  A dense gas of neutrinos and antineutrinos can undergo fast pairwise
conversions near the decoupling regions of core-collapse supernovae and in
compact binary neutron star mergers. The flavor dependent neutrino heating can
play a role in sustaining convection inside the hot and dense matter. In this
paper, we study the unexplored effect of the bulk velocity of matter on fast
pairwise conversions and demonstrate that depending on the direction and the
magnitude of the bulk velocity, neutrino flavor conversions could be
significantly enhanced or suppressed. The bulk velocity of matter, which is
usually neglected in the context of neutrino oscillations, can reach values of
one-tenth of the speed of light in astrophysical environments. We find that
bulk velocities much smaller than the maximum allowed velocities can
substantially change the neutrino flavor conversion rate. The demonstration of
possible enhancement of neutrino flavor conversion rate due to the bulk
velocity of matter also raises several important issues that are of relevance
in the supernova mechanism. Future studies with realistic velocity profiles
could elucidate the possible implications on the phenomenology of core-collapse
supernovae and neutron star mergers.
