臺灣博碩士論文加值系統

本研究以實驗方法來探討圓柱表面流場於雷諾數為 1.56×105 ~ 5.32×105間之三維非定常特性。實驗主要在臨界雷諾數下，分別對預臨界區、單分離泡區、和雙分離泡區內，以及各區域之交界處選定幾組雷諾數來進行圓柱表面側向壓力之量測。並利用小波轉換搭配模態分離法，輔以相關性分析，得以量化分析圓柱表面流場於臨界雷諾數下之三維特性。
首先實驗所得之側向壓力訊號進行相關性係數分析，對流動分離線以及分離泡發生處，於圓柱表面上位置的結果作一比較，可歸納出圓柱表面流場在不同位置之三維特性。研究結果發現，三維性最強烈之處為紊流再接觸現象發生之位置，其次為紊流分離，最後則為層流分離。實驗在任一雷諾數下，其圓柱上游仍未靠近分離線之流場，均屬於二維流場。
至於圓柱之瞬時頻率分析，首先利用經驗模態分離法，將渦流溢放擾動訊號濾出，再利用小波轉換得其瞬時溢放頻率，可發現於分離泡發生之位置，其瞬時頻率皆呈現一不連續分佈，且擺盪劇烈。只有在預臨界區內，分離線下游處之流場二維特性較強，渦流溢放頻率呈連續分佈且落於溢放主頻附近。

This study is intended to investigate the characteristics of unsteady three-dimensional flows around a circular cylinder at Reynolds numbers between 1.56×105 ~ 5.32×105. The experiment was carried out to measure pressure fluctuations along the spanwise direction on the circular cylinder. The purpose of this study is to quantify the characteristics of the three-dimensionality of the circular cylinder flow in the critical regime by means of correlation analysis and wavelet transformation.
Correlation coefficients of pressure distributions along the spanwise direction on the circular cylinder were obtained in the regions of turbulent reattachment, the laminar and turbulent separations and separation bubble. The results obtained in the turbulent reattachment region show strongest three-dimensionality, compared to the findings obtained around turbulent and laminar separations. Upstream of laminar separation, flow near the circular cylinder behaves most two-dimensional like at any Reynolds number studied.
In order to analyze the instantaneous frequency components contained in pressure fluctuations, the raw signals obtained by pressure transducers were firstly reduced by the empirical mode decomposition method (EMD). Subsequently, the reduced signals were analyzed by the wavelet transformation to obtain the instantaneous behavior of vortex shedding along the spanwise direction of the circular cylinder. Where the separation bubble occurred, the instantaneous vortex shedding frequency varies discontinuously and fluctuates violently. The instantaneous vortex shedding frequency obtained in the region downstream of the separation line shows a continuous distribution.

中文摘要………………………………………………………………...……I
英文摘要……………………………………………………………………..II
誌謝……………………………………………………………….…………IV
目錄………………………………………………………………………..…V
表目錄…………………………………………………………..…………...IX
圖目錄……………………………………………………………….……….X
符號說明………………………………………………………...…...……XIV

第一章 序論……………………………………………………………….…1
1.1 研究動機與目的………………………………………………………1
1.2 文獻回顧………………………………………………………………2
1.2.1 圓柱流場………………………………………………………….2
1.2.2三維圓柱流場……………………………………………………..4

第二章 實驗設備與模型…………………………………………………….6
2.1 內政部建研所環境風洞………………………………………………6
2.2 圓柱模型………………………………………………………………6
2.3 壓力轉換器……………………………………………………………7
2.4 熱線測速儀量測系統…………………………………………………8
2.5 資料擷取系統…………………………………………………………9
2.6 輔助儀器設備…………………………………………………………9
2.6.1 手提式壓力校正器……………………………………………….9
2.6.2 雷射墨線儀……………………………………………………...10
2.6.3 二維移動機構…………………………………………………...10

第三章 實驗方法與步驟…………………………………………………...11
3.1 風洞實驗參數分析…………………………………………………..11
3.1.1 雷諾數…………………………………………………………...11
3.1.2 無因次頻率……………………………………………………...12
3.1.3 壓力係數………………………………………………………...12
3.1.4 阻力係數………………………………………………………...13
3.2 三維圓柱表面壓力量測……………………………………………..13
3.3 溢放頻率量測………………………………………………………..14
3.4 訊號處理……………………………………………………………..14
3.4.1 相關性係數……………………………………………………...15
3.4.2 小波轉換………………………………………………………...15
3.4.3 經驗模態分離法………………………………………………...17

第四章 實驗結果與討論…………………………………………...………18
4.1 圓柱流場之三維特性………………………………………..………18
4.1.1 預臨界區……………………………………………………...…19
4.1.2 單分離泡區…………………………………………………...…20
4.1.3 雙分離泡區…………………………………………………...…20
4.2 預臨界區………………………………………………………..……21
4.2.1 表面壓力擾動與相關性分析………………………...…………21
4.2.2 瞬時溢放頻率…………………………………………...………22
4.3 單分離泡區……………………………………………………..……23
4.3.1 表面壓力擾動與相關性分析……………………...……………23
4.3.2 瞬時溢放頻率……………………………………...……………24
4.4 雙分離泡區……………………………………………………..……24
4.4.1 表面壓力擾動與相關性分析………………………...…………24
4.4.2 瞬時溢放頻率……………………………………………...……25
4.5採用經驗模態分離法與小波分析之幫助.……………………..……26

第五章 結論…………………………………………………………...……28
5.1 圓柱流場三維非定常特性………………………..…………………28
5.2 未來建議………………………………………………….………….30

參考文獻…………………………………………………………………….31

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