  We study, by numerical simulations, the entrainment process in a supersonic,
radiative jet flow, during the evolution of Kelvin-Helmholtz instabilities, in
the context of the the acceleration problem of molecular bipolar outflows,
observed in Giant Molecular Clouds. Our results show that a large fraction of
the initial jet momentum can be transferred to the ambient medium by this
process. We therefore analyze in detail the instability evolution and compare
some of the main observational properties of molecular outflows with those of
the entrained material that we observe in our simulations. In particular, we
find a good agreement for the mass vs. velocity distribution and for the
outflow collimation structure, especially when a light jet is moving into a
denser ambient medium. This is probably the case for (obscured) optical jets
driving powerful molecular outflows in the denser environment of the inner
regions of molecular clouds.
