  Stability of coarse particles against gravity is an important issue in dense
suspensions (fresh concrete, foodstuff, etc.). On the one hand, it is known
that they are stable at rest when the interstitial paste has a high enough
yield stress; on the other hand, it is not yet possible to predict if a given
material will remain homogeneous during a flow. Using MRI techniques, we study
the time evolution of the particle volume fraction during the flows in a
Couette geometry of model density-mismatched suspensions of noncolloidal
particles in yield stress fluids. We observe that shear induces sedimentation
of the particles in all systems, which are stable at rest. The sedimentation
velocity is observed to increase with increasing shear rate and particle
diameter, and to decrease with increasing yield stress of the interstitial
fluid. At low shear rate ('plastic regime'), we show that this phenomenon can
be modelled by considering that the interstitial fluid behaves like a viscous
fluid -- of viscosity equal to the apparent viscosity of the sheared fluid --
in the direction orthogonal to shear. The behavior at higher shear rates, when
viscous effects start to be important, is also discussed. We finally study the
dependence of the sedimentation velocity on the particle volume fraction, and
show that its modelling requires estimating the local shear rate in the
interstitial fluid.
