風剪應力驅動紊流場表面條痕形成的自持機制

Sustaining Mechanism of Surface Streaks in Wind-Driven Turbulent Shear Flow

郭時棣(Shih-Di Kuo)

指導教授 ： 蔡武廷

國立臺灣大學/工學院/工程科學及海洋工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202001191

摘要

在高風速的自由液面流場下，表面流向速度將會產生分區：高速區域成流向狹長條痕狀，低速區則會間歇性的以點狀散播在高速條痕末端使之分岔。數值模擬之結果顯示，此現象藉由二種機制產生：一為波浪參與之朗謬爾環流；一為無波浪參與的純表面風驅剪切應力引導之紊流場。藉由強紊流區域可視化的方法，本篇研究分析表面順流向高速條痕以及水面下暗藏之紊流擬序結構之關係及特性，其中採用的條件平均法擷取流場特徵，可以有效去除重要事件中心以外的雜訊及干擾。由於在流場中可以清楚見得強紊流區域大致由準流向擬序結構組成，故除了使用物理訊號作為篩選條件外，亦採用幾何方法識別流場中分別的結構並加以平均。平均的結果顯示了單一結構的幾何特性及與條痕的空間關係，提供了條痕與結構自持機制之線索。利用以上結果，參考壁層流場的結構機制，我們接著提出對於水氣介面剪力驅動流場之條痕結構自持機制的猜想，說明在表層流場中看到的速度分布特徵--高速條痕及間歇性低速區域--的成因，以及羅列準流向結構借助於速度分布之自持及強化的要素。延續前述之結構辨識，我們進一步將結構依照幾何特性區分，並找出此流場中結構的主要幾何型態。該幾何型態與背景渦度場有所關聯，並得以類比於壁層流場之馬蹄渦結構，提供高速條痕強力的邊界下沉流，使得高速條痕持續綿延。

關鍵字

擬序渦旋結構 ； 自由液面 ； 應力驅動 ； 水面特徵 ； 自持 ； 條件平均法 ； 擬序結構分類

並列摘要

The study is dedicated to discover the interactions between surface streak and the underlying coherent structures. The ocean under high wind speed is likely to have a streaky appearance. The surface signature arises from spanwise velocity convergence and divergence and streamwise velocity. Two mechanisms join in such velocity distribution by inducing coherent motion of the flow, and the numerical simulations with different boundary conditions have succeeded in separating them. In this study, We analyze a numerical result of free-surface with suppressed surface motion to understand the mechanism by wind shear. Swirling strength is utilized to visualize the turbulent flow and understand the coherent structure hidden under the streaks. With conditional averaging, the event of interest is extracted, and the noisy turbulent signal is suppressed. In addition to Variable-interval-space-averaging skills, we introduce an averaging scheme based on structure identification and succeed with better results. Averaging from the flow field gives us a clue to the self-sustaining mechanism of streaks and structures. We apply a classification scheme of the coherent structure by its geometry, and find out that the curvature of the structure is one of the causes of downwellings. The structure is strengthened by streaks and profile and provokes convergence near-surface to generate high-speed streak. We propose a complete hypothesis and describe the interaction between structure and the velocity distribution near-surface of the shear-driven free-surface flow.

並列關鍵字

Coherent Structure ； Free-surface ； Stress-driven ； Surface signatures ； Self-sustaining ； Conditional average ； Coherent structure classification

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