當鈍體受到風力作用時，由於物體的形狀或是雷諾數的大小，流場會產生不同位置的分離及尾跡產生，而本文將以數值模擬方法來分析通過圓柱體的複雜流場。探討的雷諾數區間則是於次臨界流及超臨界流下之流場，即雷諾數為6.1x10^4和10^6。
    本文採用了不同的紊流模式，包含RNG k-epsilon、LES、Laminar model來進行模擬，並嘗試不同的邊界尺寸來改善其邊界對於流場模擬的影響。文中亦和實驗值與其他數值模擬的參考文獻進行比較，其平均風壓係數的分佈及阻力係數、升力係數和史托赫的計算值。
    研究結果顯示，當流場為次臨界流時，流體剛好在層流與紊流轉換的一個狀態，所以使用Laminar和適用高雷諾數的RNG k-epsilon模式都無法得到好結果，而使用LES模式在平均風壓分佈及風力係數的計算上，都能和實驗有相近的結果。於此雷諾數下建議可使用User Define Functions (UDF)來進行適合的公式來修改計算式，避免結果偏差於軟體之設定，或是要求的細密網格超過可接受的時間成本。
    另外在超臨界流下，使用二維的RNG k-epsilon已可以得到和實驗值十分吻合的結果。而若要清楚的將流場可視化，則須配合適當的計算域大小及優良網格繪製，避免邊界過於狹隘造成流場壓縮。由於邊界的影響在風壓分佈上並不明顯，建議在進行後處理計算前可以先於繪圖軟體檢視流場的合理性，可節省試誤的時間。

When the wind passed through a curve shaped bluff body, at different Reynolds number, separation occurs at different location. In this study, numerical method is used to simulate the flow field around a circular cylinder at both subcritical and supercritical Reynolds number region, 6.1x10^4 and 1.0x10^6, respectively. Besides turbulence models such as RNG k-epsilon and LES, Laminar model were also used in this study. Several combinations of numerical simulation domain and grid structures were studied to ensure better numerical result. The simulated mean pressure distribution, drag coefficient, lift coefficient and Strouhal number were compared with data reported in literatures.

    When the flow is in the sub-critical region, the thin boundary layer on cylinder surface is laminar and later transits to turbulent after separation. Therefore, the Laminar model was used. The Laminar model produces good results up to the point of separation but cannot correctly simulate the wake flow.  Comparing to the experimental data, LES model produces better results. In the future, the User Define Functions (UDF) mode should be activated to introduce appropriate modification of the Fluent’s general purposed parameter setting When the flow is the super-critical region, two-dimensional RNG k-epsilon model can produce results in good agreement with experimental data. It was found that, appropriate computational domain and sufficient fine grid are needed in order to obtain good pictures of the flow field visualization.

目錄
第1章 緒論.........................................- 1 -
    1-1 前言.......................................- 1 -
    1-2 研究動機 ..................................- 1 -
    1-3 研究方法 ..................................- 2 -

第2章 文獻回顧	................................- 3 -
2-1 背景說明	................................- 3 -
2-2 相關研究	................................- 3 -

第3章 理論基礎 ...................................- 6 -
3-1 流體特性概述	................................- 6 -
3-2 鈍體氣動力現象	................................- 8 -
3-3 風力係數	................................- 10 -
3-4流體經過圓柱體的流場特性.......................- 11 -

第4章 計算流體力學介紹............................- 14 -
4-1 計算流體力學	................................- 14 -
4-2 網格繪製	................................- 15 -
4-3 應用軟體介紹	................................- 16 -
   4-3-1 數值方法.................................- 16 -
   4-3-2 紊流模式.................................- 20 -
   4-3-3 Wall Function............................- 26 -
   4-3-4 邊界條件.................................- 30 -

第5 章 條件設定及計算過程	......................- 31 -
     5-1 數值模擬步驟說明 .......................- 31 -
     5-2 流體流經圓柱於次臨界流區之雷諾數 .......- 32 -
        5-2-1相關條件設定........................- 32 -
     5-3流體流經圓柱於超臨界流區之雷諾數 ........- 40 -
         5-2-1相關條件設定.......................- 40 -
     5-4 文獻的設定參考..........................- 44 -

