Symmetry breaking of rigid/flexible plates in bluff body wakes generates rotation and drift

Bluff body wakes have historically been important for understanding nature and aiding industry. For Reynolds numbers above approximately $Re\approx 10$, a recirculation bubble develops behind the bluff body. If a solid or elastic appendage is attached to the bluff body, it may exert a torque and a side force on the body. We use theory, numerical simulations and experiments to investigate and explain this phenomenon. More specifically, numerical simulations are carried out for a freely falling cylinder with an attached splitter plate for $Re \approx 50$. Experiments of a fixed cylinder with an attached elastic filament are preformed using a vertical soap-film tunnel for $Re \approx 2000$. Both experiments and simulations reveal that if a body has an appendage smaller than or of the same order as the body it is attached to, the body rotates and drifts. We explain our findings with a simple model and discuss the implications for propulsion.