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    題名: 以LES模擬圓頂屋蓋結構在平滑逼近流場中的氣動力特性
    其他題名: LES simulation of aerodynamic characteristic of hemispherical dome in smooth approaching flow
    作者: 劉皓汝;Liu, Hao-Ju
    貢獻者: 淡江大學土木工程學系碩士班
    鄭啟明;Cheng, Chii-Ming
    關鍵詞: 大跨度;半球;數值模擬;Hemi-spherical dome;Aerodynamics;LES;CFD
    日期: 2013
    上傳時間: 2013-04-13 11:47:58 (UTC+8)
    摘要: 近年來建築設計不只往高度上發展,同時也追求跨度的延展,隨著建築技術進步與材料強度的提升,屋蓋的跨度由數十米發展為數百米,重量也由每平方公尺減少至幾十公斤的薄膜結構。而設計此類大跨度屋蓋結構,風的載重是影響其安全性與舒適度的重要考量之一,本研究即以計算流體力學模擬大跨度圓頂屋蓋在平滑流場中受風特性,並與文獻之風洞實驗結果比較。
    本研究內容分為兩部分:第一部份為網格選定,在嘗試幾種不同計算域切割與網格繪製後,做一個簡單的初步試算,與實驗結果比較,選定一套網格後加密模型周圍計算域後,進行第二部分模擬。第二部分為半球在Re=6.6×104和Re=2×106兩個雷諾數流場受風模擬,每個雷諾數在計算域入流5D的地面又分為兩種不同邊界條件設定:可滑動界面(slip wall)和不可滑動界面(non-slip wall)。
    模擬結果顯示,低雷諾數流場半球子午線上平均與擾動壓力分佈與實驗值誤差較大,主要原因有二:一是透過可視化發現半球尾跡區之分離泡超過網格加密區塊,過大的網格造成其模擬的精準度不足;二是流體在半球表面分離之前所形成的邊界層為層流邊界層,分離之後方轉為紊流邊界層,使用Fluent的基本設定是否能夠完整模擬如此複雜的流體變化尚有待證實。
    高雷諾數的流場模擬顯示,不同地面邊界層設定所得結果相近,半球子午線上除迎風面擾動壓力分佈略大於實驗值,其他位置平均與擾動壓力與實驗都很吻合,擾動風壓頻譜亦如是,顯示數值模擬所得之半球體空氣動力特性與風洞實驗結果吻合。但是風壓分佈機率模擬結果比實驗值分佈集中,說明其風壓極值不足以作為局部構件設計的參考依據。由數值模擬所得之水平向昇力係數頻譜可觀察到數個峰值,顯示半球體後方並無單一的渦旋剝離頻率。透過可視化看到流體流經半球後方渦旋並無明顯交替作用,但會隨著昇力係數的變化左右擺動。
    This study uses LES to simulate the aerodynamic characteristics of hemispherical dome in a smooth approaching flow field. The accuracy of numerical simulation was verified firstly by comparing with the wind tunnel measurements. Then the details of the aerodynamics of the dome were presented in this thesis. Prior to study the dome aerodynamics, several schemes of the grid system and numerical parameters were examined to determine the optimal ones for this study. The numerical simulation in this thesis can be categorized into two parts: the aerodynamics of dome in two Reynolds numbers, Re=6.6×104 and 2×106.
    In the case of Re=6.6×104, the mean and RMS pressure coefficients on the center meridian are noticeably deviated from experiment data. The numerical error may be caused by two reasons. The first probable source of error is that the separation bubble in the wake region extent beyond the mesh refined area. The second one is more subtle. At subcritical Reynolds number, the boundary layer developed over the dome surface is of laminar nature; it transits to become turbulent flow after separated from dome surface. Whether the basic setting of CFD tool, ANSYS-FLUENT, is apt to such a complex numerical simulation is to be confirmed.
    As for the second case, Re=2×106, the mean and RMS pressure coefficients on the center meridian agree well with experiment data except near the front stagnation area. The power spectral densities of the numerical simulated pressure fluctuations also agree with wind tunnel measurements satisfactory. Only the probability densities of the numerical simulation exhibit deviations from the wind tunnel data. It indicates that although the current numerical simulation scheme can reproduce the hemi-spherical dome’s aerodynamic quite well; it is still insufficient to generate the small scale turbulence that contributes to the pressure peaks. The lift force spectrum exhibits multiple peaks; which indicate the complexity of the vortex shedding in the horizontal plane. The time history of the vorticity further demonstrate that there exists no clear interaction between two separated free shear layers of the two opposite side of the dome; however, the wake flow show rather periodic sway synchronized with the variation of lift force coefficient.
    顯示於類別:[土木工程學系暨研究所] 學位論文

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