低雷諾數振盪有限圓柱尾流域類源流型

本論文係利用數值模擬與實驗方法探討在低雷諾數均勻流中作簡諧振盪之有限長圓柱的流場行為。研究的雷諾數的定義為Re=ρUL/μ，物理意義是流體的慣性力與黏滯力的比值，範圍介於70~500，其中𝜌為流體密度，U為流體速度，D為圓柱直徑，μ為動力黏滯係數(Dynamic Viscosity)。數值計算使用商用軟體Ansys Fluent進行流場模擬；而實驗在拖曳式甘油槽中進行PIV(Particle Image Velocimetry)定量量測。根據本研究團隊之成果(Chen12021)，有限長圓柱於特定的流場條件下，由於縱向流場分離(Longitudinal Flow Separation, LFS)與橫向流場分離(Transverse Flow Separation, TFS)的交互作用下，於圓柱的二分切平面(Bisectional Cross-section Plane, BCP)上之尾流區域的流線投影會出現類源流(Source-like pattern，Chang et al.22021)的現象。本論文研究之目的，希望能界定出有限長圓柱尾流區域會出現類源流現象之參數條件。數值計算的參數範圍如下: Re=70~500；圓柱長與直徑比，AR (Aspect Ratio)，分別為1.5、2、3、4、5；圓柱橫向振幅A/D=0.4；圓柱振盪之頻率在0.7~1.3倍無限長圓柱渦脫落頻率。計算之結果歸納出類源流現象出現之區域。吾人藉由源流強度(Source Strength, SS)等參數，藉以探討類源流場的特性，且修正先前研究，並以高斯球面檢驗源流中心位置的流動情形。實驗採用在甘油槽中拖曳圓柱的方式，以消除傳統水洞壁面邊界層的影響。吾人以跟隨圓柱同步平移的高速攝影機記錄流場，使用PIV技術獲得BCP上流場的瞬時速度分布及流線，並和數值模擬的結果進行比對。研究結果顯示，AR=1.5~5各圓柱在Re=90~300的範圍中皆會出現類源流的現象，皆為類線狀源流(Line-like source)，其中AR=1.5與AR=5的出現範圍較小。然而AR=3與AR=4有較大的範圍出現類源流的現象。實驗與數值模擬流場比對，可以證實類源流的現象確實存在，且源流中心位置近乎相同。

關鍵字

有限長圓柱；低雷諾數；類源流場；振盪圓柱；圓柱尾流

並列摘要

The article is to report a simple harmonic vibrational circular cylinder of finite-span in low Reynolds number flow by experiments and numerical simulations. The study is under the flow condition between Reynolds number 70~500. The definition of Reynolds number is Re=ρUL/μ, which possess the physical meaning of the ratio of inertial force to viscous force, where 𝜌 stands for fluid density, Ufor fluid velocity, D for the diameter of the circular cylinder, and μ for dynamic viscosity. The software used for numerical simulation is Ansys Fluent. Experiments are operated in a trailing sink. According to the results of this research team(Chen12021), due to the interactions of the Longitudinal Flow Separation(LFS) and the Transverse Flow Separation(TFS), the streamline projection on the Bisectional Cross-section Plane(BCP) (Chang et al.22021) of the finite-span circular cylinder will appear a Source-like phenomena under specific flow field conditions. The article seeks to find out the range that the source-like pattern appears in the wake region of a finite-span circular cylinder, by operating numerical simulations in the conditions of Re=70~500, and aspect ratio(AR)1.5, 2, 3, 4, 5, respectively. With the transverse amplitude of A/D=0.4, and 0.7~1.3 times of the vortex shedding frequency of infinite-span circular cylinder, summarizes the area which the source-like pattern appears. Uses parameters such as Source Strength (SS) to specify the characteristics of the source-like patterns, and corrects the mistakes in the previous study. Furthermore, examine the flow field around source center by a Gaussian spherical surface. The experiments are carried out in a trailing sink, which can eliminate the influence of the boundary layers of walls in traditional water holes. Film the flow field by a high-speed camera, then use PIV technology to depict streamlines and velocity vectors of the flow field, and compare the results with which in the numerical simulations. Source-like patterns can be found in all wake regions of AR1.5~5 cylinders, within the condition of Re=90~300. AR1.5 and AR5 cylinders have smaller range that source-like patterns appear, while AR3 and AR4 cylinders have wider. After comparing flow patterns of both numerical simulations and experiments, it can be proven that the source-like flow patterns exist, and the center of source positions come very close between them.

並列關鍵字

finite-span circular cylinder ； low Reynolds number ； source-like pattern ； vibrational circular cylinder ； wake region of a cylinder

參考文獻

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