Abstract:

The VOF (volume of fluid) method was used to numerically simulate the free-rising process of bubble in still water, and the evolution of bubble wake with different initial diameters was studied. The results show that there are three motion states of bubble wake: symmetrical shedding, unstable shedding and periodic shedding during the bubble freely rising, and the unstable shedding is transitional state between symmetrical shedding and periodic shedding. The shape of bubble changes from spherical to ellipsoidal as it rises. The symmetric shedding of bubble wake occurs when the bubble is in the ellipsoidal shape state and rises straightly. As the bubble continues to rise, when the long axis of the bubble and the horizontal direction form an angle, the wake will change from symmetrical shedding to a transition state (unsteady shedding). Eventually, the bubble wake changes a motion state of periodic shedding. The critical Reynolds number of transition of the wake motion state of bubble with an initial diameter of 2.4－3.7 mm is difference and increases with the increase of the bubble initial diameter. The frequency of periodic shedding of the bubble wake with an initial diameter of 2.4－3.7 mm is 31－39 Hz, and the frequency decreases with the increase of the initial diameter of bubble.

Key words: numerical simulation, VOF method, bubble, wake, frequency of shedding

摘要：

利用VOF（volume of fluid）方法对静止水中气泡的自由上升过程进行了数值模拟，研究了初始直径不同的气泡自由上升时尾流的演变过程。研究发现气泡自由上升过程中其尾流有对称脱落、过渡态和周期性脱落三种运动状态，其中过渡态是介于对称脱落和周期性脱落之间的非稳定性脱落。气泡上升过程中其形状由球形转变为椭球形。尾流的对称脱落发生在气泡处于椭球形状且沿直线上升的过程；随着气泡的继续上升，当气泡的长轴与水平方向产生夹角时，尾流就会由对称脱落转变为过渡态（非稳定性脱落）；最终，尾流会转变为周期性脱落。2.4 ~3.7 mm气泡尾流运动状态发生转变的临界Reynolds数不同，且随气泡初始直径的增大而增加。直径为2.4 ~3.7 mm气泡的尾流周期性脱落的频率为31 ~39 Hz，且频率随气泡直径的增大而减小。

关键词: 数值模拟, VOF方法, 气泡, 尾流, 脱落频率