Physical models and vortex dynamics of swimming and flying: a review

The swimming of aquatic animals and flying of insects and birds have fascinated physicists and biologists for more than a century. In this regard, great efforts have been made to develop new features and promote their applications in underwater and air propulsion. However, many challenges remain in understanding these forms of physical processes. Five key physical models are summarized to show how researchers use numerical and experimental methods to understand physiology, movement ecology and evolution from the viewpoint of fluid mechanics. They are morphological model, flexibility model, kinematics model, tethered/free model and force measurement model. Then, the latest progresses on the vortex dynamics of some simplified models and even high-fidelity models are presented, including the forming, growth, interaction, role and influence factors of the vortical structures. Some other aspects in swimming and flying, including stability, manoeuvrability and acoustics, are also briefly reviewed. Finally, the major challenges and several open issues in this field are highlighted.