Dynamics of bubble migration in a square channel flow of a viscoelastic fluid

Abstract

Cross-stream migration of a deformable bubble is investigated computationally in a pressure-driven channel flow of a viscoelastic fluid via interface-resolved simulations. The flow equations are solved fully coupled with the Giesekus model equations using the front-tracking method and extensive simulations are performed for a wide range of flow parameters to reveal the effects of bubble deformability, fluid elasticity, shear-thinning, and fluid inertia on the bubble migration dynamics. Migration rate of a bubble is found to be much higher than that of a solid particle under similar flow conditions mainly due to free-slip condition on its surface. It is observed that direction of bubble migration can be altered by varying shear-thinning of the ambient fluid. With a strong shear-thinning, the bubble migrates towards the wall while it migrates towards the center of the channel in a purely elastic fluid without shear-thinning. An onset of elastic flow instability is observed beyond a critical Weissenberg number, which in turn causes a path instability even for a nearly spherical bubble. An inertial path instability is also observed once bubble deformation exceeds a critical value. Shear-thinning is found to be suppressing the path instability in a viscoelastic fluid with a high polymer concentration whereas it reverses its role and promotes path instability in a dilute polymer solution. It is found that bubble migration towards wall induces a secondary flow with a velocity that is about an order of magnitude higher than the one induced by a solid particle under similar flow conditions.