應用流場可視化與經驗模態分解探討數值模擬風浪紊流場;Using flow visualization and empirical mode decomposition to study numerical wind-wave turbulent flow

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Title:	應用流場可視化與經驗模態分解探討數值模擬風浪紊流場;Using flow visualization and empirical mode decomposition to study numerical wind-wave turbulent flow
Authors:	呂冠鴻;Lu,Guan-hung
Contributors:	水文與海洋科學研究所
Keywords:	經驗模態分解;自由液面紊流邊界層;流場資料可視化;flow visualization;empirical mode decomposition;Langmuir circulation
Date:	2012-07-05
Issue Date:	2012-09-11 18:25:57 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究以可視化之技術來呈現數值模擬資料，探討風剪驅動之自由液面紊流邊界層的水面流場特性，並試圖以影像處理之技術分離不同物理機制機制所造成的影響。首先開發一搭配OpenGL的三維流場資料可視化即時互動軟體進行水面與其下的流場呈現，以了解流場於時空下的演化。其中發現如溫度、沿流向速度、氣體通量（gas flux）皆出現了沿流向之條痕結構，其是由於水面下方的紊流場，即Langmuir circulations所導致形成之結構。而在觀察水面結構的時候，亦發現由於流場受到不同物理機制，包括了重力波、表面張力波、Langmuir circulations的影響，而使得水面形成了多種不同尺度的結構。欲探討個別機制對水面流場之影響，故利用了影像處理之技術進一步對水面流場之結構作不同尺度之分解，此技術包含了Huang等（1998）所發展的經驗模態分解法（empirical mode decomposition）與條件相位平均法（conditional phase average method）之概念，藉此量化不同物理機制所造成的影響。其結果發現：於沿流向速度場的條痕結構與氣體通量場之條痕結構極為相似，或許可以作為於現場實驗難以量測的氣體通量之量化的依據或參考；另外，於氣體通量場的分解結果亦發現因下方紊流而造成的氣體通量佔總氣體通量不少的分量，顯示風浪交互作用所產生的水面下之紊流在海氣交換的過程中扮演了一個重要的角色。An interactive visualization tool and an image procession technique are developed to explore the coherent structures within a turbulent boundary layer bounded by a wind-driven gravity-capillary wave. The flow filed is adopted from direct numerical simulation conducted by Tsai and Hung (2007). The interactive visualization tool is developed using OpenGL, and is capable of visualizing time-evolving, three-dimensional flow field. The image procession method is a combination of conditional phase averaging and the method of empirical mode decomposition developed by Huang et al. (1998). With the help of the visualization tool, coherent structures with various length scales and induced by different physical processes are identified, including the gravity wave, the capillary wavelets and the Langmuir circulations. The streamwise vortices of Langmuir circulations are found to induce elongated streaks on the surface distributions of temperature, gas flux and streamwise velocity. The developed image procession technique is then applied to decompose the surface signatures formed by the various underlying flow processes, and to quantify their contributions. The decomposed streaky signatures in the surface distribution of streamwise velocity are found to highly correlate with those in gas flux distribution. This suggests a basis to quantify the gas flux, which is difficult to be measured directly, using other measureable quantities. In addition, the decomposed streaky signatures are found to dominate the distribution of gas flux, indicating Langmuir circulations play a significant role in air-water interfacial transport.
Appears in Collections:	[Graduate Institute of Hydrological and Oceanic Sciences] Electronic Thesis & Dissertation

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