Please use this identifier to cite or link to this item: `http://hdl.handle.net/11455/2263`
 標題: 靜態與震盪縱列雙圓柱之流場實驗研究Experimental Study of Stationary and Oscillating Circular Cylinders in a Tandem Arrangement 作者: 呂邦超Lu, Pang-Chao 關鍵字: bistable;雙穩定;critical spacing;tandem arrangement cylinders;lock-on boundary;臨界間距;縱列雙圓柱;鎖定邊界 出版社: 機械工程學系 摘要: 本文以實驗量測探討縱列雙圓柱尾流渦漩剝離的特性及互鎖的現象。實驗主要是量測尾流渦漩剝離頻率、表面壓力、阻力、雙穩定現象、及尾流渦漩結構，並利用煙線可視化技巧，來觀察近尾流場的結構。 實驗是在開放式風洞中進行，測試模型為直徑20-60mm之圓柱，雷諾數範圍為4500-15000之間。首先針對靜態縱列雙柱體研究，結果發現，渦漩剝離頻率與雙柱體無因次間距(L/D)有密切的關係。在臨界間距(L/D=3.88-4.22)時，有兩個頻率交替發生，此時流場的特徵為存在兩種穩定狀態的狀況，稱為雙穩定流場。在臨界間距下利用頻譜分析得知，高頻部分是由上游柱體形成的渦漩造成，低頻部分則是由下游柱體形成的渦漩造成。雙頻交替的產生並無週期性。 動態的探討上除了與靜態量測相同外，藉由改變上游柱體之震盪頻率及震盪振幅，探討在不同震盪模式下流場發生鎖定現象與臨界間距的關係。結果發現，不同間距所對應的鎖定邊界，隨著間距的增加，逐漸往右邊移動，接近單一柱體鎖定邊界。另外針對不同震盪頻率仍可找到適當的振幅，使得流場仍有雙穩定現象的產生。 比較本文中靜態與動態縱列雙柱體的研究結果，對於縱列雙柱體之雙穩定現象現象及渦漩剝離機制可以有更進一步的了解。The vortex shedding for two identical circular cylinders in a tandem arrangement have been studied experimentally. The experiments were performed in an open-type wind tunnel for Reynolds number raging from 4500-15000. In addition to the case of two stationary cylinders, the oscillating case involves a rotationally oscillating upstream cylinder. The vortex shedding frequency, lock-on regime, surface pressures, drag forces, and vortical structure of the near wake are described. At the critical spacing where the dual vortex shedding frequencies take place have been observed for the stationary case well as for the oscillating case. The occurrence of dual shedding frequencies indicates switching of the flow patterns between two stable states and is referred to as the bistable flow. Moreover, the two shedding frequencies appeared intermittently, in which the lower frequency results from the vortex shedding behind the downstream cylinder and the higher frequency from the upstream cylinder. In the oscillating case, research is emphasized on the control of the bistable flow. This is attained by rotationally oscillating the upstream cylinder while holding the downstream cylinder stationary. The vortex shedding from a rotationally oscillating cylinder is found to lock-on to the external forcing at certain oscillation frequency and amplitude. As the spacing between the cylinders is increased, the lock-on boundary moves toward higher frequencies, and progressively close to that for a single oscillating cylinder. The bistable flow could also occur when the vortex shedding is locked-on to the oscillating upstream cylinder. The effects due to vortex lock-on and bistable shedding are reported and discussed in detail. URI: http://hdl.handle.net/11455/2263 Appears in Collections: 機械工程學系所

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