在日常生活中，經常可以發現到許多渦漩誘發振動(Vortex Induced Vibration, VIV)的現象。當結構體沈浸在流動的流場中，在結構體後端會產生漩渦脫落(Vortex shedding)的現象，這會使系統紊亂度提高，繼而於結構體的上部和下部後方交替連續的在結構體後方形成渦漩流動，而此類流場多屬於紊流。
在計算流體力學(Computational Fluid Dynamic, CFD)中，針對流體在高流速下的紊流行為，已有許多的紊流模型相繼出現，例如混合長度模型及k-ε模型。然而這些以RANS方程式(Reynolds-Averaged Navier-Stokes equation)為基礎發展而來的模型往往是異常複雜的，並且需要利用物理實驗資料作修正，也不能模擬非穩態渦流。就目前我們感興趣的流場是非穩態、流況複雜的情形下，大渦漩模擬(Large Eddy Simulation, LES)會是個不錯的選擇。
本論文目的主要是研究LES中的佔有核心地位的幾種SGS模型(Sub-Grid Scale model)，利用LES建立二維圓柱紊流流場模擬，延伸模擬非穩態及複雜幾何結構體流場的基礎能力，並驗證LES對紊流流場模擬的整體表現，量化實驗無法直接或不易量測的參數如壓力場、結構體的升力與阻力係數、渦漩頻率、紊流統計性質等等，以期深入研究其物理機制。經研究後發現，SGS model的選用對模擬結果並不會有明顯的差別，僅對時間平均 分量速度分佈的影響較為明確。此外，二維圓柱紊流模擬在捕捉某些流場特性的表現並不盡人意，甚至在雷諾數高於1500時會產生不正確的結果，日後採用三維數值模擬是必要的作法。

關鍵字：計算流體力學、大渦漩模擬、SGS模型

The phenomena of Vortex Induced Vibration (VIV) could be observed frequently in the daily life. In the downstream region of a structure immersed in the flow field, it usually performs the pattern of the vortex street with the highly turbulent viscosity, which is caused by the vortex shedding from separating shear layers of the structure.
In Computational Fluid Dynamic, there have been many models, such as mixed length model and k-ε model, used for turbulent flow simulation at highly Reynolds numbers. However, the mathematic theory of these models based on the Reynolds-Averaged Navier-Stokes equation (RANS), often represent extremely complicated form and need to be corrected with the experiment database. Also, they are not appropriate to be used to regenerate the unsteady flow field. Currently, Large Eddy Simulation (LES) is a proper choice, due to the capacity of simulation with the unsteady and complicated properties of turbulent.
The primary purpose of this thesis is to investigate the core part of LES, Sub-Grid Scale models (SGS models), and to build two dimension simulation of circular cylinder turbulent flow. On the one hand, this study could be extending the ability of simulation of unsteady, complicated geometric structure flow. On the other hand, the uneasily estimated or unestimated parameters, such as pressure field, drag and lift coefficient, vortex frequency, turbulent statistic properties, etc., are quantified to verify the overall performance of the LES with turbulent flow simulation, and study the physics of turbulent flow in depth. The study shows that the effect with several types of SGS models is insignificant for flow statistics, except for the time-averaged resolved cross-flow velocity distribution. In addition, two dimension simulation is unsatisfying to represent certain flow characteristic properties, even result in incorrect outcome as Reynolds number is higher than 1500. Three dimension numerical simulations should be top priority in future work.

Keyword: CFD, LES, SGS models

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