Eliot G. Drucker

Morphology and experimental hydrodynamics of fish fin control surfaces

Over the past 520 million years, the process of evolution has produced a diversity of nearly 25000 species of fish. This diversity includes thousands of different fin designs which are largely the product of natural selection for locomotor performance. Fish fins can be grouped into two major categories: median and paired fins. Fins are typically supported at their base by a series of segmentally arranged bony or cartilaginous elements, and fish have extensive muscular control over fin conformation. Recent experimental hydrodynamic investigation of fish fin function in a diversity of freely swimming fish (including sharks, sturgeon, trout, sunfish, and surfperch) has demonstrated the role of fins in propulsion and maneuvering. Fish pectoral fins generate either separate or linked vortex rings during propulsion, and the lateral forces generated by pectoral fins are of similar magnitudes to thrust force during slow swimming. Yawing maneuvers involve differentiation of hydrodynamic function between left and right fins via vortex ring reorientation. Low-aspect ratio pectoral fins in sharks function to alter body pitch and induce vertical maneuvers through conformational changes of the fin trailing edge. The dorsal fin of fish displays a diversity of hydrodynamic function, from a discrete thrust-generating propulsor acting independently from the body, to a stabilizer generating only side forces. Dorsal fins play an active role in generating off-axis forces during maneuvering. Locomotor efficiency may be enhanced when the caudal fin intercepts the dorsal fin wake. The caudal fin of fish moves in a complex three-dimensional manner and evidence for thrust vectoring of caudal fin forces is presented for sturgeon which appear to have active control of the angle of vortices shed from the tail. Fish are designed to be unstable and are constantly using their control surfaces to generate opposing and balancing forces in addition to thrust. Lessons from fish for autonomous underwater vehicle (AUV) design include: 1) location of multiple control surfaces distributed widely about the center of mass, 2) design of control surfaces that have a high degree of three-dimensional motion through a flexible articulation with the body, 3) the ability to modulate fin surface conformation, and 4) the simultaneous use of numerous control surfaces including locating some fin elements in the downstream wake generated by other fins. The ability to manufacture an AUV that takes advantage of these design features is currently limited by the nature of available materials and mechanical drive trains. But future developments in polymer artificial muscle technology will provide a new approach to propulsor design that will permit construction of biomimetic propulsors with conformational and articulational flexibility similar to that of fish fins.

Function of pectoral fins in rainbow trout: behavioral repertoire and hydrodynamic forces

SUMMARY Salmonid fishes (trout, salmon and relatives) have served as a model system for study of the mechanics of aquatic animal locomotion, yet little is known about the function of non-axial propulsors in this major taxonomic group. In this study we examine the behavioral and hydromechanical repertoire of the paired pectoral fins of rainbow trout Oncorhynchus mykiss, performing both steady rectilinear swimming and unsteady maneuvering locomotion. A combination of kinematic analysis and quantitative flow visualization (using digital particle image velocimetry) enables identification of the propulsive roles played by pectoral fin motions. During constant-speed swimming (0.5 and 1.0 body length s-1), the pectoral fins remain adducted against the body. These fins are actively recruited, however, for a variety of maneuvering behaviors, including station holding in still water (hovering), low-speed (i.e. non-fast-start) turning, and rapid deceleration of the body during braking. Despite having a shallow pectoral-fin base orientation (the plesiomorphic teleost condition), trout are capable of rotating the fin base over 30° during maneuvering, which affords the fin an impressive degree of kinematic versatility. When hovering, the pectoral fins are depressed beneath the body and twisted along their long axes to allow anteroposterior sculling. During turning and braking, the fins undergo spanwise rotation in the opposite direction and exhibit mediolateral and dorsoventral excursions. Water velocity fields and calculated momentum flows in the wake of the pectoral fins reveal that positive thrust is not generated during maneuvering, except during the retraction half-stroke of hovering. Relatively large laterally directed fluid force (mean 2.7 mN) is developed during turning, whose reaction powers yawing rotation of the body (4-41° s-1). During deceleration, the wake-force line of action falls below the center of mass of the body, and this result supports a long-standing mechanical model of braking by fishes with ventrally positioned paired fins. Despite its traditional categorization as a propulsor of limited functional importance, the salmoniform pectoral fin exhibits a diverse locomotor repertoire comparable to that of higher teleostean fishes.

Experimental hydrodynamics of fish locomotion: functional insights from wake visualization.

