  Because of its key role in feedback in star formation and galaxy formation,
we examine the nature of the interaction of a flow with discrete sources of
mass injection. We show the results of two-dimensional numerical simulations in
which we explore a range of configurations for the mass sources and study the
effects of their proximity on the downstream flow. The mass sources act
effectively as a single source of mass injection if they are so close together
that the ratio of their combined mass injection rate is comparable to or
exceeds the mass flux of the incident flow into the volume that they occupy.
The simulations are relevant to many diffuse sources, such as planetary nebulae
and starburst superwinds, in which a global flow interacts with material
evaporating or being ablated from the surface of globules of cool, dense gas.
