Predicting the slowly converging dynamics of asymmetric vortex wakes

Hydrofoils with symmetric oscillations can generate asymmetric vortex wakes. This surprising asymmetry has been widely reproduced, but a simple metric to predict its onset has remained elusive. Here, using a combination of vortex modeling and water channel experiments, we show that vortex wake deflection is well-predicted by the ``relative dipole angle''. In addition to offering a predictive physics-based metric, our results show that a hydrofoil's wake can converge much more slowly than previously thought (200+ oscillation cycles), and that the wake's asymmetry is more than a memory of the hydrofoil's initial condition - it is an instability inherent to the vortex street.