Biorobotic Waterfowl Flipper with Skeletal Skins in a Computational Framework: Kinematic Conformation and Hydrodynamic Analysis

Abstract

Cormorants (Phalacrocoraxe), types of aquatic birds, utilize the compliance/flexibility of the flippers and exploit hydrodynamic/biomechanic processes to accomplish diverse operations. Particularly, the flipper-propelled locomotion exhibits traits such as super-redundancy and large deformations, necessitating depiction of both movements of the rigid skeletons as well as local deformations of the soft tissues. However, there are few well-established kinematic/hydrodynamic framework models and constitutive equations for such rigid–flexible intrinsically coupled biosystems. Herein, combined with a skeletal skinning algorithm to handle the deformation of a flexible body attached to a rigid body, a numerical computation framework for an in-depth fluid–structure interaction is presented, which enables the capture of viscoelastic and anisotropic characteristics of a highly compliant 3D rigid–flexible coupled model in a low-Reynolds-number flow. Considering the biorobotic cormorant flipper with a nonuniformly distributed stiffness as a representative, the challenging issue of controlling a biomechanically compliant flipper to synthesize realistic locomotion sequences, including rigid skeleton movements and soft tissue deformations, is addressed. Furthermore, a numerical computational hydrodynamic analysis is performed to demonstrate that the cormorant flipper can generate 5 N fluid force and 0.45 N m fluid moment during the turning operation in 0.8 s, which is consistent with the former experimental results.