration, and then biasing to original side; (3) gap-flow switching spontaneously from one side to the other, and in the intermediate time where existing another short duration stable flow pattern (which can be termed an intermediate flow pattern).
Furthermore, the correlation between two temporal frequency of wide and narrow wakes were studied completely. As we known, Fast Fourier transform (FFT) and wavelet transform which provided with the time-mean and instantaneous frequency contents respectively, and the spectrum analysis results of them were compared in detail.
|Appears in Collections:||土木工程學系所|
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