Do vortex streets remember their birth?

Even foils with symmetric oscillations can generate asymmetric wakes. As a result, a foil can feel a net lateral force despite having laterally-symmetric kinematics. Is this asymmetry simply a lingering memory of initial conditions, or something more? We answered this question with a physical model that is the first to 1) predict wake deflection a priori based on kinematic inputs, and 2) explain the dynamics of wake deflection, e.g. the evolution of a vortex wake from a deflected to straight state. We used water channel experiments to both validate the model and demonstrate for the first time that wake deflection angle can decay slowly, sometimes over 200+ oscillation cycles. We found that wake deflection is a supercritical pitchfork bifurcation governed by a parameter that we named the “relative dipole angle”, and we show how a model of this bifurcation offers design and control guidelines for foil-based applications like fish-inspired robots.