  We develop a technique to study relativistic perturbations in the generalised
brane cosmological scenario, which is a generalisation of the multi-fluid
cosmological perturbations to brane cosmology. The novelty of the technique
lies in the inclusion of a radiative bulk which is responsible for bulk-brane
energy exchange, and in turn, modifies the standard perturbative analysis to a
great extent. The analysis involves a geometric fluid -- called the Weyl fluid
-- whose nature and role have been studied extensively both for the empty bulk
and the radiative bulk scenario. Subsequently, we find that this Weyl fluid can
be a possible geometric candidate for dark matter in this generalised brane
cosmological framework.
