上升气泡运动、变形与诱导流动的实验研究
EXPERIMENTAL STUDY ON THE MOTION, DEFORMATION, AND INDUCED FLOW OF RISING BUBBLES
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摘要: 通过向静水中连续注入空气形成频率可控的上升气泡串, 采用阴影成像与体视粒子图像测速技术对气泡的运动、变形以及诱导流场进行三维测量. 基于阴影成像测量结果研究了气泡轨迹、运动速度以及界面波动特征; 提出了基于两个非正交方向的投影图像进行气泡三维重构的数学方法, 并基于该方法得到了气泡界面的三维形貌; 研究了不同空气注入流量对气泡上升速度、气泡空间分布以及气泡诱导流场的影响; 通过条件平均方法, 研究了单个气泡对周围速度脉动的影响. 研究发现, 气泡轮廓近似为椭球形, 其瞬时长宽比呈现周期性的波动, 波动频率与理论预测值符合较好; 在空气注入流量较大时, 气泡的运动轨迹呈现较大的随机性, 并且在水平截面内的分布趋于各向同性; 随着高度增加, 气泡上升速度与诱导流场速度首先快速增长, 然后趋于稳定. 在本实验考虑的空气流量范围内, 单个气泡对脉动速度场的诱导作用受流量影响较小; 因此, 泡诱导湍流可以近似地看作单个气泡诱导作用的线性叠加.Abstract: The rising bubbles with controllable releasing frequency were generated by continuously injecting air into the quiescent water. The motion, deformation, and induced flow of these bubbles were measured in a three-dimensional (3D) manner using shadowgraphy and stereoscopic particle image velocimetry (SPIV). This work investigates the characteristics of the bubble motions and the interface oscillations based on the shadowgraphy data. The bubble images from SPIV are employed to reconstruct their three-dimensional geometries via a newly proposed method. The bubble motions, the spatial distributions of bubbles, and the bubble-induced flows are compared for different airflow rates. The conditional average method is employed to statistically extract the fluctuating flow field induced by a single bubble. It is found that the typical shape of the bubble is approximately ellipsoidal, and its aspect ratio exhibits periodic oscillation, with the dominant frequency in good agreement with the theoretical prediction. When the airflow rate is high, the trajectory of the bubbles exhibits stronger randomness, and the distribution within the horizontal cross-section tends to be isotropic. As the height increases, both the rising speed of the bubble and the induced flow velocity present a temporary rapid increase before stabilizing. Within the range of the airflow rate considered in this experiment, the inducing effect of each bubble is less sensitive to the airflow rate. Accordingly, the bubble-induced turbulence could be viewed as the superposition of the induced flows of individual bubbles.