Flow states and transitions of an active nematic in a three dimensional channel

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.