本研究針對無攻角之二維方柱，於方柱迎風面中央射出一均勻噴流，以雷諾數與和噴流與橫風之注射比為變數，探討橫風雷諾數在1600 ~ 3300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，藉由煙線可視化技術觀察方柱周圍流場特徵，並利用長曝光攝影技術分析迎風面噴流及方柱尾流之平均流場特性。使用熱線風速儀量測方柱尾流區渦漩逸放及其引致之上游流場不穩定性，並分析頻率、紊流統計特性與尾流寬度。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：噴流注射比小於1.0時，由方柱迎風面射出之噴流呈現震盪模態；在注射比大於1.2時，噴流隨機偏向某一邊，稱為偏折模態；注射比在1.0 ~ 1.2之區間極為不穩定，為一模態轉換之臨界區域。以長曝光照片分析迎風面噴流之平均流場特性，在震盪模態時噴流所形成之迎風面寬度隨注射比增加，但在模態轉換之過程劇烈下降，模態轉換為偏折噴流後，迎風面寬度再隨噴流注射比之增加而上升。熱線風速儀輸出之渦漩逸放頻率與方柱上游之主頻值相同，且訊號波呈反相，若配合煙線可視化的判斷，顯示下游渦漩逸放在方柱某一側生成時，會將上游噴流推向反方向；史卓數在噴流震盪模態時會隨注射比增加而增加，在模態轉換過程隨注射比增加而降低，在偏折噴流模態，史卓數再隨注射比增加而逐漸增加，尾流寬度則為相反之趨勢。在注射比大於1.2時，由於噴流模態轉換，使得尾流寬度達到最寬，故渦漩逸放頻率最低，史卓數因此達到最小值。

The surface flow characteristics and wake instability of a two-dimensional square cylinder controlled by an upstream-surface jet injection were experimentally investigated. Flow visualization was performed by using the smoke wire technique and smoke-injection through the jet. The wake and jet instability characteristics were detected simultantaneously by using two one-component hot-wire anemometers. The wake width was evaluated by considering the distance between two peaks appearing in the belocity distributions measured in the wake. The flow field around the jet injection area on 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 subject to the influences of the injection ratio R and Reynolds number Rew. At R < 1.0, the jet presented left-and-right oscillation motion (which was denoted as the oscillation mode). At R > 1.2, the jet deflected and attached to the upsurface of the cylinder (which was designated as the deflection mode). In-between the values of 1.0 and 1.2, the jet presented unsteady behavior—it sometimes oscillated and sometimes was deflected (which was denoted as the transition mode). The wake width in the oscillation mode decreased with increasing R, increased abruptly in the transition mode, and decreased with increase of R again in the deflection mode. The oscillation frequency in the wake varied inversely with did the the wake width. The oscillation signals detected in the wake and the jet had a 180o phase difference. Assisted by the flow visualization, the oscillation motion of the jet was induced by the oscillation motion of the vortical flow structure in the wake.

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