本研究針對攻角為零度之二維方柱，於方柱迎風面中央射出一均勻逆向噴流，以雷諾數和噴流對橫風速度比(簡稱為注射比)為變數，探討橫風雷諾數在1600 ~ 4200之區間時，方柱周圍流場結構、尾流區渦漩逸放特性以及噴流之動態行為。在一氣動力風洞中，藉由煙霧可視化技術觀察並分析迎風面噴流及方柱尾流之流場特徵。使用熱線風速儀量測方柱尾流區渦漩逸放頻率及其引致之上游流場不穩定性。以PIV技術量化迎風面噴流結構，並與流場可視化之特徵比對。流場可視化的結果顯示，方柱迎風面噴流之特徵模態與注射比具有密切之關聯性。注射比小於1.0時，由方柱迎風面射出之噴流呈現搖擺模態，其頻率與尾流渦漩逸放頻率相同；在注射比於1.0至4.0之間，噴流隨機偏向某一邊且貼附於方柱迎風面，噴流氣柱高度受尾流渦漩逸放影響產生週期性的高低變化，稱振盪偏折模態；在注射比於4.0至8.0之間，噴流噴出後隨機偏邊且不再貼附於方柱迎風面，因噴流形成一包覆方柱之流場，其兩側邊界之距離遠大於方柱之寬度，使得方柱尾流兩側之剪應力層不再交互作用，致使原有之週期性渦漩逸放之現象不復可見，上游流場亦不具特徵頻率，稱為無振盪偏折模態；在注射比大於8.0時，噴流噴出後會隨機偏向一邊，且無週期性的擺動，在強勁的噴流影響下，尾流渦漩結構變得零碎，其渦漩逸放現象仍舊存在但不具週期性，導致上游亦無特徵頻率，稱為噴流型式模態。在搖擺模態及振盪偏折模態時，渦漩逸放頻率隨噴流對橫風速度比與雷諾數增加而增加。此外，經流場觀察及訊號分析，判斷噴流氣柱之搖擺現象乃尾流渦漩逸放現象所引致之行為。

The surface flow characteristics and the wake instability of a two-dimensional square cylinder controlled by up-wind jet injection were experimentally investigated. The characteristic flow patterns were observed by flow visualization. The wake and the jet instability characteristics were detected simultaneously by using a two one-component hot-wire anemometer. The flow field around the upstream cylinder surface was measured by a particle image velocimeter (PIV). The results of flow visualization showed that the features of flow were strongly subjected to the influences of the injection ratio R and Reynolds number Rew. At R < 1.0, the jet presented up-down flapping motion (which was denoted as the flapping mode). The flapping frequency coincided with the vortex shedding frequency. At 1.0 < R < 4.0, the jet deflected and attached to the upsurface of the cylinder, the jet column oscillated back-and-forth periodically (which was designated as the deflection with oscillation mode). The oscillating frequency of the jet column was the same as the vortex shedding frequency. At 4.0 < R < 8.0, the jet deflected without oscillation and did not attach to the upstream surface of the cylinder (which was designated as the deflection without oscillation mode). At R > 8.0, the jet presented unsteady up-down flapping motion (which was denoted as the jet-type mode). The main frequency in the flapping and the oscillation modes increased with increasing R and Rew. The flapping motion of the jet was induced by the oscillation motion of the vortical flow structure in the wake.

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