  Active transport such as fluid flow is sought in molecular communication to
extend coverage, improve reliability, and mitigate interference. Flow models
are often over-simplified, assuming one-dimensional diffusion with constant
drift. However, diffusion and flow are usually encountered in three-dimensional
bounded environments where the flow is highly non-uniform such as in blood
vessels or microfluidic channels. For a qualitative understanding of the
relevant physical effects inherent to these channels, based on the Peclet
number and the transmitter-receiver distance, we study when simplified models
of uniform flow and advection-only transport are applicable. For these two
regimes, analytical expressions for the channel impulse response are derived
and validated by particle-based simulation. Furthermore, as advection-only
transport is typically overlooked and hence not analyzed in the molecular
communication literature, we evaluate the symbol error rate for exemplary
on-off keying as performance metric.
