In an aquatic environment, the control of stability and maneuvering of animals is determined by the morphology of the control surfaces. The position, mobility, hydrodynamic characteristics of the control surfaces are associated with swimming performance. For cetaceans (whales, dolphins, porpoises), the pectoral flippers are mobile, hydrofoils. The flippers have a cross-sectional design, similar to engineered hydrofoils for lift generation. The flippers contribute to lateral turning, diving, surfacing, braking, trim control, and reduction of recoil from propulsive movements. Flippers are a modification of the pectoral appendages. They enclose a skeletal framework based on the terrestrial mammalian forelimb. Flipper shape can vary from elongate, wing-like appendages with tapering tips to short, rounded paddle-like structures. Although cetacean flippers are the quintessential example of evolutionary homology and discussed in textbooks in comparative anatomy, evolution, paleontology and functional morphology, little is known of the diversity of flipper geometry within the Cetacea and the hydrodynamic characteristics and function of these control surfaces.

In 2007, the National Science Foundation gave a grant (IOS - 0640185) to the team of Frank Fish (West Chester University), Laurens Howle (Duke University), and Mark Murray (United States Naval Academy) to investigate the three-dimensional geometry and hydrodynamic performance of cetacean flippers with differing morphologies for an integrated, collaborative approach using biological and engineering analyses.

Left to right: Frank Fish, Laurens Howle, and Mark Murray

The three-dimensional geometry of cetacean flippers is being analyzed from specimens obtained from stranded cetaceans. The three-dimensional shape of the flippers is analyzed using X-ray computer tomography (CT-scans). The two- and three-dimensional geometry of the flippers is compared between species to assess hydrodynamic adaptation. Based on the CT-scan-generated geometry, scale physical models of the flippers are fabricated. The models are tested in a water tunnel at whale-appropriate Reynolds number over a range of angles of attack (-30^o to 30^o) and whale-appropriate sweep angles to measure lift and drag. The morphometric and hydrodynamic data will be combined using principal components analysis (PCA) to define ecomorphological groups related to locomotor performance.

This study provides the first hydrodynamic analysis of cetacean flipper design. Application of engineering techniques to study biological structures will provide a greater understanding of the relationship between performance and morphology for large and hard to study aquatic animals.

This project emphasizes the union of biological and engineering approaches to answer comparative questions regarding the design of relevant hydrodynamic structures. The research will increase our understanding of the functional morphology of whales and dolphins in relation to swimming performance and ecology. Because of the large size of cetaceans, there has been precious little data relating morphology and locomotor performance. Functional morphologists interested in aquatic locomotion of animals will gain valuable insight into maneuverability at high Reynolds number and drag and lift performance in differing biological designs. This work is tied to an understanding of fluid dynamic control mechanisms. There is a strong possibility that the hydrodynamic results of the work on flipper morphology can be applied in the design of watercraft. Demonstration of significant practical applications on the association of natural morphologies and performance is likely to attract considerable interest from engineers through the biomimetic approach.