The standard ballistic aggregation model assumes that the particles that are added to a growing agglomerate follow straight-line trajectories. The finite viscosity of real fluids, however, biases the flow of liquid around, rather than through, the agglomerate. The result is that the particles, which follow streamlines in the fluid, will tend to be deposited near the periphery of the growing agglomerate. A modification is made to the ballistic aggregation model to account for this effect, and the geometry of the resultant structures is characterized, The simulation results suggest that even under conditions where convection dominates particle motion, branched open structures are likely to be formed.