  We create Bose-Einstein condensates of 87-rubidium in a static magnetic trap
with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic
trap center we are able to control the condensate evolution. We observe a
change in the frequency of the center-of-mass oscillation in the harmonic
trapping potential, in analogy with an increase in effective mass. For fluid
velocities greater than a local speed of sound, we observe the onset of
dissipative processes up to full removal of the superfluid component. A
parallel simulation study visualizes the dynamics of the BEC and accounts for
the main features of the observed behavior.
