CAREER: Three-dimensional Flow Mechanics and their Relationship to Fin Stiffness Patterns in Caudal-fin-based Propulsion

Biological swimmers largely surpass man-made marine propellers in efficiency and agility. As a result, there is a strong interest in understanding the mechanics of biological underwater propulsion to create bio-inspired propellers that can match nature’s performance characteristics. Fish and marine mammals have flexible fins with nonuniform stiffness patterns that undergo three-dimensional bending deformations in their flapping motion.

The mechanics of these deformations are tightly coupled with the flow and generate complex 3D flow features that are not well understood. This project will study flow mechanics generated by the deformation of caudal (rear) fins in the thunniform mode of locomotion, used by nature’s most efficient swimmers. It will lay the groundwork for the development of bio-inspired propulsion systems for unmanned underwater vehicles that are more efficient and agile.

Improved performance characteristics will break capability and cost barriers to the use of unmanned underwater vehicles for a wide range of applications, including environmental monitoring, maintenance of industrial infrastructure, ocean cleanup efforts, aquaculture and improved defense systems. The project will also provide opportunities for a group of undergraduate students to participate in research and enhance K-12 summer programs with the goal of broadening participation in STEM.

In fish and marine mammals, the use of fins with non-uniform and tunable mechanical properties, in combination with advanced sensorimotor control, is thought to be a main driver of performance. Caudal fin flexibility affects performance through two mechanisms: (i) resonant mechanics and Strouhal number matching, and (ii) determining the hydrodynamic shape that the fin presents to the flow.

The effect and flow mechanisms associated with the latter, which is dependent on stiffness distribution in addition to mean stiffness value, remain poorly understood. The goal of this research is to understand the 3-D fluid mechanisms and fluid-structural mechanics generated by caudal fins with non-uniform stiffness distributions, and to determine how these mechanisms can be leveraged to enhance efficiency and agility in bio-inspired propellers.

A new approach will be established that combines hardware-in-the-loop optimization and soft robotics to find optimal 2-D stiffness distributions of the caudal fin, both for propulsion and maneuvering. This will then be assessed to elucidate the 3-D flow mechanisms that lead to optimality. The project will also provide experiential learning experiences to a group of undergraduate students at the University of Maryland, as well as provide modules, at-home experiments and virtual lab tours to K-12 students through UMD’s Women in Engineering Program.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.