臺灣博碩士論文加值系統

本篇論文針對汽車外流場之氣流特性，採用FLUENT軟體來分析汽車外流場的氣動參數以及噪音參數之情形。研究內容包括：首先針對基本氣動力流場與噪音預測之數值方法做基本的比對驗證，其後再接續分析較複雜的結構外型以及流場環境，對不同外型的後照鏡以及不同外型的導流板之汽車外流場，深入研究氣流壓力、速度與噪音流場等關鍵參數，對汽車行駛之穩定性與氣動誘發噪音進行比較及討論，並分析其優劣處。研究結果顯示：使用DES紊流模式之噪音預測分析之結果雖然在精度上略遜於LES紊流模式，但由於其計算時間較少，因此在工程應用或是噪音初步評估上亦是可行之方式；改變後照鏡表面外型使得噪音頻譜有所變化，後照鏡表面越平滑，噪音值表現更佳；最後，研究中也提出一具有高速行駛穩定性與最低噪音之最佳化設計的導流板。

This thesis plans to use a code that combines fluid and acoustic simulation. The parameters in the near field of sound are computed by FLUENT that is a CFD solver with various turbulent models and the far field sound intensity, that are governed by the Ffowcs-Williams and Hawkins equation (FW-H). There are three steps in the task. The first stage is to demonstrate the numerical package to predict aero-noise of the incompressible flow, through simple 3D geometry sample models are used for validation. The second stage is to reform the numerical package accuracy and efficient, and apply to side view mirror and road vehicle. The present simulation results are found to be in good agreement with the published experimental results. At engineering environment, DES model is an acceptable technique except LES model predicting aero-noise. Aero-noise of rough side view mirror is louder than smooth one.The results indicate that the installation of an appropriately angled tail fin reduces the aerodynamic lift coefficient, and hence improves the vertical stability of the vehicle under high speed driving conditions. Furthermore, it is shown that the use of an end plate reduces the noise profile behind the car. The present steady also proposed an optimal tail fin / end plate configuration, which possesses enhanced aerodynamic characteristics and aero-acoustic properties, and hence improves driver safety and comfort.

摘要..........................................................I
Abstract.....................................................II
誌謝........................................................III
目錄.........................................................IV
表目錄.....................................................VIII
圖目錄.......................................................IX
1. 緒論.......................................................1
1.1 研究動機與背景............................................1
1.2 文獻回顧..................................................9
1.2.1 氣動性能實驗量測方法....................................9
1.2.2 氣動性能數值分析方法....................................9
1.2.3 噪音實驗量測方法.......................................10
1.2.4 噪音數值分析方法.......................................12
1.2.5 數值氣動噪音分析之噪音改良應用.........................15
1.3 研究方法.................................................18
1.4 預期遭遇之困難...........................................19
2. 數值分析..................................................20
2.1 統御方程式...............................................20
2.2 紊流模式.................................................21
2.2.1 k-ε紊流模式............................................22
2.2.2 Spalart-Allmaras紊流模式...............................23
2.2.2.1 Spalart-Allmaras 模式的傳輸方程式....................23
2.2.2.2 紊流黏度的建立.......................................24
2.2.2.3 紊流產物的建立.......................................24
2.2.2.4 紊流消散項的建立.....................................25
2.2.3 Detached Eddy Simulation (DES)紊流模式.................25
2.2.4 Large eddy simulation(LES)紊流模式.....................26
2.2.4.1 濾波Navier-Stokes方程式..............................26
2.2.4.2 次網格模式(Sub Grid Models)..........................27
2.2.4.3 Smagorinsky-Lilly模式................................27
2.3 遠場噪音數值模式.........................................28
2.3.1 Lighthill’s聲學近似模式...............................28
2.3.2 Ffowcs-Williams and Hawkins方程式......................29
2.4 數值方法.................................................30
2.4.1 擴散項(Diffusion Terms) ...............................31
2.4.2 對流項(Advection Terms)................................31
2.4.3 矩陣方程式求解.........................................31
2.4.4 速度與壓力間之耦合算法(Velocity-pressure Coupling
Algorithm) ..................................................32
3. 研究構型、格點與流場特徵描述..............................34
3.1 幾何構型與格點分佈.......................................34
3.2 邊界條件.................................................34
3.3 物件規格.................................................38
3.4 噪音探測點...............................................44
4. 後照鏡個案分析與討論......................................47
4.1 數值穩定度...............................................47
4.1.1 程式驗證...............................................47
4.1.2 格點數選擇.............................................54
4.1.3 疊代次數選擇...........................................54
4.2 後照鏡驗證組分析與討論...................................56
4.3 後照鏡案例1分析與討論....................................69
4.4 後照鏡案例2分析與討論....................................73
5. 汽車後導流板個案分析與討論................................77
5.1 汽車後導流板案例1分析與討論..............................77
5.2 汽車後導流板案例2分析與討論..............................80
5.3 汽車後導流板案例3分析與討論..............................84
5.4 汽車後導流板案例4分析與討論..............................88
5.5 汽車後導流板案例5分析與討論..............................94
5.6 汽車後導流板案例6分析與討論.............................100
6. 結論與建議...............................................108
6.1 後照鏡個案結論..........................................108
6.2 汽車後導流板個案結論....................................109
6.3 建議....................................................111
參考文獻....................................................113
符號索引....................................................117
附錄........................................................119
1. 聲源介紹.................................................119
1.1 壁面偶極聲源之輻射......................................119
1.2 壁面偶極子源效力........................................119
2. 聲的量度與聽覺特性.......................................120
2.1 聲壓級、聲強級與聲功率級................................120
2.2 分貝的相加與相減........................................121
2.3 聲音的頻譜與計權聲級....................................121
作者簡介....................................................124

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