Swirl motions occurring in a bottom blowing water bath of an aspect ratio between approximately 0.2 and 1.0 were observed by eye inspection and with a high-speed video camera. The aspect ratio is defined as a ratio of the bath depth to the bath diameter. Effects of reduced pressure on the period, amplitude and some related aspects of the swirl motions were investigated. Measured values of these quantities for gas injection under the reduced pressure on the bath surface agreed with their respective measured values for gas injection with the same volumetric gas flow rate under the atmospheric pressure on the bath surface. Empirical equations for these quantities were proposed.
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