本研究針對無傾角之二維方柱，以雷諾數與旋轉角為變數，探討方柱周圍流場及尾流區渦漩逸放之流場行為。將方柱模型置於一拖曳式水槽中，利用質點軌跡視流法觀察方柱從啟動渦漩至渦漩逸放之流場衍化過程，並以質點影像速度分析測得流場量化之結果。使用熱膜風速儀量測方柱尾流區的渦漩逸放頻率，並將結果與特徵模態相結合。方柱的時間平均流場共有五種特徵模態，以旋轉角14°±1°為分界，分成低角度(次臨界)區及高角度(超臨界)區。在低角度區，因黏滯效應的因素，方柱兩個側邊之流場顯現出多種渦漩的模態，並歸納成三種模態。並以5°為分界，區分成I區及III區；以Rew = 2259±30區分成I區及II區。在高角度區，分成兩種模態，因高角度的分離效應，使得兩個側邊的流場呈現簡單的分離泡模態；並以Rew = 3330±30區分成IV區及V區。IV、V區之模態，又可區分為旋轉角15°< α < 38°與38° < α < 45°兩種模態，在15° < α < 38°時，B面前端會有分離泡但在38° < α < 45°時，因旋轉角夠大，B面的前端分離泡因而消失，在45°時，流場變為楔型流，在A、B面上分離泡完全消失。這些模態的流場包括迎風面之分離流；方柱側面呈現的雙分離泡、單分離泡及貼附流；方柱尾流區之迴流泡等。根據熱膜風速儀輸出的時序圖與頻譜圖，可得知方柱在14°角附近時渦漩逸放頻率最高，因此史卓數也達最大值。

The evolution processes of the surface flow and wake behind an impulsively started square cylinder at different incidence angles have been studied experimentally in a water towing tank. Particle tracking flow visualization method (PTFV) and particle image velocimetry (PIV) were used to obtain clear flow images for Reynolds numbers between 103 and 104. The incidence angles were also varied from 0 to 45 degrees. Vortex shedding characteristics were measured by a single-component hot-film anemometer for Reynolds numbers. Flows around the square cylinder were identified in five categories. The incidence angles between 13 to 15 degrees exhibited as a dividing line, between which low-incidence angle region and high incidence-angle region. The low incidence-angle region was divided into three modes. The region for which incidence angle below 5 degree and Reynolds number lower than 2259±30 were identified as region I. Reynolds numbers above 2289 were identified as region II. The region where incidence angle was between 5 and 13 degrees are identified as region III. Flow characteristics in three categories prominently different. The high-incidence angle region were divided into two modes. Reynolds number below 3300 were identified as region IV and Reynolds number above 3360 were categorized as region V. Region IV and V have two modes based on incidence angle α. These angles were identified as 15 < α < 38 and 38 < α < 45. At incidence angle 45 degree, a symmetric wedge flow is observed on the two surfaces facing windward. Flow modes of dual bubble, single bubble, attached flow and recirculation bubble were identified from PIV study. From hot-film anemometer study vortex shedding frequency was highest for square column at incidence angle about 14 degree.

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