  The flux flow properties of epitaxial niobium films with different pinning
strengths are investigated by dc electrical resistance measurements and mapped
to results derived within the framework of a theoretical model. Investigated
are the cases of weak random pinning in as-grown films, strong random pinning
in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove
array milled by focused ion beam. The generic feature of the current-voltage
curves of the films consists in instability jumps to the normal state at some
instability current density $j^\ast$ as the vortex lattice reaches its critical
velocity $v^\ast$. While $v^\ast(B)$ monotonically decreases for as-grown
films, the irradiated films exhibit a non-monotonic dependence $v^\ast(B)$
attaining a maximum in the low-field range. In the case of nanopatterned films,
this broad maximum is accompanied by a much sharper maximum in both,
$v^\ast(B)$ and $j^\ast(B)$, which we attribute to the commensurability effect
when the spacing between the vortex rows coincides with the location of the
grooves. We argue that the observed behavior of $v^\ast(B)$ can be explained by
the pinning effect on the vortex flow instability and support our claims by
fitting the experimental data to theoretical expressions derived within a model
accounting for the field dependence of the depinning current density.
