肥皂膜為物理上最接近二維的流體，我們的實驗主要探討單一與兩個相同的圓柱體在垂直流動的肥皂膜中的動力行為，圓柱體被設計為在肥皂膜上只有旋轉的自由度，我們觀察到單一圓柱體在肥皂膜中有穩定(Steady)、擺動(Fluttering)及轉動(Tumbling)三種運動模式，我們透過干涉條紋及影像拍攝來討論圓柱體在流場中的運動，並且建立一個簡易的模型來描述其運動的行為。在我們的實驗中，雷諾數可以用來描述圓柱體在運動過程中的轉變。對於在肥皂膜中水平放置的兩個圓柱體，我們觀察其具有穩定-穩定(steady-steady)、穩定-轉動(steady-tumbling)及轉動-轉動(tumbling-tumbling)三種主要的運動模式，且在特定的流量範圍內，三種的運動模式可透過外力交互產生，而我們也發現了在旋轉動-轉動的狀態下，具有兩種不同的相位。此外，在兩個圓柱體的實驗中，我們並沒有觀察到擺動的行為。

Soap films are considered very good physical approximations to the two-dimensional (2D) fluids. We present experimental results for the dynamics of freely-rotating rigid cylindrical rods in a vertically flowing soap film. We study various auto-rotational motions for single and double rods in the soap film. The observed dynamics of the rigid rods are discussed through their complicated interaction with the surrounding fluid flow, including hydrodynamic pressure and drag.
In this thesis, we first report scientific findings in the past made with the experiments in 2D soap film and auto-rotational motions, and then give a brief introduction of our apparatus and experimental principles. By putting the rigid cylindrical rods in a vertically flowing soap film, we systematically observe the behavior of freely-rotating rods with different experimentally adjustable parameters, such as lengths of rods and widths of films. Finally, the detailed description of our results is given by means of flow pattern and image analysis.
For single rod experiment, there are three distinct, stable dynamical states. The first is a Steady (S) state: the rod is immobile and perpendicular to the flow direction. The second is a Fluttering (F) state: the rod executes a fluttering motion in a manner like a bird flapping its wings. The third is a Tumbling (T) state: the auto-rotational motion of the rod. We attempt to explain why the rod gets these motions by using experimental technique to visualize the flow, such as optical interference technique. We establish a simple model to describe the behavior of single rod and the transitions between the three motions.
We not only discuss single rod experiment but also study further the hydrodynamically coupled interaction between two such rods. We will show the existence of more dynamical states and describe the coupling rods’ motions. For the case of horizontal double rods experiment, the two identical rods are put at the same height in the flowing soap film with small interval and the center of the gap is at the middle of the film. The three main states are observed, including steady-steady (2S), steady-tumbling (ST) and tumbling-tumbling (2T) states. We also discover the coupling interaction between left-right rods to result in two distinct phases (in-phase and out-of-phase) as they are in 2T modes. Unfortunately, the experimental data of horizontal double rods is insufficient to interpret these two different phenomena.
Perhaps, this interesting question can be answered by changing the separate interval or getting flow pattern of two tumbling rods. These will be our future works. In addition, the case of vertical double rods experiment is also another interesting topic and we believe that there will be more interesting phenomena.

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