  We investigate the stability of self-propelled particle flocks in the
Taylor-Green vortex, a steady vortical flow. We consider a model where
particles align themselves to a combination of the orientation and the
acceleration of particles within a critical radius. We identify two distinct
regimes, if alignment with orientation is dominant the particles tend to be
expelled from regions of high vorticity. In contrast if anticipation is
dominant the particles accumulate in areas of large vorticity. In both regimes
the relative order of the flock is reduced. However we show that there can be a
critical balance of the two effects which stabilises the flock in the presence
of external fluid forcing. This strategy could provide a mechanism for animal
flocks to remain globally ordered in the presence of fluid forcing, and may
also have applications in the design of flocking autonomous drones and
artificial microswimmers.
