Quasi-steady aerodynamics predicts the dynamics of flapping locomotion

The propulsion of a flapping wing or foil is emblematic of bird flight and fish swimming. Previous studies have identified hallmarks of the propulsive dynamics, which are attributed to unsteady effects such as the formation and shedding of edge vortices and wing-vortex interactions. Here we show that the key behaviors are captured by a quasi-steady aerodynamic model that predicts forces from motions without needing to solve for the flows. We address the forward dynamics induced by up-and-down heaving motions of a thin plate with a nonlinear model which involves lift and drag that vary with speed and attack angle. Simulations reproduce the well-known transition for increasing Reynolds number from a stationary state to a propulsive state, where the latter is characterized by a Strouhal number that is conserved across broad ranges of parameters. These findings extend the phenomena of unsteady locomotion that can be explained by quasi-steady modeling, and they broaden the conditions and parameter ranges over which such models are applicable.