DRUCKER, E.G.: Experimental hydrodynamics of fish locomotion: functional insights from wake visualization

The fins of fishes are sophisticated control surfaces that play important roles in stabilization and propulsion of the body during locomotion. Despite a century of active investigation into the mechanisms of fin function, there remains one critical area in which progress has been limited: analysis of the physical interaction between the animal’s propulsor and the aquatic medium. This paper reviews current biomechanical research employing a new flow visualization technique, digital particle image velocimetry (DPIV), adopted from the field of fluid mechanics. This method allows experimental measurements of water velocity flows in the wake of freely swimming animals, and thereby enables direct investigation of the hallmark of fluid force production (vorticity). Emerging from recent applications of DPIV in research on fish swimming are findings with general significance for the study of aquatic locomotion: (1) DPIV is an effective tool for measuring the reaction forces on swimming animals. From rates of momentum transfer and vortex geometry, both the magnitude and orientation of wake forces can be determined to gain insight into the mechanisms of propulsion. (2) Three-dimensional analysis of wake flow is critical. Flow patterns in perpendicular sections of the wake reveal a hydrodynamic force balance on steadily swimming fish, a result validating the DPIV approach. (3) Fishes can adjust the structure and strength of their wake in response to varying locomotor demands. Multiple fins are employed simultaneously and independently to control locomotor force during both unsteady maneuvering and steady swimming across a range of speeds. For future work, DPIV holds considerable promise for illuminating the dynamics of biological fluid flow and for allowing functional interpretation of evolutionary trends in propulsor design.