  We discuss a minimal generalization of the incompressible Navier-Stokes
equations to describe the solvent flow in an active suspension. To account
phenomenologically for the presence of an active component driving the ambient
fluid flow, we postulate a generic nonlocal extension of the stress-tensor,
conceptually similar to those recently introduced in granular media flows.
Stability and spectral properties of the resulting hydrodynamic model are
studied both analytically and numerically for the two-dimensional (2D) case
with periodic boundary conditions. Future generalizations of this
momentum-conserving theory could be useful for quantifying the shear properties
of active suspensions.
