A single gas bubble rising in a narrow vertical tube is investigated via a numerical model on a 3-D axisymmetric computational domain. The transient governing equations are solved by a finite volume scheme with a two-step projection method. The interface between the liquid and gas phase is tracked by a coupled level set and volume-of-fluid (CLSVOF) method. A surface tension modeling method, which preserves the jump discontinuity of pressure at the interface, is employed. The flow structure and terminal velocity obtained in the numerical simulation are in excellent agreement with experimental measurements. Special attention is paid to the bubble oscillations during the initial stage of ascent. It has been found that the bubble bottom undergoes severe oscillations while the nose maintains a stable shape. A parametric study is performed to identify the factors controlling the oscillations at the bubble bottom.

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