Two kinds of swirl motions are observed in a cylindrical bath with centric bottom gas injection. One is induced byapproximately periodical generation of bubbles at the nozzle exit. The other is caused by hydrodynamic instability of a large scale ring vortex enclosing the bubbling jet. The two swirl motions occur for HL/D_??_1 and HL/D_??_2, respectively, where HL is the bath depth and D is the bath diameter. Air-water, mercury-nitrogen and Wood's metalnitrogen systems are used in the present cold model experiments. The following three subjects for the former type of swirl motion are treated.
(1) Time from the start of gas injection to the initiation of swirl motion, being called the starting time of swirl motion.
(2) Time required by the swirl motion to damp out after the stopage of gas injection, being termed the damping time of swirl motion.
(3) Amplitude of swirl motion at the side wall of the vessel.
Empirical correlations of the starting time and damping time are derived. The amplitude of swirl motion exhibits a peak value at HL/D between 0.4 and 0.5.