本研究探討一個方形截面柱體在不同旋轉角時，瞬間啟動，其表面動態流場結構之演化過程以及尾流區非穩態渦漩的行為與起源。方柱模型放置於拖曳式水槽中，採用質點影像軌跡流場觀察法及質點影像速度儀，偵測瞬間啟動方柱表面的動態流場結構以及尾流區非穩態渦漩的演化。質點影像軌跡流場可視化的結果顯示，方柱在不同旋轉角的流場行為可分為四種演化過程，這些演化過程包含了複雜的物理現象，諸如方柱表面的主要渦漩、次要渦漩以及在方柱後方產生的迴流。將尾流區渦漩逸放的頻率作無因次化分析，可獲得渦漩逸放頻率與史卓數、洛斯柯數、雷諾數及旋轉角之間的關係。最後，以PIV技術，得到量化的流場結構，使得拓樸結果更清晰顯現。重要貢獻有三點：(1)瞭解瞬間啟動方柱表面動態流場結構的演化過程及複雜的模態；(2)將尾流渦漩逸放的建構回溯至啟動後表面非穩態流場的演化；(3)使用拓樸法則分析流場的動態現象，增進對流場結構與演化過程的瞭解。

The evolution processes of the surface flow and wake vortical structure of an impulsively started square cylinder at different incidence angles had been studied experimentally in a water towing tank. Particle tracking flow visualization method (PTFV) and the particle image velocimetry (PIV) were used to obtain clear flow images for Reynolds numbers between 103 and 104 as the incidence angles varied from 0o to 45o. The cross-section profile of the square cylinder is 6 cm × 6 cm and the aspect ratio of the finite square cylinder is about 9. Four categories of flow patterns were found by particle tracking flow visualization method (PTFV). They were closely related to the inclination angle of the square cylinder. The functional relationships among the Strouhal, Roshko, and Reynolds numbers in the inertial effect-dominated regimes were obtained. Topological critical points, separatrices, and alleyways were identified and discussed to elucidate the unsteady structure of the instantaneous streamline patterns. Further more, the effects of topological flow patterns on the wake properties were presented and discussed together with the vortex evolution of an impulsively started square cylinder. Stable vortex shedding in the wake was established after the initial period of complex vortex evolution around the square cylinder.

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