Large deformable bubbles in wall-bounded turbulence: effects of inertial and viscous forces

We investigate numerically the effect of density and viscosity difference on a swarm of large deformable bubbles dispersed in a turbulent channel flow. For a given shear Reynolds number, Reτ = 300, and a constant bubble volume fraction, Φ ≃ 5.4%, we performed a campaign of direct numerical simulations (DNS) of turbulence coupled with a phase field method (PFM), employed to describe the interfacial phenomena. For each simulation, we vary the Weber number (ratio of inertial to surface tension forces), the density ratio (ratio of bubble density to carrier density) and the viscosity ratio (ratio of bubble viscosity to carrier viscosity). Our results show that breakage and coalescence phenomena are not influenced by density contrasts, while they are significantly influenced by viscosity differences. Increasing the bubble viscosity damps turbulence fluctuations, strongly prevents large deformations and thus reduces the breakage events. The bubbles shape, on the contrary, depends on both density and viscosity ratios. An increase of the bubble density increases the occurrence of large deformations of the bubble surface, while an increase of bubble viscosity reduces their occurrence. These effects are mostly visible for larger Weber, so in case of weaker surface forces.