Efficient thrust enhancement by modified pitching motion

Thrust and/or efficiency of a pitching foil (mimicking a tail of swimming fish) can be enhanced by tweaking the pitching waveform. The literature, however, show that non-sinusoidal pitching waveforms can enhance either thrust or efficiency but not both simultaneously. With the knowledge and inspiration from nature, we devised and implemented a novel asymmetrical sinusoidal pitching motion that is a combination of two sinusoidal motions having periods T1 and T2 for the forward and retract strokes, respectively. The motion is represented by period ratio $\mathrm{\mathbb{T}} = {T_1}/T$ , where T = (T1 + T2)/2, with $\mathrm{\mathbb{T}} > 1.00$ giving the forward strokes (from equilibrium to extreme position) slower than the retract strokes (from extreme to equilibrium position) and vice versa. The novel pitching motion enhances both thrust and efficiency for $\mathrm{\mathbb{T}} > 1.00$ . The enhancement results from the resonance between the shear-layer roll up and the increased speed of the foil. Four swimming regimes, namely normal swimming, undesirable, floating and ideal are discussed, based on instantaneous thrust and power. The results from the novel pitching motion display similarities with those from fish locomotion (e.g. fast start, steady swimming and braking). The $\mathrm{\mathbb{T}} > 1.00$ motion in the faster stroke has the same characteristics and results as the fast start of prey to escape from a predator while $\mathrm{\mathbb{T}} < 1.00$ imitates braking locomotion. While $\mathrm{\mathbb{T}} < 1.00$ enhances the wake deflection at high amplitude-based Strouhal numbers (StA = fA/U∞, where f and A are the frequency and peak-to-peak amplitude of the pitching, respectively, and U∞ is the freestream velocity), $\mathrm{\mathbb{T}} > 1.00$ improves the wake symmetry, suppressing the wake deflection. The wake characteristics including wake width, jet velocity and vortex structures are presented and connected with $S{t_d}( = fd/{U_\infty })$ , ${A^{\ast}}( = A/d)$ and $\mathrm{\mathbb{T}}$ , where d is the maximum thickness of the foil.

First report of fish trace fossils (Undichna) from the Middle Devonian Achanarras Limestone, Caithness Flagstone Group

Two newly-discovered specimens of the fish locomotion trace Undichna (U. britannica and Undichna isp.), are described from the Middle Devonian Achanarras Limestone Member (Caithness Flagstone Group, NE Scotland). Fish trace fossils have not previously been reported from the Achanarras Limestone Member, despite decades of study of the unit as a key locality for fish body fossils. The traces comprise discontinuous sinusoidal grooves; one showing multiple parallel incisions, created by the fins of an acanthodian fish swimming close to the substrate. The apparent absence of trace fossils attributable to infaunal or epifaunal benthic organisms suggests that the sediment at the bottom of the lake was relatively inhospitable. The low ichnodiversity of the Achanarras Limestone Member is likely due to low oxygen levels in the depositional environment.Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Nonlinear analysis of compliant robotic fish locomotion

Compliant robotic fish can achieve a better swimming performance than rigid-bodied robotic fish. Therefore, this article investigates the swimming behavior of the compliant robotic fish based on a new swimming model that combines the large-amplitude elongated-body theory with decoupled natural orthogonal complement matrices. The simulation reveals that the multi-order resonances are generated in tail-beat amplitude, forward speed, stride length, and transport efficiency when the compliant robotic fish is driven at the corresponding frequency. Moreover, the resonant effects demonstrate the nonlinear behaviors as the driving torque increases. A control strategy for resonance utilization is presented to improve the performance capabilities. The potential influence factors for resonant effects are also discussed, showing that the tail-generated hydrodynamic force significantly impacts the resonant effect. These nonlinear characteristics can provide important guidelines for the motion control of the compliant robotic fish.

Role of Transient Characteristics in Fish Trajectory Modeling

In this experiment, we analyzed live fish (silver carp) trajectories recorded in an experimental vertical-slot fishway. Combined with a numerical simulation, we demonstrated that randomness shown in fish trajectories might not merely be attributed to the fish’s random choices in its swimming, but could also be a consequence of adaption to the bulk unsteady turbulent flow structures. Simple superposition of a fish trajectory on the time-averaged flow field obtained either by interpolating on discrete point measurements or numerical simulation data is not an ideal method for description of fish movement. How to model the fish paths in transient flow and the necessity of simultaneous recording of the flow field and the fish locomotion are challenging topics. We also discussed the possible integration of currently existing methods to promote the development of fish trajectory modeling.

