Effect of orifice geometry on bubble dynamics and induced flow structures in quiescent fluids

Bubble flows from underwater orifices are fundamental to gas–liquid operations, although the influence of orifice geometry on bubble dynamics and induced flows remains underexplored. Shadowgraphy and laser-induced fluorescence particle image velocimetry are employed to investigate bubbles released into a quiescent liquid from circular and elliptical orifices with aspect ratios AR = 1–4. Elliptical orifices produce smaller bubbles with higher aspect ratios and greater morphological complexity. These features result from anisotropic contact angles along the orifice edge, which induce non-uniform capillary forces and strong deformation at detachment. This mechanism drives high-amplitude zigzag trajectories, distinct from the spiral paths observed with circular orifices. A force-budget analysis attributes the enhanced lateral drift to rotation-induced forces. In the wake, circular orifices sustain coherent counter-rotating vortices, whereas elliptical orifices promote irregular shedding and multiscale structures. The induced turbulence spectra follow an approximate $-2$ scaling. Furthermore, flows from elliptical orifices exhibit a higher fractal dimension of the turbulent/non-turbulent interface and stronger entrainment, with a marked increase in the engulfment flux. These results quantify the mechanisms by which orifice geometry determines bubble dynamics and the developing flow field.