Unsteady bio-fluid dynamics in flying and swimming

Flying and swimming in nature present sophis-ticated and exciting ventures in biomimetics, which seeks sustainable solutions and solves practical problems by emu-lating nature's time-tested patterns, functions, and strate-gies. Bio-fluids in insect and bird flight, as well as in fish swimming are highly dynamic and unsteady; how-ever, they have been studied mostly with a focus on the phenomena associated with a body or wings moving in a steady flow. Characterized by unsteady wing flapping and body undulation, fluid-structure interactions, flexible wings and bodies, turbulent environments, and complex maneuver, bio-fluid dynamics normally have challenges associated with low Reynolds number regime and high unsteadiness in modeling and analysis of flow physics. In this article, we review and highlight recent advances in unsteady bio-fluid dynamics in terms of leading-edge vor-tices, passive mechanisms in flexible wings and hinges, flap-ping flight in unsteady environments, and micro-structured aerodynamics in flapping flight, as well as undulatory swimming, flapping-fin hydrodynamics, body–fin interac-tion, C-start and maneuvering, swimming in turbulence, collective swimming, and micro-structured hydrodynamics in swimming. We further give a perspective outlook on future challenges and tasks of several key issues of the field.