雙穴蝕氣泡破裂流場交互作用之試驗研究

Abstract

本研究利用二組L型透明圓管放置於U型平台上旋轉，個別產生單一穴蝕氣泡，並將其中一顆氣泡藉由連通管移至另一試管之特定位置上。藉由調整兩顆氣泡間的距離，並使用壓力震波將試管內二顆氣泡震破，藉此分析二顆氣泡的破裂行為，在氣泡破裂後會於氣泡中間形成液體帶狀區，形成低流速、高壓力情況。 本研究使用高速攝影機，以每秒8000到10000張連續拍攝氣泡破裂過程的影像並做分析，分析各種不同間距氣泡破裂流場交互作用的特性。並應用質點影像測速法，進行流場交互作用的演變過程量測與細部速度場量測。根據PIV計算結果顯示，此液體帶狀區如輻射狀往周邊散開，不但影響破裂後的流場運動，同時也產生二氣泡之間的流體紊亂情況。氣泡受壓力震波後，氣泡之間的壓力波相互影響而導致流場紊亂，流場紊亂處有Rayleigh-Taylor不穩定、Kelvin-Helmholtz不穩定和Richtmyer-Meshkov不穩定現象發生。 實驗結果發現，距離參數γ值越小，壓力場之變化對氣泡外觀、推力及交互作用愈明顯。當γ值遠大於氣泡半徑時，二氣泡間破裂現象不互相牽引，等同於單一氣泡運動的過程。此外，二氣泡破裂的複雜現象可清楚展示與分析。

In this study, two cavitation bubbles were generated by rotating a U-tube filled with water. Cavitation bubbles are generated separately in two different L-tubes. One of the cavitation bubbles was then moved to a specific stand-off distance from the other bubble through a connection tube. Adjusting these two bubbles to the desired stand-off distance, a pressure pulse was then triggered to induce the bubble collapse flow. After the bubble collapsed, a liquid zone with low velocity and high pressure was produced. A high speed camera, recording 8000 to 10000 frames per second, was adopted to take the sequential images of the bubble collapse flows in this study. The characteristics of the interaction of the bubble collapse flows were analyzed based on the recorded sequential images. The detailed velocity fields of the interactions of the collapsed bubble flows were measured by particle image velocity (PIV) method. According to PIV measurements, results revealed that the formed liquid zone propagated radially. The movement of the liquid zone not only influences the motion of the flow field, but also produces disturbed flow between the bubbles. After the pressure imposed on the bubbles, the effect of the interaction of the pressure waves between two bubbles led to the disturbed flow field, resulted in the Kelvin-Helmholtz , Richtmyer-Meshkov and Rayleigh-Taylor instabilities. Experimental results showed that the effect of changes in the pressure field on the bubble appearance, thrust and interactions increased as the distance parameter γ value decreased. While the distance parameter γ value was much larger than the radius of the bubble, the bubble collapsing phenomenon would not be affected by the drag of each bubble. The phenomenon was equivalent to the motion process of a single bubble. In addition, the complex phenomena of the collapsing flows of two cavitation bubbles were presented and analyzed clearly.