Flow physics of a flexible inertial swimmer in disturbed flows

Swimming and flying animals operate in complex flow environments filled with vortices, gusts, and unsteady currents. They continuously adapt to harness these flow structures to enhance locomotion. In this study, we investigate how a flexible swimmer interacts with structured environmental heterogeneity, focusing on the influence of features such as vortex streets on propulsion. The swimmer is modeled as a rigid flat plate with a torsional spring at the leading edge, enabling passive pitching in response to prescribed heaving and external flow disturbances.

A range of flow conditions is simulated using methods spanning from linearized thin airfoil theory to high-fidelity viscous flow simulations. In particular, we compare results from immersed boundary method-based simulations with predictions from potential flow simulations, impulse theory, and linearized models to highlight differences in the underlying flow physics. The results reveal the primary flow physics and how viscous effects and nonlinearities influence wake structure, force generation, and kinematic response. This study clarifies when higher-order modeling becomes essential and underscores the capabilities and limitations of linear and inviscid approaches in disturbed flow environments.