Collaborative Research: Unsteady Ground Effect: How Solid Boundaries Affect Bio-Inspired Propulsion

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Many animals fly or swim near the surface of water or the ocean floor. These boundaries create unsteady, three-dimensional, and asymmetrical flows that can increase the animal's flying/swimming speed and efficiency. Despite the wide implications of these benefits, there are no reliable models for near-boundary bio-locomotion. Accurate models could reshape the way biologists think about the flight strategies of birds migrating over open water or the evolutionary pressures on the shapes of bottom-dwelling fish. A better understanding of near-boundary lifestyles could help ecologists better predict the fragility of near-ground ecosystems to over-fishing, loss of habitat, or changing climate. Better models would also reshape the way engineers design bio-inspired vehicles that operate near boundaries. The key to improving these models is a better understanding of the complex flows governing near-boundary swimming and flying. The proposed work aims to shed light on these complex flows and bring them into public view using multimedia and an outreach program for middle school girls. The goal of this project is to understand the unsteady flow mechanisms governing near-boundary swimming. The project will support the first systematic study of unsteady ground effect through a combination of water channel experiments and inviscid flow simulations. As a result, classic steady ground effect theory will be generalized to the modern interdisciplinary flow phenomena present in swimming and flying animals. These flow phenomena will be explored by testing the effects of Strouhal number, reduced frequency, aspect ratio, undulation, and asymmetric kinematics on near-ground swimming. The research will focus on four specific aims: (i) developing scaling laws for the forces and energetics of near-ground swimming, (ii) mapping near-ground three-dimensional flow interactions, (iii) determining the role of undulatory kinematics in near-ground swimming, and (iv) understanding how asymmetrical kinematics alter near-ground flows. By including both experiments and inviscid simulations, the research will identify which effects are Reynolds-number dependent and which are driven purely by inviscid vortex dynamics. This research will provide key insights into the near-ground lifestyles of fish and will aid in the design of novel bio-inspired underwater vehicles.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.