本研究針對無攻角之二維方柱，於方柱背風面中央射出一均勻順向噴流，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1628 ~ 4212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方柱周圍之流場特徵。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與方柱側邊分離剪流層之不穩定性，並分析頻率以及方柱周圍流場訊號的相位關係。實驗結果顯示，方柱背風面噴流之特徵模態與注射比具有密切之關聯性，觀察出四種流場模態。以雷諾數2144為例，注射比小於1.2時，由方柱背風面射出之噴流於方柱背面左右搖擺，噴流本身被方柱後方的渦漩逸放所主宰，噴流搖擺的頻率與方柱尾流渦漩逸放的頻率相同，換算為史卓數均為0.135，方柱周圍流場結構與無噴流時相似，稱為尾流主導模態(wake dominated regime)；在注射比於1.2至3.3之間，噴流不再貼附於方柱背風面，根部貫穿靠近方柱的渦漩結構，尾端會擺動但不具週期性，使得方柱尾流兩側之分離流場不再交互作用，尾流區中原有之週期性渦漩逸放之現象不復可見，側邊分離剪流層及上游流場亦不具特徵頻率，稱為過渡模態(trasitional regime)；在注射比於3.3至5.1之間，方柱噴流又開始出現週期性搖擺，且尾流區亦有渦漩逸放，而因尾流寬度減小渦漩尺寸變小導致渦漩逸放特徵頻率隨著注射比增加而增加，在此區段的史卓數比在尾流主導模態時大很多，且數值隨著注射比增加而增加，但此時兩側分離剪流層及上游流場已穩定，稱為臨界模態(critical regime)；在注射比大於5.1時，在強勁的噴流影響下，噴流貫穿方柱後方主要的渦漩結構，牽引方柱兩側分離的剪流層於兩側形成駐渦，使尾流寬度明顯減小，噴流本身及尾流皆不具特徵頻率，兩側分離剪流層及上游流場也已穩定，稱為噴流主導模態(jet dominated regime)。在尾流主導模態時，渦漩逸放頻率隨雷諾數增加而增加，噴流擺動頻率不隨注射比增加而改變；臨界模態時，渦漩逸放及噴流擺動頻率隨注射比與雷諾數增加而增加。

The flow behavior around a square cylinder subject to the modulation of a planar jet issued from the cylinder’s rear surface was studied by the laser-assisted smoke flow visualization method and hot-wire anemometer measurement for Reynolds number between 1628 and 4212. The temporally evolving smoke flow patterns around the square cylinder were revealed by the smoke flow visualization. The frequency characteristics of the instability waves around the square cylinder were synchronously detected by hot-wire anemometers. Four characteristic flow modes were observed within different ranges of injection ratios. For Reynolds number 2144, at low injection ratios (R < 1.2 ), which was denoted as the wake dominated regime, the jet swings periodically leftward and rightward on the rear surface. The motions of jet flapping were mainly dominated by the vortex shedding in the wake. The Strouhal numbers of the jet flapping and wake oscillation presented the same values of 0.135, which was the same as the wake instability Strouhal number of a natural square cylinder. At moderately low injection ratios (1.2 < R < 3.3), which was named the transitional regime, the leading section of the jet penetrated the wake a short distance. The jet oscillated a little without periodicity. The vortex shedding in the wake vanished. At the moderately high injection ratios (3.3 < R < 5.1), which was termed the critical regime, smaller wake width was observed. The reduction in the wake width induced smaller vortices and higher vortex shedding frequency when compared with wake dominated regime. At high injection ratios (R > 5.1), the jet had large momentum to entrain wake fluid and therefore stabilized the periodic wake instability. Two standing vortices appeared in the near wake aside the high-momentum jet. The width of the wake substantially decreased. Neither periodic jet oscillation nor vortex shedding in the wake was observed.

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