在圓柱旋轉流中，寬高比與雷諾數等參數的改變，以及選擇不同的邊界，都會影響著流體的行為模式。並且一旦超越了臨界雷諾數之後，流體的行為將不再只有單一個模式。為了探討這類型的流體，本論文將著重在選擇不同的邊界下，討論寬高比與臨界雷諾數的關係，將這些結果與學者所採用的邊界作比較。具體作法將固定上下圓盤為轉速相等，但方向相反，藉由選擇側面為自由表面與固體邊界。本研究主要利用數值套裝軟體ANSYS FLUENT，模擬圓柱旋轉流在二維軸對稱模型與三維模型的情況，並且藉由改變轉速，發現臨界雷諾數。
結果顯示，在自由表面時，寬高比與臨界雷諾數首先隨著寬高比增加臨界雷諾數下降，到了最低點的臨界雷諾數後上升；在固體邊界時，寬高比與臨界雷諾數在定性上與自由表面一樣，但整體往右平移1個單位的寬高比，而在定量上，臨界雷諾數約上升三百多；最後發現學者的邊界在定性上與自由表面和固體邊界相似，但整體的趨勢與自由表面的趨勢，往左平移約0.1到0.5個單位的寬高比，臨界雷諾數介於兩者之間。

The swirling flow between two coaxial counter-rotating disks may not be the only one solution given different aspect ratios of the rotating disks or different Reynolds numbers. Therefore, in the present study, it discusses the relation between aspect ratio and critical Reynolds number choosing different boundary conditions and compares these results with academic. In particular, there are two cases about free surface and solid boundary respectively with two coaxial counter-rotating disks. Specifically, ANSYS FLUENT simulations are performed the motions of the flow constructing 2D axisymmetric and 3D models and are found the critical Reynolds number by changing the angular speeds.
The results show that critical Reynolds number first decreases and then increases as the aspect ratio increases in free surface case. Moreover, it is found that case of the solid boundary and the motion of the flow is similarity qualitatively with free surface case. In quantitative, it is overall right shift about aspect ratio of 1 unit and the critical Reynolds number is higher three hundred in solid boundary. Finally, it is found that the boundary academic chose appears like the cases of free surface and solid boundary in qualitative. But it shows that it is overall left shift about aspect ratio of 0.1 to 0.5 unit and the critical Reynolds number is between them quantitatively.

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