Fighting the flow: the stability of model flocks in a vortical flow

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.