以實驗探討雙眼牆渦漩之形成與演變過程

Abstract

本論文以水工實驗探討雙眼牆渦漩之形成與演變過程。利用盆池渦漩(Bathtub vortex)形成一個近似穩態的渦漩，再以快速改變流量的方式造成渦漩強度(Intensity)及範圍的變化(Strength)。實驗的結果顯示在瞬間減少流量的情況下，可在渦漩的演變過程中觀察到徑向擴張的現象；而快速增加流量時，於環向速度的演變過程中，可測量到雙眼牆的雙速度峰值。實驗上發現，形成雙速度峰值最小半徑比rc* > 1與渦度比ζc*> 4.8，這指出徑向擴張與強度變化是形成雙眼牆結構重要因素，此與Kuo et al. (2004)研究結果相似。然而在自然界中雙眼牆的產生除了渦漩的強弱變化之外，地形影響及不對稱入流也都會產生雙眼牆結構。實驗上，也利用了拖曳地形的方式產生不對稱入流，使得渦漩產生雙眼牆結構，這是在文獻中未曾被探討過的。這些所觀察到的雙眼牆現象與大氣中颱風渦漩有一定程度的相似。

In this thesis, the formation and transition process of double eyewall vortices were investigated in a rotating tank with background vorticity. After a quasi-steady bathtub vortex is generated, the vortex is intensified and strengthened caused by a sudden increase and decrease of suction rate. It is observed that the core region of vortex evolves into a transition state, expanding the core radius resultant of the sudden decrease of the suction rate. This is followed by the formation of double eyewall structure identified by the double-peaked velocity distribution in the azimuthal component due to the sudden increase of the suction rate. According to the experimental results, the criterion of formation of double eyewall vortex are with radius ratio rc* > 1 and vorticity ratio ζc* > 4.8. Note that the radius expansion, change of intensity and change of strength are significantly parameters to form the double eyewall structure, which are similar to the results of Kuo et al. (2004). Furthermore, double eyewall phenomea can be also induced by the interaction of topography and nonsymmetrical in-flow. Preliminary results show the phenomenon of the formation of double eyewall structure by towing a topography toward a vortex. Similar experimental results have not been observed in literatures yet. Also, these phenomena are similar to what were observed in typhoon vortices.