  We use active nematohydrodynamics to study the flow of an active fluid in a
3D microchannel, finding a transition between active turbulence and regimes
where there is a net flow along the channel. We show that the net flow is only
possible if the active nematic is flow aligning and that - in agreement with
experiments - the appearance of the net flow depends on the aspect ratio of the
channel cross-section. We explain our results in terms of the when hydrodynamic
screening due to the channel walls allows the emergence of vortex rolls across
the channel.
