Experimental investigation of ventilation bubble dynamics around a vertically moving cylinder under reduced ambient pressure

Ventilation bubble is widely used to reduce friction drag of object travelling in fluid. A large number of experimental studies are performed on the ventilation bubble in water tunnel, while little is known about it around vertically moving cylinder under reduced pressure. We aim to investigate dynamics of the ventilation bubble in vertically moving case. To support the study, a specialized experiment set-up is designed, based on which images of ventilation bubble around vertically moving cylinder under reduced ambient pressure can be captured and experiments are conducted to study the influence of velocities and flow rate on the ventilation bubble dynamics. In detail,We first describe the development of ventilation bubble and details of re-entrant jet. In addition to that, two types of re-entrant jets are observed, and the maximum velocity of re-entrant jet is obtained. Besides, four modes of development of ventilation bubbles' closure patterns are concluded. Finally, geometric features of ventilation bubble are obtained through image processing which are then described and analysed in detail.The above results contribute to the research on the control of vertically moving cylinder through ventilation bubble.