有限長多圓柱流場實驗之研究

Experimental Study of Flow About Multiple Cylinders of Finite Span

陳正修(Cheng-Hsiu Chen)

指導教授 ： 朱錦洲

共同指導教授 ： 張建成(Chien-Cheng Chang)

國立臺灣大學/工學院/應用力學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202003476

摘要

流體流過鈍物是流體動力學中探討已久的問題。1903年12月17日萊特兄弟駕駛自行研製的飛機飛行者一號成功的在人類歷史上首次重於空氣的航空器持續而且受控的動力飛行。提供飛機升力的機翼，受到均勻入流在縱方向產生壓力差造成升力。自此，流體圍繞鈍物之研究不斷出現。大部分的工程應用的設施為圓柱體，沉浸於空氣流場。了解流體圍繞圓柱相當於了解這些設施的流場。
本實驗主要在探討多圓柱繞流流場。實驗以甘油當作工作流體，在雷諾數30-120的範圍內，量測多圓柱在不同排列下所受阻力，升力，漩渦脫離頻率，流場情況，渦度分布，等等。將這些實驗測得資料整理歸納，找出不同量測項目之間的關聯性。同時，數值模擬搭配來驗證理想狀況與現實情況的差距，找出差距原因並加以討論。透過本實驗希望能在往後工程設施的設計上能提供分析及預測，提高流場的控制性。
初步的實驗結果有對於流場速度場的驗證。另外對於單圓柱的頻率與其他實驗結果相比有吻合；而模擬在單圓柱方面有了全面的驗證，包括阻力，最大升力，頻率。雙圓柱則是將流場結構，阻力，頻率等等做了驗證。根據實驗的結果顯示當在圓柱後面擺設控制圓柱有助於降低前圓柱的阻力，擺設方法與圓柱大小依據不同雷諾數與圓柱直徑有關。本實驗最終目的是利用最小的圓柱(最少的材料)來有效降低前方圓柱的阻力。目前發現當二圓柱串聯排列時，二圓柱
總和阻力小於單一圓柱。當後方圓柱漸漸往上提後，二圓柱總阻力增加外，前方圓柱之阻力甚至大於單一圓柱。
本實驗從單圓柱流場觀察出圓柱二側有一對強度一樣，一正一負的強渦度線，從圓柱二側出發後擴散至下游。此強渦度對圓柱受力有顯著的影響，而此影響可以透過串聯與交錯排列圓柱看出。

關鍵字

漩渦脫離 ； 實驗量測 ； 雙圓柱 ； 有限長圓柱 ； 數值模擬

並列摘要

Fluid flow through blunt objects is a long-discussed issue in fluid dynamics.
The Wright brothers made the first controlled, sustained flight of a powered, heavier-than-air aircraft with the self-made Wright Flyer on December 17, 1903. The wing that provides lift to the aircraft is subjected to uniform inflow and creates a pressure difference in the longitudinal direction and cause the lift. Since then, the study of fluids around blunt objects has continued to emerge. Most of the facilities for engineering applications are cylinders, immersed in air flow field. Understanding the fluid surrounding cylinders is equivalent to understanding the flow field of these facilities.
Our experiment is mainly about exploring the flow field around a multiple cylinders in different arrangement. The measured data of these experiments are then analyzed to find the relation between different arrangement of the cylinders. At the same time, numerical simulation is used to compare with the difference between the ideal situation and the realistic situation, to find out the reason that causes deviation and discuss it. Through this experiment, we hope to be able to analysis and prediction in the design of future engineering facilities and improve the controllability of the flow field.
The preliminary experimental results have verified the velocity field of the flow field. The frequency of the single cylinder is consistent with other experimental results; while the simulation has a comprehensive verification of the single cylinder, including drag, maximum lift, and frequency. The double cylinders have verified the flow structure, resistance, frequency. According to the results of the experiment, it is shown that placing a control cylinder behind the cylinder helps to reduce the resistance of the front cylinder. The arrangement method and the size of the cylinder depend on the Reynolds number and the diameter of the cylinder. The ultimate goal of this experiment is to use the smallest cylinder (the least material) to effectively reduce the resistance of the front cylinder.
It is found that when two cylinders are arranged in tandem, the two cylinders total resistance is less than a single cylinder. When the rear cylinder is gradually raised, the total resistance of the two cylinders increases, and the resistance of the front cylinder is even greater than that a single cylinder. We have found that the two strong vorticity line started from the cylinders side and diffuse to downstream mainly effects the force exerted on the cylinder, and this effect can be found through tandem cylinder arrangement and staggered cylinder arrangement.

並列關鍵字

Vortex Shedding ； Experimental Measurement ； Double Cylinders ； Finite Length Cylinder ； Numerical Simulation

參考文獻

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