Abstract Despite enormous progress during the last twenty years in understanding the mechanistic basis of aquatic animal propulsion—a task involving the construction of a substantial data base on patterns of fin and body kinematics and locomotor muscle function—there remains a key area in which biologists have little information: the relationship between propulsor activity and water movement in the wake. How is internal muscular force translated into external force exerted on the water? What is the pattern of fluid force production by different fish fins (e.g., pectoral, caudal, dorsal) and how does swimming force vary with speed and among species? These types of questions have received considerable attention in analyses of terrestrial locomotion where force output by limbs can be measured directly with force plates. But how can forces exerted by animals moving through fluid be measured? The advent of digital particle image velocimetry (DPIV) has provided an experimental hydrodynamic approach for quantifying the locomotor forces of freely moving animals in fluids, and has resulted in significant new insights into the mechanisms of fish propulsion. In this paper we present ten “lessons learned” from the application of DPIV to problems of fish locomotion over the last five years. (1) Three-dimensional DPIV analysis is critical for reconstructing wake geometry. (2) DPIV analysis reveals the orientation of locomotor reaction forces. (3) DPIV analysis allows calculation of the magnitude of locomotor forces. (4) Swimming speed can have a major impact on wake structure. (5) DPIV can reveal interspecific differences in vortex wake morphology. (6) DPIV analysis can provide new insights into the limits to locomotor performance. (7) DPIV demonstrates the functional versatility of fish fins. (8) DPIV reveals hydrodynamic force partitioning among fins. (9) DPIV shows that wake interaction among fins may enhance thrust production. (10) Experimental hydrodynamic analysis can provide insight into the functional significance of evolutionary variation in fin design.

EXPERIMENTAL HYDRODYNAMICS AND EVOLUTION: FUNCTION OF MEDIAN FINS IN RAY- FINNED FISHES

Abstract The median fins of fishes consist of the dorsal, anal, and caudal fins and have long been thought to play an important role in generating locomotor force during both steady swimming and maneuvering. But the orientations and magnitudes of these forces, the mechanisms by which they are generated, and how fish modulate median fin forces have remained largely unknown until the recent advent of Digital Particle Image Velocimetry (DPIV) which allows empirical analysis of force magnitude and direction. Experimental hydrodynamic studies of median fin function in fishes are of special utility when conducted in a comparative phylogenetic context, and we have examined fin function in four ray-finned fish clades (sturgeon, trout, sunfish, and mackerel) with the goal of testing classical hypotheses of fin function and evolution. In this paper we summarize two recent technical developments in DPIV methodology, and discuss key recent findings relevant to median fin function. High-resolution DPIV using a recursive local-correlation algorithm allows quantification of small vortices, while stereo-DPIV permits simultaneous measurement of x, y, and z flow velocity components within a single planar light sheet. Analyses of median fin wakes reveal that lateral forces are high relative to thrust force, and that mechanical performance of median fins (i.e., thrust as a proportion of total force) averages 0.35, a surprisingly low value. Large lateral forces which could arise as an unavoidable consequence of thrust generation using an undulatory propulsor may also enhance stability and maneuverability. Analysis of hydrodynamic function of the soft dorsal fin in bluegill sunfish shows that a thrust wake is generated that accounts for 12% of total thrust and that the thrust generation by the caudal fin may be enhanced by interception of the dorsal fin wake. Integration of experimental studies of fin wakes, computational approaches, and mechanical models of fin function promise understanding of instantaneous forces on fish fins during the propulsive cycle as well as exploration of a broader locomotor design space and its hydrodynamic consequences.

Wake Dynamics and Locomotor Function in Fishes: Interpreting Evolutionary Patterns in Pectoral Fin Design

Abstract The great anatomical diversification of paired fins within the Actinopterygii (ray-finned fishes) can be understood as a suite of evolutionary transformations in design. At a broad taxonomic scale, two clear trends exist in the morphology of the anteriorly situated pectoral fins. In comparing basal to more derived clades, there are general patterns of (i) reorientation of the pectoral fin base from a nearly horizontal to more vertical inclination, and (ii) migration of the pectoral fin from a ventral to mid-dorsal body position. As yet, the functional significance of these historical trends in pectoral fin design remains largely untested by experiment. In this paper we test the proposal that variation in pectoral fin structure has an important influence on the magnitude and orientation of fluid forces generated during maneuvering locomotion. Using digital particle image velocimetry for quantitative wake visualization, we measure swimming forces in ray-finned fishes exhibiting the plesiomorphic and apomorphic pectoral fin anatomy. Our experiments focus on rainbow trout (Oncorhynchus mykiss), a lower teleost with pectoral fins positioned ventrally and with nearly horizontally inclined fin bases, and bluegill sunfish (Lepomis macrochirus), a relatively derived perciform fish with more vertically oriented pectoral fins positioned mid-dorsally on the body. In support of hypotheses arising from our prior wake studies and previously untested models in the literature, we find that the pectoral fins of sunfish generate significantly higher forces for turning and direct braking forces closer to the center of mass of the body than the pectoral fins of trout. These results provide insight into the hydrodynamic importance of major evolutionary transformations in pectoral fin morphology within the Actinopterygii.