結構疲勞破壞因結渦至振動(VIV)產生振盪導致。在過去的幾年裡，VIV 一直在被研究，主要聚焦於“鎖定”現象發生期間的振盪響應。儘管了解振動位移和氣動力作用在結構上以避免這種現象是至關重要，但了解近尾流中的行為也很重要。
本研究使用低速風洞和粒子追蹤測速系統(PIV)，通過實驗研究了在 VIV 引起的氣彈力不穩定性、鎖定和非鎖定狀態下圓柱尾流的渦流結構。由熱線測速系統 (HWA) 量測得速度數據作為確認 PIV 結果可靠度的基準。相位平均技術用於使流場可視化結果更清晰，並應用 Q 準則來檢測尾流中的渦流。另外執行靜態實驗，其結果用於比較動態實驗。研究表明，在共振時，渦旋脫落頻率與結構固有頻率相匹配，此時圓柱的位移最大。隨著圓柱體的振動，自由流和振動圓柱體之間的相對速度發生變化。發現這種變化會影響渦流強度並對尾流中的渦流行為產生很大影響，特別是對渦流中心到中心線的距離以及每個連續渦流之間的距離。因此，渦流型態對圓柱體的振盪很敏感。此外，通過對流場進行統計分析，探討非靜止圓柱的紊流特性。分解相位平均速度，識別真實的速度擾動。

The instability of structures under the presence of vortex-induced vibration (VIV) can generate oscillations which may cause fatigue damage to the structure. In past years, VIV has been studied and its main focus has concerned the oscillation response during the “lock-in” phenomenon. Even though it is crucial to know the vibrating displacement and forces acting on the structure to avoid this phenomenon, understanding the behavior in the near-wake flow can be important too.
This study experimentally investigates the vortical structures in the wake of a circular cylinder during aeroelastic instability induced by VIV, at lock-in and at non-lock-in state, using a low-speed wind tunnel and a particle image velocimetry (PIV) system. The velocity data from a hot-wire anemometry (HWA) system is used as a benchmark to confirm the reliability of the PIV results. The phase average technique is used to have a clearer visualization of the flow field and the Q-criterion is applied to detect the vortices in the wake. Static experiments were also done, and the results are used for comparison. This study reveals that at resonance, the vortex shedding frequency matches that of the structural natural frequency, and the displacement of the vibrating cylinder is the largest. As the cylinder oscillates, the relative velocity between the freestream and the vibrating cylinder is altered. This alteration is found to affect the vortex strength and to have a great influence on the behaviors of the vortices in the wake, specifically on the distances of the vortex center from the centerline and the distance between each consecutive vortex. Therefore, the vortex patterns are sensitive to the oscillation of the cylinder. In addition, the turbulent characteristics of the non-stationary cylinder are explored by performing a statistical analysis of the flow. The phase-averaged velocity is decomposed, and the true velocity fluctuations are identified.

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