摘要或動機

陸地上的金字塔無時無刻沒有受到氣體的包覆，這個實驗透過風洞模擬金字塔模型，在流體中的壓力變化，並藉由煙線來觀察流體的移動情形，可具體看到當流體經過物體週遭時流場的改變。經由測量壓力，可更深入的探討風速與壓力的關係。在實驗中，可發現當流體迎面而來，在金字塔後方形成的流場變化中，以渦流最易觀察；且透過壓力的測量，發現模型的前、後方，會有明顯的壓差，會對模型造成力矩，可能會使金字塔結構不穩定。越往金字塔上方壓差越小，因此其受阻力所產生的合力矩應較同底面積、同高的長方體小，故金字塔可以長久在沙漠中屹立不搖。此外，流體流過模型兩側會產生分離的情況，分離點的位置會影響渦流區的大小；一般而言，分離點發生在物體的越後方，尾流的尺寸越小，壓差所造成的阻力越低，實驗中發現流速對分離點的位置影響不大，金字塔的擺放形狀卻會對分離點的位置產生明顯的影響，所以越流線型的物體，其受到壓差所產生的阻力越小。希望透過以上的研究，能夠對金字塔周圍複雜的流場有更深入的了解。The pyramid on land is constantly surrounded by gas. Using the wind tunnel, we can investigate the distribution of pressure surrounding the model of pyramid; furthermore, we can discuss about the relationship between pressure and wind velocity. In addition, with the smoke wire, we can visually observe the dynamic flow filed. In our experiment, we discovered that when fluid comes, there will be obvious difference of pressure between the front and the back of the pyramidal model, causing a torque to the model. At higher parts of the model, there is less difference of the pressure, so the resulting torques of a pyramid should be less than that of a cuboid. Therefore, a pyramid can stand in the desert for a long time. Besides, when the fluid flows through the two sides of the model, it will separate, and the position of the separation points will influence the size of the turbulence area. Generally speaking, the farther the separation points are to the back, the smaller the size of the turbulence area, and the less the drag caused by the difference of pressure. With the study above, I expect to have more understanding about the complex flow field of pyramid.