Many fish swim using an undulating motion of their bodies. The muscle activity that bends the body and produces these movements during steady, continuous swimming can cost a significant amount of energy. But some fishes, such as rainbow trout, can adopt a special swimming behavior that likely enables them to save their own energy by extracting energy from nearby water vortices.
In a fluid environment, vortices are swirls of water or air often released (or “shed”) from stationary objects and other living creatures, including other fish, that are in the path of an oncoming flow. Trout use water vortices that come their way from upstream sources to their advantage by adjusting their typical swimming behavior to produce a ‘slalom’ movement between vortices. Body bends increase in amplitude and curvature, and the tail beats at a frequency that matches the frequency at which vortices are shed upstream. The pattern of muscle activity along the body also changes, where only muscles close to the head are active. This differs from typical undulating motion where muscles contract all along the body, starting from the head and moving toward the tail to produce a traveling body wave that pushes the fish forward. Researchers hypothesize that these changes in muscle activity and body motion help the trout position its body so that it interacts with the vortices in a specific way. The exact nature of this interaction is still under investigation, but one explanation is that the fish controls the angle of its body so that local flow from the vortices produces a continuous upstream force on the body. Scientist James Liao uses the analogy, “…we hypothesize that trout use their body like a sail to tack upstream.”
The general concept of taking advantage of altered fluid flows behind other objects to reduce the energetic cost of motion is found in human behaviors too, for instance, in cyclists that draft behind one another to save energy.
“Aquatic animals swimming in isolation and in groups are known to extract energy from the vortices in environmental flows, significantly reducing muscle activity required for locomotion. A model for the vortex dynamics associated with this phenomenon is developed, showing that the energy extraction mechanism can be described by simple criteria governing the kinematics of the vortices relative to the body in the flow. In this way, we need not make direct appeal to the fluid dynamics, which can be more difficult to evaluate than the kinematics. Examples of these principles as exhibited in swimming fish and existing energy conversion devices are described. A benefit of the developed framework is that the potentially infinite-dimensional parameter space of the fluid–structure interaction is reduced to a maximum of eight combinations of three parameters. The model may potentially aid in the design and evaluation of unsteady aero- and hydrodynamic energy conversion systems that surpass the Betz efficiency limit of steady fluid dynamic energy conversion systems.” (Dabiri 2007:L1)
“Fishes moving through turbulent flows or in formation are regularly exposed to vortices. Although animals living in fluid environments commonly capture energy from vortices, experimental data on the hydrodynamics and neural control of interactions between fish and vortices are lacking. We used quantitative flow visualization and electromyography to show that trout will adopt a novel mode of locomotion to slalom in between experimentally generated vortices by activating only their anterior axial muscles. Reduced muscle activity during vortex exploitation compared with the activity of fishes engaged in undulatory swimming suggests a decrease in the cost of locomotion and provides a mechanism to understand the patterns of fish distributions in schools and riverine environments.” (Liao et al. 2003:1566)
“Simultaneous visualization of a two-dimensional, horizontal slice through the columnar vortices generated by the D-section cylinder (15) using digital particle image velocimetry (DPIV) (19) and the movements of trout with high-speed video revealed that trout slalom between vortices rather than through them…If the flow is decomposed into a downstream…and a lateral (z axis) component, slaloming between vortices occurs when trout move against the downstream flow but with the local lateral flow. Slaloming through oncoming vortices requires opposing the local lateral flow. Actuated foils generate more thrust if they slalom through rather than between Kármán vortices (20), but they require more power input to oppose the instantaneous local flow. Trout slalom between rather than through vortices and, thus, minimize power input rather than maximize thrust output. To quantify the movement of trout near vortices, we described the phase relation between the lateral motions of points along the body relative to the arrival of drifting vortices…When the center of a vortex drifted down to the center of mass (COM) of the fish, the COM was at its maximum lateral excursion away from the vortex, indicated by a phase relation of 180°. For all points anterior to the COM, the body moved away from an oncoming vortex (<180°), whereas body points posterior to the COM move toward the vortex (>180°). A phase relation of 0° or 360° would indicate that the fish had intercepted the center of a vortex.” (Liao et al. 2003:1567)
“…an array of mechanosensory cells distributed along the body of most fish, potentially enables them to detect pressure discontinuities so as to select favourable hydrodynamic conditions in the flow” (Sutterlin & Waddy 1975; Braun & Coombs 2000). (Beal et al. 2006:385.)