ERI: Free surface and flexibility effects in partially-submerged bioinspired propulsion

Organisms often have specialized strategies for movement at the water’s surface. Their locomotion addresses unique constraints including surface waves, splashing, and reduced force production in air, which is a thousand-fold lower density than water. These behaviors present unique design opportunities for underwater vehicles, with applications including water-to-air take-off, maneuvering, and near-surface station-holding. Despite the prevalence of these biological examples, new information is needed about how bioinspired flapping propulsion performs while partially submerged. This work focuses on understanding the influence of the water’s surface on partially submerged propulsive performance. Beyond underwater vehicles, this work also helps explain the biomechanics of near-surface locomotion and aquatic jumping. This project will train an interdisciplinary team of undergraduate students in hands-on research problem solving. The project will also support a bioinspired design workshop for undergraduate anthropology and design students and an engineering workshop for women high school students in Southern California. This work experimentally investigates the roles of and interactions between the free surface and propulsor flexibility during partially submerged bioinspired flapping. Specific objectives are to determine: (1) How propulsive performance scales with the size and speed of the surface disturbance created by the flapping motion. (2) How the free surface deforms during partially submerged flapping motions, including the effects of flexibility on the surface behaviors observed. (3) How three-dimensionality, including finite widths, spanwise bending, and surface disturbances, affects partially submerged propulsive performance. Combined, these objectives provide new understanding about the role of the free surface in partially submerged propulsion. Objectives will be addressed by testing flexible plates that are simultaneously flapping and traveling vertically out of the water. Key variables include flapping frequency and amplitude, vertical velocity, plate stiffness, and aspect ratio. Multi-axis force and torque measurements will facilitate comparisons of thrust and efficiency. Performance trends will be scaled based on the size and speed of the surface disturbance, in addition to parameterizations based on the flapping kinematics. To explain performance trends, high-speed imaging will visualize the free surface dynamics (including air cavity and wave formation, splashing, and the extents of the disturbance) and particle image velocimetry will be used to compare wakes. For cases where performance differs between aspect ratios, 3D imaging of the propulsor shape will be leveraged to explain these observations. Additionally, the project will contribute to open-access tools for volumetric flow imaging, with materials publicly disseminated through an open-source project. 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.