Size segregation, which is a robust feature of sheared granular mixtures
and geophysical mass flow deposits, is found to diminish in the presence
of a viscous fluid. We study this inhibitive effect through coupled
fluid-particle simulations of fully saturated granular flows.
Granular-fluid mixture flows are modelled according to three distinct
flow regimes -- free-fall, fluid-inertial, and viscous -- at different
angles of inclination. Each flow regime corresponds to distinct flow
dynamics and segregation behaviors. We find that segregation is indeed
weaker and slower in the presence of an ambient fluid which is more so
as the flow becomes more viscous. The ambient fluid affects segregation
in two major ways. Firstly, buoyancy reduces the contact pressure
gradients which are needed to drive large particles up, which at the
same time reduces the particles' apparent weight. On the other hand, the
streamwise drag force substantially changes the flow rheology,
specifically the shear rate profile, thereby modifying the segregation
behavior in the normal direction. Surprisingly, the fluid drag in the
normal direction is negligible regardless of the fluid viscosity and
does not affect segregation in a direct manner.