波浪邊界層流場渦漩結構之擷取

Abstract

本研究旨於探究自由傳播波浪下之流場、波狀面上之空氣紊流場邊界層內之渦漩結構。我們採用四種建立於速度應變率特徵之旋渦結構辨識方法，並與傳統與渦度向量有關之方法比較。此外，條件平均法也被運用於擷取馬蹄形渦旋結構。在兩種流場中，四種渦漩結構辨識方法皆可得到相近之結果。在自由傳播波浪下之流場，可以觀察到沿流向伸展、成對反向旋轉之渦漩結構。藉由比較渦漩結構辨識方法和與渦度向量有關之方法，可以得知以渦漩結構辨識方法來研究流場較為直覺。在波浪上之空氣紊流場中，可以觀察到馬蹄型渦漩結構及類沿流向渦漩結構，我們也觀察到了馬蹄型渦漩結構的演化並發現其紊流場存在大尺度之旋轉運動。此外，條件平均的結果顯示象限分析取樣法較VISA取樣法佳。

In this thesis we study the coherent vortical structures within the boundary-layer flow next to a wavy surface. Both aqueous flow beneath a free-propagating surface wave and turbulent air flow above a prescribed propagating wavy boundary are considered. Four vortex identification schemes based on the characteristics of velocity-strain rate are adopted to extract the vortical structures. These schemes are also compared with traditional methods based on vorticity vector. Conditional averaging technique of the flow field is also applied to extract the horseshoe vortices. In both aqueous and air flow, four vortex identification schemes derive similar results. For the aqueous flow underneath a free-propagating surface wave, counter-rotating vortex pairs that elongate along streamwise direction are observed. By comparing vortex identification schemes with methods based on vorticity vector, it is found that observing vortical structures is a more intuitive way to study the flow field. For the air flow above a prescribed propagating wavy boundary, horseshoe vortices and quasi-streamwise vortices are found, the regeneration of forward horseshoe vortices are observed, and the larger scale vortical motions in the flow field above wavy surface are discovered. In addition, the conditional averaging results shows that out of the two sampling criterions, the quadrant analysis sampling technique is more feasible than the VISA sampling technique.