  We investigate flow pattern formation and viscosity reduction mechanisms in
active fluids by studying a generalized Navier-Stokes model that captures the
experimentally observed bulk vortex dynamics in microbial suspensions. We
present exact analytical solutions including stress-free vortex lattices and
introduce a computational framework that allows the efficient treatment of
previously intractable higher-order shear boundary conditions. Large-scale
parameter scans identify the conditions for spontaneous flow symmetry breaking,
geometry-dependent viscosity reduction and negative-viscosity states amenable
to energy harvesting in confined suspensions. The theory uses only generic
assumptions about the symmetries and long-wavelength structure of active stress
tensors, suggesting that inviscid phases may be achievable in a broad class of
non-equilibrium fluids by tuning confinement geometry and pattern scale
selection.
