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  • 學位論文

應力驅動剪紊流場的擬序渦旋結構

Coherent vortical structures in a stress-driven turbulent shear flow

指導教授 : 蔡武廷

摘要


狹長的流場結構是高風速之下水面層的特徵,在水面形成由氣泡與水滴構成的條痕。儘管會被碎波間歇性阻斷,表面條痕持久與結構化的特徵顯示形成條痕的擬序渦旋與自保持機制的存在。使用高解析度的高風速之下的紊流數值模擬闡明擬序渦旋結構與它們的生成機制。流場由施加在水面邊界的剪應力驅動(水側的表面摩擦速度為1.225 公分/秒),用以模擬水面層。為了比較,也計算了較弱的剪切速率(表面摩擦速度為0.707 與 1 公分/秒)的水面層資料。採用由區域速度梯度衍伸出的渦流強度指示器辨識渦旋結構。套用兩種不同顏色以區別這些渦旋結構的旋轉方向,並實施進階變數間隔空間平均法萃取造成各式流場作用與水面特徵的渦旋結構。模擬的剪力流顯示兩種截然不同的水面特徵:幾乎等縱向間距的狹長高速條痕,與間歇性且區域化的低速點。這些水面特徵是由三種渦旋結構造成:正向的馬蹄渦(頭部朝向下游)與湧升運動及水面發散關聯。他們可以造成水面的低速點;反向的馬蹄渦(頭部朝向上游)與沉降運動關聯。他們可以強化水面的高速條痕;以及簇擁在高速條痕兩側,形成交錯、對轉的準流向渦旋結構陣列。這些擬序渦旋結構支持現有的親代渦漩與受剪切的水面交互作用而產生子代渦旋的自生成機制。不同剪切速率下的水面層呈現相似的無因次黏滯尺度條痕間距。然而,沿流向長度會隨著剪切速率增加而伸展,顯示在高風速的情況下,表面條痕將會伸長。

並列摘要


Under high wind conditions, the surface layer is characterized by an elongated flow structure which manifests itself by forming streaks of bubbles and droplets on the surface. Despite intermittent disruption by breaking waves, the persistent and structural features of these surface streaks suggest the existence of underlying coherent vortices that form the surface streaks and the self-sustaining mechanisms of the vortices. High-resolution numerical simulation of a turbulent flow subject to high shear was conducted to elucidate the coherent vortical structures and their formation mechanism. The flow is driven by imposing shear stress (surface friction velocity on the water side = 1.225 cm/s) on the boundary to mimic the surface layer. For comparison, computations were also conducted for the surface layer with weaker shear rates (with surface friction velocity = 0.707 and 1 cm/s). The vortical structures were identified by adopting an indicator of swirling strength derived from the local velocity gradient tensor. These vortical structures are marked with two different colors to distinguish the rotation direction and the vortical structures responsible for various flow processes and surface signatures can be extracted by employing the advanced variable-interval space-averaging technique. The simulated shear flow reveals two distinct surface signatures: elongated high-speed streaks that are arranged with somewhat equal cross-spacing and localized low-speed spots that appear intermittently. Three types of vortical structures were identified as responsible for these surface signatures: forward horseshoe vortices (head heading downstream) associated with upwelling and surface divergence that can cause surface low-speed spots; reversed horseshoe vortices (head heading upstream) associated with downwelling that can strengthen surface high-speed streaks; quasi-streamwise vortices that flank high-speed streaks as staggered, counter-rotating arrays. This vortical structure supports the regeneration mechanism in which pre-existing parent vortices interact with the sheared surface and induce offspring vortices. Surface layers with different shear rates exhibit similar non-dimensional streak spacing in a viscous scale; however, the streamwise extent, increases with the shear rate, indicating prolonged surface streaks under high-wind conditions.

參考文獻


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