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研究生:廖資文
研究生(外文):Tzu-wen Liao
論文名稱:鈍形體於不同展弦比下三維渦流溢放現象之影響
論文名稱(外文):Influence of aspect ratio of bluff body on three-dimensional characteristic of vortex shedding
指導教授:苗君易苗君易引用關係
指導教授(外文):Jiun-Jih Miau
學位類別:碩士
校院名稱:國立成功大學
系所名稱:航空太空工程學系碩博士班
學門:工程學門
學類:機械工程學類
論文種類:學術論文
論文出版年:2008
畢業學年度:96
語文別:中文
論文頁數:90
中文關鍵詞:三維渦流溢放小波分析法MEMS熱膜感測器
外文關鍵詞:Three-dimensionality of vortex sheddingWavelet analysisMEMS sensor
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本研究的主題為在雷諾數為 下,展弦比對於鈍形體其尾流之渦流溢放三維性之影響。在實驗中將使用兩種不同之鈍形體,分別為圓柱體與梯型鈍形體兩種,兩種不同鈍形體之特徵長度(D)為32mm,其中展弦比控制在3到12.5之間,並且利用可移動式端板(moveable end-plates)控制。在鈍形體渦流溢放的量測方面,實驗利用熱線測速儀(single Hot-wire probe)與MEMS熱膜感測器陣列同時量測流場訊號。MEMS熱膜感測器陣列是由16組感測器組成,並且服貼於模型上,其黏貼之位置於模型之分離點上游,感測器所黏貼之方向為模型之翼展方向,其長度為3D。
在開始分析數據之前,先將熱線測速儀及MEMS 熱膜感測器所量測之流場訊號以相關性分析之方式分析其相關性,以證明MEMS熱膜感測器之可靠性。再以小波分析法及二維傅立葉轉換法之方式,分析其流場之三維渦流溢放特性,將所量測之MEMS熱膜感測器訊號以小波分析法處理,其結果發現當相位差(phase-jump)發生時,同一時間下在熱線測速儀及MEMS 熱膜感測器的訊號中皆可發現發生渦流位錯之現象。
將橫跨鈍形體翼展方向之16個MEMS熱膜感測器之訊號利用二維傅立葉轉換法可清楚發現,每個感測器其波數之差異代表著三維渦流溢放特性的強度。另外,將利用短時間的相關性分析法分析16個MEMS熱膜感測器可以發現當流場之渦流位錯發生時,其側向之相關係數會大幅度的降低,因此,可以發現三維渦流溢放時間與空間之相位擾動及渦流溢放之頻率有密切相關性存在。
比較不同展弦比與不同模型之實驗結果,可以發現在兩種不同之模型下,當展弦比縮小後,其模型之側向相關係數會隨之升高;由上述之結果可以發現展弦比對於模型尾流之三維渦流溢放有顯著的影響。對於展弦比的影響,進一步利用短時間的統計分析之方式,分析16個MEMS熱膜感測器所量測之流場瞬時頻率,並將其量化表示;方式為將流場瞬時頻率以標準差之方式計算,並假設一門檻值 ,以標準差計算之值如果超過 ,將認定為流場發生三維渦流溢放現象,統計取樣時間內發生的機率,並以百分比表示;所得到之實驗結果與前述之相關性分析之結果相同,當端板之展弦比縮小時,所統計之流場瞬時頻率之變異量將隨之減少。
This study is aimed to investigate the effect of aspect ratio of bluff body on the three-dimensional characteristics of vortex shedding at Reynolds number of 104. For the present study, two bluff cylinders were employed, i. e., circular and trapezoidal cylinder models. For each of the cylinder models, its aspect ratio could be varied form 3 to 12.5, as achieved by varying the separation distance between two moveable end plates. The process of vortex shedding was detected by a single hot-wire probe positioned in the wake and an array of self-made MEMS sensors on the bluff cylinder simultaneously. The MEMS sensor array was consisted of 16 MEMS sensors, located upstream of the initiation of flow separation and aligned spanwisely in a region of 3 D in width, where D=32mm, denoting the characteristic length of the bluff bodies.
Prior to analyzing the measured data, the reliability of the signals of the MEMS sensors was confirmed by correlation analysis of the signals of the MEMS sensors and the hot wire. Subsequently, in order to examine the three-dimensional characteristics of vortex shedding, the techniques of Wavelet analysis and 2D Fourier transform were employed. Results of Wavelet analysis show that the three dimensionality of vortex shedding is highlighted by the events of phase jump, as learned from comparing the instantaneous phases of vortex shedding reduced from the signals of the MEMES sensors and the hot-wire . Consequently, these events are identified as the occurrences of vortex dislocation.
By 2D Fourier transformation of the measured MEMS sensor signals, the three-dimensionality of vortex shedding can be evidenced by significant fluctuating energy resided at the wave numbers other than zero. On the other hand, short-time correlation analysis of the signals of 16 MEMS sensors indicates that the correlation coefficient could be dropped drastically when the vortex dislocation events mentioned above were identified. The three-dimensionality of vortex shedding of concern is highlighted by remarkable temporal and spatial fluctuations in phase or frequency of vortex shedding.
By comparing the results obtained at different aspect ratios of each bluff cylinder, a trend noticed is that the time-mean correlation coefficients reduced from the signals of different MEMS sensors got higher as the aspect ratio got smaller. Clearly, this trend implies that the three-dimensionality of vortex shedding gets pronounced as the aspect ratio gets larger, as far as the range of the aspect ratios of the present study is concerned. The effect of aspect ratio on the three-dimensionality of vortex shedding is further confirmed by a short-time statistical analysis of the instantaneous vortex shedding frequency values reduced from each MEMS sensor, in order to identify the events whose standard deviation values were higher than ± 5% of the time-mean vortex shedding frequency. The results in terms of the probability, i. e., the ratio of the summation of the time intervals identified versus the sampled time, indicate that the probability indeed is decreased as the aspect ratio get smaller.
摘要 I
Abstract III
表目錄 VIII
圖目錄 X
符號說明 XVI
第一章 序論 1
1.1 研究動機與目的 1
1.2文獻回顧 2
第二章 實驗設備與模型 6
2.1 風洞設備 6
2.2 實驗模型 6
2.3 MEMS 熱膜感測器 6
2.4 壓力轉換器 8
2.5 熱線測速儀(Hot-wire anemometer) 8
2.6 資料擷取系統 8
2.6.1 NI DAQ 資料擷取系統 9
2.6.2 IOTech ADC-488/8SA資料擷取系統 9
第三章 實驗步驟與方法 10
3.1 鈍形體流場之量測 10
3.2 實驗數據之分析方法 10
3.2.1 小波分析法(Wavelet analysis) 10
第四章 實驗結果與討論 15
4.1 MEMS熱膜感測器與熱線測速儀之相關性分析 15
4.2 渦流溢放頻率與低頻擾動 17
4.3 渦流溢放之三維現象 20
4.4 影響渦流溢放之端板效應 22
第五章 結論與建議 25
5.1結論 25
5.2未來建議 26
參考文獻 27
圖表彙整 32
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