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題名 | 應用PIV與FLDV探討並列雙方柱尾流場特性之實驗研究=Experimental Study on Wake Structures Behind Two Square Cylinders Arranged Side by Side Using Piv and Fldv |
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作者 | 林呈; 林蔚榮; 白佳燕; 施致民; | 書刊名 | 興大工程學刊 |
卷期 | 14:1 2003.03[民92.03] |
頁次 | 頁1-19 |
分類號 | 440.137 |
關鍵詞 | 間隙流; 不穩定搖擺; 雙安定性; 迴流區長度; 渦流脫離頻率; Gap flow; Switching; Bistable; Formation length; Vortex shedding frequency; |
語文 | 中文(Chinese) |
中文摘要 | 本研究係利用光纖雷射杜普勒測速儀(FLDV)與質點影像測速儀(PIV)等兩套精密且非接觸式之流場定量量測系統,並配合流場可視化之定性觀測方法,探討並列雙方柱尾流流場之特性。文中針對雷諾數Re(=U0.H/v,其中U0為自由流流速,H為方柱邊,v為流體運動黏滯度)=2500~8000、淨間距比δ(=G/H,其中G為雙方柱淨間距)=0~3.0條件,分析並列雙方柱尾流場與間隙流型態及特性上之變化。為了解其流場之細部特性,特以Re=3500條件下,經由全域速度場與其時序列訊號之對應關係,對流場形態的變化濚入分析判斷,並對流場中瞬時與平均速度場分佈、上(下)柱體後方迴流區長度、及渦流脫離頻率進行詳細探討。由實際結果顯示,在此雷諾數下,其流場特性存在著間距效應。即同一雷諾數條件下,隨著δ遞減存在四種同之流場型態,分別為(1)間隙流呈穩定不偏向;間隙流朝雙柱間之中心軸向而不偏向上柱或下柱,且上(下)柱尾流場有幾近同相位與異相位脫離現象。由平均速度場結果分析顯示,上下雙柱之尾流場相對於雙柱間之中心軸向係呈現上下幾近相互對稱之特性;(2)間隙流呈不穩定搖擺;間隙流間歇性偏向上柱或下柱之搖擺事件乃為隨機發生,且隨著偏向的變化,於柱體後方所形成窄域或寬域尾流場亦隨之交互改變;(3)間隙流呈穩定偏向;於雙柱後方分別形成窄域或寬域之尾流場,且間隙流固定偏向上柱與固定偏固下柱時的平均速度場經由鏡射比對後具有幾乎相互對稱之雙安定特性;(4)間隙流呈不穩定搖擺但作用微弱;其尾流場可視為寬長比為幾近2.0之矩柱尾流場。此外,若任雷諾數增加之情形下,此四種流場型態會在δ較小時發生。 |
英文摘要 | The characteristics of wake structures behind two square cylinders arranged side by side were investigated experimentally, using particle image velocimetry (PIV), fiber laser Doppler velocimetry (FLDV), and flow visualization technique. The Reynolds number (Re), based on the free stream velocity (U0) and the width of square cylinder (H), varies from 2500 to 8000, and the net gap ration (δ=G/H, G is the net gap between two cylinders) changes from 0 to 3.0. Based upon the experimental results corresponding to different net gap ratios for Re=3500, it is found that the gap flow patterns can be classified into four categories. These categories are defined as (1) gap flow without deflection: the axis of gap flow is parallel to the central axis between two cylinders, and the mean flow field is symmetric with respect to the central axis; (2) intermittently switching gap flow: the gap flow deflects either to the upper or to the lower cylinder intermittently, and the narrow wake is formed behind the cylinder to which the gap flow deflects and the wide wake appears behind the other cylinder correspondingly; (3) bistably deflected gap flow: the gap flow bistably biases wither upward or downward; (4) weak gap flow in switching motion: the wake can be regarded as the one behind a rectangular cylinder which has the length 2 times of the width of each cylinder. Furthermore, in order to illustrate the flow characteristics of these four categories, the distribution of velocity vector, the evolutions of vortex shedding frequency in the wakes of two square cylinders, the formation lengths behind two cylinders, and the characteristics of the gap flow are all addressed in detail. |
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