In-line swimming dynamics revealed by fish interacting with a robotic mechanism
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eLife assessment
Why do fish school together? Energetic benefits have long been considered a key factor in motivating fish to swim together and tune their tail beat to exploit the whirling wake generated by conspecifics. This study clearly demonstrates that fish benefit from swimming in a two-dimensional vortical wake by locating their body in the vortical low-pressure zones that passively impart a net thrust force on their oscillating bodies. The behavioral and biofluid mechanical findings will interest comparative biomechanists, movement ecologists, evolutionary biologists, fluid mechanists, and bioinspired roboticists.
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Abstract
Schooling in fish is linked to a number of factors such as increased foraging success, predator avoidance, and social interactions. In addition, a prevailing hypothesis is that swimming in groups provides energetic benefits through hydrodynamic interactions. Thrust wakes are frequently occurring flow structures in fish schools as they are shed behind swimming fish. Despite increased flow speeds in these wakes, recent modeling work has suggested that swimming directly in-line behind an individual may lead to increased efficiency. However, only limited data are available on live fish interacting with thrust wakes. Here we designed a controlled experiment in which brook trout, Salvelinus fontinalis , interact with thrust wakes generated by a robotic mechanism that produces a fish-like wake. We show that trout swim in thrust wakes, reduce their tail-beat frequencies, and synchronize with the robotic flapping mechanism. Our flow and pressure field analysis revealed that the trout are interacting with oncoming vortices and that they exhibit reduced pressure drag at the head compared to swimming in isolation. Together, these experiments suggest that trout swim energetically more efficiently in thrust wakes and support the hypothesis that swimming in the wake of one another is an advantageous strategy to save energy in a school.
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eLife assessment
Why do fish school together? Energetic benefits have long been considered a key factor in motivating fish to swim together and tune their tail beat to exploit the whirling wake generated by conspecifics. This study clearly demonstrates that fish benefit from swimming in a two-dimensional vortical wake by locating their body in the vortical low-pressure zones that passively impart a net thrust force on their oscillating bodies. The behavioral and biofluid mechanical findings will interest comparative biomechanists, movement ecologists, evolutionary biologists, fluid mechanists, and bioinspired roboticists.
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Reviewer #1 (Public Review)
This paper focuses on the hydrodynamic interactions between in-line swimming fish by observing how real fish swim behind a robotic mechanism (a rigid NACA airfoil). After ensuring that the airfoil can generate a real-fish-like wake (reverse Von Karman Vortices), the authors found, compared to swimming alone, real fish swimming behind the airfoil will reduce tail moving frequency, synchronize tail movement with the airfoil, and experience lower pressure around the anterior of the fish. The results indicate fish do save energy and improve efficiency by swimming directly behind the thrust type of vortices. The experimental design is good and the collected data generally support the conclusions drawn. The article could, however, be improved by providing more quantitative comparisons in addition to the qualitative …
Reviewer #1 (Public Review)
This paper focuses on the hydrodynamic interactions between in-line swimming fish by observing how real fish swim behind a robotic mechanism (a rigid NACA airfoil). After ensuring that the airfoil can generate a real-fish-like wake (reverse Von Karman Vortices), the authors found, compared to swimming alone, real fish swimming behind the airfoil will reduce tail moving frequency, synchronize tail movement with the airfoil, and experience lower pressure around the anterior of the fish. The results indicate fish do save energy and improve efficiency by swimming directly behind the thrust type of vortices. The experimental design is good and the collected data generally support the conclusions drawn. The article could, however, be improved by providing more quantitative comparisons in addition to the qualitative visualizations.
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Reviewer #2 (Public Review)
This paper seeks to contribute new empirical insight into the (potential) energetic benefits of schooling. Toward this aim, the authors establish an experimental setup in which brook trout swim in a thrust wake generated by an oscillating airfoil. By combining measurements of body motion and particle image velocimetry, the authors successfully detail how brook trout respond to an incoming thrust wake.
Strengths:
• The idea of using an airfoil that oscillates in the sway and yaw direction is original and a valuable contribution to the simulation of thrust wakes using simplified mechanical systems.
• The experiments are executed with a high level of accuracy and detail, offering important insight into animal locomotion in a thrust wake. In particular, acquiring experimental data on the flow physics (velocity …Reviewer #2 (Public Review)
This paper seeks to contribute new empirical insight into the (potential) energetic benefits of schooling. Toward this aim, the authors establish an experimental setup in which brook trout swim in a thrust wake generated by an oscillating airfoil. By combining measurements of body motion and particle image velocimetry, the authors successfully detail how brook trout respond to an incoming thrust wake.
Strengths:
• The idea of using an airfoil that oscillates in the sway and yaw direction is original and a valuable contribution to the simulation of thrust wakes using simplified mechanical systems.
• The experiments are executed with a high level of accuracy and detail, offering important insight into animal locomotion in a thrust wake. In particular, acquiring experimental data on the flow physics (velocity and pressure) for this kind of problem is a major endeavor, which the authors have successfully and originally addressed.
• Performing experiments on the same animals twice is an excellent idea to explore the role of body size, without inflating the number of animals needed for experiments.Weaknesses:
• The novelty of the robotics-based experimental approach is overstated; several studies have studied the response of live fish to thrust wakes, generated by pitching airfoils or robotic fish.
• The length of the test section for the experiments is very much comparable with the body length of the animals, thereby raising doubts regarding the confounding role of wall interactions. Likewise, the role of 3D effects is elusive; experimental data are in 2D and no discussion is included about the extent to which such an approximation is valid and how it impacts quantitative measurements of vorticity and pressure included in the paper.
• Other sensory modalities (such as touch and the vestibular system) and their integration are not examined in the paper, limiting the understanding of the reader of the way in which fish appraise their surrounding to obtain hydrodynamic advantage from thrust wakes.Findings of the research can offer valuable insight into the hydrodynamic mechanisms at the basis of schooling, stimulating further interdisciplinary research at the interface of biology and engineering (fluid mechanics and robotics).
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