  The brane-world description of our universe entails a large extra dimension
and a fundamental scale of gravity that might be lower by several orders of
magnitude compared to the Planck scale. An interesting consequence of the
brane-world scenario is in the nature of spherically symmetric vacuum solutions
to the brane gravitational field equations, with properties quite distinct as
compared to the standard black-hole solutions of general relativity. We
consider the spherically symmetric accretion of matter onto brane-world black
holes in terms of relativistic hydrodynamics by assuming that the inflowing gas
obeys a polytropic equation of state. As a first step in this study, we
consider the accretion process in an arbitrary static, spherically symmetric
space-time, and show that the relativistic equations require a transition to a
supersonic flow in the solution. The velocity, temperature, and density
profiles are obtained for the case of the polytropic equation of state. We
apply the general formalism to the study of the accretion properties of several
classes of brane-world black holes, and we obtain the distribution of the main
physical parameters of the gas. The astrophysical determination of these
physical quantities could discriminate, at least in principle, between the
different brane-world models, and place some constraints on the existence of
the extra dimensions.
