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Numerical Simulation of Transient Flow Past Rectangular Cylinder and Two Cylinders in Tandem Arrangement
Tien, Cheng Ho
選取單方柱及雙方柱間距2倍、3倍、4倍、5倍方柱寬等五種排列以雷諾數100計算其流場的動態變化。結果顯示方柱間距和karman vortex street現象的發生及方柱的受力大小有密切關係。雙方柱可依其不同間距分為兩種情況，間距2倍、3倍方柱寬時，上游方柱受下游方柱影響，沒有karman vortex street發生，下游渦流脫落情況較不明顯。間距4倍、5倍方柱寬時，上下游方柱皆有karman vortex street產生，上游方柱所產生的尾流對下游方柱造成強烈的振盪。間距5倍時，下游方柱升力係數約為單方柱五倍。
The purpose of this study is to analyze the dynamical flow characteristics over a single isolated square cylinder and two identical square cylinders in tandem arrangement with variable spacing between two cylinders. For the case of the flow over two cylinders, the spacing between two cylinders ranges from 2 to 5 widths of the cylinder are examined and the flow for all cases are investigated at the Reynolds number 100. Also, the loading induced on the cylinders including the drag and the lift coefficients is determined.
In the process of the computation, the vortex sheet is first used to simulate the surface of the cylinders by enforcing zero normal component of velocity at the surface. Then, the no-slip condition at the solid surface is satisfied by producing free vorticity near the surface. Next, the Biot-Savart Law of Induced Velocities is applied to determine the velocity field within the computational domain. And finally, the transport of the free vorticity in the flow field is fulfilled by satisfying the vorticity transport equation.
In this study, our computations show that the spacing between two cylinders will have the effect on the formation of the Karman vortex trail and on the magnitude of the force exerted on the cylinders. It also shows that two different modes of flow characteristics are observed based on the different spacing between two cylinders. The first type of flow characteristics is occurred when the spacing is set to be double or triple of the width of the square cylinder. In this case, there will be no Karman vortex trail found between two cylinders, but the vortex trail is found shedding from the downstream cylinder periodically. The second type of flow characteristics is occurred when the space of two cylinders is set to be 4 times or 5 times of the width of the cylinder. In this case, both the upstream cylinder and the downstream cylinder will result in the occurrence of the Karman Vortex trail periodically. The vortex trail induced by the upstream cylinder will cause the downstream cylinder to have a very strong vibration of fluid property, such as the drag and the lift coefficients.
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