  We investigate the effect of density and viscosity differences on a swarm of
large and deformable bubbles dispersed in a turbulent channel flow. For a given
shear Reynolds number, Re=300, and a constant bubble volume fraction, Phi=5.4%,
we perform a campaign of direct numerical simulations of turbulence coupled
with a phase-field method accounting for interfacial phenomena. For each
simulation, we vary the Weber number (We, ratio of inertial to surface tension
forces), the density ratio (r, ratio of bubble density to carrier flow density)
and the viscosity ratio (e, ratio of bubble viscosity to carrier flow
viscosity). Specifically, we consider two Weber numbers, We=1.50 and We=3.00,
four density ratios, from r=1 down to r=0.001, and five viscosity ratios, from
e=0.01 up to e=100. Our results show that density differences have a negligible
effect on breakage and coalescence phenomena, while a much stronger effect is
observed when changing the viscosity of the two phases. Increasing the bubble
viscosity with respect to the carrier fluid viscosity damps turbulence
fluctuations, makes the bubble more rigid, and strongly prevents large
deformations, thus reducing the number of breakage events. Local deformations
of the interface, on the contrary, depend on both density and viscosity ratios.
The opposite effect is observed for increasing bubble viscosities. We report
that these effects are mostly visible for larger Weber numbers, where surface
forces are weaker. Finally, we characterize the flow inside the bubbles; as the
bubble density is increased, we observe, as expected, an increase in the
turbulent kinetic energy (TKE) inside the bubble, while as the bubble viscosity
is increased, we observe a mild reduction of the TKE inside the bubble and a
strong suppression of turbulence.