Role of Transient Characteristics in Fish Trajectory Modeling

In this paper, we analyzed the live fish trajectory recorded from an experiment in an experimental vertical slot fishway. Combined with a numerical simulation, we demonstrated that randomness shown in fish trajectory might not merely be attributed to fish's random choices in its swimming, also could be an adaption consequence to the bulk unsteady turbulent flow structure. Simple superposing the fish trajectory on the time-averaged flow field obtained either by interpolating on discrete point measurements or numerical simulation is not an ideal method for fish movement description in fishway engineering. How to model the fish paths in transient flow and the necessity of simultaneous recording of the flow field and the fish locomotion are challenging topics. The suggested spectrum analysis of the flow field may provide a new general method to reproduce the fish trajectory in a complex turbulent flow.

How zebrafish turn: analysis of pressure force dynamics and mechanical work

ABSTRACTWhereas many fishes swim steadily, zebrafish regularly exhibit unsteady burst-and-coast swimming, which is characterized by repeated sequences of turns followed by gliding periods. Such a behavior offers the opportunity to investigate the hypothesis that negative mechanical work occurs in posterior regions of the body during early phases of the turn near the time of maximal body curvature. Here, we used a modified particle image velocimetry (PIV) technique to obtain high-resolution flow fields around the zebrafish body during turns. Using detailed swimming kinematics coupled with body surface pressure computations, we estimated fluid–structure interaction forces and the pattern of forces and torques along the body during turning. We then calculated the mechanical work done by each body segment. We used estimated patterns of positive and negative work along the body to evaluate the hypothesis (based on fish midline kinematics) that the posterior body region would experience predominantly negative work. Between 10% and 20% of the total mechanical work was done by the fluid on the body (negative work), and negative work was concentrated in the anterior and middle areas of the body, not along the caudal region. Energetic costs of turning were calculated by considering the sum of positive and negative work and were compared with previous metabolic estimates of turning energetics in fishes. The analytical workflow presented here provides a rigorous way to quantify hydrodynamic mechanisms of fish locomotion and facilitates the understanding of how body kinematics generate locomotor forces in freely swimming fishes.

Propulsive efficiency of alternative underwater flykick techniques for swimmers

Analysis of video and speed data is used to evaluate the efficiency of human underwater flykick. The authors show that by coupling Lighthill’s theory of fish locomotion with human musculoskeletal modelling, it is possible to evaluate the effectiveness of the mechanical and hydrodynamic propulsive components of human underwater flykick. This allows the effect of subtle variances in technique to be assessed by measurement of athlete motion alone. This is demonstrated in an experimental case study of an elite athlete performing two different techniques; one more knee-based or thunniform, and the second more undulatory or carangiform/anguilliform. In finding the mean kinematics of each technique, it is first shown that maintaining stroke-by-stroke consistency of technique leads to an increase in propulsive efficiency. It is further demonstrated that in changing technique, an athlete may swim at the same kick rate but have different propulsive efficiency. This demonstrates the need to determine the energy cost in order to evaluate differing techniques. For the sprint athlete in this case study, it was shown to be more effective to swim with a thunniform technique when at higher velocities and a more anguilliform at lower velocities.

Morphometry and burst swimming in six continental fish species from Costa Rica

Introduction: Current research of fish locomotion is focused on creating better underwater vehicles and how environmental stress factors modify swimming. Objective: To study the relation of morphometric characteristics with burst swimming in six representative species of continental fishes from Costa Rica. Methods: We measured total length, standard length, height and area of the tail of 38 individuals from six species. We used a Kruskall-Wallis test and a Boxplot graphic to compare species; and a PCA test to identify body variables that influence swimming. A Non-Metric Dimensional Scaling (NMDS) test was done for species and position in the water column. Results: The fastest swimming corresponded to A. nigrofasciata (9,29cm/s), while S. salvini (1,65cm/s) was the slowest. Burst swimming speed is influenced by body size and tail type, and differed with position in the water column, being surface species the fastest. Conclusions: Morphological and ecological characteristics determine differences in burst swim.

Theory of cyborg: a new approach to fish locomotion control

Cyborg in the brain-machine interface field has attracted more attention in recent years. To control a creature via a machine called cyborg method, three stages are considerable: stimulation of neurons, neural response, and the behavioral reaction of the subject. Our main concern was to know how electrical stimulation induces neural activity and leads to a behavioral response. Additionally, we were interested to explore which type of electrical stimulation is optimal from different aspects such as maximum response with minimum induction stimulus field, minimum damage of the tissue and the electrode, reduction of the noxiousness of stimuli or pain in the living creature. In this article, we proposed a new model for the induction of neural activity led to locomotion responses through an electrical stimulation. Furthermore, based on this model, we developed a new approach of electrical neural stimulation to provide a better locomotion control of living beings. This approach was verified through the empirical data of fish cyborg. We stimulated the fish brain by use of an ultra-high frequency signal which careered by a random low frequency. According to our model, we could control the locomotion of fish in a novel and innovative way. In this study, we categorized the different cyborg methods based on the nervous system areas and the stimulation signal properties to reach the better and optimal behavioral control of creature. According to this, we proposed a new stimulation method theoretically and confirmed it experimentally.