Numerical investigation of rising bubble wake and shape variations

The unsteady and open wake and unsteady shape changes in a bubble rising on a zigzag path are investigated by performing direct numerical simulations. For the description of phase interfaces the hybrid particle-level-set method is employed. An air bubble with a volume-equivalent diameter of d_B=5.2 mm rising in water is considered. The bubble Reynolds number is set as Re_B=598. We observe a zigzagging bubble ascent path, which is caused by periodic shedding of hairpin vortices from the bubble surface to the bubble wake. Chains of up to four hairpin vortices can be observed. A twisting of the vortex chains indicates the transition from a zigzag to a spiralling ascent path. The bubble exhibits a time-averaged ellipsoidal shape and 2,2-mode shape oscillations. The shedding of hairpin vortices is accompanied by bubble-shape deformations which can be represented by traveling surface waves. This phenomenon results in a periodic asymmetric bubble deformation and is in agreement with experimental observations.