本論文在開迴路風洞中利用下游旋轉平板法產生振盪橫風，使用低於自然圓柱尾流逸放史卓數的橫風振盪史卓數，研究橫風振盪強度對圓柱尾流及氣動力性能的影響。橫風雷諾數調整為3111及4129，振盪史卓數設定為0.056及0.074。透過雷射光頁輔助流場可視化技術，觀察流場衍化過程。使用熱線風速儀及高速訊號擷取系統，偵測尾流不穩定性頻率及瞬時振盪紊流速度，搭配三重分解的方法分析統計紊流性質，其中包含紊流強度、長度尺度與時間尺度。藉由質點影像速度儀量測尾流區的速度向量場，計算流線圖、等渦度分佈圖及迴流泡特徵尺寸。利用線性壓力掃描器量測圓柱表面的壓力分佈，求得圓柱表面的壓力係數分佈及阻力係數。透過比較圓柱在振盪橫風與穩定橫風條件的結果，圓柱在振盪橫風條件下的尾流渦漩逸放史卓數比自然尾流渦漩逸放的史卓數低，此結果主要受振盪史卓數及橫風雷諾數影響。本研究求得臨界橫流雷諾數和振盪史卓數，在超過臨數值時，尾流渦漩逸放史卓數趨於一定值，然而，橫風振盪強度對渦漩逸放行為的影響並不顯著。振盪橫風造成圓柱尾流廻流泡尺寸較自然圓柱小，並且，產生較大尺寸的渦漩結構，進而提升尾流區牽引與混合的效應。上述生成的流場機制，加強尾流區動量的紊流擴散，因此，相對於自然圓柱，振盪橫風圓柱的紊流強度、時間尺度及長度尺度增加。此外，振盪橫風導致圓柱體所受到的阻力係數大幅減小。

The effects of the crossflow oscillation intensity on the wake flow and aerodynamic performance of a circular cylinder in an oscillating crossflow were experimentally studied in an open-loop wind tunnel. The study focused on crossflow oscillation Strouhal numbers that were smaller than the natural wake vortex–shedding Strouhal numbers. Crossflow oscillations were generated using a downstream rotating plate method. The crossflow Reynolds numbers were adjusted to 3111 and 4129, respectively, while the oscillation Strouhal numbers were set at 0.056 and 0.074, respectively. The crossflow pulsation intensities were varied within the range between 0.027 and 0.094. The flow evolution processes were observed using laser light–sheet assisted smoke flow visualization technique. The wake instability frequencies and instantaneous oscillating turbulent velocities were detected by a one-component hot-wire anemometer along with a high-speed data acquisition system. The wake statistical turbulence properties were analyzed using the triple–decomposition method, including turbulence intensities and Lagrangian integral length and time scales. The velocity vectors, streamline patterns, recirculation bubble geometries, and vorticity contours in the wake region of the circular cylinder were determined by averaging instantaneously captured PIV images. The pressure distributions on the cylinder's surface were quantified using a linear pressure scanner, obtaining pressure coefficient distributions and drag coefficient. The results of natural and oscillating crossflows were compared. The results of natural and oscillating crossflows were compared. The wake vortex–shedding Strouhal number was found to be lower than that of natural vortex–shedding Strouhal number, but higher than the crossflow oscillation Strouhal number. It was primarily influenced by the oscillation Strouhal number and the Reynolds number of the crossflow. Critical crossflow Reynolds and oscillation Strouhal numbers were identified, beyond which the wake vortex-shedding Strouhal number reached a constant value. The crossflow oscillation intensity did not significantly affect the wake vortex-shedding behaviour. The time-averaged geometric parameters of the recirculation bubble in the circular-cylinder wake in oscillating crossflow were significantly smaller than those in the natural crossflow. The crossflow oscillations induced large scale flow motions and hence enhanced entrainment and mixing in the wake. These mechanisms expedited turbulent diffusion of momentum in the wake, and therefore the turbulence intensity and turbulence time and length scales were enlarged when compared with those in the natural crossflow. Furthermore, the crossflow oscillations led to a substantial reduction in the drag coefficient experienced by the circular cylinder.

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