  We study the effect of droplet coalescence on turbulent wall-bounded flows,
by means of direct numerical simulations. In particular, the volume-of-fluid
and front-tracking methods are used to simulate turbulent channel flows
containing coalescing and non-coalescing droplets, respectively. We find that
coalescing droplets have a negligible effect on the drag, whereas the
non-coalescing ones steadily increase drag as the volume fraction of the
dispersed phase increases: indeed, at 10\% volume fraction, the non-coalescing
droplets show a 30\% increase in drag, whereas the coalescing droplets show
less than 4\% increase. We explain this by looking at the wall-normal location
of droplets in the channel and show that non-coalescing droplets enter the
viscous sublayer, generating an interfacial shear stress which reduces the
budget for viscous stress in the channel. On the other hand, coalescing
droplets migrate towards the bulk of the channel forming large aggregates,
which hardly affect the viscous shear stress while damping the Reynolds shear
stress. We prove this by relating the mean viscous shear stress integrated in
the wall-normal direction to the centreline velocity.
