應用 PIV 及 FLDV 於矩形柱體來流端馬蹄型渦流流場之探討

Abstract

本研究旨在以精密非接觸實驗方式，針對完全浸沒於水體且直立於底板之矩形柱體（斷面為正方形），在無攻角且雷諾數範圍在1200 ～8500之間的情況下，探討柱體來流端中心軸線上馬蹄型渦渦流流場之特性。在本實驗當中，首先利用可視化法進行定性上的實驗觀察，並且以FLDV對流場運動之特性及頻率施作詳細量測，最後在配合PIV將流場量化量測後，提供研究做數據分析，以獲得實驗最詳實且可靠之探討依據，依此來深入了解馬蹄型渦流流場之種類與特性；實驗中並量測在不同實驗條件下（其中包含雷諾數、模型之高寬比、未受模型擾動前之邊界層厚度…等），流場的速度分佈及各個渦流結構中之瞬時徑線流場，另外配合多組速度向量流場加總所得到平均流場之速度向量圖，來深入探討各個渦流之間強度的大小與變化。其中部份運用相關實驗條件的改變，來擷取更有關渦流之重要控制因數。本文研究中主要探討項目包含：(1)馬蹄型渦流流場之特性分類與流場特性之存在區間進行討論；(2)在控制不同實驗變數下，對渦流的影響做初步的研究；(3)於本文中，特別將渦流特徵頻率利用無因次轉換作深入分析，以求得馬蹄型渦流流場發生週期性之特徵頻率之相似解。最後本研究於結論當中，將研究過程所獲得之渦流流場相關性結論作歸納整理並提出具體之建議。

The laminar boundary layer on the wall ahead of a vertical rectangular pier is in a region of adverse pressure gradient, and within some distance upstream of the body this boundary layer experiences a three-dimensional separation. In the separated region, a system of vortices is developed and stretched around the base of the body like a horseshoe shape. The characteristics of horseshoe vortex system near the juncture of a vertical rectangular pier and a horizontal bottom was studied experimentally using particle image velocimetry (PIV), fiber laser Doppler velocimetry (FLDV), and particle tracking flow visualization technique. The purpose of the present study is to provide the most comprehensive classification and description of the range of horseshoe vortex system. Moreover, the characteristics of the steady type and regular type are pointed out in the present study. Several detailed flow structures in the steady type and breakaway type, regarding the position of primary vortex core, the vortex velocity profile, and the frequency of horseshoe vortex reconstruction are discussed. The range of the Reynolds number Re, based upon the free stream velocity (U0 ) and the width of vertical rectangular pier (W), is form 1200 to 8500.