Computational modeling of fluid–structure interaction in biological flying and swimming
Insects and fish in the animal world typically possess superior efficiency in locomotion, and they are capable of maneuvering in complex environments full of unpredictable disturbances. Such locomotion modes have inspired biomimetic designs of unmanned aerial and underwater vehicles. However, the fluid dynamics related to the flapping wings and fins is still a highly challenging problem as it involves three-dimensional, unsteady, massively separated flows and it also has to do with significant deformation of the flexible wings and fins. To date, the fundamental study on the role of the flexibility of the wings/fins in the animal propulsion is still very limited. In this research, our objectives are: (1) to develop a versatile computational approach to model the fluid-structure interaction (FSI) for various types of wings and fins, and (2) to study the effect of the structural deformation on the hydrodynamic performance of these biological propulsors. To achieve these objectives, we have developed an in-house FSI program. The code couples a viscous incompressible flow solver based on the immersed-boundary method and a nonlinear finite-element solver for thin-walled structures. Using the Cartesian grid, the sharp-interface immersed boundary method can treat the complex domain and moving boundaries in a straightforward manner. The solid-dynamics solver has the capability of handling large displacements and large rotations of reinforced plate structures. Using this method, we address the FSI of flapping wings/fins in three specific configurations: (1) a low-aspect-ratio elastic plate pitching in a freestream, where a scaling law of the thrust production is obtained based on the deformation mode of the plate; (2) an elastic rectangular plate performing hovering motion, where the effect of dynamic pitching due to the inertial and aerodynamic forces is investigated; and (3) a high-fidelity computational model of the cicada forewing, where the wing structure, the mechanical properties, and the wing kinematics of the real insect are measured and the FSI model is validated against the high-speed imaging data. The results from these studies may provide useful guidance for the engineering design of flexible propulsors.