第6章 結果分析及討論	......................- 45 -
     6-1流體流經圓柱於次臨界流區之雷諾數結果分析 - 45 -
         6-1-1 平均風壓係數......................- 45 -
         6-1-2 分離點及附近流場..................- 47 -
         6-1-3 阻力係數、升力係數與史托赫數......- 49 -
         6-1-4 小結..............................- 53 -
     6-2流體流經圓柱於超臨界流區之雷諾數結果分析.- 53 -
         6-2-1 平均風壓係數......................- 53 -
         6-2-2 分離點及附近流場..................- 54 -
         6-2-3 阻力係數、升力係數與史托赫數......- 57 -
         6-2-4 小結............. .................-58 -

第7章 結論與建議	..............................- 62 -
     7-1 結論 ..................................- 62-
     7-2 建議 ..................................- 63 -

參考文獻 .......................................- 65 -

圖表目錄

圖(3-1) 雷諾實驗裝置[11] ........................ - 7 -
圖(3-2)層流轉換紊流示意圖[12] ................... - 8 -
圖(3-3)分離點附近速度剖面圖[13] ................. - 9 -
圖(3-4) 次臨界流流經圓柱之流況................. . - 12 -
圖(3-5) 超臨界流流經圓柱之流況................... - 12 -
圖(3-6) 於次臨界流和超臨界流下的圓柱表面風壓分佈. - 12 -
圖(3-7) 不同雷諾數下的阻力係數分佈 .............. - 13 -
圖(3-8) 不同雷諾數下的史托赫數分佈 .............. - 13 -
圖(4-1) 網格元素形狀............................. - 15 -
圖 (4-2) Fluent對於近壁面處之Wall Function處理區域[14].. - 27 -
圖(5-1) 研究之模擬過程 ......................... - 31 -
圖(5-1)圓柱體表面網格生成至矩形邊界缺點之處..... - 33 -
圖(5-2) 改善網格細長的情況 ..................... - 33 -
圖(5-3)X-Y平面上之整體計算域網格................ - 34 -
圖(5-4)Z向上設20個網格點........................ - 34 -
圖(5-5)二維計算域之大小 ........................ - 35 -
圖(5-6)三維計算域之大小......................... - 35 -
表(5-1)次臨界流模擬之相關設定................... - 36 -
圖(5-7) RNG k-epsilon model設定................. - 37 -
圖(5-8) LES model設定........................... - 38 -
圖(5-9) Laminar model設定....................... - 39 -
圖(5-10) 10D計算域示意圖........................ - 40 -
圖(5-11) 15D計算域示意圖........................ - 41 -
圖(5-12) 10D網格示意圖 ......................... - 42 -
圖(5-13) 15D網格示意圖 ......................... - 42 -
表(5-2)超臨界流模擬之相關設定................... - 43 -
表(5-3)各文獻之參考設定 ........................ - 44 -
圖(6-1) 次臨界流通過圓柱之平均風壓係數.......... - 46 -
圖(6-2) 10D-LES 速度分佈圖 ..................... - 47 -
圖(6-3) 15D-LES 速度分佈圖 ..................... - 48 -
圖(6-4) Laminar 速度分佈圖 ..................... - 48 -
表(6-1) 本研究與實驗值之比較 ................... - 49 -
圖(6-5) 10D-LES 阻力、升力係數歷時與頻譜........ - 50 -
圖(6-6) 15D-LES 阻力、升力係數歷時與頻譜 ....... - 51 -
圖(6-7) Laminar 阻力、升力係數歷時與頻譜 ....... - 52 -
圖(6-8) 超臨界流通過圓柱之平均風壓係數 ......... - 54 -
圖(6-9) 4.5D-LES 總壓分佈圖 .................... - 55 -
圖(6-10) 10D-LES 總壓分佈圖 .................... - 56 -
圖(6-11) 15D-LES 總壓分佈圖 .................... - 56 -
表(6-2)文獻中的參考實驗值及數值模擬結果 ........ - 57 -
表(6-3) 本研究之係數計算結果 ................... - 58 -
圖(6-12) 參考文獻[8]所整理之阻力係數分佈圖 ..... - 58 -
圖(6-13) 4.5D-LES 阻力、升力係數歷時與頻譜 ..... - 59 -
圖(6-14) 10D-LES 阻力、升力係數歷時與頻譜 ...... - 60 -
圖(6-15) 15D-LES 阻力、升力係數歷時與頻譜 ...... - 61 -

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