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  • 學位論文

具背景效應下單一渦旋之生成、演變及環境熱效應之探討

Generation, Transition and Thermal Effect of a Swirling Vortex with Background Vorticity

指導教授 : 朱錦洲
共同指導教授 : 張建成(Chien-Cheng Chang)

摘要


本論文包括三部分,分別是單一渦旋流場由單胞結構演變至雙胞之過程、似龍捲漏斗雲形成之探討與渦旋熱效應之研究。 第一部分為探討旋轉水槽中,單一渦旋流場由單胞結構演變至雙胞之過程。實驗上以巧妙的機制,藉由虹吸管的方式自水槽中抽吸部分流體後移開所導致。在渦旋到達類似二維的正壓渦旋前,整個演變的過程可區分為三個階段:(1)在汲取階段產生單胞渦旋;(2)在虹吸管移離水面瞬間,高速旋轉流體將空氣自自由液面捲入至穿越整層流體而達底層,導致雙胞渦旋的產生;(3)由邊界層往渦心幅合的流體,逐步將雙胞結構的流場由旋轉軸的底層推往液面,產生像倒圓錐形狀一塊獨立的環流區域。最終渦旋演變為穩定的近二維正壓旋渦,而包圍渦心的外圍流場則保持著地轉平衡狀態。在單胞渦旋時,吾人以Burgers的旋渦模型來描繪出此種流場結構;然而在雙胞渦旋時,Sullivan和Bellamy-Knights的旋渦模型卻無法完整地描繪出此種流場結構。實驗上,藉由定性的染料施放法及定量的粒子追跡測速法可準確的得到流場中流體的細部資訊,諸如渦旋流場結構、速度及渦度分布圖,及自由液面下沉量等。如此一連串複雜的流場行為可幫助我們解釋部分尚未發現或未確定的龍捲風概念性模型。 第二部分是似龍捲漏斗雲形成之探討。吾人以吸入法在水槽中心產生單一渦旋,在生成渦旋的同時,從渦旋上下、層注入紅、綠色染料來模擬及顯影似龍捲漏斗雲的形成與成長過程。藉由酒精、水、染料和鹽來調配不同比重之紅色染料(Specific weight, ρ = 1.10、1.05及0.97),而綠色染料比重固定為1.04。實驗顯示(1)當在染料比重小於水時,會產生正的浮力,增加渦旋強度,使其達到底部的時間較久;(2)當染料比重接近於水時,渦旋強度較不受影響;(3)在染料比重略比水來得重時,會產生負的浮力,減弱渦旋強度,使其縮短到達底部的時間。經由實驗的探討可幫助我們解釋部分漏斗雲形成的原因與演變之過程。 最後第三部分為渦旋熱效應之研究。藉由水工模擬實驗,探討具有背景渦度下渦漩熱效應現象,吾人將著重在熱邊界層的形成、熱邊界層與渦旋之交互作用與渦旋過冷、熱帶之影響。實驗上引進新穎之全域速度與溫度場量測方法—粒子影像速度與測溫法(Particle Image Velocimetry/Thermometry, PIV/T),可同時量測流場中二維剖面的速度與溫度分布。實驗結果顯示(1)冷、熱邊界層確實於冷、熱底板的生成,且溫度梯度自周圍往中心漸增;(2)冷、熱邊界層之存在會影響渦旋之熱力結構,冷邊界層易造成渦旋底部的穩定,有抑制渦旋的效果,而熱邊界層容易形成渦旋底部的不穩定,有增強渦旋的效果;(3)渦旋強度與β平面下之路徑變化會受到溫度效應影響;冷渦旋較常溫渦旋之路徑偏北,常溫渦旋又較熱渦旋之路徑偏北,且熱渦旋之渦旋強度大於常溫渦旋,而常溫渦旋又大於冷渦旋。經由初步的實驗探討可幫助我們進一步瞭解及重視自然界中渦旋與環境熱效應之問題,如颱風與龍捲風。

並列摘要


The thesis consists of three parts: transition of a swirling vortex from one-celled to two-celled structure in a rotating tank, evolution of funnel-like vortex structure and the thermal effect on vortex motion. In part I, we investigate the transition of a swirling vortex from one-celled to two-celled vortex structure in a rotating tank. The main idea is to initiate the flow by siphoning fluid out of the tank and then lift the siphoning mechanism off the water in a short period of time. Before it reaches a state of quasi-two-dimensionality, the core region of the vortex can be roughly divided into three stages. (1) A stage of siphoning induces the formation of the one-celled vortex. (2) A stage of downward jet impingement triggers the transition of the vortex into the two-celled one. (3) A stage of detachment of the inner cell leads to a cup-like recirculation zone, which is pushed upward by an axial flow from the boundary layer. This eventually develops into a stable quasi-two-dimensional barotropic vortex. The core region is enclosed by an outer region which is in cyclostrophic balance. In siphoning stage, the flow pattern can be well fitted by Burgers' vortex model. However, in the post-siphoning stage, the present data shows a flow pattern different from the existing two-celled models of Sullivan and Bellamy-Knights. Flow details, including flow patterns, velocity profiles and surface depressions were measured and visualized by particle image velocimetry, particle tracking velocimetry and the dye-injection method with various colors. The one-celled and two-celled flow structures are also similar to the conceptual images of the one and two-celled tornadoes proposed in the literature. In part II, a swirling vortex is visualized by red/green dye-injection from top/bottom in order to simulate the formation and transition of funnel-like structure. In particular, the time needed for touch down of the vortex is considered. The specific weight of the red dye will be adjusted by alcohol, water and salt (ρred = 0.97, 1.05 and 1.10); however, the specific weight of the green dye is fixed (ρgreen = 1.04). It is shown that (1) when the specific weight of the red dye is smaller than water, a positive buoyant force will be produced and the strength of vortex will be increased, resulting in taking more time for the vortex to touch down; (2) when the specific weight of the red dye is near to that of water, the time needed for touch down will be little influenced; and (3) when the specific weight of the red dye is larger than water, a negative buoyant force will be produced and the strength of vortex will be decreased, resulting in taking less time for the vortex to touch down. From experimental investigation, we can make an analog and interpret characteristics of tornado-funnel structures in nature. In part III, the formation of thermal boundary layer, the interaction of thermal boundary layer and vortex, and their effects on the vortex passing the cold/hot belt are investigated. A PIV/T (Particle Image Velocimetry/Thermometry) technique is developed for measuring quantitatively velocity and temperature simultaneously. It is shown that (1) a cold/hot boundary layer will be generated in the thermal plate and the temperature gradient will gradually increase from the ambient to center of plate; (2) the dynamics of a swirling vortex including, the strength and path of the vortex, will be influenced by the cold/hot boundary layer; and (3) a colder vortex moves to the north compared to the trajectory of the vortex in the environment of room-temperature. On the other hand, a hotter vortex moves to the south under the thermal effect. A hotter vortex will be stronger than a colder vortex. These results may shed some light on the thermal effects on vortices in nature, such as tornadoes or typhoons.

參考文獻


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