Meeting Abstract
Flapping flexing foils and fins face fluid forces that contain significant unsteady terms. To explore how structural mechanics and unsteady flow forces interact to determine lift and thrust performance on heaving and pitching elastic foils we coupled a computationally efficient two-dimensional unsteady vortex method with a finite element method to compute locomotor forces over a wide range of kinematics (frequency as well as heave and pitch amplitude and phase) and structural mechanics (size and stiffness) for flapping foils. We show that the coefficients of thrust and lift in airfoils rarely depend on the coupling of unsteady fluid forces whereas, because of increased fluid density, flow forces are an important determinant of the instantaneous deformation of foils. Thus fluid-structure coupling is crucial in aquatic systems and less so in aerial systems. In airfoils, even in regions of structural resonance, fluid loading never contributes more than 20% to the development of locomotor forces. In contrast, and depending on foil stiffness, that fraction is far greater in water. In both systems, the emergent bending dynamics and foil forcing can be tuned to produce maximum forces.
