  We report a new lift force model for intruders in dense, granular shear
flows. Our derivation is based on the thermal buoyancy model of Trujillo &
Hermann[L. Trujillo and H. J. Herrmann, Physica A 330, 519 (2003).], but takes
into account both granular temperature and pressure differences in the
derivation of the net buoyancy force acting on the intruder. In a second step
the model is extended to take into account also density differences between the
intruder and the bed particles. The model predicts very well the rising and
sinking of intruders, the lift force acting on intruders as determined by
discrete element model (DEM) simulations and the neutral-buoyancy limit of
intruders in shear flows. Phenomenologically, we observe a cooling upon the
introduction of an intruder into the system. This cooling effect increases with
intruder size and explains the sinking of large intruders. On the other hand,
the introduction of small to mid-sized intruders, i.e. up to 4 times the bed
particle size, leads to a reduction in the granular pressure compared to the
hydrostatic pressure, which in turn causes the rising of small to mid-sized
intruders.
